Carnotaurus

Not to be confused with Carnosaurus. Carnotaurus is a genus of theropod dinosaur that lived in South America during the Late Cretaceous period, about 69 to 66 million years ago. Its only species is Carnotaurus sastrei.

We know about Carnotaurus from one well-preserved skeleton. This makes it one of the best-understood theropods from the Southern Hemisphere. The skeleton was found in 1984 in the Chubut Province of Argentina, in rocks of the La Colonia Formation. Carnotaurus belongs to the Abelisauridae, a group of large theropods. These dinosaurs were the big hunters in the southern lands of Gondwana during the Late Cretaceous. It is often placed in the Brachyrostra, a group of short-snouted animals found only in South America.

Carnotaurus was a lightly built, bipedal predator. It walked on two legs. It grew 7.5 to 8 metres (24.6 to 26.2 feet) long and weighed 1.3 to 2.1 metric tons. It had two thick horns above its eyes. No other carnivorous dinosaur is known to have had horns like these. It also had a very deep skull on a muscular neck. Its front legs were small and vestigial, while its back legs were long and slender. Skin impressions show small scales, about 5 mm across, with large bumps along the sides of its body. There are no signs of feathers.

Scientists think the horns and strong neck may have been used when rival animals fought each other. It is still unclear how Carnotaurus hunted. Some studies suggest it could take down very large animals such as sauropods. Other studies found that it mostly hunted smaller animals.

Discovery

The only skeleton (holotype MACN-CH 894) was dug up in 1984 by a team led by Argentinian scientist José Bonaparte. The same team also found the spiny sauropod Amargasaurus. This was the eighth trip in a project called "Jurassic and Cretaceous Terrestrial Vertebrates of South America," which began in 1976 and was supported by the National Geographic Society. The skeleton was in good condition, and its bones were still joined together. However, weathering had worn away the back two-thirds of the tail, much of the lower leg, and the hind feet. It came from an adult and was lying on its right side, with its neck bent back over its body. Its skin left many marks in the rock, so a second trip went back to the site and found several more patches of skin. During fossilization, the skull was squashed, and the premaxillae were pushed back onto the nasal bones.

The skeleton was collected on a farm called "Pocho Sastre" near Bajada Moreno in the Telsen Department of Chubut Province, Argentina. It was trapped inside a large hematite concretion, which is a very hard kind of rock, so it took a long time to prepare. In 1985, Bonaparte named it as a new genus and species, Carnotaurus sastrei. The name means "meat-eating bull," which refers to the animal's horns, which looked like a bull's. The specific name honors Angel Sastre, who owned the ranch where it was found. A full description of the skeleton was published in 1990. It was the second member of the family Abelisauridae to be discovered, after Abelisaurus. For many years, it was the best-understood member of its family and the best-understood theropod from the Southern Hemisphere. In the 21st century, scientists described other well-preserved abelisaurids, including Aucasaurus, Majungasaurus and Skorpiovenator. These helped scientists rethink parts of the body of Carnotaurus. The holotype skeleton is displayed at the Argentine Museum of Natural Sciences in Bernardino Rivadavia, and replicas can be seen in this and other museums around the world. A life-sized sculpture by Stephen and Sylvia Czerkas was once on display at the Natural History Museum of Los Angeles County. The museum ordered it in the mid-1980s, and it is probably the first life restoration of a theropod to show accurate skin.

Description

Carnotaurus was a large meat-eating dinosaur with a light body. The only known skeleton was about 7.5 to 8 m (24.6 to 26.2 ft) long. This made it one of the largest abelisaurids. Ekrixinatosaurus and possibly Abelisaurus are known from very incomplete remains. They may have been similar in size or larger. A 2016 study found that only Pycnonemosaurus, at 8.9 m (29.2 ft), was longer. Carnotaurus was estimated at 7.8 m (25.6 ft). Several studies estimated its weight at about 1,300 to 2,100 kg.

Carnotaurus was very specialized. Its skull, vertebrae and forelimbs were especially unusual. Its pelvis and hind legs were more like those of the older, more basal Ceratosaurus. Both were long and slender. The left femur (thigh bone) is 103 cm long but only about 11 cm wide on average.

The skull was about 59.6 cm (23.5 in) long. It was shorter and deeper for its size than in any other large meat-eating dinosaur. The snout was fairly wide, and the jaws curved upward. A pair of thick, cone-shaped horns, about 15 cm (5.9 in) long, rose above the eyes. The forelimbs were also shorter, compared to body size, than those of any other large meat-eating dinosaur. This included tyrannosaurids.

Carnotaurus was the first theropod dinosaur found with many fossil skin impressions. These showed small scales that did not overlap. Larger bumps ran along the sides of the neck, back and tail. There is no evidence that it had feathers.

Classification

Carnotaurus is one of the best-understood genera of the Abelisauridae. This is a family of large meat-eating dinosaurs that lived in the ancient southern supercontinent Gondwana. Abelisaurids were the main predators in the Late Cretaceous of Gondwana. They took the place of the carcharodontosaurids and filled the role that the tyrannosaurids had in the northern continents. Many features of this family were more extreme in Carnotaurus than in any other abelisaurid. These included a shorter skull, shorter arms, and unusual neck and tail bones.

Scientists still debate how the members of the Abelisauridae are related. However, Carnotaurus is consistently shown to be one of the most derived members of the family. Its closest relative might have been Aucasaurus or Majungasaurus. A 2008 review suggested that it was not closely related to either. Instead, it proposed Ilokelesia as its sister taxon. In 2009, Juan Canale and colleagues created the new clade Brachyrostra. It included Carnotaurus but not Majungasaurus, and many studies have followed this classification since. Carnotaurus also gives its name to two subgroups, the Carnotaurinae and the Carnotaurini. Not all paleontologists accept these groups. Below is a cladogram published by Canale and colleagues in 2009.

Paleobiology

Carnotaurus is the only known meat-eating two-legged animal with a pair of horns on its frontal bone. Scientists are not completely sure what the horns were for. They may have been used in fights with others of its kind or to catch prey. They may also have been used to show off to mates or to recognise members of the same species. Greg Paul (1988) thought the horns were used for butting. Gerardo Mazzetta and colleagues (1998) suggested it used them like rams. Fernando Novas (2009) found several adaptations for hitting with the head. For example, its short skull may have let it move its head more quickly by reducing its moment of inertia. Other studies suggest rivals pushed slowly against each other instead of hitting quickly. Rafael Delcourt (2018) proposed slow headbutting, like the marine iguana, or blows to the neck and sides, like the giraffe. Mazzetta and colleagues (1998) also proposed that the horns might have hurt or killed small prey. This would be the only reported case of horns used as hunting weapons in animals. Mazzetta and colleagues studied the jaws. They suggested it could make quick bites but not strong ones. Quick bites are more useful for capturing small prey, as crocodiles do. Its skull and lower jaw were quite flexible (kinesis), somewhat like snakes. This may have helped it swallow small prey whole. Its bite force was estimated at about 3,341 newtons in 2009. A 2022 study estimated about 3,392 newtons at the front of the jaws. François Therrien and colleagues (2005) argued its bite was twice as strong as that of the American alligator. The alligator may have the strongest bite of any living tetrapod. They also compared its jaws to those of Komodo dragons and suggested it mainly hunted large animals. Robert Bakker (1998) found it mainly ate very large prey, especially sauropods. He noted that its skull features, including the occiput, had also evolved in Allosaurus.

Mazzetta and colleagues (1998, 1999) thought that Carnotaurus was a fast runner. Its thigh bone seemed strong enough to handle the bending moments of running. Scott Persons and Phil Currie (2011) argued that its tail was built to hold a very large caudofemoralis muscle. This muscle attaches to the fourth trochanter and pulls the thigh bone backwards. Because of this, it could have been one of the fastest large theropods. Smaller epaxial muscles, including the longissimus and spinalis, controlled tail movement and balance. This may have made sharp turns harder. Cerroni and Paulina-Carabajal (2019) studied the space inside its skull using a CT scan. They estimated a reptile encephalization quotient that was larger than that of Majungasaurus but smaller than that of tyrannosaurids. The pineal gland, which produces hormones, might have been small. The olfactory bulbs, which handled the sense of smell, were large. The optic lobes, which handle sight, were relatively small. This suggests that smell may have been better developed than sight. This is the opposite of modern birds. The flocculus, which helps keep the gaze steady, was large. This may mean quick head and body movements were common. Hearing may have been poor, as shown by the short lagena of the inner ear.

Age and paleoenvironment

At first, the rocks where Carnotaurus was found were placed in the upper part of the Gorro Frigio Formation. Scientists thought they were about 100 million years old, from the Albian or Cenomanian stage. Later, they were found to belong to the much younger La Colonia Formation. A 2021 study dated them to the Maastrichtian to early Danian ages, around 69 to 64 million years ago. Carnotaurus was the most recent South American abelisaurid known. By the Late Cretaceous, South America was already separated from both Africa and North America.

The La Colonia Formation is found on the southern slope of the North Patagonian Massif. Most fossils, including Carnotaurus, come from its middle section. This area likely formed in estuaries, tidal flats or coastal plains. The climate had both dry and wet seasons. Common animals found there include ceratodontid lungfish, turtles, plesiosaurs, crocodiles, dinosaurs, lizards, snakes and mammals. Other dinosaurs include Koleken inakayali, which is closely related to Carnotaurus, the saltasauroid titanosaur Titanomachya gimenezi, an unnamed ankylosaur and an unnamed hadrosauroid. Some snakes belong to the families Boidae and Madtsoiidae, such as Alamitophis argentinus. Turtles include at least five taxa: four from Chelidae (Pleurodira) and one from Meiolaniidae (Cryptodira). Plesiosaurs include two elasmosaurs, Kawanectes and Chubutinectes, and a polycotylid, Sulcusuchus. Mammals include Reigitherium bunodontum, once thought to be the first South American docodont, and Coloniatherium cilinskii. There are also possible gondwanatherians or multituberculates, such as Argentodites coloniensis and Ferugliotherium windhauseni. Remains of an enantiornithine and, possibly, a neornithine bird have also been found.