Stereospondyli

Stereospondyli Zittel, 1888

Capitosauria Yates & Warren, 2000

Capitosauroidea Säve-Söderbergh, 1935 Cyclotosauridae Shishkin, 1964

Cyclotosaurus Fraas, 1889

cf. Cyclotosaurus sp.

Figs. 4, 5 View Fig .

Material.— WMNM P97556, a portion of the palatine ramus of the left pterygoid; WMNM P64371, a left humerus; all from clay pit #III of the August Lücking GmbH & Co. KG, northwest of the village of Bonenburg, North Rhine-Westphalia, Germany; Contorta Beds of the Exter Formation, Rhaetian, Triassic.

Description.—WMNM P97556 ( Fig. 4; SOM 1: fig. 1): the pterygoid fragment is dorsoventrally flattened and weakly concave on the ventral side, while it is convex on the dorsal side. The medial edge, which forms the posterolateral bor- der of the interpterygoid vacuity, is straight, with only the most proximal portion weakly medially recurved. On the other hand, the lateral edge, which contributes to the border of the subtemporal window, is weakly convex. This convexity, resulting from a lateral enlargement called “transverse flange” ( Damiani 2001), probably switches to a concavity in the most proximal portion of the palatine ramus. The ventral surface of the bone is concave, but the concavity is accentuated in the medial portion, at around 1 cm from the edge, forming a sort of longitudinal groove ( Fig. 4A 1 View Fig ). The lateral-most portion of the ventral surface is rather flat but weakly ornamented ( Fig. 4A 1 View Fig ). The dorsal surface of the bone is smooth and convex along the longitudinal axis Fig. 4A 2 View Fig ). The ornamentation consists of a few longitudinal ridges, most evident in the medial-most portion of the ventral surface.

A 1 rostral lateral A 2 rostral lateral transverse flange 10 mm

WMNM P64371 ( Fig. 5 View Fig ; SOM 1: figs. 2, 3): A detailed morphological and a preliminary histological description of this humerus was published by Konietzko-Meier et al. 2019). The humerus shows the typical cyclotosaur morphology, with a dorsoventrally flattened and short shaft and weakly developed supinator process (SOM 1: figs. 2, 3).

A preliminary histological description of WMNM P64371 was published by Konietzko-Meier et al. (2019). Here we summarize the previous study and update the histology with the new details about the growth pattern. The shape of the cross section is triangular, with the apex of the triangle representing the radial crest ( Fig. 5A View Fig 1 View Fig ). The entire bone section is highly porous, mostly as a result of advanced remodelling ( Fig. 5A View Fig 1 View Fig ). The inner part of the bone is crushed, and thus it is difficult to determine whether an empty medullary cavity was present. Only a very small area of the trabeculae in the center of section seems to be exclusively of secondary origin ( Fig. 5A View Fig 2 View Fig ). Most trabeculae in the deep parts of the section still consist of some primary bone in the interstitial areas between the secondary bone, and the amount of primary bone increases towards the periphery. There are distinctive growth marks in the cortex, consisting of zones, annuli, and lines of arrested growth (LAGs). Zones are made up by a poorly organized parallel-fibered matrix complemented by a large amount of longitudinal vascular canals. The organization of the matrix increases toward the bone surface. In ontogenetically older zones, vascular canals are usually enlarged by resorption, transitioning to erosion cavities. Next to the outer bone surface, the vascular canals are represented by simple canals or primary osteons. In the annuli, the tissue is always highly organized parallel-fibred matrix and less vascularized compared to the zones. The first annulus followed by the first LAG is in the inner part of the section, marking the first period of slow growth. After the first LAG, there are nine regular cycles visible, each consisting of a highly vascularized, thick zone followed by a thin, avascular annulus terminating in a LAG Fig. 5A View Fig 2 View Fig ). The regular pattern breaks down after the tenth LAG, where a set of closely spaced growth marks, consisting of dark lines, is visible ( Fig. 5A View Fig 3 View Fig ). The exact number is difficult to determine, but at least nine lines are visible, with the last line being more pronounced (LAG XI, Fig. 5A View Fig 2 View Fig , A 3 View Fig ). Beyond this set, there are three highly vascularized zones. However, these are thinner than those in the deeper cortex Fig. 5A View Fig 3 View Fig ). This pattern indicates further bone growth. The thickness of zones decreases in each cycle. Each zone is followed by an annulus terminating in a double LAG (LAGs d. XII to d. XIV, Fig. 5A View Fig 4). Next to the bone surface, there is one last LAG ( Fig. 5A View Fig 3 View Fig , LAG XV).

Remarks.—The morphology of the palatine ramus of the pterygoid indicates that it belongs to Capitosauria ( Schoch 2008; Arbez et al. 2017; Marzola et al. 2017). However, the presence of ornamentation on the ventral side allows to exclude several capitosaurids listed in Schoch (2008: character #49). The subsequently described Stanocephalosaurus amenasensis Dahoumane et al., 2016 ( Nedjari et al. 2010) shows an ornamented pterygoid but can be excluded from consideration since its palatine ramus is not concave, the ornamentation is “honeycomb-like’’ instead of straight ridges, and its geological age is too late (early Middle Triassic). Cyclotosaurus naraserluki ( Marzola et al., 2017) shows ventral ornamentation on the palatine ramus of pterygoid, as in the Bonenburg fossil, and is almost coeval (see above). Thus, the shape and ornamentation of WMNM P97556 allow its identification as the palatine ramus of the left pterygoid and assignment to Cyclotosaurus sp. We assume this to be the same biological species as that represented by humerus WMNM P64371 (see below).

Based on the morphological comparison with other temnospondyl humeri, Konietzko-Meier et al. (2019) assigned humerus WMNM P64371 to Cyclotosaurus sp. As noted, the humerus represents the typical Cyclotosaurus morphology with a dorsoventrally flattened, short shaft and weakly developed supinator process (SOM 1: fig. 1). Furthermore, the bone histology of WMNM P64371 excludes amniotes from consideration (for discussion and details see Konietzko-Meier et al. 2019).

Capitosauroidea indet.

Figs. 6–16 View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig View Fig .

Material.—WMNM P97559, a partial right dentary with remains of the precoronoid and the presplenial (Bone Bed 2b); WMNM P97557, a right angular in matrix (Bone Bed 2a); WMNMP97791,a partial left clavicle(Bone Bed2a);WMNM P98583, a portion of the left lateral wing (sensu Schoch 1999) of an interclavicle (Bone Bed 2a); WMNM P97555, a dermal bone indet. (Bone Bed 3); WMNM P97553, the proximal portion of a right femur (Bone Bed 2a); WMNM P97790, an almost complete left femur (Bone Bed 2a); all from clay pit #III of the August Lücking GmbH & Co. KG, northwest of the village of Bonenburg, North Rhine-Westphalia, Germany; Contorta Beds of the Exter Formation, Rhaetian, Triassic.

Description.— WMNM P97559 ( Fig. 6 View Fig ; SOM 1: fig. 4): The dentary is dorsoventrally higher in the posterior portion, and it becomes lower in the rostralmost portion, just before the symphysial region. The specimen is weakly mesially recurved and is not complete, since it lacks the symphysial region and the posteriormost part of the dentary, and only the precoronoid and presplenial remain of all the bones normally attached on the lingual side. The lingual side holds a deep groove in the posterior half of the bone ( Fig. 6A View Fig 1 View Fig ). This groove is the Meckelian canal, which in a complete jaw would have been covered by the coronoids and splenials lacking here, but the sutural surface of attachment of these bones is visible in the area dorsal to the groove and just below the dental margin. The precoronoid and the presplenial are fragmentary and lack both the anterior and posterior to portions. The anteriormost part of the dentary, just before the symphysial region, is much narrower than the posterior one, and so is the groove. The labial side presents a deep groove, termed “horizontal groove” ( Welles 1947) that crosses the surface diagonally, from the lowermost distal extremity up to the uppermost mesial one, close to the marginal tooth row ( Fig. 6A View Fig 2 View Fig ). Dorsally to this groove, the surface of the dentary is rather smooth, with the exception of damage produced by diagenetic alteration and weathering. Ventral to the groove, the surface is weakly ornamented. The ornamentation consists of a few ridges parallel to the long axis of the bone ( Fig. 6A View Fig 2 View Fig ). The dorsal region of the dentary holds 25 tooth sockets, all of which are elliptical with the main axis labiolingually oriented and all of the same size ( Fig. 6A View Fig 3 View Fig ).

WMNM P97557 ( Fig. 7 View Fig ; SOM 1: fig. 5): This sub-triangular bone is an angular which has the anterior part, the one contacting the dentary, highly elongated. The dorsoventrally tallest part of the bone is close to the posterior end, in correspondence with the suture between the dentary and the surangular ( Fig. 7 View Fig ; SOM 1: fig. 5). This specimen has been prepared only labially, with the lingual face still being covered by matrix. The labial side is entirely ornamented, and the ornamentation appears as an arrangement of pits and ridges, with the ridges forming a reticular anastomosing pattern surrounding polygonal pits. The ridges are arranged radially and converge towards the region positioned ventrally to the tallest part of the bone. Proceeding distally from this region, the ridges progressively stop anastomozing and start being parallel to each other, especially in the anteriormost area.

WMNM P97791 ( Figs. 8 View Fig , 9 View Fig ; SOM 1: fig. 6): The ventral surface of the clavicular blade shows an ornamentation with an anastomozing pattern of ridges enclosing pits ( Fig. 8A View Fig 1 View Fig ). The dorsal surface of the blade is smooth in temnospondyls, but in WMNM P97791 this surface remains covered by sediment ( Fig. 8A View Fig 2 View Fig ). In lateral view, the anteriormost portion of the base of the ascending process ( Rinehart and Lucas 2016) is visible ( Fig. 8A View Fig 2 View Fig ). The medial edge of the specimen shows a distinctive stepped morphology ( Fig. 8A View Fig 3 View Fig ). However, thin sectioning of the specimen revealed diagenetic fracturing as the explanation of this morphology.

The clavicle shows a diploë structure, typical for the dermal bone of temnospondyls with the ornamented external surface ( Fig. 9A View Fig 1 –A View Fig 3 View Fig ). In the external cortex, the primary tissue comprises highly organized parallel-fibered bone. The region of the cortex representing the structures of the ornamentation is almost avascular or with only irregularly arranged small vascular canals ( Fig. 9A View Fig 2 –A View Fig 5 View Fig ). Most of the canals show lamellar infilling, primary or secondary, while only very few canals are still simple. In the ornamented portion, the apices of the ridges all shifted medially with the growth of the bone, but still maintain the same distance between each other ( Fig. 9A View Fig 2 View Fig , A 3 View Fig ). The external cortex gradually transits to the medial region of the diploë. The size of the internal spaces increases with the decreasing amount of primary tissue present between cavities ( Fig. 9A View Fig 6 View Fig , A 7 View Fig ). However, the remains of a primary tissue are visible deep in the bone, and the trabeculae show the regular arrangement in rows. Only the most central part is composed of irregularly arragend secondary trabeculae. The medial region is bounded by the inner cortex, but the border here is also not sharp. The inner cortex is highly vascularized, and annuli and zones are easily distinguishable ( Fig. 9A View Fig 8 View Fig , A 9 View Fig ). The tissue in the annuli is avascular with a highly organized parallel-fibered matrix, whereas the matrix in the zones is less organized, and the longitudinal vascular canals are well arranged in rows ( Fig. 9A View Fig 8 View Fig , A 9 View Fig ). In the central-most region of the inner cortex, large cavities are present as a result of resorption and remodeling.

WMNM P98583 ( Fig. 10 View Fig ; SOM 1: fig. 7): The ventral side of the interclavicle fragment is almost entirely ornamented with the exception of a portion of the lateral rim, where the clavicle once overlapped the interclavicle. The ornamentation is formed by thick, branched ridges radiating from the center of the interclavicle. In the center itself, the ridges form polygonal pits ( Fig. 10 View Fig ). The dorsal surface is mostly smooth with the exception of weak radial striations.

WMNM P97555 was sectioned to observe its bone histology ( Fig. 12 View Fig ). The internal structure of the specimen is highly crushed but the histological characters are partially preserved. The external cortex is very thin and preserved only in the region of ornamentation ( Fig. 12A View Fig 2 View Fig , A 3 View Fig ). The primary matrix is parallel-fibered with numerous interwoven structural fibers. Numerous longitudinal vascular canals, consisting of primary and secondary osteons, are arranged in rows ( Fig. 12A View Fig 2 View Fig , A 3 View Fig ). In the external-most portion, there are no vascular canals, but growth marks are visible, showing the shift of the ridges during growth. Detailed analysis of the growth marks is not possible due to the poor state of preservation. The medial region is built by secondary trabeculae separated by large cavities. In cores of some of these trabeculae, the remains of primary tissue with vascular canals can still be observed ( Fig. 12A View Fig 4, A 5 View Fig ). It is important to note that the border between the external and medial regions is relatively sharp. The medial cortex is very thin and highly fragmentarily preserved ( Fig. 12A View Fig 6 View Fig , A 7 View Fig ). Similar to the external cortex, there is also a sharp border with the medial region. The matrix of the internal cortex is more highly organized than that of the external cortex, and the tissue is almost avascular, showing only a few very small vascular canals.

WMNM P97553 ( Figs. 13 View Fig , 14 View Fig ; SOM 1: fig. 8): Femur heavily corroded by exposure on the sea floor and diagenetic processes, possibly compromising the morphology. The anteroventral surface of the shaft of the bone is weakly concave and smooth, except for the presence of the musculus pubisischiofemoralis internus insertion ( Fig. 13A View Fig 2 View Fig ) (Eltkin and Langer 2014). In the proximal-most portion of this surface, there is a ridge which is the result of cracking and does not correspond to any anatomical structure. The

WMNM P97555 ( Figs. 11 View Fig , 12 View Fig ): The bone is strongly weathered and is still partially embedded in sediment ( Fig. 11A View Fig 1 View Fig , A 2 View Fig ). Since the specimen shows ornamentation with thick ridges, it is identifiable as a dermal bone. However, due to poor preservation, it is impossible to determine what dermal bone it is.

posterodorsal region shows the highly weathered remains of the internal trochanter, located on the ventral-most area of this surface ( Fig. 13A View Fig 1 View Fig ). In the proximal portion of the posterodorsal region, there is the intertrochanteric fossa Fig. 13A View Fig 1 View Fig ). The distal half of the bone is broken off just proximal to the proximal end of the adductor crest.

WMNM P97553 was sectioned close to the natural fracture. The histological cross section would anatomically have an oval shape, but because of crushing, the long sides of the oval are concave, resulting in a figure-eight shape Fig. 14A View Fig 1 View Fig ). The inner region of the bone is crushed, so it is hard to estimate the original shape of the medullary region Fig. 14A View Fig 2 View Fig , A 3 View Fig ). The transition between medullary region and cortex seems to be gradual. The size of erosion cavities decreases gradually from very large in the central-most part of the section to smaller in the primary cortex. In the middle portion of the cortex, cavities become larger again, then return to smaller sizes in the external-most cortex ( Fig. 14A View Fig 4, A 5 View Fig ).

The cortex is characterized by lamellar-zonal bone tissue, with eleven annuli and zones easily distinguishable ( Fig. 14A View Fig 1 View Fig , A 4, A 5 View Fig ). In the avascular annuli, the primary matrix is made up by highly organized parallel-fibered bone, with thick collagen fibers. The annuli are separated by zones hosting only one row of vascular canals. In the deeper part of the cortex, the vascular canals are usually altered by remodeling, represented by the secondary osteons or erosion cavities. Towards the bone surface, the vascular canals become smaller and less remodelled and mostly are primary osteons or simple vascular canals. The matrix in the zones is a poorly organized parallel-fibered bone matrix. In the cortex the distinct decrease of the growth rate expressed by thinning of the growth rings is not observed, as well as the relatively low remodeling when compared to the second femur indicate that the bone belonged to a still actively growing individual.

WMNM P97790 ( Fig. 15 View Fig , 16 View Fig ; SOM 1: fig. 9): A nearly complete femur, the proximal and distal articulation facets are damaged by corrosion, however. The incomplete preservation of the proximal and distal ends necessitates caution in estimating the femur’s size in the living animal. The shaft is long and almost straight in ventral and dorsal view. The torsion angle between the proximal and distal end is approximately 10°. The bone is dorsoventrally flattened (with the exception of the midshaft), which results in a ratio between the long and short midshaft diameter of approximately 1:1.27 ( Fig. 15 View Fig ). At midshaft, the bone is almost round in cross-section and shows a pronounced curvature so that the dorsal edge is convex and the ventral edge is concave ( Fig. 15 View Fig ). On the anteroventral side, the shallow indentation of the fossa poplitea is visible ( Bystrow and Efremov 1940) ( Fig. 15A View Fig 1 View Fig ). In posterodorsal view, the distal end of the femur possesses a triangular indentation, the fossa tendinalis ( Bystrow and Efremov 1940), which has its maximum width at its most distal point while narrowing more proximally ( Fig. 15A View Fig 2 View Fig ). These indentations separate the posterior articulation area for the fibula and the anterior articulation area for the tibia. Ventrally, the adductor crest emanates from the anterior articulation area for the tibia ( Fig. 15A View Fig 5 View Fig ). It extends proximally into the internal trochanter, aligning with the anterior margin of the shaft.

The femur (WMNM P97790) was sectioned at mid-shaft ( Fig. 15A View Fig 1 View Fig ). The shape of the section is circular, with an elevation representing the adductor crest ( Fig. 16A View Fig 1 View Fig ). Apart from a thin outermost layer, the entire section is highly remodeled and spongious ( Fig. 16A View Fig 1 View Fig ). The remains of the primary cortex consist of lamellar-zonal bone with ten visible growth cycles ( Fig. 16A View Fig 2 View Fig ). The annuli are made of well organized parallel-fibered bone, whereas the zones show a less organized matrix ( Fig. 16A View Fig 2 View Fig ). The zones mostly consist of longitudinal primary osteons, but in the deeper cortex, secondary osteons and erosion cavities are more common ( Fig. 16A View Fig 2 View Fig ).

Remarks.—Based on the labiolingually elliptical shape of the tooth sockets, it is possible to narrow down the identification of WMNM P97559 to either Capitosauroidea or Chigutisauridae ( Warren and Davey 1992) . However, the morphology points more towards Capitosauroidea, since the posterior region is enlarged compared to the anterior one, whereas in Chigutisauridae , the area just posterior to the symphysial region is enlarged ( Schoch 1999; Warren 1981). Among Capitosauroidea, it is difficult to assign this specimen to a genus. This is because the most similar taxon is the Middle Triassic Mastodonsaurus giganteus Jaeger, 1828 , but the precoronoid and the presplenial seem to terminate more posteriorly in this taxon, leaving a larger portion of the dentary exposed on the lingual side ( Schoch 1999) than in WMNM P97559.

Since Cyclotosaurus was certainly present in Bonenburg, this would be the most logical attribution for WMNM P97559, but there are some anatomical inconsistencies. First, the groove on the labial side is parallel to the marginal tooth row in Cyclotosaurus ( Sulej and Majer, 2005) but diagonal in WMNM P97559. Second, the lingually exposed portion of the dentary is smaller in Cyclotosaurus ( Sulej and Majer 2005) than in the Bonenburg specimen. However, only one other dentary of Cyclotosaurus has been described ( Sulej and Majer 2005), therefore intraspecific variability cannot be excluded. It is not possible to confirm or exclude the Cyclotosaurus attribution of the specimen, nor to link it to any of the other fossils described herein. For these reasons, the specimen is assigned in open nomenclature to Capitosauroidea indet.

The ornamentation of the angular WMNM P97557, with its reticular pattern of ridges and polygonal pits, is a character only shared by Capitosauroidea and Metoposauroidea (Shishkin 1987; Sulej and Majer 2005; Sulej 2007; de Buffrénil et al. 2016). Since the ornamentation is more robust than in Metoposauroidea, the specimen is assigned to Capitosauroidea. The type of ornamentation resembles that of clavicle WMNM P97791, while it shows differences to the fragment of interclavicle WMNM P98583 and the dermal bone indet. WMNM P97555, that show deeper and larger valleys between sharper ridges (see Discussion). Thus, the angular may belong to a different species than WMNM P98583 and WMNM P97555, but at the same time it is impossible to confirm or exclude its attribution to Cyclotosaurus or any other of capitosauroid. For these reasons, the specimen is assigned in open nomenclature to Capitosauroidea indet.

With its anastomozing pattern, the ornamentation of the clavicle fragment WMNM P97791 is typical of capitosauroids and metoposaurids. Nonetheless, the ridges of this specimen are relatively robust, more similar to those of capitosauroids Schoch 1999; Sulej and Majer 2005). Furthermore, the microstructure with its high porosity seems to be more typical of capitosauroids when compared to Metoposaurus with its relatively compact bones ( Kalita et al. 2022). Based on morphological and histological data, we assign clavicle WMNM P97791 to Capitosauroidea indet., with a high probability of it belonging to the same taxon as the angular described above, given the similarity of their ornamentation. On the other hand, the differences in the dermal sculpturing from the fragment of interclavicle WMNM P98583 and the dermal bone indet. WMNM P97555 are evident and could reflect a taxonomic distinction (see Discussion). We cannot exclude the possibility that the clavicle WMNM P97791 and the angular WMNM P97557 pertain to the same Cyclotosaurus species represented by the fragmentary pterygoid WMNM P97556 and the humerus WMNM P64371.

The ornamentation of the interclavicle fragment WMNM P98583 formed by ridges, grooves, and polygonal pits is typical of Capitosauroidea (Shishkin 1987; Sulej and Majer 2005; de Buffrénil et al. 2016). Nevertheless, the thickness of the ridges, the depth of the grooves, and the wide distance between ridges is quite significant, especially in comparison with other specimens described herein. The exception is the Capitosauroidea indet. dermal bone WMNM P97555, which shows a robust ornamentation as well (see Discussion). The interclavicle fragment WMNM P98583 is thus assigned to Capitosauroidea indet., and it might pertain to a different taxon than the clavicle WMNM P97791 and the angular WMNM P97557.

The ornamentation type of WMNM P97555 is characteristic of both Capitosauroidea and Metoposauroidea, but the histology excludes metoposauroid affinities since their bone is more compact ( Kalita et al. 2022). The coarse nature of the bone ornamentation of WMNM P97555 suggest that it represent the same species of indeterminate capitosauroid as the fragmentary interclavicle WMNM P98583 (see above and Discussion) .

The femoral fragment (WMNM P97553) does not show the clear torsion between the proximal and distal end that is typical of Trematosauria, but it shows the less pronounced torsion characteristic of Capitosauroidea ( Schoch 1999; Sulej and Majer 2005). Also, Plagiosauridae can be excluded since they show an adductor crest arising from the posterior articulation area of the fibula, while in this specimen the adductor crest arises from the internal trochanter and terminates in the distal posterior articulation area for the tibia. This character and the less dorsoventrally flattened ends indicate a similarity of the specimen to Paracyclotosaurus davidii Watson, 1958 , rather than Mastodonsaurus giganteus (Jaeger, 1828) ( Watson 1958; Schoch 1999). The specimen herein described can thus be assigned to Capitosauroidea, with more similarities to Paracyclotosaurus than to Mastodonsaurus . Histological data suggest similarity with humerus WMNM P64371 and femur WMNM P97790 (see below), however again due to our scanty knowledge of the variation of histological characters among capitosaurs, it is difficult to state if these bones represent the same capitosauroid taxon at different ontogenetic stages or two separate taxa (see Discussion for more details).

Identification of WMNM P97790 as a capitosauroid femur emerges as the sole viable option from comparative analysis, given that the humerus in Stereospondyli features a short, flattened shaft with widened ends distally and posteriorly. The specimen shows resemblance with femur WMNM P97553 (see above). Regardless of its exact affinity, this specimen represents the largest Stereospondyli femur preserved in the fossil record. The limited availability of stereospondyl limbs, exemplified by the scarcity of described Brachyopoidea femora (only three worldwide), poses challenges to comparing specimen WMNM P97790 to other Stereospondyli . Nevertheless, the simplified limb bone morphology (compared to terrestrial archosaurs and synapsids) observed in Stereospondyli results from their adaptation to an aquatic lifestyle and aligns well with the characteristics exhibited by WMNM P97790. The femur of Mastodonsaurus giganteus reveals an elongated straight shaft with broadened proximal and distal ends, the latter being more expanded. In the dorsal view, the femur exhibits axial symmetry, convexity, and relative smoothness, except for the distal triangular fossa tendinalis. Conversely, the ventral aspect is concave, hosting a fossa poplitea and an asymmetrical adductor crest. These features demarcate articulation regions for the fibula and tibia in Mastodonsaurus , akin to those observed in WMNM P97790. Despite similarities, distinctions emerge in the dorsoventrally flattened articulation areas of M. giganteus compared to WMNM P97790. Notably, the adductor crest in M. giganteus originates from the distal posterior fibular articulation extending to the proximal central internal trochanter, while in WMNM P97790, it arises from the anterior tibial articulation extending to the anterior internal trochanter. The overall morphological resemblance between M. giganteus and WMNM P97790 suggests a probable affinity with Capitosauroidea.

Additionally, the preliminary study of the histology reveals similarities with the small femur WMNM P97553 and the cf. Cyclotosaurus sp. humerus WMNM P64371 (see above and Discussion). However, due to the rudimentary knowledge about histological variability among capitosaur ( Konietzko-Meier et al. 2019; Teschner et al. 2023) and very scanty morphological data for cyclotosaur postcrania ( Sulej and Majer 2005), it is not possible to confirm the Cyclotosaurus affinity of the femur, and it is thus assigned to Capitosauroidea indet. A dedicated study of the morphology and histology of WMNM P97790 is the subject of a forthcoming paper by one of the authors (LG).