Riojavenatrix lacustris, Isasmendi & Cuesta & Díaz-Martínez & Company & Sáez-Benito & Viera & Torices & Pereda-Suberbiola, 2024

Riojavenatrix lacustris sp.nov.

lsid: urn:lsid:zoobank.org:act:F5E10E99-9BD8-4384-9FD1-36575E24B05D

Etymology: Latin for ‘related to a lake’, which declines from the word ‘ lacus ’ (lake).

Holotype: A partial skeleton of a single individual, including: CPI 1637, left femur; CPI 1638, left tibia and astragalus; CPI 1639A– B, left fibula; CPI 1640, left metatarsal III; CPI 1641A–B, right ischium; CPI 1642, right tibia; CPI 1643, right femur; CPI 1644, possible fragment of metatarsal II; CPI 1645, left phalanx III-1; CPI 1646, left phalanx III-3; CPI 1647, left phalanx I-2; CPI 1648, left phalanx IV-2 (or IV-3); CPI 1675A–B, left pubis; CPI 1676, left calcaneum; and CPI 1677, fragment of a dorsal neural arch.

Type locality and horizon: Virgen del Villar-1 site, La Rioja, Spain; the Enciso Group is uppermost Barremian–lower Aptian ( Suarez-Gonzalez et al. 2013, 2015), but the type locality is most likely to be lower Aptian within the Enciso Group.

Diagnosis: A medium- to large-sized spinosaurid theropod with the following unique combination of characters within Spinosauridae : (i)alateromediallythickandtriangularpubicbootindistalview,with a straight posterolateral margin (similar to Ichthyovenator , although in Ichthyovenator this margin is concave, but it is absent in Baryonyx , Suchomimus , and FSAC-KK 11888); (ii) an anteroposteriorly expanded ischial boot with an anterodorsally oriented tip and an angular anterodistal surface (absent in Ichthyovenator and FSAC-KK 11888; similar to Megalosaurus bucklandii ); (iii) a narrow, restricted and relatively deep articular groove on the proximal surface of the femur, which is anteromedially–posterolaterally inclined (distinct from those of Baryonyx and Suchomimus ); (iv) a medial femoral condyle with the long axis exhibiting only slight posteromedial orientation (distinct from those in Baryonyx , ‘ Spinosaurus B’ (Nr. 1922 X 45), Suchomimus , and FSAC-KK 11888); (v) presence of a vertical ridge on the medial margin of the ascending process of the astragalus (potential autapomophy, but it could also be a character in Spinosauridae because no other spinosaurid astragalus has been described to date); (vi) height of ascending process of the astragalus more than twice the height of the astragalar body (potential autapomophy, but it could also be a character in Spinosauridae because no other spinosaurid astragalus has been described to date); (vii) presence of an anterior depression with a dorsally located foramen on the lateral surface of the calcaneum (autapomorphy; absent in other spinosaurids); and (viii) absence of a longitudinal groove on the medial surface of phalanx I-2 (potential autapomophy, but it could also be a character in Spinosauridae or even in Megalosauroidea; given that this element is not preserved in other spinosaurids or megalosauroids, a synapomorphy cannot be excluded).

Description

Axial skeleton

Dorsal vertebra ( Fig. 3 View Figure 3 ): This piece consists of the mid- and posterior portions of the base of the neural arch, the bases of the transverse processes, and the neural spine of a mid- or posterior dorsal vertebra (CPI 1677). The neural arch does not preserve any of the zygapophyses. The thickness of the neural spine is constant across its anteroposterior length, but anteriorly, the interspinous ligament scar is present and slightly thickens the neural spine transversely ( Fig. 3A View Figure 3 ). The right spinodiapophyseal fossa is smooth, whereas the left one is more rugose. The transverse processes are horizontal in lateral view and slightly inclined dorsolaterally ( Fig. 3B View Figure 3 ).

Anteriorly, the spinoprezygapophyseal laminae would have bounded a deep and dorsoventrally elongate spinoprezygapophyseal fossa, which is filled with matrix. This was inferred considering the distance between both laminae, the posterior extension of the fossa, and the considerable missing anterior portion of the spine ( Fig. 3A View Figure 3 ). On the posterior side, another fossa is present, interpreted here as the spinopostzygapophyseal fossa ( Fig. 3C View Figure 3 ). This fossa is also dorsoventrally elongate and deep, delimited by the spinopostzygapophyseal laminae. Ventrally, these laminae merge to form the hyposphene ( Fig. 3C View Figure 3 ). Underneath the left transverse process, a fragment of the posterior centrodiapophyseal lamina is centrally positioned, extending ventrally to the centrum ( Fig.3B View Figure 3 ).Posterior to this lamina,the postzygocentrodiapophyseal fossa is present on the right side of the neural arch ( Fig. 3B View Figure 3 ). This fossa is also present on the left side, but it is less pronounced.

Appendicular skeleton

Pelvic girdle

Pubis ( Fig. 4A–J View Figure 4 ): Two fragments of the left pubis are preserved (CPI 1675A–B). The proximal fragment preserves the articular surface for the ischium, the surface of the acetabulum, and part of the iliac articulation ( Fig.4A–E View Figure 4 ). The iliac articulation is broad, strongly rugose, and somewhat convex in proximal view ( Fig. 4E View Figure 4 ). Adjacent to the medial margin, an anteroposteriorly elongate concavity is present on this articular surface.However,this area is not well preserved and might be an artefact of abrasion. The iliac peduncle is not complete anteriorly, but it broadens towards the acetabular surface ( Fig. 4E View Figure 4 ). The medial surface of the iliac peduncle is straight, becoming convex near the acetabular surface in proximal view. Posterior to the iliac articular facet, the acetabular surface is a small triangular concavity, which is located medially on the proximal surface and is posteromedially inclined ( Fig. 4E View Figure 4 ). The ischial peduncle projects posteriorlyandtapersposterodistallytowardstheischialarticulation ( Fig.4D View Figure 4 ). The ischial articulation is not completely preserved at the proximal end. It is triangular in posterior view and posteromedially directed ( Fig. 4D View Figure 4 ). The obturator notch is distally opened, relatively small anteroposteriorly, and subcircular ( Fig. 4B, C View Figure 4 ).

The distal fragment of the left pubis ( Fig. 4F–J View Figure 4 ) lacks part of the symphysis and the medial portion of the distal expansion ( Fig. 4J View Figure 4 ). The shaft of the distal fragment is straight and shows a teardrop-shaped cross-section at the level of the symphysis. The pubic apron projects as a blade from the shaft ( Fig. 4J View Figure 4 ). The symphysis is almost entirely missing, but the preserved distal part is medially projected. Furthermore, it is sigmoidal in medial view, with a large distal extension. Based on the breakage surface, its development, and distalmost extension of the preserved apron, the pubic apron would have almost reached or slightly extended along the pubic boot ( Fig. 4J View Figure 4 ). The pubis is slightly expanded anteriorly on its distal end, and it has a greater posterior projection ( Fig. 4G, I View Figure 4 ). In distal view, the pubic boot is mediolaterally broad, especially at its centre ( Fig. 4H View Figure 4 ). Its anterior and medial surfaces are convex, whereas the lateral surface is gently concave. The posterior process of the pubic boot tapers posteriorly.Indistalview,thepubicbootistriangularinshape ( Fig.4H View Figure 4 ).

Ischium ( Fig. 4K–T View Figure 4 ): The right ischium preserves its proximal end (including the iliac peduncle), part of the shaft and the ischial boot (CPI 1641A–B). Both lateral and medial surfaces of the iliac peduncle are convex ( Fig. 4Q View Figure 4 ). At the level of the surface of the acetabulum, the medial surface is flat and the lateral one is slightly concave, whereas near the pubic peduncle, the medial surface becomes concave and the lateral surface convex ( Fig. 4Q View Figure 4 ). In the posterior margin of the iliac peduncle, there is a dorsoventrally directed crest, forming a bulge near the dorsal margin. This bulge can be observed in medial and lateral views ( Fig. 4L–N View Figure 4 ). Anterior to this crest, the medial surface shows some proximodistally oriented furrows.

The iliac peduncle is mediolaterally expanded, with an oval contour in proximal view ( Fig. 4Q View Figure 4 ). Although it is abraded, the articular surface of the iliac peduncle is concave posteromedially. A protuberance is present at the centre of the concavity, bounded anteriorly and medially by a groove. This concavity is anterolaterally delimited by a transversely oriented edge ( Fig. 4Q View Figure 4 ). The surface of the acetabulum bears a pronounced concavity that becomes shallower anteriorly. In proximal view, the acetabular surface has a medial crest, partly broken. This crest extends anteriorly from the anteromedial edge of the iliac articulation ( Fig. 4Q View Figure 4 ).

The preserved cross-section of the ischial diaphysis is somewhat oval, with a rounded crest, posterolaterally directed, running longitudinally on the lateral surface of the shaft ( Fig. 4O, R, S View Figure 4 ). The posterior surface of the diaphysis is concave proximally and flattens towards the ischial boot, becoming a crest at the boot ( Fig. 4S View Figure 4 ). Both medial and anterior surfaces of the shaft are flat ( Fig. 4O, P View Figure 4 ). The ischial boot, triangular in medial or lateral view, is not strongly expanded anteroposteriorly and exhibits an angular anterodistal surface ( Fig. 4P, R View Figure 4 ). The posterior half of the lateral surface is convex, whereas the anterior half shows a concavity caused by the anterior process of the ischial boot ( Fig. 4R View Figure 4 ). This anterior process is connected to the diaphysis by a blade that becomes gradually less prominent proximally ( Fig. 4R View Figure 4 ). The blade is laterally inclined distally and becomes vertical proximally. The medial surface of the ischial boot is flat and shows some longitudinal striations that run proximally to the shaft ( Fig. 4P View Figure 4 ). In distal view, the ischial boot shows a triangular outline owing to the tapering of the anterior process ( Fig. 4T View Figure 4 ).

Hindlimb

Femur ( Fig. 5 View Figure 5 ): The right femur preserves only its proximal end (CPI 1643), and the left femur consists of most of the diaphysis and the distal articular end (CPI 1637). The femoral head is gently convex and subcircular in medial view ( Fig. 5D View Figure 5 ). Its proximodistal axis is slightly longer than the anteroposterior one. The femoral head is medially oriented and anteromedially directed at an angle of ~20° ( Fig. 5J View Figure 5 ). The proximal surface of the femur is posterolaterally inclined. There is an anteroproximally inclined groove (i.e. the oblique ligament groove) on the posterior surface ( Fig. 5C View Figure 5 ). This groove separates the femoral head from the shaft; therefore, the latter is well offset medially. Medial to this groove, the posterior lip of the femoral head is well developed and extends slightly beyond the posterior surface of the head ( Fig. 5C, J View Figure 5 ). The femoral head has a concave ventral surface. This surface becomes broader anteriorly, making the femoral head overhang the medial surface of the femoral diaphysis ( Fig. 5A, C View Figure 5 ). On the anterior side of the femoral head, there is an oblique ridge that extends from the femoral head to the proximal end of the diaphysis ( Fig. 5A View Figure 5 ). In proximal view, a deep, broad, and oblique articular groove can be noticed. This is more pronounced anteromedially and becomes shallower and narrower posterolaterally. In the same view, the femoral head is laterally confluent with the greater trochanter, which is incomplete and narrower than the head ( Fig. 5J View Figure 5 ).

The left femur lacks the proximal end and the diaphysis proximal to the fourth trochanter. The preserved length of the left femur measures ~ 515 mm. The shaft of the femur is rather straight in anterior and posterior views, but it is anteriorly bowed in lateral and medial views ( Fig. 5E–H View Figure 5 ). The diaphysis is oval in cross-section, the mediolateral axis being larger than the anteroposterior one, with a maximum circumference of 322 mm. The anterior surface of the shaft is convex and slightly pinched in the middle ( Fig. 5E View Figure 5 ). The pinch extends 190 mm distal from the proximal fracture. Medial and lateral to this pinch, the surface becomes flat. The lateral surface is convex and becomes flat next to the distal expansion of the bone ( Fig. 5F View Figure 5 ). The medial surface is also convex but leads to a shallow and large concavity, which is located posterior to the medial epicondyle and extends until half of the medial condyle ( Fig. 5H View Figure 5 ). The posterior surface of the femoral shaft is flat and faces posterolaterally ( Fig. 5G View Figure 5 ).

The fourth trochanter is located posteromedially in the proximal part of the preserved shaft. Only its most distal end is preserved, consisting of a well-developed and prominent longitudinal crest that becomes broader proximally ( Fig. 5G View Figure 5 ). Medially, there is a shallow and smooth groove; and laterally, there is a shallow, smooth, and broad concavity, which is excavated in the posterior surface of the diaphysis in lateral view.

The distal end of the femur is slightly more medially than laterally expanded in posterior view ( Fig. 5G View Figure 5 ). The medial epicondyle or medial distal crest is found on the mediodistal surface. This medial epicondyle is rounded, low, and not well developed ( Fig. 5E View Figure 5 ). On the anterior surface of the medial epicondyle, the attachment for the muscle femorotibialis externus is a small rugose patch. The extensor groove is broad and slightly V-shaped in distal view ( Fig. 5I View Figure 5 ) and becomes even shallower and broader proximally ( Fig. 5E View Figure 5 ). The flexor groove is rather broad, deep, and U-shaped in distal view ( Fig. 5I View Figure 5 ). The flexor groove is bounded by two ridges medially and laterally, proximal to the condyles ( Fig. 5G View Figure 5 ). The medial ridge runs vertically from the proximal margin of the medial condyle, whereas the lateral ridge is more prominent and oblique, originating at the proximal end of the tibiofibular crest. The medial crest merges with the shaft proximally.

The anterior surface of the lateral condyle is rounded. The medial condyle is slightly more flattened and anterolaterally oriented in the anterior surface ( Fig. 5E View Figure 5 ). At the distal end, the condyles are robust, being wide mediolaterally. Both medial and lateral condyles project distally to an almost equal extent in anterior view ( Fig. 5E View Figure 5 ). In the same view, the lateral condyle projects distally and slightly laterally, whereas the medial condyle is solely distally projected. The medial condyle is teardrop-shaped in posterior view ( Fig. 5G View Figure 5 ). In distal view, the lateral condyle is rounded and the medial one is more elliptical with the long axis slightly posteromedially oriented ( Fig. 5I View Figure 5 ). The distal condyles are not separated by an anteroposterior pronounced groove in the distal surface, but there is a central shallow groove that connects to the tibiofibular crest ( Fig. 5I View Figure 5 ). The tibiofibular crest is broad and positioned on the posterior surface of the lateral condyle ( Fig. 5G, I View Figure 5 ). In posterior view, the crista tibiofibularis is teardrop-shaped and oblique with respect to the long axis of the lateral condyle ( Fig. 5G View Figure 5 ). The tibiofibular crest is somewhat trapezoidal in distal view. This is laterally bounded by a broader groove, and medially, a narrow and deep groove separates the crest from the distal margin of the condyle or the flexor groove. Between both grooves, there is a posterolaterally elevated ridge in distal view ( Fig. 5I View Figure 5 ).

Tibia ( Figs 6A–J View Figure 6 , 7A–E View Figure 7 ): The right tibia (CPI 1642) preserves only its proximal portion ( Fig. 6A–D, I View Figure 6 ). The left tibia (CPI 1638) is complete, including the articulated astragalus ( Figs 6E–J View Figure 6 , 7A–E View Figure 7 ). The right tibia lacks the cnemial crest and the medial condyle. However, the lateral condyle and the proximal end of the fibular crest are well preserved.

The left tibia shows many furrows on the medial surface close to the proximal end. Furthermore, these are also present on the anterior margin of the medial malleolus. The shaft of the left tibia is straight and fairly robust, with a length of ~ 810 mm ( Fig. 6E–H View Figure 6 ). The cross-section of the tibial diaphysis is oval, where the mediolateral axis is the largest. At mid-shaft, the diaphysis narrows laterally and becomes wider medially. In lateral view, the anterior surface of the tibia is concave proximally, becoming convex distally. The posterior surface is rather straight close to the proximal end and concave distally. The anterior surface is flat and the posterior surface gently convex along the whole shaft. The lateral surface is convex, whilst the medial margin is flat proximally and becomes concave at mid-shaft.

The cnemial crest of the left tibia is rounded ( Fig. 6E, F View Figure 6 ). Dorsally, it expands mediolaterally and flexes laterally in anterior view. The cnemial crest projects from the anterior surface of the diaphysis beyond the proximal articular surface of the tibia ( Fig. 6E, F View Figure 6 ). The lateral surface of the cnemial crest bears a longitudinal tuberosity near its anteroposterior corner ( Fig. 6F, J View Figure 6 ). In proximal view, the lateral condyles are offset from the cnemial crest by a poorly developed incisura tibialis ( Fig. 6I, J View Figure 6 ). The lateral condyles are large, rounded, and broad. The preserved portions of both medial condyles suggest that they were also large. There is no process extending anterolaterally from the lateral condyle in either tibia, but there is a notch present between the medial and lateral condyles in the right tibia ( Fig. 6I View Figure 6 ). In proximal view, the posterior outline of the left tibia is nearly straight owing to the lack of the medial condyle, whereas the medial surfaces are smoothly convex in both tibiae.

The fibular flange is a low longitudinal crest with a slightly broadened base ( Fig. 6E–G View Figure 6 ). It is situated proximally on the lateral surface of the diaphysis, but it is distally located to the proximal end of the tibia. The outline of the base of the fibular crest is oval, but narrow, in lateral view ( Fig. 6F View Figure 6 ). This is separated from the proximally located lateral process associated with the fibular crest. In the right tibia, the proximal end of the lateral process associated with the fibular crest also reaches the proximal end of the tibia ( Fig. 6B, E–G View Figure 6 ). These processes are rounded and proximodistally oriented ( Fig. 6B, F View Figure 6 ).

The distal end of the tibia is expanded mediolaterally, being anteroposteriorly narrower. Both the lateral and medial malleoli extend somewhat equally distally, with the lateral malleolus being larger and slightly more distally projected than the medial one ( Figs 6E, G View Figure 6 , 7A, C View Figure 7 ). This makes the distal surface almost horizontal, being ~15° with respect to the horizontal or ~75° with respect to the long axis of the tibial shaft. The lateral malleolus is rounded and the medial one more angled in posterior view ( Figs 6G View Figure 6 , 7C View Figure 7 ). In medial view, the medial malleolus shows a flat surface bounded posteriorly by a longitudinal ridge, which is mediodistally inclined ( Figs 6G, H View Figure 6 , 7C, D View Figure 7 ). The medial malleolus is transversely wider and more robust than the lateral one. The facet for the fibula shows longitudinal furrows and occupies a large portion of the anterior surface of the lateral malleolus ( Figs 6E, F View Figure 6 , 7A, B View Figure 7 ). The astragalar facet of the tibia is high (~ 150 mm), broad, and triangular and occupies more than half of the distal end of the anterior surface of the tibia ( Figs 6E View Figure 6 , 7A View Figure 7 ). This facet is delimited proximomedially by the supraastragalar buttress. The latter is developed as a well-developed and bluntly rounded ridge. It originates mediodistally and it is almost vertical basally, becoming a low oblique ridge proximally. The supraastragalar buttress is more pronounced distally and gradually fades proximolaterally ( Figs 6E View Figure 6 , 7A View Figure 7 ).

Fibula ( Fig. 6K–T View Figure 6 ): The left fibula (CPI 1639A–B) is almost complete but lacks part of the distal diaphysis. The fibula is a slender bone with a rather thin shaft and with an estimated length of 750 mm. The proximal end is strongly expanded anteroposteriorly and slightly expanded mediolaterally. Furthermore, the fibula is deformed, displaying an S-like shape at ~ 340 mm from its proximal end. Below the S-like deformation, the shaft of the fibula is somewhat twisted, a feature that might be associated with the deformation ( Fig. 6K–N View Figure 6 ).

The proximal end of the fibula is more posteriorly than anteriorly expanded ( Fig. 6L, N View Figure 6 ). The posterior margin of the proximal fibula becomes thinner posteriorly to form a thin, blade-shaped margin ( Fig. 6O View Figure 6 ). This margin is more rounded than the anterior one in medial view, which is more acute in lateral view ( Fig. 6L, N View Figure 6 ). The proximal margin is slightly concave at the level of the medial fossa and becomes convex near the anterior and posterior margins in lateral view ( Fig. 6L, N View Figure 6 ). In proximal view, the proximal margin of the fibula is comma-shaped, with the anterior margin being wider than the posterior one ( Fig. 6O View Figure 6 ).

The medial fossa reaches the proximal margin and occupies half the expansion of the proximal end of the tibia. It is shallow, inverted teardrop-shaped, and centrally positioned in medial view ( Fig. 6N View Figure 6 ). Anteriorly, the medial fossa is bounded by an elevated edge, which creates the anterior margin of the proximal end and broadens proximally. The posterior margin of the medial fossa is posteriorly opened ( Fig. 6N View Figure 6 ). Below the medial fossa, the medial surface is planar. In the shaft, a posteromedially located low ridge runs ventrally and becomes oblique below the iliofibularis tubercle, bounding an anterior groove on the medial surface. At the level of this tubercle, a shallow depression can be noticed on the medial surface of the shaft ( Fig. 6N View Figure 6 ). Below the oblique ridge, a proximally well-developed groove is present, which extends parallel to the diaphysis distally. The posterior edge of this groove consists of a medially raised edge. Distally, the groove fades at the distal end of the preserved shaft when the diaphysis expands ( Fig. 6N View Figure 6 ).

At the proximal expansion of the fibula, the lateral surface is convex anteriorly, becoming almost flat in the middle of the lateral side and again slightly convex posteriorly ( Fig. 6L View Figure 6 ). Some longitudinal furrows are present on the anterolateral surface of the fibula. Posteriorly, the proximolateral surface of the fibula has a longitudinal and vertically oriented depression. Another broader, but slightly shallower depression is present on the anterolateral surface, at the same level as the posterior one ( Fig. 6L View Figure 6 ). The fibular diaphysis has a strongly convex lateral surface, but it is more planar at the level of the iliofibularis tubercle.

The insertion for the iliofibularis muscle is located on the anterior margin of the fibular shaft, approximately at one-third from the proximal surface ( Fig. 6K, L, N View Figure 6 ). It is low and rugose in anterior view and is formed by two protuberances, which are bounded laterally by a longitudinal, shallow, and rugose groove ( Fig. 6K, L, N View Figure 6 ).

The distal end is expanded anteroposteriorly and transversely. At the beginning of the distal expansion, both anterior and posterior edges are sharp. Distally, the anterior part is rounded ( Fig. 6P View Figure 6 ), whereas the posterior margin is sharp edged and slightly inclined laterally ( Fig. 6R View Figure 6 ). The medial surface for the articulation with the tibia is flat proximally ( Fig. 6S View Figure 6 ). However, in distal view, the anterior portion of the medial surface appears convex, becoming concave posteriorly ( Fig. 6T View Figure 6 ). On the lateral surface of the distal expansion, an oblique ridge starts anteroproximally and runs posterodistally down to the ventral end ( Fig. 6Q View Figure 6 ). Posteroproximally, overlying this ridge, there is a shallow and broad depression. The distal outline of the fibula is teardrop-shaped. This surface shows a depression anteromedially and becomes convex posteriorly ( Fig. 6T View Figure 6 ).

Astragalus ( Figs 6E–H View Figure 6 , 7A–E View Figure 7 ): The left astragalus is almost complete (CPI 1638), lacking only part of the anterodistal margin of the medial condyle of the astragalar body. The astragalus is L-shaped in lateral view, with a relatively tall ascending process ( Figs 6E, F View Figure 6 , 7A, B View Figure 7 ). The astragalus measures 155 mm in height and 122 mm mediolaterally. In anterior view, the astragalar body shows an hourglass shape, with concave distal and proximal surfaces ( Figs 6E View Figure 6 , 7A View Figure 7 ). However, the distal concavity is broader and less pronounced than the dorsal one. In distal view, the astragalar body is rectangular ( Fig. 7E View Figure 7 ). The medial condyle of the astragalus is expanded anteroposteriorly. There is a shallow distal groove, becoming broader and slightly deeper laterally. The groove is bounded anteriorly by a parallel low crest that is slightly more prominent laterally and almost reaches the astragalus–calcaneum contact ( Fig. 7E View Figure 7 ). In anterior view, there is another groove, horizontal and shallow, developed between the condyles, at the mid-height of the astragalar body ( Fig. 7A View Figure 7 ). This groove has its maximum depth medially, gradually gets shallower laterally, and extends until the lateral rim. There is a bulge on the groove. Between the astragalar body and the ascending process, there is a depression; hence, the ascending process is offset from the astragalar body ( Fig. 7B View Figure 7 ). This depression is broader and more marked laterally, showing a triangular shape in proximal view. The facet for the fibula is not preserved. The medial side of the posterior margin of the astragalar body is more proximally projected than the lateral side ( Fig. 7C View Figure 7 ). This posterior margin is gently convex over most of its extension, but laterally it becomes shorter and slightly concave. The astragalar body is smooth on most of its anterior surface. Nonetheless, the area that articulates with the calcaneum is rugose.

The medial condyle is more anteriorly projected than the lateral one in ventral view; hence, the astragalus is narrower laterally ( Fig. 7E View Figure 7 ). Besides the anteroposterior expansion of the astragalar body, the distal condyles are oriented anterodistally. The distal condyles of the astragalus are rounded anteroposteriorly ( Fig. 7E View Figure 7 ).

The ascending process of the astragalus is laminar, triangular, and proximolaterally oriented ( Fig. 7A View Figure 7 ). The height of the ascending process is twice the height of the astragalar body, with the former measuring ~ 103 mm and the latter 52 mm. The ascending process is located slightly laterally, and it does not reach the lateral margin of the astragalar body ( Fig. 7A View Figure 7 ). It arises at around one-fifth from the medial margin of the astragalar body. The process is transversely broad, with its base extending mediolaterally ~65% of the transverse width of the astragalar body. The medial margin of the ascending process shows a well-defined ridge that is perpendicular to the mediolateral axis of the astragalar body and arises from the anterodorsal surface of the astragalar body ( Fig. 7A View Figure 7 ). This ridge is parallel to the ventral part of the astragalar buttress of the tibia. The ridge fades where the ascending process tilts laterally at an angle of 40–45° with respect to the long axis of the tibia. Proximally, this margin gradually becomes oriented more transversely. The lateral margin of the ascending process is rather straight and vertical ( Fig. 7A View Figure 7 ). The proximal end of the ascending process is placed at three-quarters of the transverse width of the astragalar body from the medial side. The ascending process does not contact the supraastragalar buttress ( Fig. 7A View Figure 7 ).

Calcaneum ( Fig. 7F–K View Figure 7 ): The left calcaneum (CPI 1676) is complete, with a height of 63 mm. Its anteroposterior width measures 72 mm, whereas its mediolateral width is 29 mm. The contact of the calcaneum with the astragalus is slightly sinuous, and the tibia overlaps the calcaneum. The calcaneum is reniform anteriorly and laterally ( Fig. 7H, I View Figure 7 ). In lateral view, the anteroproximal margin is more proximally elevated than the posteroproximal margin ( Fig. 7I View Figure 7 ). The proximal rim is concave for the articulation with the fibula, and the proximal articular surface is proximally directed. The articulation for the distal end of the fibula is well developed ( Fig. 7F View Figure 7 ). The distal profile is strongly convex in lateral view ( Fig. 7I View Figure 7 ). In proximal view, the calcaneum is anteriorly broad, narrowing posteriorly, with a sharp edge that is oriented posterolaterally ( Fig. 7F View Figure 7 ). This edge broadens distally and has a crescent shape in order to accommodate the lateral malleolus of the tibia ( Fig. 7J View Figure 7 ). The lateral surface has a broad vertical groove and another anteriorly located depression. The latter is as deep as the groove, triangular, and bears a foramen at its proximalmost point ( Fig. 7I View Figure 7 ). Between the groove and the depression, a triangular convexity is present. The medial surface of the calcaneum shows a concave facet for the lateral malleolus and an anteriorly located groove to articulate the lateral condyle of the astragalus. The groove is posteriorly inclined and parallel to the anterior margin of the calcaneum ( Fig. 7K View Figure 7 ).

Metatarsal III ( Fig. 8A–F View Figure 8 ): The left metatarsal III is complete (CPI 1640). The diaphysis is straight, measuring 364 mm proximodistally, and with a subrectangular cross-section at mid-shaft ( Fig. 8C–E View Figure 8 ). Its borders are rounded, except for the medial surface, which is planar. Its proximal end is anterodorsally and posteroventrally expanded, especially posteroventrally projected in lateral view ( Fig. 8C, E View Figure 8 ).

In proximal view, metatarsal III has an hourglass-shaped outline, the anterodorsal part of which is more lateromedially expanded than the posteroventral one ( Fig. 8F View Figure 8 ). The articular surface for metatarsal II is proximally located in the medial surface of metatarsal III ( Fig. 8E View Figure 8 ). It is concave, densely striated, and reaches the middle of the diaphysis. The articular surface for metatarsal IV is posteroventrally concave and convex anterodorsally close to the proximal end of metatarsal III ( Fig. 8C View Figure 8 ). This surface is also striated, as in the articulation for metatarsal II. The medial articular surface is bounded anterodorsally and posteroventrally by two elevated edges that fade distally. The posteroventral edge is more pronounced than the anterodorsal one. On the anterodorsal surface, there is a marked scar ( Fig. 8B View Figure 8 ). Another scar is located on the posteroventral surface, below the proximal expansion ( Fig. 8D View Figure 8 ).

The distal condyle is divided posteroventrally by a shallow groove ( Fig. 8A View Figure 8 ). On the anterodorsal surface, there is no marked hyperextensor pit; however, a subtle depression is present adjacent to the condyle ( Fig. 8A, B View Figure 8 ). The distal condyle is subrectangular in distal view, with the medial side being anterodorsally and posterooventrally larger than the lateral side ( Fig. 8A View Figure 8 ). The medial collateral ligament pit is much deeper and larger than the lateral one. Both of them are suboval ( Fig. 8C, E View Figure 8 ).

Phalanx I-2 ( Fig. 8G–K View Figure 8 ): CPI 1647 is interpreted as the ungual phalanx of digit I based on the small size of the element (it measures 63 mm proximodistally) and its significant curvature. This (left) phalanx is complete. It is long, narrow, distally pointed, and strongly arched in lateral and medial views ( Fig. 8H, J View Figure 8 ). The phalanx I-2 is triangular in cross-section but has an oval articular surface in proximal view ( Fig. 8K View Figure 8 ). The articular surface for the articulation of phalanx I-1 is concave. This surface is slightly pointed dorsally in proximal view, and the proximodistal lip is not well developed. Ventrally, this surface is rounded. The flexor tuberosity, placed on the ventral side of the articular surface, is not very pronounced, but it is more developed than the lip ( Fig. 8K View Figure 8 ). Both medial and lateral surfaces are convex proximally, but the lateral surface is slightly flatter ( Fig. 8H, J View Figure 8 ). The dorsal surface of phalanx I-2 is convex, and the ventral side is comparably flatter ( Fig. 8G, I View Figure 8 ). There are no ventral or flexor fossae. Both medial and lateral margins are softly convex; hence, this phalanx is symmetrical. Only the lateral surface bears a longitudinal groove. This is well developed and faces at about two-thirds proximally, becoming progressively shallower ( Fig. 8H View Figure 8 ).

Phalanx III-1 ( Fig. 8L–Q View Figure 8 ): CPI 1645 is interpreted as a proximal phalanx because it lacks a proximal keel. Furthermore, it can be assigned confidently to digit III based on the rather symmetrical distal condyles and not as asymmetrical proximal articular surface. The left phalanx III-1 is virtually complete, measuring 118 mm in length, with the flexor tubercle partly eroded and lacking the ventral part of the medial condyle. The height and width of the phalanx change proximodistally, being higher proximally. Between both articular surfaces, the neck constricts the phalanx, especially ventrally, such that the phalanx is wider than tall at mid-shaft ( Fig. 8N, Q View Figure 8 ).

The dorsal margin is smooth and gently convex, becoming flat and inclined distally. Above the proximal margin of the collateral ligament pits is the extensor fossa ( Fig. 8M View Figure 8 ). It is deep and oval, with the longest axis directed transversely. The ventral surface is also slightly mediolaterally convex, but proximodistally concave in lateral and medial views ( Fig. 8M, P, Q View Figure 8 ). Proximally, there is a proximolaterally oriented oblique groove that separates the flexor tubercle into two processes. The proximal articular surface is devoid of any keel separating the articular facets. This surface is weakly concave and subcircular, with a pronounced, medially inclined proximodorsal lip. This surface is partly deformed ( Fig. 8O View Figure 8 ).

Close to the proximal end, there is a proximoventral fossa on both medial and lateral sides ( Fig. 8N, Q View Figure 8 ). These are rugose and roughly triangular. Distally, the lateral collateral ligament pit is subcircular and deeper than the medial one. The medial collateral ligament pit extends more proximally and is oval ( Fig. 8N, Q View Figure 8 ).

Both condyles are equally developed ( Fig. 8L View Figure 8 ). The long axis of the lateral condyle is inclined slightly laterally and the medial condyle is somewhat inclined medially. The lateral condyle is slightly more dorsally and ventrally projected, whereas the medial condyle is slightly more distally projected ( Fig. 8M, P View Figure 8 ). The condyles are separated by a shallow intercondylar sulcus. In distal view, the condyles are mediolaterally expanded on the ventral surface. In ventral view, the lateral condyle ends more abruptly proximally or, at least, it is more pronounced than the medial one ( Fig. 8P View Figure 8 ).

Phalanx III-3 ( Fig. 8R–W View Figure 8 ): CPI 1646 can be attributed confidently to digit III because of the symmetry of the distal condyles and the proximal articular surface. The specimen is not a proximal phalanx because it presents a keel on the proximal surface and is here identified as phalanx III-3 based on the position of the collateral ligament pits, which are dorsally displaced, and the lack of a dorsal extensor fossa. The left phalanx III-3 is complete and almost symmetrical axially. Compared with phalanx III-1, this is smaller, measuring 67 mm in length, but stouter and proportionally broader transversely. It is taller and wider proximally and distally. The neck of phalanx III-3 especially constricts the pedal element.

The dorsal surface is convex, but concave in lateral or medial view ( Fig. 8T, W View Figure 8 ). There is no extensor fossa on the dorsal surface of this phalanx ( Fig. 8S View Figure 8 ). The ventral surface is flat, but it is also concave in medial or lateral view ( Fig. 8T, W View Figure 8 ). Two rounded and considerably broad proximoventral crests are present, and they are bounded medially and laterally by a rugose area ( Fig. 8T, V, W View Figure 8 ). Proximally and centrally positioned on the ventral surface, there is a process. The proximal end of the process is missing, but the preserved part is broad, rounded, and proximodistally oriented. The proximal articular surface is triangular in outline in proximal view, with a well-developed and centred proximodorsal lip ( Fig. 8U View Figure 8 ). A vertical median keel separates the lateral and medial articular facets. Both articular facets are equally developed and subsymmetrical.

In medial and lateral views, the proximoventrally located fossae are less developed in phalanx III-3 than in phalanx III-1 ( Fig. 8T, W View Figure 8 ). On the medial side, the fossa is restricted to a triangular rugose patch. On the lateral margin, this fossa is oval and very small. Both collateral ligament pits are more dorsally placed, in comparison to phalanx III-1. These pits are deep, subcircular to oval, and almost equally developed.

The distal condyles are not symmetrical. The medial condyle is more dorsoventrally developed compared with the lateral one ( Fig. 8R View Figure 8 ). The long axis of the lateral condyle is laterally oriented, and it is medially directed in the medial condyle. In ventral view, the lateral condyle ends more abruptly proximally ( Fig. 8V View Figure 8 ).

Phalanx IV-2 (or IV-3) ( Fig. 8X View Figure 8 –AB): There is a proximal fragment of a left pedal phalanx IV-2 or IV-3 (CPI 1648). This identification is based on high asymmetry of the proximal articular surface, which indicates that it would be from the digit IV. Furthermore, the presence of a keel on the proximal articular surface and the relatively large size of the element suggest that it would not be the distalmost non-ungual phalanx of digit IV. This fragment broadens and increases in height proximally. The neck of the phalanx would also contract the bone ( Fig. 8X, Y View Figure 8 , AA, AB). At this level, it is triangular in cross-section with rounded margins.

The dorsal surface of this phalanx is convex, but it is concave in lateral or medial view ( Fig. 8Y View Figure 8 , AB). The preserved ventral surface shows a smooth fossa, which is bounded by two proximoventral ridges ( Fig. 8 View Figure 8 AA). The lateral ridge is broader than the medial one. Close to the proximal rim, there is a rugose surface. At the proximal rim, there is a centred proximal process. In proximal view, the proximal articular surface is triangular and bears a median keel that separates both articular surfaces for the condyles ( Fig. 8Z View Figure 8 ). The proximodorsal lip is highly pronounced and medially directed. The lateral articular facet is smaller but deeper than the medial one. The lateral articular facet is triangular (almost right-angled triangular) in proximal view, and the medial one is oval. Dorsal to the proximoventral ridges, there are no proximoventral fossae. In this area, a triangular rugose surface is present on both sides ( Fig. 8Y View Figure 8 , AB).

PHYLOGENETIC RESULTS

Matrix from Rauhut and Pol (2021)

The phylogenetic analysis resulted in 10 000 most parsimonious trees, with a length of 5386 steps (CI = 0.180; RI = 0.614). The strict consensus (Supporting Information, Supplementary Material S2, Fig. S1 View Figure 1 ) shows a rather well-resolved topology of general non-tetanuran relationships, including the monophyletic Ceratosauria. Although Tetanurae is also solved as monophyletic, the non-Coelurosauria tetanuran clades are poorly solved, with more completely known taxa, such as Allosaurus Marsh, 1877 , Asfaltovenator Rauhut & Pol, 2019 , Concavenator Ortega et al., 2010 , or Spinosaurus , placed within a large and complex polytomy with several taxa and clades of different degrees of instability within Tetanurae. Piatnitzkysauridae , megalosaurines ( Megalosaurus Buckland, 1824 , Torvosaurus gurneyi Hendrickx & Mateus, 2014 , Torvosaurus tanneri Galton and Jensen, 1979 , and Wiehenvenator Rauhut et al., 2016 ); some megaraptorans; metriacanthosaurids; some carcharodontosaurids; tyrannosaurids, and Coelurosauria are found as monophyletic groups within this large polytomy in Tetanurae. However, Riojavenatrix is located in this high-grade polytomy, branching early with the majority of other members of Carnosauria ( Allosauridae + Megalosauridae clade, currently redefined by Rauhut and Pol 2021) and with all spinosaurids.

Therefore, we focus on the outcomes of the iterative PCR and other methodologies of pruning in order to analyse the unstable taxa that affect this complex polytomy and to determine whether Riojavenatrix is one of these unstable taxa, to evaluate the causes of this uncertainty, and to use this information to find possible solutions.

The reduced strict consensus from iterPCR ( Fig. 9 View Figure 9 ) was finished after a total of seven iterations. A total of 27 terminal taxa and one clade (megaraptorans) were pruned as unstable taxa, where Riojavenatrix was included (pruned in iteration 4). In iteration 3, several clades within the tetanuran polytomy were established as monophyletic, such as Megalosauroidea. Riojavenatrix was branched in a polytomy with other spinosaurids within Megalosauroidea. In the final reduced consensus, for the interrelationships of spinosaurids, Camarillasaurus , Irritator Martill et al., 1996 , Riojavenatrix , Sigilmassasaurus , and Vallibonavenatrix ( Fig. 9 View Figure 9 ) were found as unstable taxa in the MPTs owing to the fragmentary nature of the majority of members of Spinosauridae , with many characters coded as missing data. With these taxa removed, Spinosauridae are split into two groups, Baryonychinae ( Baryonyx + Suchomimus ) and Spinosaurinae ( Ichthyovenator + Spinosaurus ). In order to evaluate the position of Riojavenatrix , a reduced consensus with manual pruning (command punnelsen) of the unstable spinosaurids, except Riojavenatrix , was carried out. The results (in Supporting Information, Supplementary Material S2, Fig. S2 View Figure 2 and S 3 View Figure 3 ) show that the polytomy is still a high-degree one and that spinosaurids still remain located within this polytomy. The alternative positions of the pruned taxa ( Camarillasaurus , Irritator , Sigilmassasaurus , and Vallibonavenatrix ) are as follows: (i) all of them as early branching tetanurans; (ii) all of them in a dichotomy with Riojavenatrix ; (iii) Irritator related to Suchomimus in a dichotomy; (iv) Irritator located in a dichotomy with Baryonyx ; and (v) Vallibonavenatrix related to Leshansaurus Li et al., 2009 in a dichotomy.

Using prunnelsen, to improve the establishment of clades for the node of Tetanurae (node 226 in TNT, considered as a polytomy of degree 38) and calculating prunings until three taxa, we can see that the most common spinosaurids pruned are Irritator , Riojavenatrix , and Vallibonavenatrix . These are the same results as in the iterPCR methodology, where these taxa were also considered as unstable taxa. The manual pruning of these taxa shows a reduced strict consensus, with a monophyly of Baryonyx and Suchomimus , where these three pruned taxa have several alternative positions. Riojavenatrix could be located as an early branching baryonychine or in a dichotomy with Suchomimus within Baryonychinae (Supporting Information, Supplementary Material S2, Fig. S2 View Figure 2 and S 3 View Figure 3 ). The agreement subtree was not obtained by TNT owing to the numerous alternative results when dichotomies are forced.

The alternative position of the unstable taxa in spinosaurids ( Irritator , Riojavenatrix , and Vallibonavenatrix ), based on the results in the pruning methodology of TNT, were forced in several constraints (see Supporting Information, Supplementary Material S2). The constraint obtained by forcing Riojavenatrix as a baryonychine is rather well supported, requiring only three additional steps (length of 5389, 200 trees retained). When forcing the position of Riojavenatrix within Baryonychinae , the high-degree polytomy is mostly resolved, and this recovered most ‘traditional’ monophyletic groups within Tetanurae. One of the groups recovered as monophyletic is Spinosauridae , whose monophyly is also recovered in the reduced consensus tree from the iterPCR ( Fig. 9 View Figure 9 ). However, apart from the forced monophyly of baryonychines with Riojavenatrix , other monophyletic groups within Spinosauridae are not established. In this result, the sister taxa of Spinosauridae are a dichotomic group of Lourinhanosaurus Mateus, 1998 and Monolophosaurus Zhao & Currie, 1993 . When forcing Riojavenatrix and Suchomimus together, the support has a similar result, also requiring only three additional steps (length of 5389, 200 trees retained). However, the position of this forced group has numerous alternative positions within Spinosauridae , probably for the splitting up of Baryonychinae .

The Bremer support analysis (all taxa included) shows many clades with minimal support. Tetanurae, including the high-degree polytomy, has a Bremer support of one. However, monophyletic groups, such as Megalosauridae , have high values of Bremer support (three and five). The results of support for jackknife and bootstrap analyses show that most of the groups have values <50%. Spinosaurids are all grouped together here, but with a low jackknife support value of two. In bootstrap, Baryonyx , Irritator , and Suchomimus are grouped together, with a support value of four, but the other spinosaurids are branched in the polytomy of tetanurans, which has a bootstrap value of 27 (see Supporting Information, Supplementary Material S2).

As was found out by the iterPCR and other methods of pruning, there are numerous unstable taxa in the matrix. This taxon instability could be one of the reasons for these low measures of support. Therefore, the position of all the unstable spinosaurids pruned in the reduced consensus by the iterPCR (see above) was ignored during another round of the jackknife and bootstrap support analyses. The results reveal that Spinosauridae improved their support values (jackknife values: 20 for Spinosauridae , 70 for Baryonychinae , and 37 for Spinosaurinae ; bootstrap values: 8 for Spinosauridae , 52 for Baryonychinae , and 23 for Spinosaurinae ; see Supporting Information, Supplementary Material S2). The appendicular skeleton has rarely been described in spinosaurids so far. The placement of Riojavenatrix within Spinosauridae in the phylogenetic results allows us to discuss those appendicular characters that might diagnose Spinosauridae and support the placement of Riojavenatrix within the clade. For that, appendicular characters have been mapped on the reduced consensus trees using MESqUITE and in the strict consensus in TNT.

Riojavenatrix shares the following characters with megalosauroids within Carnosauria: faint scar of the muscle femorotibialis externus [character (ch.) 692; shared with Cryolophosaurus and some early branching theropods]; weak medial epicondyle of the femur (ch. 694; plesiomorphic condition present in Herrerasaurus Reig, 1963 , Dilophosaurus wetherilli Welles, 1970 , some coelophysoids, and some coelurosaurs); metatarsals <50% of tibial length (ch. 746; present in Afrovenator Sereno et al., 1994 , Eustreptospondylus Walker, 1964 , Riojavenatrix , and Spinosaurus , and some other theropods outside megalosauroids).

After mapping appendicular characters of Riojavenatrix on the reduced consensus trees and the previous anatomical comparison, the following unique combination of characters supports its placement as spinosaurid: the pubic apron extended from the middle of the pubic shaft (ch. 646; present also in Ichthyovenator ; but absent in Spinosaurus and unknown in Baryonyx and Suchomimus ; also present in megalosaurids and Allosaurus ); deep oblique ligament groove on posterior surface of femoral groove (ch. 678; shared with most theropods, but absent in Afrovenator , Megalosaurus , and Torvosaurus tanneri ); femoral head strictly directed medially in the anteroposterior plane (ch. 680; potential synapomorphy of Spinosauridae within megalosauroids, although also present in Afrovenator ; outside megalosauroids, it is also present in allosauroids and Coelurosauria).

Synapomorphies in Baryonychinae that also are present in Riojavenatrix are as follows: posteromedially oriented long axis of medial condyle of femur in distal view (ch. 693; absent in Spinosaurus and other theropods, but unknown in Ichthyovenator ; also recovered in other taxa outside Spinosauridae ); incomplete ossification in the fibular crest (ch. 710; present in Suchomimus , but unknown in Baryonyx ; absent in other megalosauroids); bluntly rounded vertical ridge as a buttress for astragalus in the tibia (ch. 719; present in Suchomimus and Riojavenatrix , and distinct from the rest of the megalosauroids and carnosaurs).

Synapomorphies in Spinosaurinae that also are present in Riojavenatrix are as follows: ascending process of astragalus offset from astragalar body by a pronounced groove (ch. 739; present in Spinosaurus and Riojavenatrix , absent in Suchomimus , and unknown in other spinosaurids; also present in Wiehenvenator within megalosauroids, and in some coelurosaurs).

Within Spinosauridae , baryonychines have the following synapomorphies that are absent in Riojavenatrix : absence of an expanded pubic boot (ch. 639; shared with Afrovenator within megalosauroids, some coelophysoids, some ceratosaurians, and alvarezsauroids); lateral malleolus projects far distal to medial malleolus (ch. 717; present in Suchomimus ; absent in Spinosaurus and Riojavenatrix , but is poorly coded in the matrix); anterior portion of fibular proximal end mediolaterally wider than the posterior portion (ch. 724; present in Baryonyx and other megalosauroids, but distinct from Spinosaurus and Riojavenatrix , some allosauroids, and some Coelurosauria).

Within Spinosauridae , spinosaurines have following synapomorphies that are absent in Riojavenatrix : large and ovoid pubic obturator foramen (ch. 652; shared with Ichthyovenator and Spinosaurus , also present in metricanthosaurids and Concavenator ); low proximal projection of cnemial crest (ch. 702; present in Spinosaurus , but unknown in Ichthyovenator and Baryonyx ; and absent in Suchomimus , other megalosauroids, allosauroids, coelophysoids, and some ceratosaurians); low and rounded fibular crest (ch. 713; present in Spinosaurus and other theropods, such as Asfaltovenator , some ceratosaurians, and coelurosaurs; but absent in Suchomimus and Riojavenatrix within spinosaurids); presence of a deep oval fossa on medial surface of fibula (ch. 730; potential synapomorphy of spinosaurines within megalosauroids, but it is unknown in Ichthyovenator ; also present in allosauroids); fibular facet on astragalus reduced and facing laterally (ch. 734; present in Spinosaurus , but absent from Riojavenatrix and other megalosauroids and carnosaurs, except for Mapusaurus Coria & Currie, 2006 , but unknown in other spinosaurids).

The potential autapomorphy of Riojavenatrix within spinosaurids based on the phylogenetic analysis is as follows: height of ascending process of the astragalus more than twice the height of astragalar body (ch. 735; present in Riojavenatrix but not in Suchomimus and other megalosauroids; also present outside spinosaurids in Allosaurus , Tyrannosaurus Osborn, 1905 , Albertosaurus Osborn, 1905 , and some derived coelurosaurs).

Matrix from Mateus and Estraviz-López (2022)

The alternative phylogenetic analysis, using the matrix proposed by Mateus and Estraviz-López (2022), resulted in 307 MPTs, with a length of 1123 steps (CI = 0.550; RI = 0.573). The strict consensus ( Fig. 10A View Figure 10 ) shows a well-resolved topology in most of the monophyletic groups of theropods. Regarding results in non-spinosaurid groups, Piatnitzkysaurus Bonaparte, 1979 is branched earlier in a polytomy of Carnosauria, and it is not recovered as a megalosauroid, unlike the results of Carrano et al. (2012). In Megalosauroidea, there is only a degree 5 polytomy formed by the megalosaurids Afrovenator , Eustreptospondylus , Duriavenator Benson, 2008 , and Dubreuillosaurus Allain, 2005 (Afrovenatorinae in the study by Carrano et al. 2012) and the dichotomy of Megalosaurinae. Megalosaurinae ( Torvosaurus + Megalosaurus ) is a monophyletic group in these results. Within Spinosauridae , baryonychines ( Baryonyx + Suchomimus ) are recovered in a dichotomy. The rest of the spinosaurids (including the new taxon Riojavenatrix ) form a degree 7 polytomy within the group, where the node of Baryonychinae is also included.

Only three taxa are pruned from the iterPCR of this consensus as unstable and collapsing nodes in the strict consensus: Eocarcharia Sereno & Brusatte, 2008 , Monolophosaurus , and Afrovenator . Other taxa are considered equally unstable, always forming polytomies even if they are pruned. These equally unstable taxa are as follows: Baryonyx , Iberospinus , Ichthyovenator , Irritator , Riojavenatrix , Spinosaurus , Suchomimus , and Vallibonavenatrix . Most of these taxa have scored characters that support alternative positions in different trees; therefore, these characters should be re-evaluated to analyse the instability of these taxa. In addition, most spinosaurids show numerous missing data in their coding, and if they were to be scored, they might help to resolve the positions ( Pol and Escapa 2009). The IterPCR script analyses the missing data of each unstable taxon and evaluates their ancestral condition. If the missing characters have different optimization in the ancestral node of the problematic taxon in different MPTs, it implies that their scoring (if possible) might help to solve the instability of the taxon ( Pol and Escapa 2009). This is the case for Riojavenatrix , where if ≤113 of all missing data were scored, they might help to resolve its position (for the list of these characters obtained in the iterPCR script results, see Supporting Information, Supplementary Material S2). However, all these characters are associated with missing bones or missing anatomical elements in the material for Riojavenatrix . If new material for this taxon become known in future, all these characters should be re-evaluated and scored in subsequent phylogenetic approaches to define a stable position for Riojavenatrix .

The agreement subtree ( Fig. 10B View Figure 10 ) shows a dichotomy between Suchomimus and Baryonyx , which is the only dichotomy that is consistent in all the MPTs, and they are the only taxa that have been not considered as unstable and pruned. All the other spinosaurids are equally considered as unstable, and there are six possible combinations of agreement subtrees (see list of pruned taxa inSupporting Information, Supplementary Material S2). This result is consistent, in part with the iterPCR one, but the latter analysis considers all the spinosaurids as equally unstable (including Suchomimus and Baryonyx ). Following the methodology for pruning trees incorporated in TNT (prunnelsen), node 31 was improved if Afrovenator was pruned, node 30 was improved if Monolophosaurus was pruned, and finally, node 29 was improved if Eocarcharia was pruned. The results from iterPCR and prunnelsen are similar regarding the unstable taxa and pruning ( Afrovenator , Monolophosaurus , and Eocarcharia ), because both methodologies have similar rules for stopping ( Pol and Escapa 2009). However, equally unstable taxa in the polytomy (spinosaurids) are not taken into consideration in the pruning methodology of TNT, whereas they are in the iterPCR. Regarding the agreement subtrees, its algorithm continues pruning taxa until the subtree is dichotomous, and for that, this method does not prune the same most unstable taxa, like the other methods mentioned, and keeps Suchomimus and Baryonyx as stable taxa because they form a dichotomy in all the subsets. Therefore, although Iberospinus and Riojavenatrix are left in the agreement subtree, they should still be considered as unstable taxa, as was inferred from the iterPCR.

The Bremer support analysis (all taxa included) shows many clades with minimal support. This is the case for Spinosauridae , which show a Bremer support of two. Jackknife and bootstrap analyses have values <50% in some groups, especially within Megalosauroidea. Spinosaurids are supported by 35 (jackknife) and 14 (bootstrap) values ( Fig. 10A View Figure 10 ).

The unambiguous synapomorphies (see Supporting Information, Supplementary Material S2) from the strict consensus for spinosaurids are as follows: presence of webbing at base of neural spines in dorsal vertebrae (ch. 179); accessory centrodiapophyseal lamina in dorsal vertebrae (ch. 180); and expanded infraprezygapophyseal fossa in dorsal vertebrae (ch. 181). However, these characters are missing data in Riojavenatrix . The ambiguous synapomorphies that are shared by Riojavenatrix and others spinosaurids are as follows (for the complete list, see Supporting Information, Supplementary Material S2): large and oval obturator foramen of pubis (ch. 285; present in Ichthyovenator and Riojavenatrix ); expanded and triangular morphology of distal end of ischium (ch. 297; present in Ichthyovenator , Riojavenatrix , and Vallibonavenatrix , but absent in Baryonyx or Suchomimus ); posteromedial orientation of medial condyle of femur in distal view (ch. 312; present in Baryonyx , Riojavenatrix , and Suchomimus ); bluntly rounded vertical ridge on medial side as buttress for astragalar in the tibia (ch. 320; present in Riojavenatrix and Suchomimus ); fibular flange is not extended to proximal end of the tibia (ch. 322; present in Riojavenatrix and Suchomimus ); and almost double height of the ascending process with respect to the height of astragalar body [ch. 331; present in (ch. state 2): Riojavenatrix and Suchomimus ].

CO M PA R I SO NS W I T H SP I N O S AU R I DA E A N D OT H E R T H E RO P O D S

Results from phylogenetic analysis support a placement of Riojavenatrix withintheSpinosauridae.Thisnewtaxonhasaunique combination of spinosaurid characters supporting this placement, according to the phylogenetic analysis performed with the matrix of Rauhut and Pol (2021): (i) a pubic apron that extends from the middle of the pubic shaft; (ii) a deep oblique ligament groove on the posterior surface of the femoral groove; and (iii) femoral head strictly directed medially in the anteroposterior plane.

Within Spinosauridae , Riojavenatrix shares three synapomorphies with members of Baryonychinae in the first phylogenetic analysis: (i) the postermedially oriented long axis of the medial condyle femur in distal view; (ii) the incomplete ossification in the fibular crest; and (iii) the bluntly rounded vertical ridge as buttress for astragalus in the tibia. However, Riojavenatrix shares only one synapomorphy with members of Spinosaurinae : the ascending process of astragalus offset from astragalar body by a pronounced groove.

Its placement within Spinosauridae is supported by six ambiguous synapomorphies according to the results from the phylogenetic analysis performed with the matrix of Mateus and Estraviz-López (2022): (i) a large and oval pubic obturator foramen; (ii) expanded and triangular morphology of the distal end of the ischium; (iii) the posteromedially oriented long axis of the medial condyle femur in distal view ( Baryonychinae synapomorphy based on the first phylogenetic analysis using the matrix of Rauhut and Pol 2021); (iv) the bluntly rounded vertical ridge as buttress for astragalus in the tibia ( Baryonychinae synapomorphy based on the first phylogenetic analysis using the matrix of Rauhut and Pol 2021); (v) fibular flange is not extended to the proximal end of the tibia owing to incomplete ossification ( Baryonychinae synapomorphy based on the first phylogenetical analysis using the matrix of Rauhut and Pol 2021); and (vi) the height of the ascending process of the astragalus being double the height of the astragalar body. Therefore, this unique combination of spinosaurid synapomorphies based on two phylogenetic analysis implies that Riojavenatrix can be regarded confidently as a spinosaurid.

The fossil record of Spinosauridae is, in most cases, fragmentary, with only a few well-known taxa. Comparison with the spinosaurid fossil record is limited by the absence of material overlapping between Riojavenatrix and the European Ceratosuchops and Riparovenator , the African Cristatusaurus and Sigilmassasaurus , and the South American Angaturama Kellner and Campos, 1996 , Irritator , and Oxalaia Kellner et al., 2011 . Nevertheless, the fossil remains of Riojavenatrix lacustris overlap with enough material of other African, Asian, and European spinosaurids ( Charig and Milner 1997, Sereno et al. 1998, Allain et al. 2012, Ibrahim et al. 2014, Sánchez-Hernández and Benton 2014, Malafaia et al. 2018, 2020a, Mateus and Estraviz-López 2022) to allow us to make comparisons and evaluate the possible synonymy between them, especially between the herein described taxon and the other Iberian spinosaurids.

Pelvic girdle

Pubis: Riojavenatrix preserves an obturator notch in the pubis, like several tetanurans and more derived theropods ( Hutchinson 2001). The preserved diaphysis of the pubis in Riojavenatrix is straight and similar to that of Baryonyx (NHMUK VP R9951), Iberospinus ( Mateus and Estraviz-López 2022) , Ichthyovenator (MDS-Savannakhet BK10-11), and Suchomimus (MNN GDF500), and unlike the curved shaft of FSAC-KK 11888. In Iberospinus , a longitudinal groove extends along the pubic shaft, a feature that is also present, albeit subtly, in Ichthyovenator ( Mateus and Estraviz-López 2022) and in Baryonyx (NHMUK VP R9951). This groove is not present in the preserved shaft of Riojavenatrix , because it is present at the proximal end of the pubic shaft. The pubic apron of Riojavenatrix would have reached almost to the pubic boot and extends much further distally than in Baryonyx (NHMUK VP R9951), Ichthyovenator (MDS-Savannakhet BK10-11), and Suchomimus (MNN GDF500). The distal position of the pubic apron of Riojavenatrix also differs from that of FSAC-KK 11888, because in the latter it does not reach the pubic boot. In distal view, the triangular-shaped distal end of the pubis of Riojavenatrix resembles that of Ichthyovenator (MDS-Savannakhet BK10-11) and megalosaurids ( Fig. 11A, B View Figure 11 ). However, in Ichthyovenator (MDS-Savannakhet BK10-11) the lateral surface is much more concave ( Fig. 11B View Figure 11 ), giving it an L-shaped distal outline ( Allain et al. 2012), in comparison to the more triangular pubic boot of Riojavenatrix in distal view ( Fig. 11A View Figure 11 ). This triangular-shaped distal end of the pubis is related to a strong mediolateral expansion of the anterior part of the pubic boot, which was proposed as a feature of Baryonychinae ( Sereno et al. 1998, Allain et al. 2012). However, the pubic boots of FSAC-KK 11888, Baryonyx (NHMUK VP R9951), and Suchomimus (MNN GDF500) are much narrower mediolaterally throughout their whole anteroposterior length in distal view than those of Ichthyovenator (MDS-Savannakhet BK10-11) and Riojavenatrix ( Fig. 11A–E View Figure 11 ). In Suchomimus (MNN GDF500) and Baryonyx (NHMUK VP R9951), a subtle L-shape can be noticed in distal view, whereas it is more oval in FSAC-KK 11888, differing from Riojavenatrix ( Fig. 11A, C–E View Figure 11 ).

Ischium: The iliac peduncle of the ischium of Riojavenatrix shows a planoconcave articulation, a feature shared with Baryonyx (NHMUK VP R9951), FSAC-KK 11888, and Suchomimus (MNN GDF500). According to Allain et al. (2012) and Malafaia et al. (2020a),the articulation of the ischium with the ilium is peg-and-socket in Ichthyovenator and Vallibonavenatrix . Nevertheless, the morphology of these iliac peduncles in the latter taxa is similar to that of other spinosaurids (i.e. planoconcave); and they do not have the well-developed and deeply excavated fossa on the iliac peduncle, referred to as the peg-and-socket articulation, featured in abelisauroids and carcharodontosaurians ( Sereno et al. 2004, Carrano et al. 2012). Therefore, all spinosaurids have a planoconcave articulation of the ischium to the ilium, a feature also seen in Riojavenatrix . In proximal view, the iliac articulation is less anteroposteriorly elongate in Riojavenatrix than in Baryonyx (NHMUK VP R9951) and FSAC-KK 11888, resembling that of Ichthyovenator (MDS-Savannakhet BK10-12–13), Suchomimus (MNN GDF500), and Vallibonavenatrix (MSMCa-1–3). The latter two also have a medially positioned and anteroposteriorly oriented crest with an adjacent groove on the medial surface of the acetabulum ( Malafaia et al. 2020a). In Baryonyx (NHMUK VP R9951), a similar anteroposteriorly oriented groove is present. This is also lateral and medially bounded by two low, rounded, and parallel ridges that, together with the groove, fade anteriorly. Despite Riojavenatrix having a similar crest, it lacks the adjacent groove. In lateral view, the preserved portion of the ischial diaphysis and the distal portion of the Riojavenatrix ischium are more slender than those of FSAC-KK 11888, Ichthyovenator (MDSSavannakhet BK10-12–13), Suchomimus (MNN GDF500), and Vallibonavenatrix (MSMCa-1–3). The anterior margin of the shaft and the distal portion of the Riojavenatrix ischium are transversely proportionately as thick as in Suchomimus (MNN GDF500) and Vallibonavenatrix (MSMCa-1–3). Nevertheless, the ischial boot expands further anteroposteriorly with respect to the shaft in Riojavenatrix than in FSAC-KK 11888, Ichthyovenator (MDS-Savannakhet BK10-12–13), Suchomimus (MNN GDF500), and Vallibonavenatrix (MSMCa-1–3) ( Fig. 11F–J View Figure 11 ). In FSAC-KK 11888, the distal expansion is posteriorly directed, differing further from the Riojavenatrix ischium. The ischial boot is rounded in lateral view in FSAC-KK 11888 and Suchomimus (MNN GDF500), whereas it is more triangular in Ichthyovenator (MDS-Savannakhet BK10-12–13), Vallibonavenatrix (MSMCa-1–3), and Riojavenatrix ( Fig. 11F– J View Figure 11 ). Nevertheless, the anterodistal surface of the ischial boot is only angular in Riojavenatrix . Furthermore, the anterior tip of the ischial boot in Riojavenatrix resembles that of Megalosaurus (OUMNH J.13565; Benson 2010, fig. 15G–L). This tip is not as marked in Ichthyovenator (MDS-Savannakhet BK10-12–13), and it is not present in FSAC-KK 11888 and not preserved in Suchomimus (MNN GDF500) and Vallibonavenatrix (MSMCa- 1–3) ( Fig. 11F–J View Figure 11 ).

Hindlimb

Femur: The femur of Riojavenatrix shows several similarities with megalosauroid theropods ( Fig. 12 View Figure 12 ), such as the rounded medial epicondyle and the small rugose patch for the attachment of the muscle femorotibialis externus ( Carrano et al. 2012). Furthermore, like other spinosaurids, the medial condyle of the left femur of Riojavenatrix has a posteromedial orientation ( Benson 2010), and not posterolateral as previously suggested by some authors (e.g. Benson 2010, Carrano et al. 2012, Malafaia et al. 2018). Nevertheless, the posteromedial displacement in the new taxon is remarkably much less marked than in Baryonyx (NHMUK VP R9951), FSAC-KK 11888, ‘ Spinosaurus B’ (Nr. 1922 X 45; Stromer 1934), and Suchomimus (MNN GDF500) and more similar to CMP-3b/211 ( Fig. 12A–F View Figure 12 ). Indeed, this condition is somewhere between that of these spinosaurids and other megalosauroids, such as Megalosaurus (NHMUK PV 31806; Benson 2010: fig. 16I–J). The distal end of the Riojavenatrix femur also differs from Baryonyx (NHMUK VP R9951) and FSAC-KK 11888 in the distal extension of the lateral condyle. This distal end projects further distally than the medial condyle in Baryonyx ( Charig and Milner 1997, Carrano et al. 2012) and the right femur of FSAC-KK 11888. In Riojavenatrix , instead, the femoral condyles are almost equally projected, a feature also seen in CMP-3b/211, ‘ Spinosaurus B’ (Nr. 1922 X 45), and Suchomimus (MNN GDF500). Furthermore, the femoral condyles in FSAC-KK 11888 are notably narrower ( Fig. 12A, F View Figure 12 ), and its fourth trochanter is hypertrophied ( Ibrahim et al. 2014). The main axis of the lateral condyle of Baryonyx (NHMUK VP R9951) and FSAC-KK 11888 is more anteroposteriorly directed and narrower than in Riojavenatrix ( Fig. 12A, D, F View Figure 12 ). In Riojavenatrix , the lateral condyle is almost as rounded and broad as that of Suchomimus (MNN GDF500), with a similar orientation, and it is virtually identical to CMP-3b/211 ( Fig. 12A, B, E View Figure 12 ). The extensor groove is also more developed in Baryonyx (NHMUK VP R9951), ‘ Spinosaurus B’ (Nr. 1922 X 45), and FSAC-KK 11888 than in Riojavenatrix , and this is more similar to that of Suchomimus (MNN GDF500) and CMP-3b/211 ( Fig. 12A–F View Figure 12 ). The flexor groove in Baryonyx (NHMUK VP R9951) is also wider and deeper in comparison to Riojavenatrix .

The head of the femur of Riojavenatrix is subcircular, as in CMP-MS-0/22, CMP-3b/211, Baryonyx (NHMUK VP R9951), and Suchomimus (MNN GDF500), and it differs from that of FSAC-KK 11888, which is more oval in medial view. The articular groove of the proximal surface of the right femur resembles that of CMP-MS-0/22 and CMP-3b/211, owing to the latter ones being relatively narrow and deep ( Malafaia et al. 2018) ( Fig. 12G–J View Figure 12 ). However, in Baryonyx (NHMUK VP R9951) and Suchomimus (MNN GDF500), the proximal articular grooves are relatively broader and not as restricted, at least, posterolaterally ( Fig. 12I–J View Figure 12 ). Furthermore, this articular groove is more anteriorly located and anteroposteriorly oriented in Riojavenatrix ( Fig. 12G View Figure 12 ). Riojavenatrix also lacks the longitudinal V-shaped groove present on the medial surface of the shaft of Baryonyx ( Charig and Milner 1997) . In the FSAC-KK 11888 femur, the crista tibiofibularis is more posteriorly projected in distal view than in Riojavenatrix ( Fig. 12F View Figure 12 ). The CMP-MS-0/22 and CMP-3b/211 femora differ almost exclusively from those of Riojavenatrix in the bowing of the femoral shaft, with CMP-3b/211 being straighter in lateral view, and with a slightly broader tibiofibular crest.

Tibia: The supraastragalar buttress of Riojavenatrix is a vertical ridge located on the medial side, a feature also shared with Chilantaisaurus Hu, 1964 ( Benson and Xing 2008), CMP-3c/188, FSAC-KK 11888, and Suchomimus ( Rauhut 2003, Benson and Xing 2008, Carrano et al. 2012, Malafaia et al. 2018). The shaft of the left tibia of Riojavenatrix is straight, whereas in CMP-3c/188 it is medially curved distally owing to the medially projected medial malleolus ( Malafaia et al. 2018). In Suchomimus (MNN GDF500), the tibial shaft is laterally bowed, a feature also observed, but to a lesser extent, in FSAC-KK 11888. Unlike the tibia in Riojavenatrix , the shaft of the Camarillasaurus tibia is ‘G-shaped’ owing to a deep groove present posteriorly on its lateral surface ( Sánchez-Hernández and Benton 2014). This structure is also described in the left tibia of FSAC-KK 11888 ( Samathi et al. 2021). However, this could be the result of preservation or pathology ( Samathi et al. 2021). Riojavenatrix differs further from CMP-3c/ 188 in lacking the concavity situated posterior to the fibular crest that bears a tibial foramen ( Malafaia et al. 2018). Suchomimus (MNN GDF500) has a proximodistally oriented groove that is parallel and anterior to the fibular flange. In Camarillasaurus , this is a subtle depression and has a distally located foramen ( Sánchez-Hernández and Benton 2014). These are not present in Riojavenatrix . The cnemial crest follows the same pattern of Camarillasaurus, FSAC-KK 11888, and Suchomimus , where the tibia narrows towards the anteriormost part of this structure ( Samathi et al. 2021). However, in Riojavenatrix , the tip of the cnemial crest is more laterally directed, differing from that of Camarillasaurus (MPG-KPC8), FSAC-KK 11888, ‘ Spinosaurus B’ (Nr. 1922 X 45), and Suchomimus (MNN GDF500) ( Fig. 13A, C–E View Figure 13 ), and being more similar to other non-spinosaurid theropods, such as Allosaurus (USNM 4734; Gilmore 1920: fig. 49), Condorraptor Rauhut, 2005 , Fukuiraptor Azuma & Currie, 2000 (FPMN 9712220; Azuma and Currie 2000: fig. 13C), Megalosaurus (NHMUK PV 31809; Benson 2010: fig. 17G), Sinraptor Currie & Zhao, 1993 (IVPP 10600; Currie and Zhao 1993: fig. 22I), and Tyrannosaurus ( Brochu 2003) . In addition, the cleft between both tibial condyles is narrower in Riojavenatrix than in Camarillasaurus (MPG-KPC8) (‘intercondylar groove’ of Sánchez-Hernández and Benton 2014), ‘ Spinosaurus B’ (Nr. 1922 X 45), and FSAC-KK 11888, and it is more similar to that present, for instance, in Suchomimus (MNN GDF500) and Megalosaurus ( Benson 2010) ( Fig. 13B–E View Figure 13 ). Furthermore, in Riojavenatrix the lateral process of the lateral condyle of the tibia is proximodistally longer and narrower than the shorter and triangular one of Camarillasaurus (MPG-KPC8).

Distally, the supraastragalar buttress is more marked and thinner in Riojavenatrix than in CMP-3c/188. This crest smoothly inclines laterally in CMP-3c/188 and rises from the medial edge of the tibia, whereas it is more angular and with its distalmost end being located on the anterior side of the tibia in Riojavenatrix . This feature is similar in Suchomimus (MNN GDF500), but the tibia is slightly damaged at this point. The distal portion of the supraastragalar buttress is more inclined in FSAC-KK 11888 than in Riojavenatrix . The facet for the ascending process of the astragalus of the Riojavenatrix tibia is not as broad as in CMP-3c/188 and Suchomimus (MNN GDF500), occupying slightly more than half of the anterior surface of the tibia distally, but it extends almost as high as in the latter taxa. The mediolateral extension of this facet in Riojavenatrix is certainly more similar to FSAC-KK 11888, but in the latter the apicalmost point of the facet is more centrally placed. On the posterior side, the CMP-3c/188 tibia shows a marked concavity between the lateral malleolus and a vertical triangular crest ( Malafaia et al. 2018), which is a slight depression in Riojavenatrix . The angle between the tibial malleoli is also much higher in CMP-3c/188 than in Riojavenatrix , being more angular, straighter, and almost equally directed distally in Riojavenatrix . In this feature, the tibia of Riojavenatrix resembles the specimen FSAC-KK 11888 and somewhat that of Suchomimus (MNN GDF500). However, the lateral malleoli of FSAC-KK 11888 and Suchomimus (MNN GDF500) are slightly more distally projected than in Riojavenatrix . In posterior view, the distal margin between the tibial malleoli of Riojavenatrix is straight, as in Suchomimus (MNN GDF500). In FSAC-KK 11888, there is a concavity, as in CMP-3c/188 ( Malafaia et al. 2018), which is more pronounced in the latter specimen. The distal expansion of the Riojavenatrix tibia is not as pronounced as in Suchomimus , because the lateral malleolus is larger and projects further laterally in Suchomimus (MNN GDF500) than in Riojavenatrix . This projection in Riojavenatrix resembles that of CMP-3c/188 and FSAC-KK 11888.

Fibula: In comparison to other spinosaurids, the fibula of Riojavenatrix is more slender. As in other megalosauroid theropods, the fibula of Riojavenatrix shows a shallow medial fossa (sensu Carrano et al. 2012) or lacks the medial depression (sensu Benson 2010 and Rauhut et al. 2016), differentiating them from most of other neotheropods. In proximal view, the Riojavenatrix fibula shows a comma-shaped outline. However, this is posteriorly straighter than in Wiehevenator (WMN P27479 and 27502) and it thins posteriorly more than in Baryonyx (NHMUK VP R9951; note that the better-preserved Baryonyx fibula is here regarded as a left one), Camarillasaurus (MPG-KPC unnumbered), and FSAC-KK 11888. Moreover, the fibula is more C-shaped in proximal view in Baryonyx (NHMUK VP R9951; crescentshape of Charig and Milner 1997) and FSAC-KK 11888. In medial view, the posterior margin of the proximal end in Baryonyx is more proximally projected than the anterior edge in Riojavenatrix . In Baryonyx (NHMUK VP R9951), it is also more elevated but less pronounced, and distint from Camarillasaurus (MPG-KPC unnumbered) and FSAC-KK 11888, which have an almost even proximal end. The medial fossa is deeper in FSAC-KK 11888 and proportionally shallower in Baryonyx (NHMUK VP R9951) compared with Riojavenatrix . Instead, Riojavenatrix has a medial fossa on the fibula that resembles that of Suchomimus (MNN GDF500). Nevertheless, the distal end of the fossa is more acute in Riojavenatrix than in Suchomimus (MNN GDF500), but more rounded than that of Baryonyx (NHMUK VP R9951) and FSAC-KK 11888. This fossa extends more distally in Baryonyx (NHMUK VP R9951) and FSAC-KK 11888 than in Riojavenatrix , which is similar to Suchomimus (MNN GDF500). The proximal expansion of the fibula is more abrupt in Riojavenatrix than in FSAC-KK 11888, especially the posterior blade. This blade is blunter in Riojavenatrix than in Baryonyx (NHMUK VP R9951). Below the medial fossa, the iliofibularis tubercle is not especially marked in Riojavenatrix and thus similar to the condition observed in FSAC-KK 11888 and Suchomimus (MNN GDF500). In Baryonyx (NHMUK VP R9951), this is even less noticeable and lacks the groove present in the herein described taxon.

Astragalus: The distal condyles of the astragalus are anterodistally projected in Riojavenatrix with an angle similar to other tetanurans ( Sereno et al. 1996, Carrano et al. 2012). In addition to the herein described one, only one other, still undescribed, spinosaurid astragalus is known (MNN GDF, unnumbered and referred to Suchomimus according to Rauhut 2003). The ascending process of the latter astragalus is taller than that of Allosaurus ( Sereno et al. 1998) . In Suchomimus , the ascending process of the astragalus is higher than the astragalar body, a feature also proposed for Spinosauridae among early branching tetanurans ( Carrano et al. 2012). Indeed, the ascending process for the astragalus is 1.6 times taller than its body in that referred to Suchomimus ( Carrano et al. 2012) . The ascending process of the astragalus of Riojavenatrix is taller than its body, like the one referred to Suchomimus . Nevertheless, this is even taller in Riojavenatrix , doubling the height of the astragalar body. Owing to the absence of other spinosaurid astragali, it is difficult to compare the Riojavenatrix astragalus with other taxa. Therefore, it is not possible to assess whether the vertical ridge on the medial margin of the ascending process of the astragalus is an autapomophy of Riojavenatrix or a synapomorphy among Spinosauridae .

Calcaneum: The Riojavenatrix calcaneum is smaller in size than those of Baryonyx (NHMUK VP R9951) and Iberospinus (ML1190-31) ( Fig. 14 View Figure 14 ). In particular, this calcaneum is mediolaterally narrower and higher than that of Iberospinus (ML1190-31) ( Fig. 14A, B View Figure 14 ). Moreover, the calcaneum of Riojavenatrix bears a foramen located in an anterolateral depression, which is so far present only in this taxon. The calcaneum of Baryonyx (NHMUK VP R9951) differs from the one of Riojavenatrix in having a centrally located large depression and several, but smaller, foramina, and from Iberospinus (ML1190-31) because the latter lacks any of them ( Fig. 14A–C View Figure 14 ). Furthermore, the astragalar facet of Baryonyx bears two fossae ( Charig and Milner 1997) that are not present in Riojavenatrix .

Pes: Pedal elements are scarce and usually fragmentary among Spinosauridae . The hourglass-shaped proximal end of the third metatarsal of Riojavenatrix is also present in other tetanurans ( Carrano et al. 2012). This metatarsal is robust in Riojavenatrix , resembling the shape of those of Chilantaisaurus ( Benson and Xing 2008) , Sinraptor ( Currie and Zhao 1993) and Torvosaurus ( Britt 1991) . In addition, unlike avetheropods, the cross-section of the shaft of the metatarsal III of Riojavenatrix is rectangular, similar to early branching tetanuran theropods ( Carrano et al. 2012). Charig and Milner (1997) mentioned the presence of two distal ends of metatarsal bones belonging to Baryonyx , indicating that they could belong to metatarsal II, III, or IV. Despite these being fragmentary, they have been identified here as the distal ends of the metatarsals II and III. The metatarsal that is not ginglymoid would correspond to the distal end of metatarsal III and the other to the distal end of metatarsal II, owing to the latter showing a ventral groove and being mediolaterally broad in distal view compared with the distal ends of metatarsals IV. Furthermore, the metatarsal assigned to digit II is too dorsoventrally low for it to be considered to be from digit IV. In distal view, the third metatarsal of Riojavenatrix is not ginglymoid, a condition also present in other megalosauroids, such as Baryonyx (NHMUK VP R9951), Megalosaurus ( Benson 2010) , and other theropods ( Norell et al. 2001). In Riojavenatrix , the distal condyle is much more mediolaterally broad than in the metatarsal III of Baryonyx (NHMUK VP R9951). Furthermore, this is square-shaped and dorsoventrally larger than mediolaterally wide in Baryonyx (NHMUK VP R9951). Moreover, the ventral margin of the distal condyle of the metatarsal III of Riojavenatrix is concave, whereas it is flat in Baryonyx (NHMUK VP R9951).

Many spinosaurid pedal unguals from Gondwanan formations have been reported, and all of them show a flat ventral side (see Stromer 1934, Novas et al. 2005, Ibrahim et al. 2014, Maganuco and Dal Sasso 2018, de França et al. 2021). Ibrahim et al. (2014) reconstructed the pes of FSAC-KK 11888, also with a flat-bottomed I-2. This is not the case for phalanx I-2 of Riojavenatrix , which is remarkably curved. Furthermore, the pedal unguals of FSAC-KK 11888 are broader than deep, differing from the preserved phalanx I-2 of Riojavenatrix . The pedal ungual phalanx of Baryonyx (NHMUK VP R9951) does not show a flat ventral side and bears a longitudinal groove on each side. The ventral surface of Iberospinus ( Mateus and Estraviz-López 2022) is flatter than that of other European spinosaurids, but its preservation does not permit a more precise comparison. A single longitudinal groove in pedal unguals is a plesiomorphy found among early branching theropods, with the exception of Abelisauroidea, in which two bifurcating grooves are present ( Carrano and Sampson 2008). The I-2 pedal ungual of Riojavenatrix shows only one longitudinal groove on the lateral side, whereas it is devoid of grooves on the medial side. Therefore, it differs from most other theropods in this feature [e.g. Alectrosaurus Gilmore, 1993 ( Mader and Bradley 1989); Bambiraptor Burnham et al., 2000 ; Deinonychus Ostrom, 1969 ; Tyrannosaurus ( Brochu 2003) : FMNH PR2081; and an indeterminate carcharodontosaurian from Portugal ( Malafaia et al. 2019); Fig. 15 View Figure 15 ]. But also, it differs from all the spinosaurid ungual phalanges described so far, which have a longitudinal groove on the medial side (see Stromer 1934, Charig and Milner 1997, Novas et al. 2005, Ibrahim et al. 2014, Maganuco and Dal Sasso 2018, de França et al. 2021, Mateus and Estraviz-López 2022). Therefore, the presence of a longitudinal groove on the lateral surface of pedal phalanx I-2 and the absence of it on the medial side might be an autapomorphy of Riojavenatrix . Nevertheless, the absence of pedal unguals that can be attributed confidently to digit I in other spinosaurids, or even in Megalosauroidea, prevents this assertion with certainty, because it can also be a synapomorphy of any of the latter groups.

H I STO LO G I C A L A N A LYSI S

Bone sections show, in general, a 5- to 10-mm-thick, moderately thin cortex of primary compact bone, which grades into a large medullary region completely filled by bony trabeculae ( Fig. 16A, B View Figure 16 ). The bone microstructure is too poorly preserved for a precise histological assessment, owing to the intense diagenesis and low-grade metamorphism experienced by the sediments of the Enciso Group (see ‘ Geographical and geological setting ’). Therefore, the bone matrix microstructure and the type of primary vascularity are not recognizable. However, some general patterns of the bone microstructure can be identified in the recovered material. In this sense, clusters of Haversian systems are clearly identifiable in the innermost region of the compacta ( Fig. 16E View Figure 16 ), where at least two overlapping generations of secondary osteons are visible. Isolated secondary osteons and large resorption rooms incompletely filled by lamellar bone (i.e. incipient secondary osteons) are also present on the peripheral regions of the compacta. When present, they tend to be organized in circular rows, in the same manner as that seen in other groups of dinosaurs ( Company 2011, Cerda et al. 2017).

Other histological structures recorded in the cortex consist of well-defined lines of arrested growth, which result in a cortical stratification. They are more evident in the external region of the compacta, where the secondary remodelling is less invasive ( Fig. 16B–D View Figure 16 ). The presence of these growth lines indicates that the animal grew with a seasonal cyclicity, probably annual ( D’Emic et al. 2023 and references therein). Given that the subperiosteal surface of the bones has been partly eroded away by abrasion, it is not possible to determine whether the external fundamental system, a proxy of somatic maturity composed of closely spaced resting lines, was once present in the samples.

The presence of large erosion spaces coated only by a thin layer of lamellar bone (i.e. immature Haversian osteons) and only two generations of secondary osteons, with scarce overlapping, indicates that the processes of osteonal remodelling was not complete and was still active when the animal perished. This fact, along with the presence of intense secondary reconstruction in the internal perimedullary region, but scarce towards the outer cortex, combined with the existence of cyclical growth rings, points to a somatically immature, still growing adult.