Allosaurus whitei Pickering, 1995

Malafaia, Elisabete, Dantas, Pedro, Escaso, Fernando, Mocho, Pedro & Ortega, Francisco, 2025, Cranial osteology of a new specimen of Allosaurus Marsh, 1877 (Theropoda: Allosauridae) from the Upper Jurassic of Portugal and a specimen-level phylogenetic analysis of Allosaurus, Zoological Journal of the Linnean Society 204 (1) : -

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https://doi.org/10.1093/zoolinnean/zlaf029

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Allosaurus whitei Pickering, 1995
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Allosaurus whitei Pickering, 1995

Allosaurus europaeus Mateus, Walen & Antunes, 2006 .

Holotype: YPM VP 1930 About YPM , fragment of a dorsal centrum, fragments of caudal centra, rib fragment, midshaft of right humerus, right pedal phalanx III-1, lateral tooth crown ( Marsh 1877a).

Neotype: USNM 4734, almost complete articulated skull and associated skeleton ( Gilmore 1920, Paul and Carpenter 2010, Carrano et al. 2018, Yun 2019, ICZN 2023).

Type locality and horizon: Felch Quarry 1, Garden Park, Colorado. Brushy Basin Member, Morrison Formation; Kimmeridgian– Tithonian, Late Jurassic ( Gilmore 1920, Kowallis et al. 1998).

Referred material: DINO 2560 ( Madsen 1976), BYU-VP 8901, BYU-VP 2028 ( Smith and Lisak 2001), UMNH VP 16605 ( Madsen 1976), BYU-VP 5125 ( Britt 1991), BYU-VP 13679, USNM 8423 ( Gilmore 1920), USNM 2323 ( Gilmore 1920), USNM 8335 ( Gilmore 1920), USNM 8367 ( Gilmore 1920), AMNH 5753 ( Gilmore 1920), AMNH 666 ( Gilmore 1920), AMNH 851, AMNH 507, AMNH 5767, YPM VP 14544, YPM VP 1893, YPM VP 1931, CM 11844, CM 21703, MNHNUL/ AND.001 ( Pérez-Moreno et al. 1999), MNHN / UL.AND.1–18 and 20–22 ( Figs 2–23), ML 415 (Mateus et al. 2006), MG 27804 (formerly IPFUB Gui Th4) ( Rauhut and Fechner 2005).

Locality and horizon of the referred material: All specimens are known from North America and Portugal. The North American specimens came from the Salt Wash, Brushy Basin members, and laterally equivalent levels of the Morrison Formation, ranging from the Kimmeridgian to the Tithonian in age (e.g. Gilmore 1920, Madsen 1976, Kowallis et al. 1998, Chure and Loewen 2020). The Portuguese specimens came from the Alcobaça, Lourinhã, and Bombarral formations, ranging from the Kimmeridgian to the Tithonian in age (e.g. Kullberg et al. 2013, Fürsich et al. 2021).

Locality and horizon of the Andrés specimen: The specimen was collected in the Andrés fossil site in the Pombal municipality, district of Leiria, Portugal. The Bombarral Formation where the specimens were collected is Tithonian in age (e.g. Manuppella et al. 1999, Azerêdo et al. 2010).

Emended diagnosis: (i) Maxilla with two rows of nutrient foramina on the lateral surface, one extending along the ventral margin, sometimes within a well-developed groove posteriorly, and a second row that bounds the antorbital fossa anteriorly and converges with the ventral row posteriorly; (ii) ventral margin of the jugal with a marked convexity below the postorbital process ( Chure and Loewen 2020); (iii) fossa on the basioccipital ventral to occipital condyle less than 60% of condyle width and with mostly parallel margins; and (iv) high and triangular-shaped lacrimal horn in lateral view with sharply pointed dorsal and posterior margins.

Description

Maxilla

A well-preserved but dorsally incomplete left maxilla (MNHN/ UL.AND.22) was collected at the Andrés fossil site. Almost the entire ascending process is missing, but the articular surfaces for the premaxilla, lacrimal, and jugal are preserved ( Fig. 2). The anterior maxillary body and the alveolar process (=jugal ramus sensu Hendrickx and Mateus 2014) are also completely preserved. The maxilla measures 394 mm in length and the jugal ramus has a maximum depth of 72.46 mm at the level of the sixth alveolus (Supporting Information, Table S3). It has 17 alveoli with four erupting teeth in the second, fourth, sixth, and thirteenth alveoli. This is an unusual number of maxillary teeth for Allosaurus as there are few specimens reported to have also 17 tooth positions in the maxilla ( Madsen 1976). In the tomographic slices of the specimen, it is possible to see erupting teeth in several alveoli, particularly from the anterior part of the maxilla ( Fig. 3). This image also shows an erupting tooth together with a fully erupted one in the sixth alveolus. The alveoli extend to near the level of the most posterior end of the articular surface for the palatine. The first four anterior alveoli are rounded in outline, but they become more oval, anteroposteriorly elongated and strongly mediolaterally compressed to the posterior part of the maxilla ( Fig. 2J–K). The lateral surface of the maxilla is strongly ornamented by a series of thin grooves and crests and small foramina, especially along the anterior body. Another row of large foramina is visible adjacent to the ventral margin of the maxilla extending from the anterior end to the level of the fifteenth alveolus ( Fig. 2A–B). Posteriorly, behind the level of the ninth alveolus, the ventral foramina are placed inside a groove, which is delimited dorsally by a well-developed ridge and becomes deeper posteriorly. Another row of foramina starts near the mid-length of the maxilla anterior body and extends posteriorly, bounding the anteroventral corner of the lateral antorbital fossa ( Fig. 2B).

The maxillary anterior body is relatively well developed, with a length/height ratio slightly higher than 1. In lateral view the anterior body is squared shaped with a slightly tapered anterior margin. The suture for the premaxilla in the anterior margin of the maxilla is a broad concave surface delimited by medial and lateral crests that project anteriorly. The lateral crest projects more anteriorly with respect to the medial one and has a small, forked process at mid-height ( Fig. 2E, G). Dorsally to this lateral crest there is a shallow concavity, which corresponds to the ventral border of the subnarial foramen ( Fig. 2A–B). In anterior view, a small foramen pierces the surface for articulation with the premaxilla at about its mid-height. The anterior body of the maxilla is somewhat incomplete dorsally lacking the ascending ramus and the anteromedial process. The lateral antorbital fossa occupies almost the entire lateral surface of the jugal ramus and most of the anteroposterior length of the base of the ascending ramus as is typical for most early branching allosauroids (e.g. Gilmore 1920, Madsen 1976, Currie and Zhao 1993).

In medial view, there is a continuous interdental wall, which is ornamented by thin crests and grooves ( Fig. 2C–E). The interdental wall is low (depth:anteroposterior width ratio, measured between the walls of the alveolus <1.8), as occurs in Allosaurus fragilis , Allosaurus jimmadseni , and Sinraptor dongi Currie & Zhao, 1993 , but contrasting with the condition of carcharodontosaurian allosauroids that have interdental plates more than twice as deep as wide ( Brusatte et al. 2009, 2012, Carrano et al. 2012). The interdental wall is bounded dorsally by a well-marked, straight, and horizontal paradental groove (= nutrient groove sensu Hendrickx and Mateus 2014), which extends along almost the entire length of the medial surface of the maxilla from the most anterior margin to beneath the last alveolus. Small dental foramina are visible adjacent to the paradental groove at approximately the mid-length of each alveolus. The ventral margin of the interdental wall has a series of deep notches near the mid-length of each alveolus, which become more pronounced to the posterior part of the maxilla. In the last three alveoli these notches extend to the level of the paradental groove splitting the interdental wall on separated interdental plates ( Fig. 2E). The level of the ventral margin of the interdental wall is placed well above the ventral margin of the maxillary lateral wall, so the medial surface of the lateral wall is easily visible in medial view. Dorsally to the interdental wall, the medial surface of the maxilla is smooth, but shows an undulating surface (= vertical interalveolar depressions sensu Brusatte and Sereno 2007). A well-developed sub-horizontal ridge is visible in the dorsal margin of the medial surface of the maxilla, which rises from the level of the posterior margin of the seventh alveoli ( Fig. 2C–E). This ridge is broken anteriorly and only preserves a small fragment, but probably corresponds to the maxillary medial shelf ( sensu Hendrickx and Mateus 2014). Posterior to the medial shelf, the maxilla preserves a fragment of the posterior lamina of the ascending process, which is slightly concave in medial view.

The jugal ramus is dorsoventrally low, with a slightly concave ventral margin, and strongly tapers posteriorly, ending in a thin blunted point. The lateral surface of the jugal ramus is incomplete but the dorsal margin preserves a raised blade projecting dorsally that would articulate with the anterodorsal part of the jugal. The articular surface for the palatine is a longitudinal groove extending on the mediodorsal surface of the jugal ramus from the level of the 10th alveoli to behind the level of the last alveoli ( Fig. 2C–D). This articulation is more posterior than in most other allosauroids in which it lengthens to the eighth or seventh alveolus ( Eddy and Clarke 2011). This groove is deep and bounded by well-developed longitudinal crests on the anterior end but becomes shallower posteriorly. There are at least two small foramina inside the groove for articulation with the palatine. In lateral view, the posterior part of the jugal ramus has a shallow ventral concavity extending longitudinally just dorsal to the ventral margin of the maxilla. This longitudinal concavity is interpreted as the articular surface for the anteroventral end of the jugal ( Fig. 2A–B). The dorsal surface of the jugal ramus has four small foramina distributed along the anteroventral margin of the antorbital fenestra. Posterior to the last alveolus, the maxilla is mediolaterally thin with flat and slightly striated medial and lateral surfaces, which probably would insert into a groove on the anteroventral surface of the jugal as occurs in other Allosaurus specimens (e.g. Chure and Loewen 2020).

The ascending ramus is almost completely missing with only a fragment of the base preserved. Due to this break, a smooth opening is visible in the dorsal surface of the maxillary anterior body at the level of the anterior lamina of the ascending ramus, which is interpreted as the promaxillary fenestra ( Fig. 2H–I). This fenestra leads to a large fossa within the anterior end of the base of the ascending ramus that corresponds to the maxillary sinus ( sensu Madsen 1976, = maxillary antrum sensu Brusatte et al. 2009). Posterior to the promaxillary fenestra there is a large maxillary fenestra that occupies almost the entire lateral surface of the base of the ascending process. A large recess is present in the posterior end of the ascending ramus in a position equivalent to the posteromedial maxillary fenestra ( sensu Hendrickx and Mateus 2014, = posterior maxillary fenestra sensu Eddy and Clarke 2011), but it is not connected to the maxillary fenestra ( Fig. 2F). In medial view the maxilla has a nearly vertical anterior margin, but it is convex in lateral view due to the presence of a lateral lamina that projects anteriorly and delimits a deep vertical concavity that would receive the posterior margin of the premaxilla.

Nasal

An almost complete right nasal, MNHN/UL.AND.1, was collected in a block with elements of the posterior part of a left mandible. The nasal is well preserved but was somewhat broken during fieldwork and the posterodorsal part is badly distorted especially at the level of the nasal lateral crest ( Fig. 4). It has an open internasal suture as is typical for most allosauroids (e.g. Madsen 1976). The bifurcating anterior part of the nasal consists of the premaxillary (dorsal) and maxillary (ventral) processes. These processes form the posterodorsal rim of the external naris, which has an oval, anteroposteriorly elongated shape. The dorsal ramus is incomplete, but it clearly shows a deep bifurcation that would receive the anterodorsal process of the premaxilla ( Fig. 4E). This ramus is significantly longer and more robust than the ventral one. The ventral ramus has a shallow longitudinal groove along the ventral margin that represents the surface for articulation with the maxilla. Posteriorly, this ramus has a small process projecting ventrally and is separated from the ventral margin of the nasal body by a deep notch ( Fig. 4F–G).

In lateral view, a poorly-developed lateral crest extends along the dorsal margin of the nasal ( Fig. 4A–B). A shallow concavity divides this crest on an anterior, lower protuberance and a slightly higher, more ornamented, and dorsomedially oriented posterior projection. The lateral crest ends posteriorly in a deep slot that is interpreted as the surface for articulation with the anterior ramus of the lacrimal. The lateral margin of the dorsal surface is ornamented by a series of well-developed ridges and grooves that continue up to the surface for articulation with the dorsal margin of the lacrimal ( Fig. 4A–B). The ornamentation of the lateral ridge of the nasal probably would have continued into the anterodorsal margin of the lacrimal as occurs in other Allosaurus specimens (e.g. Madsen 1976). At approximately the anteroposterior mid-length, the nasal is broken allowing observation of the internal structure with several large cavities that seems connect to each other ( Fig. 4C, J). The lateral surface of the nasal has at least two, and probably three, well-developed foramina that pierce the anteroventral surface. These foramina are placed in a smooth and shallow concavity corresponding to the extension of the antorbital fenestra into the nasal ( Fig. 4A–B). In dorsal view, the nasal is an anteroposteriorly long and transversely narrow element that slightly expands anteriorly, forming a low crest that arises near the base of the dorsal anterior ramus and extends posteriorly along about 210 mm of the dorsolateral margin of the nasal ( Fig. 4K–L).

The medial surface of the nasal is smooth for most of the preserved anterior part and has a well-developed crest that arises from the anterodorsal margin of the dorsal ramus ( Fig. 4D–E). This crest, interpreted as the medial symphysis of the nasal, projects posteroventrally up to approximately the mid-length of the nasal. Posterior to the level of the base of the ventral ramus, the medial symphysis has a flat medial margin, which is interpreted as the surface for articulation with the opposite nasal ( Fig. 4E). The medial symphysis delimits a shallow and broad concavity that occupies almost the entire anterior end of the nasal medial surface. Most of the posteroventral part of the nasal is broken, but it preserves part of the anterior margin and the suture for the maxilla. This suture is a shallow groove extending along the ventromedial surface of the nasal and is bounded dorsally by a low ridge that is subparallel to the nasal ventral margin ( Fig. 4D–E). The suture for the maxilla continues into the ventral margin of the ventral ramus, where it forms a shallow longitudinal groove that is separated from the nasal body by a small notch. Posterodorsally to this notch the medial surface of the nasal has a deep groove with a half-moon shape that may correspond to a pneumatic recess ( Fig. 4F–G).

The posterior part of the nasal is a dorsoventrally thin blade that projects ventromedially. The dorsal surface of this blade is slightly concave and is bounded laterally by a prominent longitudinal ridge. The ventral surface has a series of thin longitudinal ridges extending along the flat posterior end of the nasal, which is interpreted as the area where the nasal would overlap the frontal ( Fig. 4I). The posterolateral end of the nasal has a process that projects posteriorly and delimits a deep groove that would receive the anterior ramus of the lacrimal. This process is broken, but the preserved fragment suggests that it would be relatively robust. Posterior to the lacrimal process, the nasal has an almost flat surface extending along the lateroventral margin, which is interpreted as the articular surface for the prefrontal.

Lacrimal

Almost complete right, MNHN/UL.AND.2, and left, MNHN/ UL.AND.3, lacrimal bones were found in the Andrés fossil site ( Fig. 5). The right lacrimal is the most complete lacking only a small section of the ventral end of the preorbital ramus. The left lacrimal also lacks a small part of the ventral end of the preorbital ramus. In lateral view, these elements have an inverted V-shape, with an almost straight ventral process (preorbital ramus) and an anteriorly tapered dorsal process (rostral ramus) projecting anteroventrally. The preorbital and rostral rami make an acute angle between each other, this angle (measured between the anterior margin of the preorbital ramus and the ventral margin of the rostral ramus) being approximately 66º in the left lacrimal and 69º in the right element. The preorbital ramus articulates ventrally with the jugal and the rostral ramus articulates with the nasal laterally and with the ascending process of the maxilla ventrally. The rostral ramus is long, almost as long as the preorbital ramus, but it is dorsoventrally narrow and transversely thin. The anterior part of the rostral ramus projects laterally delimiting a shallow longitudinal concavity that extends along the ventral surface, representing the contact with the nasal. The dorsal margin of the anterior end of the rostral ramus has a small longitudinal notch, which is interpreted as the surface for articulation with the ascending ramus of the maxilla. Between the surfaces for articulation with the nasal and the articulation with the maxilla, the rostral ramus of the lacrimal has a broad, longitudinal canal that pierces the dorsal surface with an anteroposterior orientation, which corresponds to the nasolacrimal duct ( Witmer 1997a, Rauhut 2003a). This canal projects posteriorly along the ventral margin of the rostral ramus of the lacrimal and connects with a set of pneumatic foramina that pierce the posterior margin of the preorbital ramus ( Fig. 6).

The preorbital ramus is almost straight and has a mostly circular cross-section at mid-height, but sharply expands distally forming a fan-like, transversely thin, and anteroposteriorly broad blade. In posterior view, it has distinct medial and lateral layers separated by a deep sulcus, whereas the anterior margin is mostly rounded in the left lacrimal but has a small concave surface ventrally in the right one. The posterior margin of the lateral layer of the preorbital ramus is slightly convex, with a low projection extending into the orbital fenestra, whereas the anterior margin is almost straight with a very subtle bulge ( Fig. 5). The ventral end of the preorbital ramus is mostly flat in lateral view. In the left lacrimal, a small notch is visible adjacent to the posteroventral margin of the ventral expansion, which is interpreted as the suture for the jugal ( Fig. 5B–C). The dorsal extension of the suture for the jugal in the preorbital ramus is difficult to interpret, but a small vertical ridge extending anterodorsaly on the lateral surface is interpret as the posterior limit of this suture. Dorsally, the lateral surface of the preorbital ramus shows a very rough area near the base of the cornual process, which occupies almost the entire dorsal end of the lateral surface and is bounded by low posterior and anterior crests ( Fig. 5A–C). The anterior crest projects posteroventrally, connecting with the ridge that forms the posterior margin of the suture for the jugal. The posterior crest projects posteriorly and extends slightly into the orbital fenestra. The posterior surface of the preorbital ramus has a deep vertical groove, inside which two foramina are visible with the dorsal foramen slightly larger than the ventral one ( Fig. 5J–M). This posterior groove and associated foramina correspond to the naso-lacrimal canal ( sensu Eddy and Clarke 2011) or to the lacrimal duct ( sensu Currie and Zhao 1993) and connects with the canal that exits in the anterior end of the rostral ramus, as described above. Small foramina are also present on the anterior surface of the preorbital ramus (three on the right lacrimal and one on the left). On the right lacrimal these foramina are placed inside a shallow, but well-marked vertical groove, whereas in the left lacrimal the anterior surface of the preorbital ramus is rounded and without any visible groove ( Fig. 5N–R). In medial view, the preorbital ramus has a well-marked vertical ridge projecting anterodorsally and delimiting a narrow vertical groove at approximately the mid-height of the ramus ( Fig. 5D–F). This groove is similar to the medial vacuity described in the lacrimal of Sinraptor dongi ( Currie and Zhao 1993) .

The dorsal surface of the lacrimal has a well-developed and strongly ornamented cornual process, which is triangular in lateral view, longer than deep, and with a somewhat tapered dorsal margin. The lateral and medial surfaces of this process have several well-marked vertical crests and grooves resulting in an extremely rough appearance ( Fig. 5G–I). These crests and grooves also extend along most of the dorsal margin of the rostral ramus. The dorsolateral surface of the lacrimal has two large pneumatic recesses, the posterior one is vertically elliptical and pierces the base of the cornual process, whereas the anterior recess is horizontally elliptical and pierces the posterior end of the rostral ramus ( Fig. 5A–C). The posterior recess on the left lacrimal is much larger than that on the right lacrimal. In the latter, a vertical and relatively thin blade separates the recesses, whereas in the left lacrimal they are separated by a wider surface. The morphology of the recesses is also distinct on the right and left lacrimal elements. The anterior recess on the right lacrimal is subdivided by a thin, vertical septum, which is not present in the left lacrimal ( Fig. 5B–C). In addition, the right lacrimal has a third smaller foramen at the anterior end of the rostral ramus that is not present in the left one. The lacrimal bones are strongly pneumatic, with several interconnected chambers within the body of the lacrimal, particularly in the region of the cornual processes but also along at least half the length of the rostral ramus ( Fig. 6). These chambers are probably the lacrimal diverticulum that forms part of the paranasal sinuses generally present in most theropod dinosaurs (e.g. Witmer 1997a, Witmer and Ridgely 2008, Gold et al. 2013). In anterior view, two grooves separated by a thin crest at the base of the cornual process represent the articulation for the prefrontal ( Fig. 5J–M). The medial surface of the lacrimal is mostly smooth except around the cornual process.

Prefrontal

An almost complete left prefrontal, MNHN/UL.AND.5, was collected in Andrés ( Fig. 7). This is a T-shaped element that comprises a long ventral ramus and a shorter dorsal ramus. The ventral ramus is a blade-shaped and transversely thin process that strongly tapers ventrally. The dorsal ramus is also a transversely thin and distally tapered process that projects anteroventrally making an acute angle with the dorsal end of the ventral process. The prefrontal articulates with the lacrimal laterally and ventrally, with the frontal posteromedially, and with the posterior process of the nasal anteriorly. This element, together with the lacrimal form the anterodorsal margin of the orbital fenestra. In lateral view, a prominent vertical ridge projects from the dorsal end of the prefrontal into the dorsolateral surface of the ventral ramus. This ridge delimits a deep and funnel-like concavity that would receive the prominent triangular process on the medial surface of the lacrimal near the junction of the ventral and dorsal rami ( Fig. 7C–D). The prefrontal connects with the lacrimal also along most of the dorsoventral length of the ventral process ( Fig. 7N–O). Medially, a shallow and broad concavity opens between the dorsal and ventral rami. Inside this concavity, there are two low crests projecting anterodorsally and a small foramen adjacent to the dorsalmost crest ( Fig. 7A–B). Dorsally and posterodorsally to the medial concavity, the prefrontal has strongly rough margins corresponding to the suture for the frontal posteriorly and for the nasal anteriorly along the dorsal ramus.

The dorsal ramus has a well-developed blade extending ventrally, which forms a deep anteroposterior slot near the junction of both rami. In dorsal view, the dorsal ramus is smooth, slightly concave, and transversely thin, indicating a reduced exposure of the prefrontal in the skull roof. The dorsal ramus is broken and lacks a small part of the mid-section, but a shallow longitudinal concavity is visible near the posterior end, which probably represents the anterior surface for articulation with the nasal ( Fig. 7A). Posteriorly, the dorsal surface of the prefrontal has a transversely thin, triangular, and strongly striated process projecting dorsomedially with a posterolateral-anteromedial orientation, which would fit in a deep notch on the lateral surface of the frontal. In posterior view, the prefrontal is slightly concave with a broad dorsal end that strongly tapers ventrally. The dorsal margin of the posterior surface has a series of thin vertical ridges and grooves. A small foramen pierces the dorsal end of the prefrontal posterior surface ( Fig. 7E–G). MNHN/UL.AND.5 articulates well with the lacrimal (MNHN/UL.AND.3) and with the frontal that is articulated with the braincase (MNHN/ UL.AND.21) suggesting that these elements probably belong to the same individual ( Fig. 7N–S).

Postorbital

An almost complete left postorbital, MNHN/UL.AND.4 ( Fig. 8A–J), and parts of a right postorbital, MNHN/UL.AND.20 ( Fig. 8K–L), including the ventral ramus and the most anterior portion of the squamosal process, were collected in Andrés. The partial right postorbital was collected in a block containing elements of the braincase and the dimensions of the preserved fragments are similar to those of MNHN/UL.AND.4, suggesting that these three elements probably belong to the same individual. The left postorbital has the ventral ramus slightly incomplete distally. In lateral view, this element has a T-shape, with an almost vertical and straight ventral ramus (= jugal process), a dorsal ramus projecting posteriorly (= squamosal process), and a short anterior expansion that contacts the frontal medially ( Fig. 8A–B). The jugal process is robust and anteroposteriorly broad dorsally, but strongly tapers ventrally. The ventral part of the jugal process has a shallow and broad concavity, which represents the surface for articulation with the ascending process of the jugal ( Fig. 8I–J). This concavity extends ventrally from approximately the mid-height of the process along the posteromedial surface and is delimited by blade-shaped projections of the medial and lateral margins of the ventral ramus. The squamosal process projects posteriorly making an angle of approximately 90º with the ventral ramus. In lateral view, the squamosal process is a relatively deep, but transversely thin blade with almost parallel dorsal and ventral margins anteriorly along near half of its length. At about the mid-length of the ramus, a sharp step strongly reduces the depth of the process, so it strongly tapers posteriorly. The posterior part of the squamosal process shows a well-marked dorsal concavity with a series of thin longitudinal grooves, which represents the area for articulation with the squamosal ( Fig. 8C–F). This tapered posterior end of the dorsal ramus of the postorbital would fit in a deep groove on the dorsolateral surface of the squamosal. The lateral surface of the postorbital has a well-developed, relatively high, and rugose crest extending dorsoventrally along the anterior surface of the postorbital, just adjacent to the posterodorsal margin of the orbital fenestra ( Fig. 8A–B).

In medial view, the postorbital has a deep and broad concavity, occupying the anterior end of the squamosal process, near the junction of the squamosal and jugal processes. This dorsoventrally elongated concavity is bounded by prominent crests and represents the surface for articulation with the head of the laterosphenoid ( Fig. 8C–D). A series of small foramina are visible inside this concavity. Dorsal to the surface for the laterosphenoid, the postorbital has a well-marked and transversely wide but shallow groove that corresponds to the suture for articulation with the parietal. Just anterodorsal to the articulation for the laterosphenoid, a deep and narrow groove, representing the suture for articulation with the frontal, extends dorsally along the anterodorsal margin of the squamosal process. The suture for the prefrontal is a small notch in the anterodorsal part of the postorbital, which opens on the medial surface ( Fig. 8C–F). In medial view, the ventral margin of the dorsal ramus and the dorsal part of the posterior margin of the ventral ramus are occupied by a broad concavity, which is delimited dorsally by a well-marked ridge extending longitudinally along the mid-height of the squamosal process. This concavity marks the extension of the lateral temporal fenestra into the postorbital ( Fig. 8C–D). In dorsal view, the squamosal process forms a transversely thin ridge, separated from the surface for contact with the laterosphenoid by a relatively deep groove ( Fig. 8E–F). This groove forms the floor of the anterolateral corner of the supratemporal fossa and has three small foramina inside it. In anterior view, the postorbital is concave dorsally and the anterior surface is mostly smooth and relatively broad transversely in the dorsal part. This concave dorsal surface forms the posterodorsal border of the orbital fenestra.

Frontal

A posterior part of a right frontal, MNHN/UL.AND.6, was collected during the first fieldwork campaign (1988) in Andrés ( Fig. 9). Later, in 2005, an articulated braincase was found, which preserves fragments of the posterior part of both right and left frontal bones. These last elements correspond to a significantly larger individual compared with the frontal collected in 1988 (the frontals articulated with the braincase, MNHN/ UL.AND.21, are approximately 1.5 times wider than MNHN/ UL.AND.6: see Supporting Information, Table S3). MNHN/ UL.AND.6 corresponds to the posterior part of a right frontal articulated with a fragment of the parietal ( Fig. 9). The suture between the frontal and parietal is strongly interdigitated. The frontal has a rectangular shape in dorsal and ventral views, slightly narrowing anteriorly.The dorsal surface is mostly smooth and flat, except in the posterolateral corner where a well-marked ridge extends anterolaterally from the surface for contact with the parietal. This ridge delimits a shallow concavity that corresponds to the anterodorsal border of the supratemporal fossa. The lateral margin of this concavity has a deep longitudinal groove that is interpreted as the surface for contact with the postorbital ( Fig. 9C–D). The articulation with the prefrontal in the lateral surface of the frontal anterior part is a deep, triangular-shaped groove that would receive the tapered posterior process of the prefrontal ( Fig. 9C–D). Between the articular facets for the postorbital and prefrontal, there is a narrow surface with a series of thin striae that represent the dorsal opening of the orbital fenestra. The interfrontal suture is open, but the contact between the frontals was likely firm due to the presence of a series of thin vertical ridges and grooves along most of its length. The ventral surface of the frontal is marked by a large, crescentic scar interpreted as the surface for articulation with the orbitosphenoid ( Fig. 9E–F). Medially to this scar, a groove for the olfactory bulbs extends parasagitally along the midline of the frontal. This groove is better visible in the frontals that are articulated with the braincase.

The frontal bones articulated with the braincase are broken at the level of the anterior margin of the surface for contact with the prefrontal and the anterior part is missing. The interfrontal and the frontoparietal sutures are visible along the entire preserved fragment. The frontal is almost as transversely wide (at the widest point across the level of the surface for contact with the postorbital) as are long (see Supporting Information, Table S3). It contacts with the parietal posterodorsally and with the laterosphenoid ventrally. In dorsal view, the supratemporal fossa occupies the posterior part of the frontal and extends into the anterolateral surface of the parietal. These fossae are bounded by a pair of crescentic crests that have been interpreted as the insertion of the pseudotemporalis muscle ( Chure and Loewen 2020). In anteroventral view, the frontal shows a deep longitudinal groove extending along the midline of the articulated elements, which expands anteriorly in a shallow depression of lobulated shape. These depressions represent the olfactory bulb cavities and are delimited by pair of low, but well-marked crests interpreted as corresponding to the sphenethmoid scars ( Brusatte et al. 2010a). Posteriorly, the olfactory bulbs narrow and connect with the endocranial cavity through a deep groove.

Palatine

A complete and well-preserved left palatine (MNHN/ UL.AND.7) was collected in Andrés ( Fig. 10). In lateral and medial views,the palatine is a tri-branched element formed by two anterior rami, the maxillary process and the medial symphysis (= vomeropterygoid process sensu Brusatte et al. 2008), and a posterior broad blade that forms the suture for contact with the jugal ( Fig. 10A–B). The maxillary process is a thin and tapered ramus with a shallow longitudinal groove representing the suture with the maxilla that extends along the ventral surface for approximately the anteroposterior mid-length of the palatine. The medial symphysis is a mediolaterally thin but dorsoventrally broad and L-shaped blade projecting anteroventrally. The anterodorsal margin of the symphysis is nearly straight and strongly narrowing anteriorly so it would articulate with the vomer through a tapered end ( Fig. 10A–B). The dorsomedial margin of the symphysis has a series of thin grooves and crests corresponding to the surface for articulation with the opposite palatine ( Fig. 10A–B). The medial symphysis and the maxillary process delimit an elongated and broad opening that corresponds to the fossa for the internal naris or internal narial choanae.

In lateral view the posterior part of the palatine has two bladelike and short processes, the jugal and the pterygoid processes. The jugal process is a mediolaterally thin blade, slightly expanded dorsoventrally and projecting posteriorly. This process has a series of thin, longitudinal grooves and crests along the posteromedial surface, representing the articulation for the jugal, which is delimited ventrally by a deep groove ( Fig. 10C–D). This groove separates the blade of the jugal process from a dorsoventrally thin ramus corresponding to the pterygoid process that is slightly shorter than the jugal process. In ventral view, the pterygoid process has a well-marked groove extending anterodorsally from the posterior margin and delimiting a subcircular surface that represents the area for articulation with the pterygoid. Any pneumatic recess is present on the lateral surface of the palatine. In medial view, the palatine has a deep concavity between the pterygoid process and the medial symphysis, corresponding to the ventral margin of the pterygopalatine fenestra ( sensu Eddy and Clarke 2011). In dorsal view, a deep groove extends anteroposteriorly between the jugal and the pterygoid processes ( Fig. 10E–F). The medial surface of the palatine has a deep and circular recess that pierces the posterior part near the base of the pterygoid process.

Quadrate and quadratojugal

The articular region of a right quadrate, MNHN/UL.AND.9, was collected during the first fieldwork campaign in Andrés (in 1988) and an almost complete left quadrate articulated with the quadratojugal (MNHN/UL.AND.10) was found in 2005 ( Fig. 11). These elements are approximately the same size and similar morphology suggesting that they belong to the same individual. The quadrate has a nearly vertical posterior shaft that connects the articular ventral condyles with the quadrate cotylus (= quadrate head sensu Hendrickx et al. 2014) dorsally. A broad and thin blade (pterygoid flange or pterygoid wing) projects anteromedially for most of the dorsoventral length of the shaft. This blade is slightly broken anterodorsally but is otherwise well preserved. In medial view, the pterygoid flange is delimited ventrally by two robust crests, one projecting dorsoventrally along the medial surface of the shaft (the quadrate ridge sensu Hendrickx et al. 2014) and another projecting anteriorly from the anterodorsal surface of the medial condyle ( Fig. 11D–F). These crests form an angle of approximately 70º and delimit a transversely deep concavity (medial fossa of the quadrate sensu Hendrickx et al. 2014) in the ventral part of the pterygoid flange. The quadrate cotylus, which represents the articular surface for the squamosal, has an almost circular shape and a slightly convex dorsal margin.

In lateral and medial views, the quadrate shaft is slightly concave, with a dorsal end projecting posteriorly to the level of the most posterior margin of the distal condyles. In posterior view, the shaft is bounded by two prominent longitudinal crests, the quadrate ridge that extends along the medial margin of the quadrate and other extending along the suture with the quadratojugal ( Fig. 11G–I). These crests delimit a deep vertical groove in the posterior surface of the quadrate shaft. The quadrate shaft slightly twists dorsally projecting somewhat laterally. Near the mid-height the shaft is pierced by a large foramen opening adjacent to the suture with the quadratojugal but that is almost surrounded by the quadrate ( Fig. 11G–I). Dorsally, the quadrate ridge has a deep notch that opens ventrally and connects with the foramen by a shallow groove extending dorsoventrally.

The quadrate has two well-developed articular distal condyles, with the medial one (= entocondyle) extending slightly ventral to the level of the lateral condyle (= ectocondyle). In anterior view, the condyles project somewhat dorsally. The medial condyle is significantly smaller than the lateral one. In ventral view, the condyles are oval-shaped, anteroposteriorly elongated and are separated by a shallow concavity (the intercondylar sulcus) oriented posteromedially ( Fig. 11J–L).

The quadratojugal articulates with the posterolateral surface of the quadrate shaft along nearly its entire depth. This articular surface is somewhat sigmoid, extending along the posterior surface of the quadrate shaft up to the level of the quadrate foramen and then shifting dorsally to the lateral surface. The quadratojugal is an L-shaped element with a vertical ramus for articulation with the squamosal and an anterior, nearly horizontal, jugal process ( Fig. 11A–C). These processes are positioned at an angle of approximately 90º and form the posteroventral margin of the lateral temporal fenestra. The squamosal ramus is mediolaterally thin and relatively broad anteroposteriorly. This process is broken dorsally so the morphology of the suture with the precotyloid process of the squamosal is unknown. The jugal process is a robust, distally tapered ramus projecting anterolaterally. In lateral view, the anterior part of the jugal process has a deep longitudinal groove extending along the dorsal margin, which would articulate with the quadratojugal process of the jugal. In medial view, the jugal process has a deep longitudinal concavity that extends along the ventral margin and is delimited by two crests projecting along the lateral and medial margins of the ventral surface.

Ectopterygoid

A fragment interpreted as corresponding to a partial right ectopterygoid lacking the anterior end (MNHN/UL.AND.8) was collected in Andrés ( Fig. 12). This element has a hook shape in dorsal view and has two almost parallel rami extending posteriorly and delimiting a relatively narrow and mediolaterally elongated notch that corresponds to the subtemporal fossa (= infraorbital fenestrae sensu Chure and Loewen 2020). The lateral ramus would contact the jugal and has a triangular shape, is dorsoventrally thin and strongly tapers posteriorly. In dorsal view this ramus has a shallow longitudinal groove along the lateral surface that is interpreted as the suture for the jugal ( Fig. 12A–B, G–H). The medial ramus is more rounded and thinner than the lateral one and would contact the pterygoid along the medioventral surface. The ventral margin of the ectopterygoid has a crest extending from the anterior margin of the jugal ramus to near the medial margin of the ectopterygoid, which delimits a relatively well-developed concavity that may correspond to the pneumatic recess described in other allosauroids (e.g. Currie and Zhao 1993, Eddy and Clarke 2011, Chure and Loewen 2020). In ventral view the ectopterygoid has a smoothly convex surface with a shallow rugose anterior concavity ( Fig. 12C–D).

Squamosal

A complete and well-preserved right squamosal, MNHN/ UL.AND.14, was collected in the Andrés fossil site ( Fig. 13). Also, a complete left squamosal of identical size and morphology was recovered articulated with the braincase. In lateral view, the squamosal comprises a ventral, short and blunted ramus (postcotyloid process), a mediolaterally thin blade projecting anteroventrally that represents the surface for articulation with the quadratojugal (quadratojugal process), and a dorsal process with a deep groove, which would receive the squamosal process of the postorbital ( Fig. 13A–B). A deep and circular notch opens anteroventrally between the postcotyloid process and the quadratojugal process, which corresponds to the articulation for the quadrate cotylus. The postcotyloid process is a short ramus that projects anteroventrally and slightly constricts the opening for the quadrate cotylus ventrally. The anterior margin of the quadratojugal process is strongly convex and would project into the lateral temporal fenestra, slightly constricting this opening posteriorly. The distal part of the quadratojugal process is broken and the morphology of the suture with the quadratojugal cannot be known. However, based on the preserved part it seems that this suture would be not straight, but most probably sigmoidalshaped. The lateral surface of the quadratojugal process has a series of well-marked ridges and grooves ( Fig. 13A–B). The articulation with the postorbital in the lateral surface of the dorsal ramus of the squamosal corresponds to a deep, triangular groove, which strongly broadens posteriorly and divides the ramus into a dorsal and a ventral prong across its entire length. Posterior to the groove for articulation with the postorbital, another shallower groove projects dorsoventrally adjacent to the dorsal margin of the articulation for the postorbital.

In medial view, a relatively robust ramus that corresponds to the parietal process projects anterodorsally ( Fig. 13C–D). The parietal process is thin and relatively long, with a deep groove in the dorsomedial surface, and is bounded by a vertical triangular ridge extending from the dorsal margin of the quadrate cotylus along the anteromedial margin. This ridge marks the medial margin of a deep transverse concavity that represents the posterodorsal end of the lateral temporal fenestra. The quadrate cotylus is slightly projected from the medial surface of the squamosal and is surrounded by a well-marked ridge. In posterior view, the squamosal has a fan shape, with a thin dorsal lamina that is strongly convex transversely and a mediolaterally compressed postcotyloid process, which forms a thin vertical ridge ( Fig. 13E–F). The quadrate cotylus opens ventrally with a sub-circular outline in medial and lateral views and a somewhat triangular shape in ventral view. It has slightly concave posterior and dorsal margins ( Fig. 13I).

Vomer

The vomer, MNHN/UL.AND.15, is almost complete and well preserved, but somewhat distorted, being slightly compressed mediolaterally ( Fig. 14). This is a long and thin element that would articulate with the premaxilla anteriorly, with the maxilla anterolaterally, and with the pterygoid posterolaterally ( Chure and Loewen 2020). In dorsal view, the vomer bifurcates posteriorly, forming two parallel blade-shaped and mediolaterally narrow processes that are separated by a deep notch extending to near the mid-length of the vomer. This notch has been interpreted as for receiving the vomeropalatine ramus of the pterygoid in Acrocanthosaurus atokensis Stovall & Langston, 1950 ( Eddy and Clarke 2011) and Tyrannosaurus rex Osborn, 1905 ( Madsen 1976) or for the attachment to the anterodorsal processes of the palatines in Allosaurus fragilis ( Madsen 1976) . The posterior processes (= palatine blades) are distally striated along the posterior part of the lateral surface, possibly representing the area for articulation with the pterygoid ( Chure and Loewen 2020). Anteriorly, the vomer is a dorsoventrally thin ramus that slightly expands mediolaterally and strongly tapers to the anterior part. The dorsal surface has a series of thin, longitudinal crests and grooves and a well-developed longitudinal crest extending along the mediolateral mid-width. This anterior process is somewhat ventrally deflected in lateral view ( Fig. 14A–D).

Braincase

General description: An almost complete, articulated and well-preserved braincase (MNHN/UL.AND.21) was collected in Andrés ( Figs 15–18). This specimen includes the posterior part of the frontals, the parietals, the supraoccipital, the prootics, the exoccipital-opisthotic complex, the basioccipital, the basisphenoid, and the laterosphenoid. As previously mentioned, a left squamosal is also preserved articulated with the braincase. The occipital part of the braincase is constituted by the parietals, the supraoccipital, the exoccipital-opisthotic complex, and the basioccipital ( Fig. 15A–B). The foramen magnum is bounded mostly by the exoccipital-opisthotic complex laterally and by a small portion of the supraoccipital and basioccipital dorsally and ventrally, respectively. The foramen is oval, slightly transversely wider than high. The occipital condyle is rounded and is formed mostly by the basioccipital and by a small portion of the exoccipital-opisthotic complex dorsally. The dorsal surface of the condyle is slightly concave representing the expansion of the foramen magnus into the condyle.

The posterior and medial margins of the supratemporal fenestrae are preserved. These open in the posterolateral part of the braincase, being delimited posteriorly and dorsomedially by the parietals, anteromedially by the frontals, and ventrally by the laterosphenoid ( Fig. 17). The posterior part of the braincase is somewhat compressed anteroposteriorly so the exact shape of the supratemporal fenestra is unknown, but it is apparently larger mediolaterally than anteroposteriorly, which would result in an oval-shaped opening. The right and left fenestrae are separated medially by a plate formed by the anterior end of the parietals and the posterior part of the frontals. The skull table between the supratemporal fenestrae is slightly concave in dorsal view and is delimited medially by two well-developed, crescentic crests ( Fig. 17E–G). The endocranial cavity is bounded by the laterosphenoids laterally, by the prootic ventrally, and by the parietal posteriorly ( Fig. 16).

Parietal: In occipital view, the parietals form the dorsal part of the braincase ( Fig. 15A–B, E). These elements contact the frontals anterodorsally, the laterosphenoid anteroventrally, the supraoccipital posteromedially, the exoccipital-opisthotic complex posteroventrally, and the squamosal laterally. All these sutures are easily visible despite being somewhat obscured at some points due to distortion. The interparietal suture is visible dorsally and is strongly interdigitated ( Fig. 15E–G). The frontoparietal suture is also strongly interdigitated dorsally but becomes more sinuous ventrally. The parietals have two mediolaterally broad posterior blades expanding laterally around the supraoccipital and an anterior short process for articulation with the frontals. The posterior blades of the parietals have an almost vertical orientation in posterior view and form the posterior margin of the temporal fossa. The suture between the parietals and the supraoccipital is visible and has a sinusoidal contour ( Fig. 15A–B, E). The parietal blades are sub-circular in shape, slightly deeper dorsoventrally than transversely wide, and project slightly dorsally, but do not surpass the dorsal margin of the supraoccipital. Ventrally, the parietal blades have a pair of thin and long processes that project along the dorsal margin of the paroccipital processes of the exoccipital-opisthotic complex ( Fig. 17A–C). In dorsal view the parietal has a tongue-like process that projects posteriorly and slightly overlaps the dorsal margin of the supraoccipital crest ( Fig. 17E–G). The dorsal margin of the parietals forms an anteroposteriorly thin but relatively well-developed nuchal crest (= transverse parietal crest sensu Chure and Loewen 2020 or transverse nuchal crest sensu Eddy and Clarke 2011) that projects transversally and extends slightly above the skull table. Anteriorly, the parietals have a short process for articulation with the frontals, which is constricted posteriorly by the supratemporal fossae. In dorsal view, the anterior end of the parietals is relatively broad and slightly concave between the supratemporal fossae.

Supraoccipital: The supraoccipital contacts the parietals dorsally and laterally and the exoccipital-opisthotic complex ventrally. In occipital view, the supraoccipital has an inverted T-shape with a vertical, relatively high crest (supraoccipital wedge sensu Rauhut 2003a, supraoccipital knob or tuberosity sensu Sampson and Witmer 2007, Brusatte et al. 2010a, postnuchal supraoccipital crest sensu Chure and Loewen 2020) extending along most of the dorsoventral height of the parietals, and two ventral processes projecting laterally ( Fig. 15A–B, E). These ventral processes articulate with the dorsomedial margin of the paroccipital processes of the exoccipital-opisthotic complex. Despite being strongly distorted, it is possible to infer the morphology of the supraoccipital crest, which would be massive and triangular in section. The supraoccipital crest is slightly broader dorsally and extends somewhat above the dorsal margin of the parietal blades. However, this higher position of the supraoccipital crest relative to the parietal blades is probably an artifact of preservation, and the crest would likely reach the dorsal margin of the parietal or end slightly ventral to it. The dorsal margin of the supraoccipital is narrower than the mediolateral width of the occipital condyle. Ventrally, the supraoccipital has a thin vertical process that connects to the dorsal margin of the foramen magnum. In anterior view, the ventral surface of the paroccipital process has a deep longitudinal groove delimited dorsally by a well-developed crest along the contact between the lateral process of the prootic and the exoccipital-opisthotic complex and a ventral ridge that extends along the ventral margin of the rami ( Fig. 16A–E). This groove, that corresponds to the columellar recess ( sensu Witmer and Ridgely 2010), extends into the fenestra ovalis. The ventral processes of the supraoccipital extend laterally and form a pair of shallow concavities dorsolaterally to the foramen magnus ( Fig. 15A–B, E). These concavities, that probably correspond to the remanent of the posttemporal fenestra ( sensu Chure and Loewen 2020), occupy almost the entire ventrolateral surface of the supraoccipital and connect with the opening for the middle cerebral vein placed adjacent to the contact between the supraoccipital and parietal with a curved depression.

Exoccipital-opisthotic complex: The exoccipitals and opisthotic are completely fused with any evidence of suturing, which is the typical condition in archosaurs and sauropsids in general (e.g. Currie 1997, Sampson and Witmer 2007, Bever et al. 2013). This single element has been sometimes called the exoccipital-opisthotic complex (e.g. Galton and Knoll 2006, Brusatte et al. 2010a) or the otoccipital (e.g. Sampson and Witmer 2007, Bever et al. 2013, Chure and Loewen 2020). The exoccipital-opisthotic complex forms most of the ventrolateral surface of the braincase in occipital and lateral views. It contacts the supraoccipital dorsally, the parietals laterodorsally, the basioccipital posteroventrally, and the prootic anteriorly. It also forms the dorsolateral part of the occipital condyle and most of the margin of the foramen magnus. Sutures between the exoccipital-opisthotic and basioccipital are clearly visible on both sides of the condyle ( Fig. 15A–B, E). The exoccipital has two well-developed and dorsoventrally broad paroccipital processes that project ventrolaterally (forming an angle of approximately 45º from the horizontal). These processes also extend somewhat posteriorly, with the most posterior border slightly surpassing the level of the occipital condyle. Despite the dorsal and ventral margins being subparallel along most of the length, the paroccipital processes seem somewhat expanded distally (not possible to confirm because the rami are incomplete). In anterior view, the dorsal margin of the paroccipital process is slightly concave for receiving the ventral process of the parietal. The most ventral margin of the paroccipital processes is placed near the level of the anteroventral margin of the basal tubera. The posterior surface of these processes is smooth and shallowly concave, but the anterior one is slightly inflated ( Fig. 16A–B, E). The dorsal margin of the proximal end of the paroccipital processes has a shallow concavity that is interpreted as the dorsal tympanic recess ( Fig. 15E).

The exoccipital adjacent to the neck of the occipital condyle has a deep concavity delimited dorsally by a sharp horizontal ridge extending from the dorsal margin of the condyle to the mid-height of the base of the paroccipital process ( Fig. 15A–E). This concavity corresponds to the lateral sinus of the subcondylar recess ( sensu Paulina Carabajal et al. 2021, = paracondylar recess sensu Chure and Loewen 2020, = paracondylar pocket sensu Welles 1984, Witmer and Ridgely 2010). A pair of openings, interpreted as the passage for the hypoglossal (= XII cranial) nerve and the metotic foramen, is visible inside this concavity, near the level of the dorsal margin of the condyle and there is another smaller opening in a slightly ventral position ( Fig. 15C–D). The two larger foramina are positioned almost horizontally enclosed in the exoccipital-opisthotic complex and are separated by a vertical septum. The smaller opening has an eight-shaped outline and is placed in the suture between the exoccipital-opisthotic and the basioccipital. An opening in an equivalent position has been described in the braincase of other theropods and interpreted as a pneumatopore for the subcondylar recess ( Paulina Carabajal et al. 2021). In anterior view, a projection of the prootic overlaps the base of the paroccipital processes. On the anterolateral surface of the exoccipital-opisthotic complex, near the base of the paroccipital processes and adjacent to the suture with the prootic, opens the relatively large fenestra ovalis ( Fig. 17A–D). This fenestra is bounded posteriorly by the exoccipital-opisthotic and anteriorly by the prootic. From the fenestra ovalis projects a broad and relatively deep columellar or stapedial groove extending along the ventral margin of the paroccipital process. This groove represents the pathway for the columella (= stapes) to the fenestra ovalis and may have also transmitted the internal jugular vein from the braincase ( Rauhut 2004, Smith et al. 2007). In ventral view, a stout crista metotica (= metotic strut sensu Rauhut 2004) connects the paroccipital processes to the ventral part of the exoccipital ( Fig. 18).

Basioccipital: The basioccipital contacts with the exoccipital-opisthotic complex posterolaterally and with the basisphenoid anteroventrally. This element forms the majority of the basal tubera and occipital condyle but makes a small contribution to the ventral margin of the foramen magnum. In occipital view, the ventral part of the basal tubera is wider than the occipital condyle, but the basioccipital is significantly narrower ( Fig. 15A–B, E). The basal tubera projects ventrally and is perpendicular to the horizontal plane of the ventral margin of the occipital condyle. There is a deep groove in the ventral surface of the basal tubera that projects along the suture between the basisphenoid and the basioccipital ( Fig. 15C–D). The condyle is rounded and subcircular in outline, slightly wider transversely than deep dorsoventrally (see Supporting Information, Table S3). The condyle is separated from the main body of the basioccipital by a short but robust neck and is bounded by a shallow groove that separates the articular surface from the neck. Ventrally to the occipital condyle, the basioccipital has two sharp vertical crests extending along the entire posteroventral height of the basioccipital and delimiting a deep concavity that corresponds to the subcondylar recess ( sensu Witmer 1997b or paracondylar recess sensu Chure and Loewen 2020). The ventral surface of the bacioccipital is strongly concave and has a deep pit in the ventrolateral margin inside a groove that opens adjacent to the suture between the exoccipital-opisthotic complex and the basisphenoid, which extends anteroposteriorly along the ventrolateral surface of the basisphenoid.

Basisphenoid-parasphenoid complex: The basisphenoid seems to be fused with the parasphenoid. However, the parasphenoid is very incomplete, preserving only a small fragment of the base, but almost the entire cultriform process is absent ( Figs 16A–B, E). The basisphenoid forms the anteroventral part of the braincase, but it is also incomplete anteriorly with the entirely of the basipterygoid processes missing. In lateral view, the basisphenoid shows a deep and large concavity near the ventral end of the base of the parasphenoid, which is interpreted as equivalent to the pneumatic recess for a diverticulum of the lateral/anterior tympanic system ( Rauhut 2004, Sampson and Witmer 2007). A similar recess (sometimes separated in the prootic and sobotic recesses sensu Witmer 1997b) has been described in other allosauroids ( Chure and Madsen 1996, Brusatte and Sereno 2007, Brusatte et al. 2010 a, Paulina et al. 2012, Chure and Loewen 2020). This concavity is partially covered by the preotic pendant, but it is possible to verify that it is bounded by raised ventral and dorsal crests ( Fig. 17A–D). In ventral view, a deep inverted funnel-shaped excavation that corresponds to the basisphenoid recess (sometimes called the basisphenoid sinus or basicranial fontanelle; Coria and Currie 2003) is delimited by the basisphenoid anterolaterally and by the basioccipital posteriorly. This recess has been interpreted as related to the median pharyngeal system ( Witmer 1997b, Sampson and Witmer 2007). The ventral basisphenoid recess has a thin blade projecting dorsoventrally in the mid-line of the anteroventral surface of the basioccipital ( Fig. 18). The parasphenoid preserves only a small fragment that consists in two thin blades projecting anterodorsally from the anterior margin of the basisphenoid. These blades converge dorsally and correspond to the proximal part of the cultriform process.

Prootic: The paired prootic contacts the laterosphenoid anterodorsally, the basisphenoid anteroventrally, and the parietal dorsally but sutures with these elements are mostly obscured by distortion. The prootic clearly overlaps the exoccipital-opisthotic complex posteriorly and laterally, extending as a relatively well-developed process along the base of the paroccipital processes and it seems to form most of the ventral margin of the endocranial cavity ( Fig. 16). The prootic has a pair of ventral blade-shaped processes, termed the preotic pendant ( sensu Welles 1984), projecting posteroventrally as a wing-like structure that overlaps the laterodorsal surface of the basioccipital. Other names proposed for this process (e.g. ala basisphenoidalis sensu Taquet and Welles 1977, alar process of the laterosphenoid sensu Bonaparte 1986, crista prootica sensu Currie and Zhao 1993) reflect the different interpretations for the composition of this structure. In MNHN/UL.AND.21, sutures between the prootic and basisphenoid are unclear, but it seems that the prootic overlaps the basisphenoid anteromedially and that the preotic pendant is entirely formed by the prootic ( Fig. 17A–C). In Shaochilong maortuensis ( Hu 1964, Brusatte et al. 2009), a raised ridge extending anterodorsally along the lateral surface of the preotic pendant is interpreted as the contact between the prootic and the basisphenoid, and thus the pendant is almost evenly divided between these two elements (e.g. Brusatte et al. 2010a). The specimen from Andrés has a similar ridge but it clearly does not represent this contact, which is visible more posteriorly. The preotic pendant is slightly concave in anterior view and has a series of thin vertical ridges and grooves mostly along the ventral surface. Posterolaterally, a broad tongue-like process of the prootic, sometimes termed the caudal process, overlaps the base of the paroccipital process. At nearly the mid-height of this process, the prootic has a well-marked longitudinal crest extending ventrolaterally. This crest extends towards the lateral process of the laterosphenoid and delimits a shallow concavity, which is interpreted as the anterior tympanic recess ( sensu Witmer 1997b). Laterally, the prootic consists of a thin and strongly pneumatic blade between the preotic pendant and the caudal process, which is sometimes called the prootic superficial lamina ( sensu Bever et al. 2013).

Ventrally to the caudal process of the prootic and adjacent to the suture between the prootic and exoccipital-opisthotic complex, a large and funnel-shaped fenestra ovalis pierces the lateral surface of the braincase with an anteroposterior orientation ( Fig. 17A–D). Anterodorsally to the fenestra ovalis, is another large and rounded opening that is bounded by the prootic ventrally and by the laterosphenoid anterodorsally, which is interpreted as the maxillary and mandibular (= CN V 2-3) branches of the trigeminal nerve. A smaller funnel-shaped opening interpreted as the foramen for the facial nerve (= CN VII), is placed between the fenestra ovalis and the trigeminal foramen and is bounded anterodorsally by a ridge that connects with the posterior margin of the preotic pendant. The trigeminal foramen is about twice the size of the facial foramen and is located immediately anterodorsal to the latter, being separated by a flattened septum. A small groove, which has been interpreted as for transmitting the hyomandibular branch of the facial nerve ( Brusatte et al. 2010a), extends posteroventrally from the trigeminal foramen to the margin of the fenestra ovalis ( Fig. 17A–C). This groove does not enter the fenestra ovalis, but rather is separated from it by the raised anterior rim of the fenestra. In anterior view, the prootic has a small opening within a shallow recess ventral to the endocranial cavity, which is interpreted as the exit for the abducens nerve (= CN VI). Ventrally to this recess and slightly hidden by the cultriform process is a large opening that corresponds to the exit for the carotid artery ( Fig. 16B).

Laterosphenoid: The laterosphenoid (= alisphenoid sensu Gilmore 1920) forms most of the lateral and dorsal margins of the endocranial cavity. It overlaps the anterodorsal margin of the prootic and the anteroventral margin of the parietal within the supratemporal fenestra ( Fig. 16). The laterosphenoid contribution to the supratemporal fenestra is large, but this element is only visible within the fenestra in lateral view. In anterior view, the conjoined laterosphenoids have a pair of robust processes, the capitate processes ( sensu Sampson and Witmer 2007), expanding laterally around the endocranial cavity. These processes have rounded distal surfaces, termed the distal cotylus ( sensu Madsen 1976), which would fit in a broad oval concavity of the postorbital. Ventrolaterally, the well-developed antotic crest ( sensu Sampson and Witmer 2007) extends from the ventrolateral surface of the laterosphenoid through the ventral margin of the capitate process. As occurs in most allosauroids (e.g. Brusatte et al. 2010a), the antotic crest is not continuous with the otosphenoidal crest on the ventral surface of the lateral process of the prootic, but instead these crests are separated by the fossa that houses the trigeminal foramen ( Fig. 17A–D). The antotic crest in the specimen from Andrés is well marked but relatively thin and only slightly offset laterally. The laterosphenoid connects anterodorsally and laterally with the frontal and laterally with a narrow process of the parietal. A small notch is present in the junction between the laterosphenoid, frontal, and parietal, and it apparently opens into the lateral temporal fenestra. The anterior surface of the capitate processes is slightly concave ventrally near the base. In cross-section, these processes are triangular, with ridge-shaped ventral margins. Ventrally, the laterosphenoids are somewhat distorted and the suture with the prootic and the parasphenoid is indiscernible. A notch is located on the anterior surface of the vertical ramus of the laterosphenoid near the suture with the prootic ( Fig. 16A–D). This notch houses the opening for the ophthalmic branch of the trigeminal nerve (= CN V 1). Despite the specimen being somewhat fractured in this part of the braincase, this foramen seems to open within an elongated groove.

Lower jaw

General description: The lower jaw is represented by a right articular, right and left surangulars, a left prearticular, right and left angulars, a supradentary, and a coronoid.

Articular: An almost complete and well-preserved right articular, MNHN/UL.AND.11, was recovered in Andrés ( Fig. 19). This element lacks the blade for contact with the surangular and the dorsal end of the retroarticular process. In lateral view, the articular has a rectangular shape ( Fig. 19E–F). The glenoid fossa, which would receive the medial condyle of the quadrate, is positioned on the dorsal surface of the anterior part of the articular and is strongly concave, with a semi-circular shape. In medial view, the retroarticular process in the posterior portion of the articular is a mediolaterally thin and anteroposteriorly long blade projecting dorsomedially. This process is broken dorsally therefore its precise morphology is not possible to know, but based on the preserved fragment it seems that it would have a rectangular shape. Also, the lateral surface of the articular is broken and most of the surface for articulation with the surangular is missing, except a small fragment of the anterior part. This fragment has a series of thin ridges and grooves indicating a firm, immovable sutural contact between the articular and the surangular, as occurs in Allosaurus fragilis ( Madsen 1976) . The ventral surface of the articular is a mediolaterally thin and straight blade with a shallow longitudinal anterior groove, which is interpreted as the surface for articulation with the prearticular ( Fig. 19G–H). A robust process interpreted as the pendant medial process extends medially from the dorsal margin of the articular between the glenoid fossa and the retroarticular process ( Fig. 19A–B). This process forms the floor of a deep canal that extends mediolaterally and is pierced by a small foramen that seems to connect with another small foramen in the anterior margin of the medial surface of the articular. Similar canal and foramina are described in Sinraptor dongi and are interpreted as the passage for the chorda tympani and the posterior condylar artery ( Currie and Zhao 1993).

Surangular: Two surangular bones, a right, MNHN/ UL.AND.12 ( Fig. 20), and a left, MNHN/UL.AND.13 ( Fig. 21), were collected in Andrés. The left surangular was in a block with the left prearticular and a fragment of the posterior part of a right nasal. The left surangular and prearticular preserves nearly the original anatomical position, with the prearticular only slightly displaced posteriorly. The surangular is a thin, long, and deep blade that strongly tapers anteriorly and posteriorly. It articulates with the articular, and prearticular posteromedially, with the angular ventrolaterally, with the dentary anteriorly, and with the supradentary and coronoid anteromedially. Dorsally, the surangular has a moderately robust crest that extends anteroposteriorly. This dorsal crest extends slightly medially and laterally forming the medial and lateral surangular shelves, respectively. The lateral shelf (= adductor ridge sensu Coria and Currie 2006) is a well-developed ridge delimiting a deep and oval concavity in the posterior part of the surangular ( Fig. 20A– B). This shelf has been interpreted as for insertion of enlarged pterygoideus musculature (Gauthier 1986) or representing an extensive contact for the lower infratemporal bar acting to brace the posterior jaw against lateral strain during adduction of the lower jaws ( Smith et al. 2007). The large fossa ventral to the lateral shelf probably corresponds to the area of insertion for the pseudotemporalis muscle ( Lautenschlager 2013). A relatively large foramen is visible on the posterior end inside this concavity (posterior surangular foramen), which pierces the lateral surface of the surangular with a posteroanterior orientation and it seems to connect with other foramen in the medial surface just beneath the antarticular process. Two smaller foramina are visible adjacent to the lateral shelf in a slightly anterior position relative to the posterior surangular foramen. Ventrally to the posterior concavity, the thin blade of the surangular shows a rough area consisting of a series of tiny oblique grooves and ridges, which is interpreted as the surface for contact with the angular. This surface starts near the posterior margin of the surangular, where a well-marked groove extends anterodorsally to near the level of the posterior surangular foramen. Anteriorly, the lateral shelf of the surangular is pierced by the anterior surangular foramen, which is located inside a deep, longitudinal groove ( Fig. 20A–B). This foramen has been described in other allosauroids and interpreted as the opening for the branches of the inferior alveolar nerve (e.g. Currie and Zhao 1993, Brusatte et al. 2010b).

The anterior part of the surangular is a thin, tapered, and bifurcated blade that would articulate with the dentary. The surangular also strongly tapers posteriorly, developing a blunt retroarticular process that projects posteriorly and somewhat dorsally. At approximately the mid-length of the ventral margin, the surangular has a deep and moderately wide external mandibular fenestra opening anteroventrally. In medial view, the surangular has a deep concavity, corresponding to the surangular adductor fossa, which is delimited dorsally by the well-developed medial shelf that projects along the entire length of the dorsal surface ( Fig. 20C–D). The medial shelf extends dorsally in the posterior part of the surangular, but near the mid-length becomes a mediolaterally thin blade projecting ventrally and delimiting a deep, but narrow groove adjacent to the dorsal margin of the surangular. Posteriorly, a strongly rough and fan-shaped surface projecting dorsoventrally is interpreted as the articulation for the articular. Just anterior to the surface for the articular, a robust process, corresponding to the antarticular process, extends medially from the dorsomedial surface of the surangular. The medial surface of this process has a deep and vertical groove extending along most of its depth. Between the surface for articulation with the angular and the antarticular process, a deep and broad glenoid fossa opens in the dorsal surface of the surangular ( Fig. 20E–F).

The anterior part of the medial surface of the surangular shows a deep and broad longitudinal groove extending along the dorsal surface from nearly the level of the anterior margin of the external mandibular fenestra, which corresponds to the articulation with the dentary. Posterior to the surface for articulation with the dentary, the medial shelf of the surangular has a slightly concave surface bounded dorsally by a deep longitudinal groove that represents the area for articulation with the coronoid ( Fig. 20C–D). In dorsal view, the surangular is a relatively thick, convex element with a shallow and elliptical concavity at mid-length, which is interpreted as the area for insertion of the adductor mandibulae externus muscle ( Madsen 1976, Currie and Zhao 1993, Coria and Currie 2006, Eddy and Clarke 2011).

Prearticular: A left prearticular, MNHN/UL.AND.13, was collected and is partially articulated with the surangular as previously mentioned. As these elements are still in a block only the medial surface of the prearticular is visible ( Fig. 21). The prearticular is a long, thin, and strongly bowed element that forms the posteromedial surface of the lower jaw. This element is dorsoventrally thin at mid-length, but strongly expands anteriorly and posteriorly. The ventral surface of the prearticular is slightly convex and shows a longitudinal flattened surface for articulation with the angular, which is bounded medially by a sharp ridge projecting ventrally ( Fig. 21C–D). The anterior part of the prearticular is a thin and tapered blade strongly projecting dorsally. The anterior blade has slightly striated dorsal and ventral surfaces that correspond to the sutures for contact with the coronoid and splenial, respectively. The ventral margin of the prearticular is somewhat distorted and incomplete, but a small notch is seen at the level of the base of the anterior expansion ( Fig. 21E–F), which is interpreted as a fragment of the mylohyoid foramen ( sensu Eddy and Clarke 2011, = internal mandibular foramen sensu Madsen 1976). Posteriorly, the prearticular strongly widens and extends dorsally forming a moderately broad process that would articulate with the antarticular and a thin posterior process for articulation with the articular.

Angular: Two almost complete and well-preserved angular bones, a right (MNHN/UL.AND.17) and a left (MNHN/ UL.AND.16), were recovered in Andrés ( Fig. 22). The left angular is complete, but the right one lacks a fragment of the anterior part. These elements are long and thin blades extending anteroposteriorly and together with the prearticular form the ventral margin of the posterior mandible. The angular contacts the dentary and the splenial anterodorsally, overlaps the lateral surface of the surangular posteriorly, and contacts the prearticular medially. These elements are dorsoventrally narrow along the anterior mid-length, but gradually expand to the posterior part and then taper again, ending in a narrow and strongly tapered process. In medial view, the angular forms a shallow concavity bounded posteriorly by a longitudinal low crest extending along the ventral margin, which is interpreted as the surface for articulation with the surangular ( Fig. 22A–B, E–F). The angular also articulates with the surangular posteriorly and this contact is represented by a striated surface that extends for most of the posterior part of the internal blade. Posteriorly, the angular is a dorsoventrally thin blade with a flattened area on the internal surface, which is bounded ventrally by a well-developed crest extending anteroposteriorly. This crest is interpreted as the suture for articulation with the prearticular. The articulation with the dentary and splenial in the lateral surface of the posterior blade of the angular is marked by a shallow and longitudinal groove that extends along the ventral margin ( Fig. 22C–D, G–H). The angular is concave near the mid-length, where it forms a smooth concavity that corresponds to the ventral margin of the external mandibular fenestra.

Supradentary and coronoid: Two thin, blade-shaped elements (MNHN/UL.AND.18) are interpreted as corresponding to the left supradentary and coronoid ( Fig. 23). The supradentary is broken and the posterior end is missing. This fragile element is a long and mediolaterally thin blade that expands dorsoventrally and slightly tapers anteriorly. The supradentary would probably cover the interdental plates over almost their entire length as occurs in other theropods ( Madsen 1976, Eddy and Clarke 2011, Chure and Loewen 2020). This element has been interpreted as a protection for the vascular system and interdental plates along the lingual base of the tooth row ( Madsen 1976). In lateral view, the supradentary has a somewhat sigmoidal profile with a slightly concave dorsal margin anteriorly that becomes almost straight posteriorly. The lateral and medial surfaces show thin longitudinal grooves extending along the entire preserved element. The coronoid is also a mediolaterally thin blade with a triangular posterior expansion and a long anterior ramus. The posterior margin of the coronoid has a small ventral projection extending posteroventrally. This ventral process would probably have an equivalent dorsal extension, but it is broken. Anteriorly, the coronoid strongly narrows in a dorsoventrally thin ramus, with nearly parallel margins. Just anterior to the triangular posterior process, the blade of the coronoid has a well-developed crest extending ventrally. The preserved parts of the coronoid and supradentary do not connect with one another in the broken surfaces, but it seems that they belong to the same element and that the missing fragment would be small.

YPM

Peabody Museum of Natural History

USNM

Smithsonian Institution, National Museum of Natural History

DINO

Dinosaur National Monument

UMNH

Utah Museum of Natural History

AMNH

American Museum of Natural History

CM

Chongqing Museum

MNHN

Museum National d'Histoire Naturelle

UL

University of Louisville

ML

Musee de Lectoure

MG

Museum of Zoology

IPFUB

Institute for Paleontology of the Freie Universitat

Kingdom

Animalia

Phylum

Chordata

Family

Allosauridae

Genus

Allosaurus

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