Carnotaurus

Credit: Fred Wierum.

*Carnotaurus* is an extinct genus of abelisaurid theropod that lived in South America during the Late Cretaceous.

## History

MACN-CH 894, an exceptionally preserved skeleton, is the only and holotype specimen of *C. sastrei*, found by José Bonaparte in 1984 on the Jurassic and Cretaceous Terrestrial Vertebrates of South America expedition. This expedition, the 8th in a series starting in 1976, also wielded *Amargasaurus*. It was sponsored by the National Geographic Society. Most of the front of the animal was preserved and articulate, including the posterior 2/3 of the tail, most of the lower leg and hindfeet, which were destroyed by weathering. It was an adult individual, as showed by fused sutures in the braincase. Lying on its right in the death pose, the specimen also exhibited skin impressions across most of the exposed body. A second expedition was later carried out to reinvestigate the locality, recovering more skin patches. The skull was deformed due to the fossilization process, with the rostrum displaced leftwards, the nasals pushed upwards and the premaxillae scrunched back into the nasals. The upper jaw exhibited major curvature, more in life, due to this deformation. The rostrum was most affected rather than the posterior of the skull, likely due to a stronger rigidity. In the top and bottom views, the upper jaws were shaped less like a U than the mandible, which results in a mismatch. This was caused by the deformation acting from the sides, which affects the upper, but not lower, jaws; likely due to a greater flexibility of the join flexibility in the mandible.

It was collected from the Pocho Sastre farm near Bajada Moreno, Telssen Department, Chubut Province, Argentina. Preparation progressed very slowly, since it was embedded in a hematite concretion. Bonaparte (1985) published a note presenting *C. sastrei*, briefly describing the skull and mandible. It was named from the Latin carno [carnis], meaning "flesh" and tauru, meaning "bull". In allusion to it's bull-like horns, it means "meat-eating bull", with specific epithet referencing Angel Sastre, who owned the ranch the holotype was found in. In 1990, a comprehensive description detailing the entire skeleton was publihed. It was, after *Abelisaurus*, the second abelisaurid, and by far the best understood abelisaur and Southern Hemisphere theropod for many years. In the 21st century, *Aucasaurus*, *Majungasaurus* and *Skorpiovenator*, all well-preserved abelisaurs, were named, which allowed scientists to reevaluate anatomy of Carnotaurus. Thee holotype skeleton is now on display at the Argentine Museum of Natural Sciences in Bernardino Rivadavia, and replicas are seen worldwide. Stephen and Sylvia Czerkas sculpted a life-sized *Carnotaurus* statue that was previously on display at the Natural History Museum of Los Angeles County. Ordered during the mis-1980s, it is likely the first paleoart to show accurate theropod skin.

## Description

*Carnotaurus* was large but lightly-built. The only specimen is 7.5-9 meters (24.6-29.5 feet) long, making it one of the largest abelisaurids. *Ekrixinatosaurus* and maybe *Abelisaurus* are competition, with some studies also including *Pyconemosaurus*. However, some studies use different estimation methods, which place differing sizes. It was a highly-specialized theropod, which is especially seen in the skull, vertebrae and forelimbs The pelvis and hindlimbs, however, are conservative, resembling basal *Ceratosaurus*. Both were long and slender. The femur is 103 centimeters long, having an average diameter of 11 centimeters.

The skull is 59.6 centimeters (23.5 inches) long, and is proportionally shorter and deeper than other large theropods. The rostrum is moderately broad and not tapering, curving upwards. The pair of horns were prominent, protruding obliquely above the orbits. These were formed by the frontals and probably had bony cores with a much larger keratinous sheath (Bonaparte, 1990). They were thick, conical, solid, somewhat vertically-flattened in a cross-section and were 15 centimeters (5.9 inches) long. Mauricio Cerroni et al. (2020) agreed with Bonaparte, that they supported a keratin sheat, but argued they were not that much longer than the horncores.

Like other dinosaurs, there were six major fenestrae on each side. The external naris in *Carnotaurus* is higher than long, which differs from related taxa. The antorbital fenestra is bounded by the antorbital fossa, which was small in *Carnotaurus*, as in all abelisaurids. The orbits, slightly rotated forward, suggest a degree of binocular vision The frontal is excluded from the orbit, as in all abelisaurids. The mandibular fenestra in *Carnotaurus* was large, comparatively. 4 premaxillary and 12 maxillary teeth lined each eachside of the upper jaws, with the lower having 15 dentary teeth on each side. They are long and slender, which differs from the shorter teeth in other Abelisauridae. However, Cerroni et al. (2020) stated all erupted teeth were severely damaged during recovery, later reconstructed with plater. Bonaparte (1990) notes the lower teeth were fragmentary. Therefore, reliable data concerning the teeth of *C. sastrei* is limited to replacement teeth and tooth roots still in the jaw, studied with CT scanning. The replacement teeth had low, flattened crowns, were spaced closely and inclined forwards ~45°. Bonaparte (1990) noted the lower jaw is shallow and weakly-constructed, with the dentary connected to the posterior mandibular bones by 2 points, which contrasts the robust skull. Cerroni et al. (2020) found multiple, loose, connections between the dentary and posterior jaw bones. This articulation was very flexible, but not as weak as thought. The bottom margin in the dentary is convex; it was straight in *Majungasaurus*.

With the mandible are ossified articulate hyoids. These preserved two curved, rod-like ceratobranchials that articulate with one basihyal. The latter is the only (non-avain) theropod known with this preserved. The posterior of the skull has well-developed and air-filled chambers, who surround the braincase, seen in other abelisaurids. Two chamber systems are present; the paratympanic and outgrowth chambers of the neck air sacs. Several autapomorphies are known from the skull; two horns and a very short, deep skull. The maxilla has excavations above the promaxillary fenestrae, which were excavated by the antorbital air sinus. The nasolacrimal duct exits the medial surface of the lacrimal through a canal, which has an uncertain function. Other proposed autapomorphies include a deep, long, air-fiilled excavation in the quadrate and an elongate depression on the palate's pterygoid.

The vertebral column consists of 10 cervicals, 12 dorsals, 6 fused sacrals and an unknown amount of caudals. The neck was near-straight, being especially wide towards the base and lacking the S-curve seen in most theropods. The neck's vertebrae had a double row of enlarged, upwards projecting epipophyses, which creates a smooth trough at the top of the cervical vertebrae. These reach their peak at the highest points of the spine, and towered over the unusually-low spinous processes. This likely provided a strong muscle attachment point, with a similar row seen in the tail, but formed by highly-modified caudal ribs, in a V-shape (from the front). Their inner sides create a smooth, flat, top surface on the front tail vertebrae. Each caudal rib had a forward-projecting, hook-shaped extension at the end, which connects to the previous caudal rib.

## Paleobiology

### Integument

    In 2019, a picture surfaced online of the *Carnotaurus* holotype skull being prepared in the field. The picture aimlessly floated through platforms such as Twitter, with almost no backstory present. With the help of many sources, users were able to piece together a story behind the strange photo. The photo was taken in Argentina, with a paleontologist posing over the disarticulated 3D skull of the *Carnotaurus* holotype, with him placing his hat over the dinosaurs head and a visible rocky background. The story behind the image goes as follows:
The original *Carnotaurus sastrei* specimen was unearthed as part of an expedition by the name of "Jurassic and Cretaceous Terrestrial Vertebrates of South America", which was meant to discover new taxa in South America, as little was known about both the Jurassic and Cretaceous periods. The skeleton presumably had patches of skin impressions, which are said to have covered the skull, and may have even been found on the animals chest and tail. The photo was taken in 1984. The skeleton was layered in thick Hematite concretions.

In this case, *C. sastrei*'s decayed corpse forms a nucleus. This unique combination of rock and fossil makes preparation more difficult than normal preparation. The image appears to feature Orlando Gutierrez, field assistant of José Bonaparte and technician and the head of *Carnotaurus*. The skull is covered by a hard substance of two possibilities: either the Hematite concretions, or skin impressions. Accounts from different sources claim different stories for the proposed skin impression. Dr. Mark Witton claims in an article that the 'skin' was prepared away. Some hear that the skin was discarded in the field, and others say it was discarded in the preparation lab. Dr. Darren Naish points out that the head had been separated from its body in the field which "tells us something interesting". The book "Hunting Dinosaurs", written by Louie Psihoyos states that Bonaparte had little regard for fossil material other than the skeleton itself, even sawing into fossils just to reach the skeletal elements. His statement is further solidified by him recounting Bonaparte chiseling away fossil eggshells, just to see if there was an embryo inside the egg. Some believe, however, that Louie's views of Dr. Bonaparte lacked justification; unwarranted.

Furthermore, the point that they had recognized the matrix as skin later confuses this situation a lot more. It has been brought up that the skin impressions could appear like the concretions engulfing the rest of the body. This would make it nearly impossible to recover discarded skin impressions at the dig site today. If the rock in the photo were truly skin, the face would have been covered in thick lips, with broad, blunt upturned horns and large scales would line the predators snout. Hendrickx and Bell (2021), however, mention this, but state it was prepared away in the field and likely not skin impressions. They also unearth an unseen photograph of the specimen.

In 2021, Christophe Hendrickx and Phil R. Bell described the scaly skin impressions of *Carnotaurus*. These impressions are the most complete of any theropod. Skin impressions from the shoulder, lower jaw, ribcage, thoracic, tail and possibly neck regions on the right side of the body preserves medium-large (20-65 millimeters in diameter) conical feature scales surrounded by many >14 millimeter basement scales, who are separated by interstitial tissues. The feature scales, not osteoderms, contrary to popular belief, are distributed randomly with no observable patterns, and vary along the body. Their apices are positioned differently on different areas. However, the basement scales vary; from small and elongate to large and polygonal, and circular to lenticular in the thoracic, scapular, and tail regions (respectively). Since presumed skin impressions on the face were destroyed, it is said the face was covered in scales, with possible flat scales similar to crocodilians based on the hummocky bone texture. The top of the snout had many small holes and spikes, which was a likely cornified pad. Scales probably surrounded the eyes, since the hummocky texture and longitudinal grooves on the lacrimal and postorbital are present.

The skin was made by a mosaic of polygonal non-overlapping scales dived by thin, parallel grooves. This was all around the body, minus the head. Larger bumpy structures distributed the sides of the neck, back and tail in irregular patterns. They are 5 centimeters (2 inches) tall, often have a midline ridge, become larger nearer to the top and are 8-10 centimeters (3.1-3.9 inches) apart. They are feature scales, and did not contain bone. Due to the size and active lifestyle of *Carnotaurus*, it is speculated this integument played a thermoregulatory role, which is seen in mammals and reptiles. They also suggested the skin may have protected from bites, which they ruled out due to unlikelyhood, and that it may have functioned for display, but was also ruled out since scale types are not necessarily indicative of colouration. It may have also acted as "mite pockets", which divert mites from other orifices, such as the eyes and eyes. The 2021 paper also confirmed any non-bone material on the cranium was prepared away, so it is unknown if the "soft tissue" present in popular images in circulation were soft tissue or something else entirely.

### Arms

*Carnotaurus* forelimbs are proportionally shorter than other large carnivorous dinosaurs. The forearm was only a quarter the size of the upper arm. No carpalia were in the hand, so the metacarpals attach directly to the forearm. It had four basic digits, with only the middle two ending in finger bones, with the fourth consisting of a splint-like metacarpal, possibly representing an external 'spur'. The fingers were fused and immobile, possibly lacking claws. These forearms were proportionally shorter and more robust than other abelisaurs and the fourth splint-like metacarpal being the longest hand bone. A 2009 study abelisaurid arms were vestigial, since nerve fibers responsible for stimulus transmission are reduced to the extent seen in *Dromaius* and *Apteryx*, which also have vestigial forelimbs.

The arms of *Carnotaurus* are not known to have had any function. However, several theories exist as to why the animal had such tiny, borderline useless, arms. The first theory states that the arms would have been used in mating by locked into the animals partner. The second hypothesis, created by John Conway and Darren Naish, is based almost 100% on speculation, named the "arm-waggling" theory. They state the arms were attached to the shoulder by a ball joint, making them extremely flexible. Perhaps, *Carnotaurus* has brightly ornamented arms that would have aided in getting a mate or by scaring off attackers. They state that in the forward view, both the horns and arms of *Carnotaurus* could have been seen. Since they would have assumed a default position hugging the body, the body mass would have obscured the arms until swung outward. This, in the case of a mate, would have possibly functioned like a flash display, characteristic of most birds. In the case of an attacker, the body mass would have hidden the arms until moving out, which could have confused the other animal and scared it away. Another theory, states the arms were used like an ostriches wings, a tool to help balance and steer in fast motion .

### Horns

*Carnotaurus* is the only biped animal known with a pair of horns on its frontal. However, the use of these is not known. Several have tried to interpret them as used in fighting, killing prey, same-species recognition or courtship display. Gregory S. Paul (1988) thought they were used as butting weapons, with the small orbita to minimize hurting the eyes during fighting. Gerardo Mazzetta et al. (1998) suggested it was used like rams do, having two individuals colliding head-on. Fernando Novas (2009) interpreted adaptations for delivering blows in the head, suggesting the skull may have made skull movements quicker by reducing inertia, with the muscular neck supporting strong blows. He also notes enhanced rigidity and strength in the spinal column, which may have evolved to withstand shocks brought on by the head and neck. Other studies show it did not deliver rapid head blows, rather slowly pushing with their horns. Mazzetta et al. (2009) argued these horns may have distributed force rather than sustaining blows, since they are mostly flat and strongly-built. Rafael Delcourt (2018) suggests slow headbutting or shoving, or blows to the opponent's neck and flank, as both are seen in extant animals. The latter is suggested for *Majungasaurus* in a 2011 conference paper. Gerardo Mazzetta et al. (1998) propose the horns were used to injurt/kill small prey, since they may have looked like bovid horns although the cores are blunt. However, this would then be the only example of horns used as weapons. They could have been sheathed in large colourful keratin in life. Some think such ornamentation could have been used in display, with the head angled down (horns in view) and the arms "waggling" (see here).

### Diet

    Analyses of *Carnotaurus* jaws by Mazzetta et al. (1998, 2004 and 2009) suggest it was capable of quick, but not strong, bites; since quick bites are more important than strong bites when capturing prey. They also note a high degree of kinesis within the cranium and especially the mandible, similar to modern snakes. The jaw's elasticity would have allowed it to swallow prey whole, with the anterior of the mandible being hinged, being able to move up and down. When the teeth pressed down, they would have projected forwards, allowing it to impale small prey; on the contrary, when they curved up, the backwards-projecting teeth would have ensnared prey, making escape impossible. They also found the skull could withstand forces while tugging. Thus, it probably fed on small prey with the ability to hunt larger prey, in addition. In 2009, they estimated a bite force of 3341 newtons.

However, François Therrien et al. (2006) questioned this, finding a bite force double to American alligator, the strongest bite of any living tetrapod. They also noted analogies with Komodo dragons in the flexural strength in the mandible linearly decreases nearer to the tip; the jaws were not adapted to precision capturing of small prey but rather slashing wounds towards larger prey. Consequently, this study states *Carnotaurus* must have fed upon larger animals as a possible ambush predator. Cerroni et al. (2020) argued flexibility was restricted in the mandible with the thickened skull roof and cranial joint ossification suggesting the cranium bore little or no kinesis.

Robert Bakker (1998) found it fed upon large prey, especially sauropods, since the short snout, relatively small teeth and strong occiput is similar to *Allosaurus*, who hunts sauropods. Bakker noted the upper jaw could have been used like an axe, weakening sauropods by repetitively injuring them.

### Locomotion

Mazzetta et al. (1998 and '99) presume *Carnotaurus* was a swift runner, arguing the thigh bone has a morphology that can withstand high bending moments during running, since an animal's ability to withstand these forces limits its capabilities. It would have been better than a human but not as good as an ostrich, finding a top speed of 48-56 kilometers (30-35 miles)/hour. The most important locomotor muscle in dinosaurs is the caudofemoralis, which attached to the fourth trochanter and pulls the which back when contracted. Scott Persons and Phil Currie (2011) note the caudal ribs in *Carnotaurus* do not protrude horizontally, rather angled against the vertical axis of the vertebrae, which forms a V rather than a T. This provided space for a much larger caudofemoralis, with a mass calculated at 111-137 kilograms (245-302 pounds) per leg; therefore, *Carnotaurus* may have been the fastest theropod. While the caudofemoralis was enlarged, the epaxial muscles above the caudal ribs would have been proportionally smaller, The longissimus and spinalis, responsible for tail movement and stability, are much smaller. Despite this, the caudal ribs have forward-projecting processes that interlock the vertebrae and pelvis, stiffening the tail but also diminishing the ability to make tight turns, since the hip and tail had to have simultaneously oriented - unlike other theropods.

### Senses

Cerroni and Pualina-Carabajal (2019) Ct scanned the endocranial cavity of *Carnotaurus*, finding a brain volume of 168.8 centimeters³ (though the brain filled a fraction of this measurement). They used two brain size estimates, assuming a brain 50% and 37% of the cavity. The encephalization quotient received is larger than *Majungasaurus* but lesser than tyrannosaurids. The pineal gland might have been smaller than other abelisaurids, indicated by a low dural expansion.

The olfactory bulbs were large, with the optic lobes being small. This indicates a better developed sense of smell and a poorer sense of sight (which is seen as the opposite in birds). The anterior of the olfactory tracts and bulbs curves downwards, which is only shared with *Indosaurus*. Cerroni and Paulina-Carabajal hypothesize it may have relied more on smell than other abelisaurids based on the curvature and large bulbs. The flocculus was large, indicating that it and other South American forms frequently used quick movements. In *Carnotaurus* and other belisaurids, hearing may have been poorly-developed as suggested by the short lagena. It is thought to have had a hearing range below 3 kHz.

## Classification

Carnotaurus is one of the best known abelisaurids. Though its relations to this family is debated, *Carnotaurus* is one of the most derived members, as shown consistently in cladistics. *Aucasaurus* or *Majungasaurus* may have been its closest relative. One 2008 review contrasts this, however; they propose it was not closely related to either, but instead Ilokelesia, as a sister taxon. Juan Canale et al. (2009) erected Brachyrostra to include *Carnotaurus* but not *Majungasaurus*, which has been used as a classification for many studies later. *Carnotaurus* is eponymous with two abelisaurid sybgroups: Carnotaurinae and Carnotaurini, but these are not universally accepted as valid. Carnotaurinae was defined to include all derived abelisaurids excluding *Abelisaurus*, because it is considered basal by most studies. However, a 2008 review reclassifies it as derived, instead, Carnotaurini was proposed to include *Carnotaurus* and *Aucasaurus*, and is only used by paleontologists who accept that these two are nearest relatives. Canale et al. (2009) concludes:

## Paleoecology

Originally, the rocks where *Carnotaurus* were found were identified as the Upper Gorro Frigio Formation, ~100 million years old (Albian/Cenomanian-aged). Later, however, they were reassigned to the younger La Coloni Formation 83.6-66 million years ago (Campanian and Maastrichtian-aged) Novas (2009) in a book gave the narrower timespan of 72-69.9 million years ago (Lower Maastrichtian-aged). Therefore, C. sastrei was the latest South American abelisaurid. By the Late Cretaceous, South America was isolated from Africa and North America. The La Colonia Formation exposes over the south slope of the North Patagonian Massif. Most vertebrates, including C. sastrei, come from the middle of the middle facies association, likely representing estuaries, tidal flats or coastal plain. Seasonal climate existed with dry and humid seasons. Lungfish, turtles, crocodiles, plesiosaurs, dinosaurs, lizards, snakes and mammals are the most common vertebrates. Turtles are known from ~5 genera. *Sulcusuchus*, a polycotylid, is known from this formation. *Reigitherium*, *Argentodites* and possibly a multituberculate are known mammals. One enantiornithe is known.

## References

1. ↑ https://www.skyemcdavid.com/blog/2023/carnotaurus-etymology
2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 https://en.wikipedia.org/wiki/Carnotaurus
3. ↑ https://en.wikipedia.org/wiki/Carnotaurus
4. ↑ The New Face of Carnotaurus?
5. ↑ https://www.sciencedirect.com/science/article/abs/pii/S0195667121002421
6. ↑ Paleo Profile - Carnotaurus