Spectrovenator ragei, Zaher & Pol & Navarro & Delcourt & Carvalho, 2020

Zaher, Hussam, Pol, Diego, Navarro, Bruno A., Delcourt, Rafael & Carvalho, Alberto B., 2020, An Early Cretaceous theropod dinosaur from Brazil sheds light on the cranial evolution of the Abelisauridae, Comptes Rendus Palevol 19 (6), pp. 101-115 : 103-111

Spectrovenator ragei n. sp.

( Figs 1-6)

urn:lsid:zoobank.org:act:E2C799CE-EFC0-40D8-B3AA-6A5827BFEAB4

HOLOTYPE. — MZSP-PV 833 , a partially articulated skeleton consisting of complete skull and mandibles, partially preserved cervicodorsal vertebrae and ribs, complete sacrum, partially preserved caudal series, complete pelvic girdle and hindlimbs.

ETYMOLOGY. — The specific epithet ‘ ragei ’ is a patronym honouring Dr Jean-Claude Rage for his influential contribution to our knowledge on the Paleobiogeography of continental Mesozoic vertebrates.

DIAGNOSIS. — Spectrovenator ragei n. gen., n. sp. differs from all other abelisaurid theropods by the following unique autapomorphies: posterior ramus of maxilla ornamented with vertically oriented grooves except for the smooth region anterior to the maxilla-jugal suture; lacrimal lateral surface rugose except for smooth ventral region; ventrally bowed posterior process of jugal; nuchal crest with thin and smooth dorsal margin; straight ventral margin of dentaries with deep sulcus on lateral surface; dorsal margin of surangular slightly convex; longitudinal ridge along posteroventral end of mandibular ramus.

TYPE LOCALITY. — The only known individual of Spectrovenator ragei n. gen., n. sp. was recovered at the Embira Branca Range , a locality close to the Coração de Jesus Municipality (northern Minas Gerais State, Brazil). The deposits in which the remains were recovered are referred to the Quiricó Formation, Areado Group, Sanfranciscana Basin ( Campos & Dardenne 1997; Zaher et al. 2011). The lacustrine sequence of the Quiricó Formation is constrained to the Barremian-Aptian boundary, based on stratigraphical relationships and biostratigraphical correlations, such as the presence of the gonorhynchiform teleostean Dastilbe moraesi ( Scorza & Santos 1955) , the Tucanopollis crisopollensis, Afropollis sp. and Sergipea naviformis palynozones ( Carvalho et al. 1994), as well as by the association of the ostracodans Wolburgiopsis plastica , Wolburgiopsis chinamuertensis , Ilyocypris sp. , Brasacypris sp. 1 , Harbinia symmetrica and Darwinula martinsi ( Do Carmo et al. 2004) . DESCRIPTION AND COMPARISONS

Skull

The skull of Spectrovenator ragei n. gen., n. sp. is nearly complete, missing only portions of the left and right premaxillae and part of the right mandible ( Figs 1-4). The skull is compressed anteroposteriorly in the left part of occipital region, lateromedially in the right of orbital region, and part of the left quadrate was broken and displaced anteriorly. Although the premaxillae are badly preserved, Micro-CT scan imaging revealed the presence of four premaxillary teeth and a strong premaxilla-maxilla contact (Appendix 1).

The rostrum (as preserved) corresponds to more than 50% of the total skull length ( Fig. 1 A-D; Appendix 1). The maxilla is ornamented, except below the antorbital fenestra where it bears a series of vertically oriented grooves. The anteriormost grooves, on the alveolar margin, are smaller than the posterior ones, as in Majungasaurus , Kryptops , and Rugops ( Sampson & Witmer 2007; Sereno et al. 2004; Sereno & Brusatte 2008). The maxilla-jugal contact is smooth ( Fig. 1A, B). The rostral ramus of the maxilla is relatively short, resembling the condition found in Rugops ( Sereno et al. 2004) . The ascending process of the maxilla is posteriorly slanted along its dorsal region and the antorbital fenestra is subtriangular and longer than high. The medial lamina of the maxilla is anteroposteriorly short, limiting the antorbital fossa to the anterior margin of the antorbital opening, as in most abelisaurids ( Bonaparte et al. 1990; Carrano & Sampson 2008). A promaxillary recess is present, but hidden in lateral view, a feature found in other abelisaurids ( Novas 2009). Posteriorly, the maxilla has a long anteroposteriorly directed contact with the jugal that precludes a maxilla-lacrimal contact. There are 18 maxillary teeth as in Rugops ( Sereno et al. 2004) . The medial surface of the maxilla above the alveolar margin bears fused paradental plates that are smooth ( Fig. 3E), rather than rugose as in other abelisaurids.

The nasals are thick and at least partially fused anteriorly ( Fig. 2A, B and Appendix 1). The nasals are anteriorly forked, with a long and acute maxillary process as in Rugops ( Sereno et al. 2004) . The flattened dorsal surface of the nasals is ornamented on its anterior third and smooth on its posteromedial region. The nasals bear a series of large pits and invaginations that are located medial to the ornamented ridge that form the lateral edge of the bone, a feature previously regarded as autapomorphic for Rugops ( Sereno et al. 2004) . Similar pits are also present in Carnotaurus , but this taxon lacks the invaginations and the lateral nasal ridge. The ornamentation of the dorsal surface of the nasals also resembles that of Rugops ( Sereno et al. 2004) , interpreted as indicative of overlying scales ( Delcourt 2018). The nasal-premaxilla articulation is slightly ornamented, lacking the papillate texture present in Rugops , interpreted as indicative of armoured-like dermis ( Delcourt 2018).

The lacrimal is massive, ornamented, and contacts the maxilla and nasal anteriorly, the prefrontal posterodorsally, and the postorbital and jugal ventrally ( Figs 1C, D; 2A, B). The posterodorsal antorbital fossa on the lacrimal is exposed laterally whereas the pneumatic foramen is hidden in lateral view, as Ceratosaurus ( Novas et al. 2013) , Eoabelisaurus ( Pol & Rauhut 2012) , and other abelisaurids ( Carrano & Sampson 2008). The lacrimal bears a broad rugose ridge that divides its dorsal and lateral surfaces and extends posteriorly to contact the postorbital. Differently from other abelisaurids ( Canale et al. 2009), the thin and ornamented rostral ramus of the lacrimal in Spectrovenator n. gen. forms part of the external posterodorsal border of the antorbital fossa ( Figs 1C, D; 2A, B). The lacrimal has a short posterior process that projects posteriorly toward the suborbital process of the postorbital, as in most abelisaurids ( Carrano & Sampson 2008). The prefrontals are fused to the lacrimal on the dorsal surface of the skull where the posterior process develops to contact the postorbital above the orbit. However, the lacrimal-prefrontal suture can be seen along the ventral descending process of these bones on the anteromedial region of the orbit.

The postorbital is not fused to the frontal but rather encloses, along with the lacrimal and the frontal, a small dorsally oriented fenestra located above each orbit ( Figs 1 A-D; 2A, B). The descending ramus of the postorbital is anteroventrally directed, instead of perpendicular to the dorsal ramus, and bears a moderately expanded suborbital process ( Fig. 1C, D). The dorsal ramus retains a relatively well-developed squamosal process in comparison to those of other abelisaurids ( Canale et al. 2009; Pol & Rauhut 2012).

The jugal is barely ornamented, dorsoventrally low below the orbit, and with a convex ventral margin ( Fig. 1 A-D). The anterior process separates the maxilla from the lacrimal whereas the posterior process is bifurcated and embraces the quadratojugal on its anterior third ( Fig. 1 A-D). The ascending process of the jugal is triangular and tapers distinctly on its dorsal tip. The dorsal process of the quadratojugal is anteroposteriorly broad and contacts the squamosal.

The suture between the prefrontal and lacrimal of Spectrovenator n. gen. is not visible on the dorsal skull surface and they may have been fused as in other abelisaurids( Canale et al. 2009; Sereno et al. 2004; Pol & Rauhut 2012). However, these elements are unfused along their descending processes on the anteromedial margin of the orbit. The frontals of Spectrovenator n. gen. are paired and their suture is visible dorsally as in Rugops ( Sereno et al. 2004) , but their dorsal surfaces are smooth and lack the rugosities present in other abelisaurds ( Sereno et al. 2004; Paulina Carabajal 2011a; Fig. 2A, B). The frontals are proportionately wide and short, as in other abelisaurids ( Canale et al. 2009; Paulina Carabajal 2011a), and have a median boss that extends longitudinally along the interfrontal suture. The lateral margins of the frontals are dorsoventrally thin and have a short concave orbital margin. This margin is the medial edge of the fenestra that is laterally closed by the lacrimal-postorbital contact above the orbit, opening also present in Rugops , Ekrixinatosaurus and Arcovenator ( Sereno et al. 2004; Novas et al. 2013; Tortosa et al. 2014). The frontal is not fused to the parietal, at least within the supratemporal fossa where the suture is clearly present, in contrast to other abelisaurids ( Paulina Carabajal 2011b). On this region, the dorsal surface formed by the frontals and parietal is flat, lacking the median fossa at the level of the anterior margin of the supratemporal fenestra and the narrow parietal crest separating the supratemporal fossae, both features that characterize the skull roof of most abelisaurids ( Bonaparte & Novas 1985; Sereno et al. 2004; Canale et al. 2009; Tortosa et al. 2014). The flat parietal surface between the supratemporal fossae of Spectrovenator n. gen. is even broader than that of Aucasaurus ( Paulina Carabajal 2011b) .

The posterior end of the parietal is not significantly raised above the level of the skull roof but extends posteriorly, capping the supraoccipital crest ( Fig. 2C, D). The parietal and squamosal form together a broad transversal nuchal crest, resembling but less developed than in other abelisaurids ( Sampson et al. 1998; Carrano & Sampson 2008; Paulina Carabajal 2011b). The dorsal margin of this crest is thin and smooth rather than broad and rugose as in other abelisaurids. The squamosal has a smooth lateral surface with a marked concavity and a straight postquadratic process that extends posteriorly, as in other abelisaurids ( Carrano & Sampson 2008). The descending process of the squamosal is thin and tapers gradually, forming the dorsal half of the posterior margin of the infratemporal fenestra down to its contact with the quadratojugal ( Fig. 1 A-D).

The supraoccipital has a thick and prominent median vertical crest, but this crest is not as sharp and does not project posteriorly to the level of the occipital condyle as it does in derived abelisaurids ( Canale et al. 2009). The paroccipital processes are only slightly expanded at their distal ends rather than flared as in Carnotaurus ( Paulina Carabajal 2011a) . The paracondylar recess of the otoccipital is well developed and bears two small foramina, one of which leads to a medially directed duct (hypoglossal canal) while the other extends to an anteromedially directed duct (vagal canal). The crista tuberalis is well developed and connects the posteroventral end of the paraoccipital process with the lateral margin of the basal tubera. The basioccipital is vertical, bears a slightly developed median ridge, and forms most of the basal tubera, which are wider than the basioccipital condyle, as in Ceratosaurus and abelisaurids ( Carrano & Sampson 2008). The basisphenoid covers the anterior and ventral surfaces of the basal tubera and has basipterygoid processes that are slightly recurved posteriorly, as in other ceratosaurians ( Canale et al. 2009). The ventral surface of the basisphenoid bears a deep recess that is rounded in shape and laterally enclosed by the crista ventrolateralis ( Paulina Carabajal & Currie 2017).

Mandible

The mandible has an anteroposteriorly elongated external mandibular fenestra that reaches anteriorly the level of the posteriormost dentary tooth, as in other abelisaurids ( Carrano & Sampson 2008; Figs 4, 5; Appendix 1). The dentary is straight rather than ventrally convex as in Carnotaurus and Ekrixinatosaurus ( Bonaparte et al. 1990; Calvo et al. 2004) and extends approximately along 50% of the mandibular length ( Fig. 4 A-D). There are 16 small and posteriorly recurved teeth. The lateral surface of the dentary bears a longitudinal sulcus that is located at its dorsoventral midpoint and contains aligned neurovascular foramina. Ventral to this sulcus the dentary is ornamented like the lateral surface of the skull. The posteroventral process of the dentary has a short overlapping contact with the angular ( Fig. 5A), which is shorter than in Carnotaurus or Majungasaurus ( Bonaparte et al. 1990; Sampson & Witmer 2007). The posterodorsal process of the dentary has a broad concave facet for the surangular ( Fig. 5B), as in other abelisaurids ( Pol & Rauhut 2012; Canale et al. 2009). The splenial bears an anteriorly placed ovoid foramen as in Ceratosaurus and abelisaurids ( Sampson & Witmer 2007). The angular process of the splenial is broad, ornamented, and exposed on the lateral surface of the mandibular ramus. The posterior margin of the splenial is straight, angled, and sutured to the anterior margin of the prearticular.

The surangular has a slightly convex dorsal margin and a well-developed lateral shelf with a deep fossa underneath as in Majungasaurus ( Sampson & Witmer 2007) . The posteroventral margin of the surangular, along its suture with the prearticular, is laterally everted forming a well-developed anteroposterior crest on the ventrolateral margin of the mandibular ramus ( Fig. 4 A-D). The surangular bears an anterior foramen near its contact with the dentary, a posterior foramen ventral to the surangular crest, and an intermediately positioned foramen above the posterior margin of the mandibular fenestra as in Carnotaurus and Majungasaurus ( Bonaparte et al. 1990; Sampson & Witmer 2007). The surangular forms the lateral part of the mandibular glenoid cavity along its contact with the articular. The glenoid fossa is deep and has elevated anterior and posterior buttresses ( Fig. 4C, D). The retroarticular process is anteroposteriorly long, rounded, and has a dorsomedially concave surface, as in Carnotaurus and Majungasaurus ( Bonaparte et al. 1990; Sampson & Witmer 2007).

Dentition

The upper dentition of Spectrovenator n. gen. includes 4 premaxillary and 18 maxillary teeth whereas the lower dentition includes 16 dentary teeth. The teeth vary in apicobasal height along the toothrow, reaching their maximal height at the level of the 6th to 8th maxillary teeth. The premaxillary teeth ( Fig. 3A, B) are less distally recurved than the maxillary teeth and bear denticles that are similarly large on both their mesial (2.4 denticles per mm) and distal (2.4 denticles per mm) margins, taken at the midheight of the crown. The maxillary teeth become larger at the mid section of the toothrow ( Fig. 3C) in comparison with the premaxillary teeth, although the size difference is not as large as in ceratosaurids ( Rauhut, 2004). Similarly, the largest maxillary teeth are up to 70% of the height of the dentary, in contrast to proportionately higher teeth of ceratosaurids ( Rauhut, 2004). The posterior maxillary teeth decrease in size and become more distally recurved posteriorly ( Fig. 3D, E). The maxillary teeth have denticles on their distal margin that are similar in size to those of the premaxilla (2.8 denticles per mm) but those of their mesial margin are much smaller (3.4 denticles per mm). The dentary teeth also become larger at the midpoint of the toothrow and posterior elements are more distally recurved than anterior elements ( Fig. 4).

Postcranium

The postcranial remains of Spectrovenator n. gen. reveal cervicodorsal vertebrae that are characterized by their elongated epipophyses and well-developed epipophysealprezygapophyseal laminae (EPRL), distally bifurcated cervical ribs, and transverse processes of caudal vertebrae distally anteroposteriorly expanded, as in most abelisaurids ( Fig. 6A, C; Carrano & Sampson 2008). Other abelisaurid features present in the postcranium include the preacetabular process of the ilium directed anteroventrally and with an irregular anterior margin and the cnemial crest on the tibia with an expanded distal end ( Fig. 6B, E). The astragalus is fused to the calcaneum, a feature present in other ceratosaurians ( Carrano & Sampson 2008) that differs from that of basal tetanurans ( Rauhut & Pol 2017) and non-averostrans ( Ezcurra 2017). The astragalar ascending process is tall and laminar and lacks the fusion with the fibula present in Masiakasaurus ( Carrano et al. 2011) . The metatarsus of Spectrovenator n. gen. is gracile and proportionately more elongated than in other abelisaurids ( Coria et al. 2002; Carrano 2007), lacking the reduced width of metatarsal II present in noasaurids ( Fig. 6D; Carrano & Sampson 2008). The pedal unguals of Spectrovenator n. gen. have proximally bifurcated grooves (Y-shaped) and a flexor tubercle with an associated ventral depression, as in other abelisauroids ( Fig. 6F, G; Novas et al. 2005; Carrano 2007; Novas et al. 2013).

PHYLOGENETIC ANALYSIS

The phylogenetic position of Spectrovenator n. gen. was tested using a dataset of ceratosaurian relationships that included the new taxon and integrates recently published information on abelisauroids (see Material and methods). All most parsimonious trees (MPTs) depict Spectrovenator n. gen. as a basal member of Abelisauridae , being the sister group of the clade formed by Rugops and more derived taxa ( Fig. 7). Abelisaurid apomorphies supporting the inclusion of Spectrovenator n. gen. in this clade include a large list of features: external ornamentation in the maxilla, nasal, lacrimal, and postorbital, antorbital fossa failing to extend posteroventrally along the maxilla, partially fused nasals, prefrontal at least partly fused to the lacrimal, lacrimal-postorbital contact above the orbit, lacrimal with suborbital process, presence of suborbital flange of postorbital, postorbital anteroposteriorly longer than high and with descending process anteroventrally projected, maxillary dentition with more than 12 teeth, anterior process in transverse processes in anterior caudal vertebrae, and straight dorsal margin of iliac blade. The long list of characters supporting the inclusion of Spectrovenator n. gen. in the Abelisauridae is reflected in high nodal support values for this node when the position of highly unstable and fragmentary taxa is ignored (bremer = 4; bootstrap = 94%; jackknife = 95%; Appendix 1). It should be noted that although this large list of synapomorphies does not diagnose Abelisauridae in our analysis given that Eoabelisaurus is retrieved as the most basal member of this clade. Other studies (e.g., Novas et al. 2013), however, have retrieved Eoabelisaurus as a basal abelisauroid and under these hypotheses the list of apomorphies diagnosing the clade formed Spectrovenator n. gen. and other abelisaurids would be synapomorphies of Abelisauridae .

The position of Spectrovenator n. gen. as the sister group to Rugops and more derivedabelisaurids is supported by the absence in Spectrovenator n. gen. of the following derived characters: maxillary-jugal contact larger than 40% of total maxilla length, knob-like dorsal projection of parietal, narrow sagittal crest between the supratemporal fenestrae, reduced anterior process of the lacrimal externally to the antorbital fossa, and rugose paradental plate on the maxilla. The monophyly of Rugops and more derived abelisaurids, excluding Spectrovenator n. gen. and Eoabelisaurus , also reaches high support values when fragmentary taxa are ignored (bremer = 4; bootstrap = 82%; jackknife = 89%; see Appendix 1).