PALEVO 1631-0683 Elsevier S1631-0683(19)30182-4 10.1016/j.crpv.2019.09.005 Research article General palaeontology, Systematics, and Evolution (Vertebrate Palaeontology) General Palaeontomogy, Systematics, and Evolution/Paléontologie générale, systématique et évolution (Vertebrate Palaeontology/Paléontologie des Vertébrés) Novel information on the endocranial morphology of the abelisaurid theropod Carnotaurus sastrei Nouvelles informations sur la morphologie endocrânienne de l’abélisauridé théropode Carnotaurus sastrei Cerroni Mauricio A. mauricio.cerroni@gmail.com a Paulina-Carabajal Ariana a.paulinacarabajal@conicet.gov.ar b Laboratorio de Anatomía Comparada y Evolución de los Vertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”–CONICET, Av. Ángel Gallardo 470, C1405DJR, Buenos Aires, Argentina Laboratorio de Anatomía Comparada y Evolución de los Vertebrados, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”- CONICET Av. Ángel Gallardo 470, C1405DJR Buenos Aires Argentina Instituto de Investigaciones en Biodiversidad y Medioambiente (CONICET–UNCo), Quintral 1250 (R8400FRF), San Carlos de Bariloche, Argentina Instituto de Investigaciones en Biodiversidad y Medioambiente (CONICET-UNCo), Quintral 1250 (R8400FRF) San Carlos de Bariloche Argentina 18 8 S1631-0683(19)X0009-3 985 995 © 2019 Académie des sciences. Published by Elsevier B.V. All rights reserved. 2019 Académie des sciences Full (PDF)

The endocranial morphology of the abelisaurid Carnotaurus sastrei, from the Upper Cretaceous of Patagonia, is studied using X-ray Computed Tomography (CT). The CT scans provided information that allowed the first reconstruction of the brain, inner ear and braincase pneumaticity for this South American taxon. The endocranial morphology confirms that abelisaurids share an overall conformation of the brain and inner ear. However, some traits, such as the height of the dorsal sinuses and the length of the flocculus in the cranial endocast, and a large subsellar recess in the basicranium, appear to characterize the South-American abelisaurids only. Moreover, the olfactory acuity of Carnotaurus resembles that reported for other abelisaurids (e.g., Majungasaurus, Viavenator), suggesting that the sense of smell had an important role. However, some attributes of the endocranial features of Carnotaurus (i.e. development and orientation of the olfactory bulbs and tracts) may imply particular olfactory capacities when compared with other abelisaurids.

La morphologie endocrânienne de l’abélisauridé Carnotaurus sastrei du Crétacé supérieur de Patagonie est étudiée par tomodensitométrie X (CT). Les scans de CT délivrent une information qui permet la première reconstitution de la pneumaticité du cerveau, de l’oreille interne et du crâne de ce taxon sud-américain. La morphologie endocrânienne confirme que les abelisauridés partagent une conformation d’ensemble du cerveau et de l’oreille interne. Cependant, certains traits, comme la hauteur des sinus dorsaux et la longueur du flocculus dans l’endocaste crânien, et un grand évidement de sous-fosse dans le basicrâne semblent caractériser seulement les abélisauridés sud-américains. En outre, l’acuité olfactive de Carnotaurus ressemble à celle d’autres abélisauridés (e.g., Majungasaurus Viavenator), suggérant que le sens de l’odorat a joué un rôle important. Cependant, certains attributs des traits de l’endocrâne de Carnotaurus (i.e. le développement et l’orientation des bulbes et tractus olfactifs) peuvent impliquer des capacités olfactives particulières, en comparaison de ce qui est observé chez d’autres abélisauridés.

 Abelisauridae, Paleoneurology, Reptile encephalization quotient, Pneumaticity, Cretaceous Abelisauridae, Paléoneurologie, Quotient d’encéphalisation de reptile, Pneumaticité, Crétacé presented Handled by Hans-Dieter Sues Introduction

In recent years, the knowledge on endocranial morphology of theropod dinosaurs has increased considerably through studies based on X-ray Computed Tomography (CT). So far, several analyses led to complete reconstructions of the cranial endocast and inner ear in different groups of non-avian theropods, including basal neotheropods (Paulina-Carabajal et al., 2015 and Xing et al., 2014), coelophysoids (Raath, 1977, fig. 21), ceratosaurs (e.g., Paulina-Carabajal and Filippi, 2018 and Paulina-Carabajal and Succar, 2015, Sanders and Smith, 2005; Sampson and Witmer, 2007), allosauroids (e.g., Franzosa and Rowe, 2005, Larsson, 2001, Paulina-Carabajal and Canale, 2010, Paulina-Carabajal and Currie, 2012 and Rogers, 1998), megaraptorids (Paulina-Carabajal and Currie, 2017 and Paulina-Carabajal and Porfiri, 2018), tyrannosaurids (e.g., Bever et al., 2011, and references therein; Saveliev and Alifanov, 2005 and Witmer and Ridgely, 2009) and more derived coelurosaurs (e.g., Balanoff et al., 2018 and Lautenschlager et al., 2012).

Abelisauridae is one of the most prominent families of mid-sized theropod dinosaurs that inhabited Gondwana and southern Europe in the Late Cretaceous (see Novas et al., 2013 and references therein). Remains including complete braincases are known for several taxa, such as Abelisaurus comahuensis, Carnotaurus sastrei, Skorpiovenator bustingorryi, Viavenator exxoni and Aucasaurus garridoi from Argentina (Bonaparte and Novas, 1985, Bonaparte et al., 1990, Canale et al., 2008, Coria et al., 2002 and Filippi et al., 2016), Majungasaurus crenatissimus from Madagascar (Sampson and Witmer, 2007 and Sues, 1980), Arcovenator escotae from France (Tortosa et al., 2013), and Indosaurus matleyi, Indosuchus raptorius and Rajasaurus narmadensis from India (Huene et Matley, 1933, Novas et al., 2004 and Wilson et al., 2003). However, detailed descriptions of the abelisaurid braincase are available only for a few taxa (i.e. Abelisaurus, Aucasaurus, Carnotaurus, Majungasaurus, Viavenator; Paulina-Carabajal, 2011a, Paulina-Carabajal, 2011b, Paulina-Carabajal and Filippi, 2018 and Sampson and Witmer, 2007); and the endocranial morphology is known in very few taxa, such as Aucasaurus (Paulina-Carabajal and Succar, 2015), Majungasaurus (Sampson and Witmer, 2007), Viavenator (Paulina-Carabajal and Filippi, 2018) and Indosaurus, which is known from a natural partial endocast described by Huene et Matley (1933). Some endocranial features were described for Abelisaurus (Paulina-Carabajal, 2011b), and the neurovascular pattern has been described for Carnotaurus (Paulina-Carabajal, 2011a) and Arcovenator (Tortosa et al., 2013). Jurassic ceratosaurs such as Ceratosaurus and Eoabelisaurus have complete braincases (Gilmore, 1920 and Pol and Rauhut, 2012), but only the endocranial morphology of Ceratosaurus has been studied (Franzosa, 2004 and Sanders and Smith, 2005).

Carnotaurus sastrei (Bonaparte, 1985) is a derived brachyrostran abelisaurid from the Maastrichtian La Colonia Formation, Chubut province, Argentina (Novas, 2009). The type specimen preserves a complete skull, including a complete braincase that is articulated to the rest of the skull bones. Here, we describe for the first time the cranial endocast of this taxon (Fig. 1, Fig. 2 and Fig. 3). Comparisons with other known abelisaurid cranial endocasts indicate that the overall conformation of the brain and inner ear are similar within the family, although some attributes appear to be characteristic of the South American forms (Fig. 4). The pneumaticity of the braincase is also compared with that for other abelisaurids.

 Material and methods

The skull of the holotype of Carnotaurus sastrei (MACN CH-894) was CT-scanned in 2010, at the TCba Salguero Diagnostic Center (Buenos Aires, Argentina) using a CT 64 Ingenuity Core medical tomographer. The slice thickness was of 0.62 mm and the scan energy parameters were of 119 mA and 120 kV. The virtual three-dimensional cranial endocast was obtained and visualized using software 3D Slicer version 4.8 (Fedorov et al., 2012). Final illustrations were made using Adobe Photoshop (CS6).

The CT scans allowed the reconstruction of the main encephalic structures but few neurovascular passages. The large size of the field of view (FVO) (which included the complete skull of Carnotaurus, plus the space occupied by the frontal horns) prevented the observation of the smaller structures of the braincase, such as the complete semicircular canals of the inner ear and most canals for cranial nerves (CN) and blood vessel. The lack of contrast between bone and sediment probably caused by the ferric concretion from which the skull was extracted (F.E. Novas pers. comm.) affected the quality of the resulting images.

Measurements . The total volume of the cranial endocast of Carnotaurus was calculated using the tools of software Slicer version 4.8 (Table 1). The reptilian cranial endocast does not necessary reflect the original brain surfaces, but reflects the overall shape of the brain (e.g., Hopson, 1979). Following Sampson and Witmer (2007), we will refer to the cast structures as if they were the structures themselves (e.g., “olfactory bulb” instead of “olfactory bulb cavity endocast”).

Body mass calculation . Body mass is a useful parameter to calculate the Reptile Encephalization Quotient (REQ; Hurlburt, 1996 and Hurlburt et al., 2013). We used two sources of information to estimate the body mass for Carnotaurus: one was the femoral circumference method (FCM) of Anderson et al. (1985), used here, and the other was the body mass calculated recently by Campione and colleagues (2014) using a modification of the quadrupedal equation (Campione and Evans, 2012) applied to bipedal non-avian theropods. The result of the latter analysis is approximately 1743 kg, with a 25% prediction error range of 1306–2179 kg. The FCM, calculated by W = 0.16 C f 2.73, where W is mass (g), and C f is minimum femur circumference (which in Carnotaurus is 325 mm), resulted in a total weight of 1419 kg. Both values, the minimum (1419) and the maximum (1743) body weights, were used to calculate the REQ (Table 1).

Encephalization quotient calculation . This is a measure of the relative brain size among theropods (Jerison, 1973), modified by Hurlburt (1996) as the Reptile Encephalization Quotient (REQ) based on extant reptiles. Following Hurlburt et al. (2013), we used the equation REQ = MBr/(0.0155 × MBd0.553), where MBr is the brain mass and MBd is the body mass, to calculate the REQ of Carnotaurus. We use two body mass values, one calculated here using the method of Anderson et al. (1985) and the other estimated by Campione et al. (2014) to calculate the REQs at 100%, 70% and 50% of the endocranial volume (Table 1).

Olfactory ratios . Olfactory ratios can be used to interpret the olfactory acuity in extinct animals (see Zelenitsky et al., 2009 and references therein). It is calculated as the ratio between the greatest diameter of the olfactory bulb and the longest diameter of the cerebral hemisphere, and then multiplied by 100 (Zelenitsky et al., 2009 and Zelenitsky et al., 2011).

Institutional abbreviations. MACN: Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Buenos Aires, Argentina; MAU, Museo “Argentino Urquiza”, Rincón de los Sauces, Argentina; MPCA, Museo Provincial ‘Carlos Ameghino’, Cipolletti, Río Negro; MUCPv: Museo de la Universidad Nacional del Comahue, Neuquén, Neuquén, Argentina.

 Description Endocranial morphology

The main encephalic structures of the forebrain, midbrain, and hindbrain are identifiable, as well as the bases of the largest cranial nerves and blood vessels (Fig. 1 and Fig. 2). The complete cranial endocast has a length of 179 mm from the foramen magnum to the anterior end of the olfactory bulbs, and its maximum transversal width across the cerebral hemispheres is 48.9 mm, resulting in a total volume of approximately 169.8 cm3. The endocast is anteroposteriorly long and transversely narrow, as in other studied non-coelurosaur theropods. When compared to other abelisaurids, Carnotaurus is distinguished by having the olfactory tracts and anteroventrally oriented bulbs forming a curved loop (Fig. 2A), so that forebrain and hindbrain are not sub-horizontally positioned as in Majungasaurus (Sampson and Witmer, 2007), Aucasaurus (Paulina-Carabajal and Succar, 2015) and Viavenator (Paulina-Carabajal and Filippi, 2018) (Fig. 4). Although the endocast of Indosaurus is currently lost (M. Ezcurra, pers. comm.), the sketch made by Huene et Matley (1933) shows olfactory tracts (the olfactory bulbs are missing) that are seemingly ventrally curved, reminding the condition of Carnotaurus. The angle between midbrain and hindbrain is approximately 43–44°, a value that resembles that for Majungasaurus (45°, Sampson and Witmer, 2007), but is greater than that of Viavenator (35°, Paulina-Carabajal and Filippi, 2018).

A low dural or dorsal expansion (= pyramidal peak) develops dorsally at the level of the root of CN V (Fig. 2A), as in Majungasaurus but unlike Aucasaurus and Viavenator where the dural expansion is developed at the level of the flocculus (Paulina-Carabajal and Filippi, 2018). This protuberance on the endocast indicates the presence of a dorsal longitudinal venous sinus that has been also related to the position of the pineal gland (Sampson and Witmer, 2007). However, the dural expansion in Carnotaurus is poorly projected dorsally compared to the well-developed dural expansions of the aforementioned abelisaurids (Fig. 4), and therefore the angle formed between the foramen magnum and the dural expansion in lateral view is also low, reaching a value of 40°.

The roots of almost all the cranial nerves are observed on the endocast, except for CN IV, probably CN VII (see below), and CN VIII, which is probably very small in diameter and difficult to observe, as in other studied theropods.

Forebrain–This region of the endocast includes the olfactory tracts and bulbs (CN I), cerebral hemispheres, CN II (optic nerve), the infundibular stalk and the pituitary body (= hypophysis). The olfactory tracts are elongated and curved anteroventrally, with a width of 21 mm. The olfactory bulbs are well developed, oval, and clearly separated medially (by the median septum of the mesethmoid), slightly diverging from the midline (Fig. 2B). Elongated and wide olfactory tracts and large olfactory bulbs are present in both ceratosaurs (Paulina-Carabajal and Filippi, 2018 and Sampson and Witmer, 2007) and particularly carcharodontosaurids (Larsson, 2001 and Paulina-Carabajal and Canale, 2010), in which the olfactory tracts and bulbs have nearly the same length as the rest of the endocast.

The cerebral hemispheres are well marked, aligned with the olfactory bulbs in lateral view (Fig. 2A); however, the inter-hemispherical fissure is not visible, obscured by the dorsal longitudinal sinus, as in most non-avian theropods (Hopson, 1979 and Witmer et al., 2008). Anterolateral to the cerebral hemisphere, there is a blood vessel, visible on the left side, that would exit through a foramen located in the sphenetmoid (the “vascular foramen of the olfactory tract”, see Paulina-Carabajal, 2011a, fig. 4) (Fig. 2B). Similarly positioned veins are present in Majungasaurus, named as the “venous canal” by Sampson and Witmer (2007). The roots of CNs II (optic tracts) are short, large in diameter and slightly divergent; the union of both nerves along the midline probably represents the position of the optic chiasm, although this region is not enlarged and bulbous, as in other dinosaurs (e.g., titanosaurids). The infundibular stalk is oval in section and projects ventrally contacting the pituitary body. The pituitary is posteroventrally projected and not particularly inflated, having a volume of approximately 3 cm3 (it represents approximately 1.8 % of the total volume of the endocast). This relative size of the pituitary is also observed in other abelisaurids (Paulina-Carabajal and Filippi, 2018 and Sampson and Witmer, 2007) and other non-avian theropods (Franzosa, 2004). Anterior to the pituitary is a large sphenoidal artery (Fig. 2), which exits the pituitary fossa through an anterolateral fenestra in the basisphenoid (“fenestra rostral to the pituitary fossa”, Paulina-Carabajal, 2011a, fig. 4; “foramen for or endocast of sphenoidal artery”, Sampson and Witmer, 2007, fig. 14). The passages for the cerebral branches of the internal carotid arteries are barely visible in the posterior end of the pituitary.

Midbrain–The midbrain is characterized by the low dural expansion (= pyramidal peak). As in other basal theropods, the optic lobes are not observed in the endocast; they possibly were positioned close to the midline, as in other abelisaurids such as Majungasaurus (Sampson and Witmer, 2007), Viavenator (Paulina-Carabajal and Filippi, 2018) and non-avian theropods (Franzosa, 2004, Franzosa and Rowe, 2005 and Rogers, 1998). Near the base of the infundibular stalk, there is no distinction between the roots of CNs III and IV, which probably left the endocranial cavity through a single foramen. On the lateral surface of the braincase, however, there are separate foramina for these nerves defining the contact between the orbitosphenoid and the laterosphenoid (Paulina-Carabajal, 2011a, fig. 4).

Hindbrain–This region comprises the cerebellum, the medulla oblongata, and the roots of cranial nerves V–XII. As in other abelisaurids (Paulina-Carabajal and Filippi, 2018 and Sampson and Witmer, 2007), the cerebellum is not markedly expanded laterally and is positioned just below the posterior middle cerebral veins (Fig. 2 and Fig. 4). The only discernible structure of the cerebellum in the endocast is the flocculus, which is well defined, although it is small when compared to the larger flocculi of coelurosaurian theropods and pterosaurs (e.g., Franzosa, 2004, Hopson, 1979 and Witmer et al., 2003). The flocculus in Carnotaurus is a blade-like structure, which is posterolaterally projected into the space formed by the anterior semicircular canal (Fig. 2A). This situation is strongly reminiscent of that of the South-American abelisaurids Aucasaurus (Paulina-Carabajal and Succar, 2015) and Viavenator (Paulina-Carabajal and Filippi, 2018), and contrasts with the relatively smaller flocculus described for Majungasaurus (Sampson and Witmer, 2007). The posterior middle cerebral veins extend posterodorsally. Although the foramina for these veins are not clearly observed on the braincase due to poor preservation of this region, these veins likely pierced the supraoccipital, exiting through foramina bounded between the later and the parietal, as in other theropods (Paulina-Carabajal and Currie, 2017 and Witmer and Ridgely, 2009) and extant birds (Franzosa, 2004).

The roots of CN V (trigeminal nerve) are visible on both sides of the endocast, being large in diameter and expanded laterally. Although all the branches leave the endocranial cavity through a single foramen, the ophthalmic branch (V1) bifurcates from the maxillo-mandibular branch (V2,3) and exits the braincase anteriorly through a foramen enclosed entirely by the laterosphenoid (Paulina-Carabajal, 2011a) (Fig. 2). This bifurcation indicates an almost intracranial location of the trigeminal ganglion (Sampson and Witmer, 2007). Dorsal to CN V, there is a blood vessel (poorly visible in the CT scans), which would correspond to the anterior middle cerebral vein, which possibly exits through a foramen enclosed by the laterosphenoid. The passages for CN VI (abducens nerve) are on the ventral side of the medulla and project anteroventrally, reaching the posterior portion of the pituitary body, entering the pituitary fossa and probably leaving the endocranial cavity through the fenestra for the sphenoidal artery (Paulina-Carabajal, 2011a: fig. 4). The passage for CN VII (facial nerve) is difficult to identify in the CT scans, although its root is clearly posterior to CN V (Fig. 2). This nerve is posterolaterally oriented and leaves the endocranial cavity through a foramen enclosed by the prootic. Cranial nerve VIII (vestibulocochlear nerve) is not observed in the CT scans, probably due the small diameter of these passages, as in other theropods (Paulina-Carabajal and Currie, 2017). The passages for cranial nerves IX–XI (glossopharyngeal, vagus, and accessory nerves, respectively) are united in a common canal, the metotic passage, and apparently leave the endocranial cavity through a single foramen (metotic foramen). Finally, the root of the CN XII (hypoglossal nerve) leaves posteroventrally the medulla oblongata through a single canal.

 Inner ear

The complete osseous labyrinth is difficult to trace in the CT scans, but the right inner ear is partially reconstructed and illustrated in Fig. 3. All the semicircular canals and the lagena were digitally extracted, but not the common crus. The semicircular canals are slender, with a diameter of the tube that varies between 2.5 and 3.6 mm, with the lateral semicircular canal (LSC) slightly wider. The anterior semicircular canal (ASC) is larger, somewhat curved posteriorly, and taller than the posterior semicircular canal (PSC), as in most theropods (Sampson and Witmer, 2007); unusually, the LSC appears to be relatively larger than the PSC, but this may be accentuated by the lack of the common crus. The contour of the ASC and PSC is oval, whereas the LSC is more circular. In dorsal view (Fig. 3B), the angle between ASC and PSC is about of 85°; this angle resembles that present in the abelisaurids Viavenator (85°, Paulina-Carabajal and Filippi, 2018) Majungasaurus (∼85°, Sampson and Witmer, 2007, fig. 19) and Aucasaurus (92–95°, Paulina-Carabajal and Succar, 2015). The fenestra ovalis is partially reconstructed. The lagena is conical and short, as in other abelisaurids and most theropods (e.g., Witmer and Ridgely, 2009). It has a length of approximately 6 mm.

The size, shape, angle between ASC and PSC and the general morphology of the inner ear of Carnotaurus closely resemble those of other known abelisaurids so far (e.g., Aucasaurus, Majungasaurus, Viavenator; Paulina-Carabajal and Filippi, 2018, Paulina-Carabajal and Succar, 2015 and Sampson and Witmer, 2007). This inner ear morphology is conservative and present in many mid-sized theropods, such Ceratosaurus (Sanders and Smith, 2005) and allosauroids (Franzosa and Rowe, 2005 and Rogers, 1998).

 Pneumaticity

The braincase pneumaticity is usually associated with the systems of air-filled diverticula originated from the cervical pulmonary air sacs and paratympanic systems leaving large excavations on the lateral and ventral sides of the braincase (Sampson and Witmer, 2007 and Witmer et al., 2008). Carnotaurus includes the lateral (= rostral) tympanic recess, the basisphenoidal recess and the subsellar recess (Fig. 1A), also present in other abelisaurids (Paulina-Carabajal, 2011b, Paulina-Carabajal and Filippi, 2018 and Sampson and Witmer, 2007). The lateral tympanic recess probably originated from the paratympanic system, whereas the basisphenoidal and the subsellar recesses originated from the median pharyngeal system (Dufeau, 2011 and Witmer, 1997).

In the braincase of Carnotaurus, the lateral tympanic recess is an irregular cavity on the lateral side of the basisphenoid and probably part of the prootic, just posteroventrally to the preotic pendant (Paulina-Carabajal, 2011a). The CT scans show that this recess is considerably smaller compared to the basisphenoidal and subsellar recesses (Fig. 1B). It is not clear if this space is divided into two main chambers as in Abelisaurus and Viavenator (Paulina-Carabajal, 2011b and Paulina-Carabajal and Filippi, 2018), or whether it has a connection with the basisphenoidal recess, although the evidence from the known abelisaurid braincases indicates the opposite. Sampson and Witmer (2007) reported a connection between the lateral tympanic recess and the posterior surface of the basioccipital in Majungasaurus, but this feature appears to be absent in Carnotaurus.

The basisphenoidal recess is a fairly large conical deep excavation in the ventral side of the basisphenoid, as in other abelisaurids (Paulina-Carabajal, 2011b, Paulina-Carabajal and Filippi, 2018 and Sampson and Witmer, 2007) (Fig. 1C). The basisphenoidal recess further expands posteriorly and is divided into two large lateral cavities, which strongly affect the basioccipital internally, at the base of the occipital condyle (Fig. 1B and D). Each of these paired cavities is divided into a dorsal chamber anteroposteriorly and a ventral chamber dorsoventrally. The dorsal chamber posteriorly reaches the neck of the occipital condyle; each one is separated by a median septum that partially divided the basisphenoidal recess (Fig. 1D). This condition of the posterior expansion of the basisphenoidal recess invading the neck of the occipital condyle is present in some tetanurans (Paulina-Carabajal and Currie, 2017 and Witmer, 1997), Ceratosaurus (“basioccipital sinuses”, Sanders and Smith, 2005) and the abelisaurids Aucasaurus, Ilokelesia, Viavenator (Paulina-Carabajal, 2011b and Paulina-Carabajal and Filippi, 2018) and Ekrixinatosaurus (MUCPv-294, as shown by fractures). On the other hand, the ventral chamber extends posteroventrally, invading the basioccipital, but without pneumatizing entirely the basal tuber. A similar situation is reported in Rajasaurus (Wilson et al., 2003), where the basisphenoidal portion of the basal tuber is hollow. The basisphenoidal recess of Carnotaurus communicates anteroventrally with the subsellar recess.

The subsellar recess is partially visible in left lateral and ventral views of the braincase, but is filled with sediment (Paulina-Carabajal, 2011a). The CT scans showed that the subsellar recess is well developed anterior to the basisphenoid recess and ventral to the base of the cultriform process (Fig. 1A). It is developed mainly anteroposteriorly, slightly smaller in volume than the basisphenoidal recess. Likewise, the size of the subsellar recess resembles the large recess present in Abelisaurus (Paulina-Carabajal, 2011b) and Viavenator (Paulina-Carabajal and Filippi, 2018), but differs with the smaller subsellar recess in Majungasaurus (Sampson and Witmer, 2007).

 Discussion Neurosensorial capabilities

The general shape and proportions of the cranial endocast of Carnotaurus resembles that in other abelisaurids (e.g., Aucasaurus, Majungasaurus, Viavenator), particularly in the development of the olfactory tracts and bulbs and the flocculi. However, it is worth mentioning the low dural expansion in Carnotaurus, which is the lowest compared with those of other abelisaurids such as Aucasaurus, Majungasaurus, and Viavenator (Paulina-Carabajal and Filippi, 2018, Paulina-Carabajal and Succar, 2015 and Sampson and Witmer, 2007); hence, this trait is reflected in variable development within this clade of theropods (Fig. 4). Majungasaurus exhibits a tall and well-developed dural expansion with a marked peak, similar to that present in some large theropods (e.g., Allosaurus, Tyrannosaurus), Sampson and Witmer (2007) argued the possibility that the dural peak of Majungasaurus housed a pineal gland as in extant birds. If present in Carnotaurus, a pineal gland was not as well developed as that hypothesized for Majungasaurus.

Reptile encephalization quotient. The total volume of the cranial endocast of Carnotaurus is 169.8 cm3. The values of 37% and 50% (following Hurlburt et al., 2013) as well as the resulting calculated REQs are shown in Table 1. The minimum calculated REQs of Carnotaurus, i.e. 1.4–1.9 (using body mass calculated by Campione et al., 2014) and 1.6–2.2 (using a measurement calculated here using Anderson et al., 1985 method) are much higher than those calculated for Majungasaurus (1.14–1.54) and the allosauroids Sinraptor (0.81–1.1) and Giganotosaurus (1.07–1.4), but are slightly greater than the REQs for Ceratosaurus, Allosaurus and the megaraptoran Murusraptor (Table 1). However, the REQ of Carnotaurus does not reach the higher values of tyrannosaurids (1.8–2.5).

Olfaction and olfactory acuity. The large size of the olfactory bulbs and the elongated olfactory tracts of Carnotaurus are similar in proportion to those in ceratosaurs (Franzosa, 2004 and Sampson and Witmer, 2007) and allosauroids (Franzosa and Rowe, 2005 and Larsson, 2001), indicating that olfaction was probably an important sense for this taxon, as well as most abelisaurids; and perhaps played a more important role than the sense of sight based on the reduced size of the optic lobes, a condition that markedly contrasts with that in maniraptoran theropods, including extant birds (Franzosa, 2004 and Hopson, 1979).

The olfactory tracts and bulbs in Carnotaurus are anteroventrally curved (Fig. 4A), a condition shared apparently only with Indosaurus (Huene et Matley, 1933: pl. IX 3), although in the latter the tip of the forebrain is incomplete. Conversely, Aucasaurus, Majungasaurus, and Viavenator have horizontally disposed olfactory bulbs and tracts (Fig. 4B–D). The olfactory tracts and bulbs of Ceratosaurus are anterodorsally oriented (Sanders and Smith, 2005), a condition that not only contrasts with Carnotaurus, but with all known abelisaurids. Carnotaurus has a dorsoventrally deep skull at the level of the snout, which is also reflected on the shape of the antorbital space. As was previously hypothesized (Sampson and Witmer, 2007), the nasal cavity was probably close to the opening for CN I (olfactory nerve), and thereby also the olfactory concha (Witmer, 1995). The ventrally oriented olfactory region and the deep antorbital space in Carnotaurus may reflect a difference in the location and development of the nasal concha inside the nasal cavity, which could be related with some particular olfactory capability in this taxon. However, further studies are necessary for a better understanding of the relation between these anatomical traits and their paleobiological significance.

Olfactory ratios are influenced by body size and should not be used to compare olfactory acuity directly among theropods (Zelenitsky et al., 2009). When the log-transformed olfactory ratio (1.7) and body mass (3.1–3.2; based on both estimated body masses, see § Material and methods) of Carnotaurus are plotted in the graphic published by Zelenitsky et al. (2009, fig. 2), the abelisaurid falls on predicted theropod olfactory ratio values (being just slightly higher than Majungasaurus). Allosauroids, ceratosaurians, and basal tyrannosauroids have or are close to having those predicted values for theropods of their respective body size, which suggests that their olfactory acuities represent the primitive condition for theropods (Zelenitsky et al., 2009). Unlikely, tyrannosaurids and dromaeosaurids have higher olfactory ratios than predicted for theropods of similar body size, which may indicate in this case a keener sense of smell (Zelenitsky et al., 2009). Although with a typical olfactory ratio, the particular orientation of the olfactory bulbs of Carnotaurus (and its possible relationship on the development of the nasal concha) is perhaps associated with a greater reliance on the sense of smell, than in other reported abelisaurids.

Hearing. The region of the osseous labyrinth corresponding to the lagena the sensory epithelium (= basilar papilla) of the sensory organ in life (Manley, 1973). The length of the lagena provides an estimate of the hearing sensitivity and auditory capabilities of extinct animals (Walsh et al., 2008; Witmer and Ridgely, 2009). The lagena of Carnotaurus is relatively short, being less than one quarter of the length of the inner ear, as in Viavenator (Paulina-Carabajal and Filippi, 2018), probably Aucasaurus and Majungasaurus (Paulina-Carabajal and Succar, 2015 and Sampson and Witmer, 2007), and the present in some non-coelurosaurian theropods such allosauroids and megaraptorans (Paulina-Carabajal and Currie, 2017 and Rogers, 1998). Studies by Gleich et al. (2005) estimated that large dinosaurs, for example, Allosaurus, with a body mass of approximately 1400 kg and a lagena of 8 mm long, have a best frequency of hearing range of 0.7–1.5 kHz and a high-frequency limit of hearing below 3 kHz. The estimated body mass of Carnotaurus (1419–1743 kg range) is slightly larger than that of Allosaurus (1400 kg) and the lagena reaches similar size (6 mm in length); thus, the hearing range in Carnotaurus is expected to be below 3 kHz. The shorter lagena in abelisaurids and many non-coelurosaurian theropods contrasts with the remarkably longer lagena of tyrannosaurids (Witmer and Ridgely, 2009) and therizinosaurs (Lautenschlager et al., 2012), among theropods.

Gaze stabilization. It is worth noting the difference in flocculus size among abelisaurids. The South American forms (Brachyrostra; Canale et al., 2008) share large flocculi that project posteriorly, reaching the posterior semicircular canal, whereas Majungasaurus has a markedly shorter flocculus (Fig. 4). Furthermore, Sampson and Witmer (2007) reported that the flocculi of Rugops and Indosaurus, have flocculi “only a little larger” compared to that of Majungasaurus, and regarded a small floccular process as a possible apomorphy of Abelisauridae. However, the recent evidence concerning Brachyrostra in which the flocculus is relatively larger (maybe not in volume but in length) than in the non-South American abelisaurids (Majungasaurinae; Tortosa et al., 2013) leads us to reconsider this trait. As the flocculus is important in gaze stabilization that coordinates the eye movement with the movements of the head, neck, and body (Walsh et al., 2013) and tends to be larger in animals that rely on quick movements of the head and body (Witmer et al., 2003 and Witmer et al., 2008), the difference within both clades of abelisaurids can be interpreted as different means of gaze stabilization. In this way, the South American abelisaurids (e.g., Aucasaurus, Carnotaurus, Viavenator) have relatively increased gaze-stabilization mechanisms compared with those of Majungasaurus, and perhaps Rugops and Indosaurus. However, caution is needed in using floccular size to infer ecological and behavioral traits in extinct animals, as shown by recent studies based on extant forms (Ferreira-Cardoso et al., 2017).

 Braincase pneumaticity

The braincase pneumaticity in Carnotaurus is well developed, as in other abelisaurids (Fig. 1). The lateral (= rostral) tympanic recess of Carnotaurus has proportions similar to those of Majungasaurus and Viavenator. However, the basisphenoidal recess appears to be strongly developed dorsoventrally. The ventral portion of this recess that affects the basal tubera is apparently more developed in Carnotaurus than in Majungasaurus and Viavenator. Indosaurus has basal tubera described as hollow by Wilson et al. (2003), although the feature is not illustrated, preventing further comparisons. Furthermore, the paired chamber that extends to the neck of the occipital condyle of Carnotaurus is present in the basicrania of Ceratosaurus (Sanders and Smith, 2005) and Aucasaurus, Ilokelesia, Majungasaurus, Viavenator (Paulina-Carabajal, 2011b, Paulina-Carabajal and Filippi, 2018 and Sampson and Witmer, 2007) and Ekrixinatosaurus (MUCPv-294). The pneumatization of the neck of the occipital condyle is not characteristic of ceratosaurs, but is present in a variety of tetanurans such as allosauroids (Sampson and Witmer, 2007) and megaraptorids (Paulina-Carabajal and Currie, 2017). The subsellar recess of Carnotaurus is a large cavity, which is similar in size to the South American Viavenator (MAU-Pv-LI-530; Paulina-Carabajal and Filippi, 2018), and differs from the relatively smaller subsellar recess of the Malagasy Majungasaurus (Sampson and Witmer, 2007). By contrast, the basisphenoidal and subsellar recesses in Abelisaurus (MPCA 11.098; Paulina-Carabajal, 2011b) are much larger than in any of the aforementioned abelisaurids.

Finally, both basipterygoid and paraoccipital processes of Carnotaurus are massive, a condition present in all abelisaurids (Paulina-Carabajal and Filippi, 2018). Ceratosaurus has paraoccipital processes extensively excavated by sinuses (“paroccipital process sinus”, Sanders and Smith, 2005), as in most coelurosaurian theropods (Rauhut, 2003), but not in abelisaurids.

 Conclusions

Although few species of Abelisauridae have been studied in terms of paleoneurological information, the new data provided here help to understand the variability of endocranial morphology within the clade. Furthermore, some inferences are made regarding the potential sensorial capabilities developed in the studied taxa. The CT scans also showed that the braincase pneumaticity of Carnotaurus is extensive as in other abelisaurids, characterized by relatively large basisphenoidal and subsellar recesses.

The endocranial morphology of Carnotaurus is similar to that observed in other abelisaurids. Furthermore, it shows the presence of a larger flocculus of the cerebellum in the South American forms, which also exhibit smaller and less dorsally projected dorsal longitudinal sinuses. A particular trait of Carnotaurus is the anteroventral curvature of the olfactory tracts and bulbs. This trait perhaps suggests a distinct development of the nasal concha and increased reliance on the sense of smell, which possibly was more acute than in the other documented abelisaurids. However, further studies are needed to understand the paleobiological significance of these endocranial traits.

 Acknowledgments

The authors are deeply grateful to Fernando Novas (MACN), who provided the CT scans of Carnotaurus sastrei and supported the present study (PICT2010-066), and to A. Kramarz (MACN) who allowed the study of the specimen under his care. We also thank the personnel of the Instituto Salguero, who make possible CT scanning. Jorge Calvo (MUCPv), Carlos Muñoz (MPCA) and Leonardo Filippi (MAU), who allowed them access to the specimens under their care. We also thank Fernando Novas (MACN), Santiago Hernández del Pino (IANIGLA), Martin Ezcurra (MACN), Federico Agnolín (MACN), Federico Brissón Egli (MACN) and Adriel Gentil (MACN) for their useful comments on the elaboration of the manuscript. The authors would like to thank J.I. Canale and editor H.-D. Sues for their comments and reviews, which greatly improved the manuscript. Funding was provided for CONICET https://doi.org/10.13039/501100002923 and Agencia Nacional de Promoción Científica y Tecnológica PICT2016-0481 https://doi.org/10.13039/100008725 and Sepkoski grant 2016 (to APC) .

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Digital reconstruction of the skull, cranial endocast and braincase pneumaticity of Carnotaurus sastrei (MACN-CH 894) in lateral (A and B), dorsal (C), and posterior views (D). Bone is rendered semi-transparent (A, C and D) to allow observation of internal structures. bsr: basisphenoidal recess; bt: basal tuber; en: endocast; ie: inner ear; fh: frontal horns; ltr: lateral tympanic recess; nc: nuchal crest; o: orbit; oc: occipital condyle; q: quadrate; sn: snout; skr: skull roof; ssr: subsellar recess. Scale bars equal 20 cm (A, C and D) and 5 cm (B).

Reconstitution numérique de la pneumaticité du crane, de l’endocaste crânien et du cerveau chez Carnotaurus sastrei (MACN-CH 894) en vues latérale (A et B), dorsale (C) et postérieure (D). l’os a été rendu semi-transparent (A, C et D) pour l’observation des structures internes. bsr : évidement basisphénoïdal ; bt tuber(?) basal ; en : endocaste ; ie : oreille interne ; fh : cornes frontales ; ltr : évidement latéral du tympan ; nc : crête nucale ; o : orbite ; oc : condyle occipital ; q : carré ; sn : museau ; skr : toit du cerveau ; ssr : évidement de sous-fosse. Les barres d’échelle représentent 20 cm (A, C et D) et 5 cm (B).

Digital reconstruction of the cranial endocast and inner ear of Carnotaurus sastrei (MACN-CH 894) in right lateral (A), dorsal (B), posterior (C), anterior (D), and ventral (E) views. asc: anterior semicircular canal; av: anterior vein; cer: cerebral hemisphere; ds: dorsal sinus; fl: flocculus; ie: internal carotids; lsc: lateral semicircular canal; med: medulla oblongata; met: metotic passage (for CNs IX–XI); ob: olfactory bulbs; ot: olfactory tract; pit: pituitary; pmcv: posterior middle cerebral veins; psc: posterior semicircular canal; spha: sphenoidal artery; I-XII: cranial nerves. The scale bar equals 5 cm.

Reconstitution numérique de l’endocaste crânien et de l’oreille interne de Carnotaurus sastrei (MACN-CH 894) en vues latérale droite (A), dorsale (B), postérieure (C), antérieure (D) et ventrale (E). asc : canal semi-circulaire antérieur ; av : veine antérieure ; cer : hémisphère cérébral ; ds : sinus dorsal ; fl : floculus ; ie : carotides internes ; lsc : canal semi-circulaire latéral ; med : medulla oblongue ; met : passage métotique (pour Ns IX-XI) ; ob : bulbes olfactifs ; ot : tractus olfactifs ; pit : pituitaire ; pmcv : veines médiocérébrales postérieures ; sn : museau ; psc : canal semi-circulaire postérieur ; spha : artère sphénoïdale ; I-XII : nerfs crâniens. Barre d’échelle = 5 cm.

Rendering of the right inner ear of Carnotaurus sastrei (MACN-CH 894) in lateral (A), dorsal (B), anterior (C) and posterior (D) views. The dashed lines indicate portions of the inner ear that are not reconstructed. asc: anterior semicircular canal; fo: fenestra ovalis; lag: lagena; lsc: lateral semicircular canal; psc: posterior semicircular canal. The scale bars equal 10 mm.

Rendu de l’oreille interne droite de Carnotaurus sastrei (MAC-CH 894) en vues latérale (A), dorsale (B), antérieure (C) et postérieure (D). Les lignes en tireté indiquent les portions de l’oreille interne qui n’ont pas été reconstituées. asc : Canal semi-circulaire antérieur ; fo : fenestra ovalis ; lag : lagena ; lsc : canal semi-circulaire latéral ; psc : canal semi-circulaire postérieur. Les barres d’échelle représentent 10 mm.

Comparisons of the cranial endocasts in lateral view of (A) Carnotaurus, (B) Viavenator (modified from Paulina-Carabajal and Filippi, 2018), (C) Aucasaurus (modified from Paulina-Carabajal and Succar, 2015), and (D) Majungasaurus (modified from Sampson and Witmer, 2007). ds: dorsal longitudinal sinus; fl: flocculus; ob: olfactory bulbs. Not to scale.

Comparaisons des endocastes crâniens en vue latérale de (A) Carnotaurus, (B) Viavenator (modifié d’après Paulina-Carabajal et Filippi, 2018), (C) Aucasaurus (modifié d’après Paulina-Carabajal et Succar, 2015) et (D) Majungasaurus (modifié d’après Sampson et Witmer, 2007). ds : sinus longitudinal dorsal ; fl : floculus ; ob : bulbes olfactifs. Non à l’échelle.

Selected measurements of endocranial values, Reptile encephalization quotient and olfactory ratios of Carnotaurus compared with those of other theropods.

Mesures sélectionnées de valeurs endocrâniennes. Quotient d’encéphalisation de reptile et rapports olfactifs de Carnosaurus comparés à ceux d’autres théropodes.

Table 1 Taxon EV (cm3) EV (cm3) 50% EV (cm3) 37% BdM (kg) REQ 37% REQ 50% REQ 100% OR (%) Carnotaurus 169.8 84.9 62.8 1419–1743 1.4–1.6 1.9–2.2 3.9–4.3 50 Viavenator a 141.6 70.8 52.4 – – – – 57 Majungasaurus 106.4 b 53.2 39.4 1130 b 1.14 1.54 3.08 48.3 c Ceratosaurus – – – 1.22 d 1.66 d 3.31 d 48 c Allosaurus 169–188 e 84.5–93.9 62.5–69.5 1400–2300 e 1.3–1.8 1.8–2.4 3.28 50 c Giganotosaurus 275 e 101.7 137.5 7000 f 1.07 1.4 2.9 67 c Sinraptor 95 g 47.5 35.1 1700 h 0.81 1.1 2.19 55 g Murusraptor i 148.2 74.1 54.8 1551 1.33 1.8 3.6 45–50 Tyrannosaurus 414.2 e 207.1 153.2 5654–7000 1.8–1.6 2.5–2.2 4.4–4.9 66–71 c

Paulina-Carabajal and Filippi, 2018.

Sampson and Witmer, 2007.

Zelenitsky et al., 2009.

Franzosa, 2004.

Paulina-Carabajal and Canale, 2010.

Mazzetta et al., 2004.

Paulina-Carabajal and Currie, 2012.

Christiansen and Fariña, 2004.

Paulina-Carabajal and Currie, 2017.