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Carnotaurus
Temporal range: Late Cretaceous, 72–69.9 Ma
↓
Mounted skeletal cast, Chlupáč Museum, Prague
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Family: †Abelisauridae
Genus: †Carnotaurus
Bonaparte, 1985
Species:
†C. sastrei
Binomial name
†Carnotaurus sastrei
Bonaparte, 1985

Carnotaurus (/[invalid input: 'icon']ˌkɑːrn[invalid input: 'ɵ']ˈtɔːrəs/) is a genus of large abelisaurid theropod dinosaur that lived in South America during the Late Cretaceous period, between about 72 to 70 million years ago. The only species is Carnotaurus sastrei. Known from a single but exceptionally well-preserved skeleton, it is one of the best-known theropods from the Southern Hemisphere. The skeleton was found in 1984 in the Chubut Province of Argentina in rocks of the La Colonia Formation. The name Carnotaurus means "meat-eating bull", referring to its bull-like horns. It is derived from the Latin carno [carnis] ("flesh") and taurus ("bull").

Carnotaurus was a lightly built,[1] bipedal predator, measuring 8 to 9 meters in length and weighing more than a metric ton. It was a highly specialized and distinctive theropod. Besides having thick horns above the eyes, a feature unseen in all other carnivorous dinosaurs, it was characterized by a very deep skull sitting on a muscular neck, small and possibly functionless forelimbs, and long and slender hindlimbs. Skin impressions preserved with the skeleton indicate that the skin was not covered by feathers but instead lined with rows of bumps.

The distinctive horns and the muscular neck may have been used in fighting conspecifics. Its feeding habits remain unclear: although some studies suggested it was able to hunt down very large prey such as sauropods, other studies found that it preyed mainly on relatively small animals. Carnotaurus was well adapted for running and was possibly one of the fastest large theropods.

Description

Scale diagram comparing Carnotaurus to a human

The only known individual was about 8 to 9 meters in length, making Carnotaurus one of the largest abelisaurids.[A][B] Only Ekrixinatosaurus and possibly Abelisaurus may have been similar or larger in size, though the incomplete remains of these genera make size estimations imprecise.[C][D][E] Its mass was estimated at 1,350 kg,[F] 1,500 kg[G] and 2,102 kg[H] in separate studies that used different estimation methods. Carnotaurus was a highly specialized theropod, as seen especially in characteristics of the skull, the vertebrae and the forelimbs.[I]

Skull

Skull cast, Dinosaur Discovery Museum, Kenosha, Wisconsin

The skull, measuring 59.6 cm in length, was proportionally shorter and deeper than in any other large carnivorous dinosaur.[J][E] The snout was broad, unlike the tapering snout seen in more basal theropods like Ceratosaurus, and the jaws were curved upwards.[9] As in other abelisaurids, the facial bones and especially the nasal bones were sculptured with numerous small holes and spikes.[K] In life, these bones would have been covered by a wrinkled and possibly keratinous integument.[K] Above the eyes, a prominent pair of obliquely protruding horns was present. These horns, formed by the frontal bones,[K] were thick, flattened on their upper sides and measure 15 cm in length.[10] In life, they would probably have formed the bony cores of much longer keratinous horns.[K] The proportionally small eyes[11] were situated in the upper part of the keyhole shaped orbita (eye sockets). The upper part was slightly rotated forward, probably permitting some degree of binocular vision.[K][L]

The teeth were long and slender,[11] in contrast to the usually very short teeth seen in other abelisaurids.[9] On each side of the upper jaws there were 4 premaxillary and 12 maxillary teeth,[M] the lower jaw was equipped with 15 dentary teeth per side.[N] Contrasting to the robust-looking skull, the lower jaw was shallow and weakly constructed, with the dentary (the foremost jaw bone) connected to the hindmost jaw bones by only two contact points.[11][N] The lower jaw was found with hyoid bones in life position. These slender bones, supporting the tongue musculature and several other muscles, are rarely found in dinosaurs because they were not connected to other bones and therefore get lost easily.[N][12]

Vertebrae

Sixth tail vertebra of the holotype in A) side, B) front and C) top views. Arrows show the highly modified caudal ribs.

The vertebral column consisted of 10 cervical, 12 dorsal, 6 fused sacral[E] and an unknown number of caudal vertebrae.[2] The neck was nearly straight, rather than S-curved as seen in other theropods, and also unusually wide, especially towards its base.[13] The top of the spinal column featured a double row of enlarged, upwardly directed bony processes called epipophyses, making the neck flat on top.[12][2] These processes probably provided attachment areas for a markedly strong neck musculature.[O] A similar double row was present in the tail, here formed by highly modified caudal ribs protruding in a V-shape, resulting in a flattened top of the tail.[12][14]

Forelimbs

Bones of the hand, as interpreted by Ruiz and colleagues (2011)[15]

The forelimbs were proportionally shorter than in any other large carnivorous dinosaurs including Tyrannosaurus.[P] The forearm was only a quarter the size of the upper arm. There were no carpalia in the hand, so that the metacarpals articulated directly with the forearm.[15] The hand showed four basic digits,[2] though apparently only the middle two of these ended in finger bones, while the fourth consisted of a single splint-like metacarpal that may have represented an external 'spur.' The fingers themselves were fused and immobile, and may have lacked claws.[16] Carnotaurus differed from all other abelisaurids in having proportionally shorter and more robust forelimbs, and in having the fourth, splint-like metacarpal as the longest bone in the hand.[15] A 2009 study suggests that the arms were functionless in abelisaurids, because nerve fibers responsible for stimulus transmission were reduced to an extent seen in today's emus and kiwis, which also have functionless forelimbs.[17]

Skin

Artist's impression

Carnotaurus was the first theropod dinosaur discovered with comprehensive fossil skin impressions.[18] These impressions, found beneath the skeleton's right side, come from different body parts, including the lower jaw,[18] the front of the neck, the shoulder girdle, and the rib cage.[Q] The largest patch of skin corresponds to the anterior part of the tail.[Q] Originally, the right side of the skull was also covered with several fragments of skin – this was not recognized when the skull was prepared, so that these fragments were accidentally destroyed.[18]

The skin was built up of a mosaic of polygonal, non-overlapping scales measuring approximately 5 mm in diameter. This mosaic was divided by thin, parallel grooves.[R] Scalation was similar in different body parts with the exception of the head, which apparently showed a different, irregular pattern of scales.[R][19] There is no evidence of feathers.[18] Uniquely for theropods, there were large knob-like bumps running along the sides of neck, back and tail in irregular rows. Each bump showed a low ridge and measured 4 to 5 cm in diameter. They were set 8 to 10 cm apart from each other and became larger towards the animal's top. Such bumps are unseen in other theropods, and probably represent clusters of condensed scutes, not unlike those seen on the soft frill running along the body midline in hadrosaurid ("duck-billed") dinosaurs.[Q][18] Stephen Czerkas (1997) suggested that these structures may have protected the animal's sides while fighting conspecifics and other theropods, arguing that similar structures can be found on the neck of today's Iguana where they provided limited protection in combat.[18]

Classification

 Abelisauridae 
Abelisaurid phylogeny after Canale and colleagues (2009).[20]

Carnotaurus is one of the best known genera of the Abelisauridae, a family of large theropods restricted to the ancient southern supercontinent Gondwana. Abelisaurids were the dominant predators in the Late Cretaceous of Gondwana, replacing the carcharodontosaurids and occupying the ecological niche filled by the tyrannosaurids in the northern continents.[1] Several notable traits that evolved within this family, including shortening of the skull and arms as well as peculiarities in the cervical and caudal vertebrae, were more pronounced in Carnotaurus than in any other abelisaurid.[I][O][14]

Though relationships within the Abelisauridae are debated, Carnotaurus is consistently shown to be one of the most derived members of the family by cladistical analyses.[S] Its nearest relative may have been either Aucasaurus[21][20][22] or Majungasaurus;[23][24][25] this ambiguity is largely due to the incompleteness of the Aucasaurus skull material.[I][T] A recent review suggests that Carnotaurus was not closely related with either Aucasaurus nor Majungasaurus, and instead proposed Ilokelesia as its sister taxon.[T]

Carnotaurus is eponymous for two subgroups of the Abelisauridae: the Carnotaurinae and the Carnotaurini. These groups are not universally accepted by paleontologists. The Carnotaurinae was defined to include all derived abelisaurids with the exclusion of Abelisaurus, which is considered a basal member in most studies.[26] However, a 2008 review suggested that Abelisaurus was a derived abelisaurid instead.[T] Carnotaurini was proposed to name the clade formed by Carnotaurus and Aucasaurus;[21] this group is only used by those paleontologists who consider Aucasaurus as the nearest relative of Carnotaurus.[27]

Discovery

Skull cast in side view

The only skeleton (holotype, MACN-CH 894) was unearthed in 1984 by an expedition led by Argentinian paleontologist José Bonaparte.[I] This expedition also recovered the peculiar spiny sauropod Amargasaurus.[28] It was the eighth expedition within the project named "Jurassic and Cretaceous Terrestrial Vertebrates of South America", which started in 1976 and which was sponsored by the National Geographic Society.[28][U] The skeleton is exceptionally well preserved and articulated (still connected together), with only the posterior two thirds of the tail, much of the lower leg, and the hind feet being destroyed by erosion.[U][29] It was an adult individual, as indicated by the fused sutures in the braincase.[10] The skeleton was found lying on its right side, showing a typical death pose with the neck bent back over the torso.[18] Unusually, it is preserved with extensive skin impressions.[U] In view of the significance of these impressions, a second expedition was started to reinvestigate the original excavation site, leading to the recovery of several additional skin patches.[18]

The skeleton was collected on a farm named "Pocho Sastre" near Bajada Moreno in the Telsen Department of Chubut Province, Argentina.[29] Because it was embedded in a large hematite concretion, a very hard kind of rock, preparation was complicated and progressed slowly.[29][11] In 1985, Bonaparte published a note presenting Carnosaurus sastrei as a new genus and species and briefly describing the skull and lower jaw.[29] The generic name (Latin carno [carnis] – "flesh" and taurus – "bull") refers to the bull-like horns,[30] while the specific name sastrei honors Angel Sastre, the owner of the ranch where the skeleton was found.[31] A comprehensive description of the whole skeleton followed in 1990.[2] After Abelisaurus, Carnotaurus was the second member of the family Abelisauridae that was discovered.[32] For years, it was by far the best known member of its family, and also the best known theropod from the Southern Hemisphere.[33][34] It was not until the 21th century that similar well-preserved abelisaurids have been described, including Aucasaurus, Majungasaurus and Skorpiovenator, allowing scientists to re-evaluate certain aspects of Carnotaurus anatomy.[E] The Carnotaurus skeleton is deposited in the Argentine Museum of Natural Sciences' Bernardino Rivadavia;[K] replicas can be seen in this and other museums around the world.[19] Sculptors Stephen and Sylvia Czerkas manufactured a life-sized sculpture of Carnotaurus that is now on display in the Natural History Museum of Los Angeles County. This sculpture, ordered by the museum during the mid-1980s, is probably the first life restoration of a theropod showing accurate skin.[35][18]

Age and paleoecology

Restoration of a sprinting individual

Originally, the rocks in which Carnotaurus was found were assigned to the upper part of the Gorro Frigio Formation, which was considered to be approximately 100 million years old (Albian or Cenomanian stage).[29][K] Later they were realized to pertain to the much younger La Colonia Formation,[33] and are only 72 to 69.9 million years old (Lower Maastrichtian stage).[I] Thus, Carnotaurus was the latest South American abelisaurid known.[14]

The La Colonia Formation is exposed over the southern slope of the North Patagonian Massif.[36] Most vertebrate fossils, including Carnotaurus, come from the formation's middle part (called the middle facies association).[36] This part likely represents the deposits of an environment of estuaries, tidal flats or coastal plains.[36] The climate would have been seasonal with both dry and humid periods.[36] The most common vertebrates collected include ceratodontid lungfish, turtles, crocodiles, plesiosaurs, dinosaurs, lizards, snakes and mammals.[37] Some of the snakes that have been found belong to the families Boidae and Madtsoidae, such as the Alamitophis argentinus.[38] Turtles are represented by at least five taxa, four from Chelidae (Pleurodira) and one from Meiolaniidae (Cryptodira).[39] Within the marine fossils in the area is the plesiosaur Sulcusuchus erraini of the family Polycotylidae.[39] Among the mammals in this area is Reigitherium bunodontum, which was considered the first record of a South American docodont,[36] and Argentodites coloniensis, possibly of Multituberculata.[40] In 2011, the discovery of a new enantiornithine bird was announced.[41]

Paleobiology

Function of the horns

File:Carnotaurus Sastrei.JPG
Model head, showing the horns extended by keratin

Carnotaurus is the only known carnivorous dinosaur with a pair of horns on the frontal bone.[42] The use of these horns is not entirely clear; several interpretations have revolved around use in fighting conspecifics,[11][43][6][42][O] though a use in display[42] or in killing prey[6] also has been suggested.

Greg Paul (1988) proposed that the horns were butting weapons and that the small orbita would have minimized the possibility of hurting the eyes while fighting.[11] Gerardo Mazzetta and colleagues (1998) suggested that Carnotaurus used it horns in a way similar to rams. They calculated that the neck musculature was strong enough to absorb forces that emerge when two individuals collide with their heads frontally at a speed of 5.7 m/s each.[6] Fernando Novas (2009) interpreted several skeletal features as adaptations for delivering blows with the head.[O] He suggested that the shortness of the skull might have made head movements quicker by reducing the moment of inertia, while the muscular neck would have allowed strong head blows. He also noted an enhanced rigidity and strength of the spinal column that may have evolved to withstand shocks conducted by the head and neck.[O]

Other studies suggested that rivaling Carnotaurus did not deliver rapid head blows, but pushed slowly against each other with the upper sides of their skulls.[43][42] Thus, the horns may have been a device for the distribution of compression forces without damage to the brain.[42] This is supported by the flattened upper sides of the horns, the strongly fused bones of the top of the skull, and the inability of the skull to survive rapid head blows.[42]

Gerardo Mazzetta and colleagues (1998) proposed that the horns might also have been used to injure or kill small prey. Though horn cores are blunt, they may have had a similar form to modern bovid horns if there was a keratinous covering. This would be the only reported example of horns being used as hunting weapons in animals, however.[6]

Jaw function and diet

Analysis of the jaw design of Carnotaurus by Mazzetta and colleagues (1998, 2004, 2009) suggests that the animal was capable of quick bites, but not strong ones.[42][6][7] Quick bites are more important than strong bites when capturing small prey, as shown by studies of modern-day crocodiles.[42] Furthermore, these researchers noted a high degree of flexibility (kinesis) within the skull and especially the lower jaw, somewhat similar to modern snakes. Elasticity of the jaw would have allowed Carnotaurus to swallow small prey items whole.[42] Also, the front part of the lower jaw was hinged, able to move up and down. When pressed downwards, the teeth would have projected forward, allowing Carnotaurus to spike small prey items; when the teeth were curved upwards, the now backward projecting teeth would have hindered the caught prey animal from escaping.[6] Mazzetta and colleagues also found that the skull was able to withstand forces that appear when tugging on large prey items.[42] Thus, Carnotaurus may have fed mainly on relatively small prey, but also was able to hunt large dinosaurs.[42]

This interpretation was questioned by François Therrien and colleagues (2005), who found that the biting force of Carnotaurus was twice as high as that of the American alligator, which may have the strongest bite of any living animal. Furthermore, these researchers noted analogies with modern Komodo dragons: The flexural strength of the lower jaw decreases towards the tip linearly, indicating that the jaws were not suited for high precision catching of small prey items but for delivering slashing wounds to weaken big prey items. As a consequence, according to this study, Carnotaurus must have mainly preyed upon large prey animals, possibly from ambush.[44]

Robert Bakker (1998) found that Carnotaurus mainly fed upon very large prey, especially sauropods. As he noted, several adaptions of the skull – the short snout, the relatively small teeth and the strong occiput (back of the skull) – had independently evolved in Allosaurus. These features suggest that the upper jaw was used like a serrated club to inflict wounds; big sauropods would have been weakened by repeated attacks.[45]

Locomotion

Cross section through the tail of Carnotaurus, showing the enlarged caudofemoralis muscle and the V-shaped caudal ribs

Mazzetta and colleagues (1998, 1999) presumed that Carnotaurus was a swift runner, arguing that the thigh bone was adapted to withstand high bending moments while running. The ability of an animal's leg to withstand those forces limits the top speed of that animal. The running adaptations of Carnotaurus would have been better than those of a human, although not nearly as good as those of an ostrich.[V][46]

In dinosaurs, the most important locomotor muscle is located in the tail. This muscle, called caudofemoralis, attaches to the fourth trochanter, a prominent ridge on the thigh bone, and pulls the thigh bone backwards when contracted. Scott Persons and Phil Currie (2011) noted that in the tail vertebrae of Carnotaurus the caudal ribs did not protrude horizontally ("T-shaped"), but were angled against the vertical axis of the vertebrae, forming a "V". This would have provided additional space for a caudofemoralis muscle larger than in any other theropod – the muscle mass was calculated at 111 to 137 kg per leg. Therefore, Carnotaurus could have been one of the fastest large theropods known.[14] While the caudofemoralis muscle was enlarged, the epaxial muscles situated above the caudal ribs would have been proportionally smaller. These muscles, called the longissimus and spinalis muscle, were responsible for tail movement and stability. To maintain tail stability in spite of reduction of these muscles, the caudal ribs bear forward projecting processes interlocking the vertebrae with each other and with the pelvis, stiffening the tail. As a consequence, the ability to make tight turns would have been diminished, because the hip and tail had to be turned simultaneously, unlike in other theropods.[14]

With its distinctive appearance, Carnotaurus is regularly featured in popular depictions of dinosaurs. It appears in the 1996 stop-motion film Dinosaur Valley Girls, with the model made by sculptor Thomas Dickens and based on scientific skeletal drawings and Bonapartes 1985 description.[47] Carnotaurus is the main antagonist in the 2000 Disney CGI movie Dinosaur; in this movie, it was depicted much larger than it actually was for dramatic purposes.[48] It also appears in Michael Crichton's 1995 novel The Lost World.[49] Audio Animatronics of Carnotaurus can be seen in the dark ride Dinosaur at Disney's Animal Kingdom theme park; at the time, these animatronics were the largest ever created.[50]

Notes

  1. ^ p. 38 in Bonaparte (1990)[2]
  2. ^ p. 162 in Juárez Valieri et al. (2010)[3]
  3. ^ p. 163 in Juárez Valieri et al. (2010)[3]
  4. ^ p. 556 in Calvo et al. (2004)[4]
  5. ^ a b c d p. 191 in Carrano and Sampson (2008)[5] Cite error: The named reference "cs191" was defined multiple times with different content (see the help page).
  6. ^ p. 30 in Bonaparte (1990)[2]
  7. ^ p. 187 in Mazzetta et al. (1998)[6]
  8. ^ p. 79 in Mazzetta et al. (2004)[7]
  9. ^ a b c d e p. 276–279 in Novas (2009)[8] Cite error: The named reference "n276" was defined multiple times with different content (see the help page).
  10. ^ p. 8 in Bonaparte (1990)[2]
  11. ^ a b c d e f g p. 3–5 in Bonaparte (1990)[2] Cite error: The named reference "b3" was defined multiple times with different content (see the help page).
  12. ^ p. 191 in Mazzetta et al. (1998)[6]
  13. ^ p. 255 in Novas (2009)[8]
  14. ^ a b c p. 6 in Bonaparte (1990)[2] Cite error: The named reference "b6" was defined multiple times with different content (see the help page).
  15. ^ a b c d e pp. 256–261 in Novas (2009)[8] Cite error: The named reference "n256" was defined multiple times with different content (see the help page).
  16. ^ p. 1276 in Ruiz et al. (2011)[15]
  17. ^ a b c p. 32 in Bonaparte (1990)[2] Cite error: The named reference "b32" was defined multiple times with different content (see the help page).
  18. ^ a b pp. 264–299 in Novas (2009)[8] Cite error: The named reference "n264" was defined multiple times with different content (see the help page).
  19. ^ pp. 188–189 and 202 in Carrano and Sampson (2008)[5]
  20. ^ a b c p. 202 in Carrano and Sampson (2008)[5] Cite error: The named reference "cs202" was defined multiple times with different content (see the help page).
  21. ^ a b c p. 2 in Bonaparte (1990)[2] Cite error: The named reference "b2" was defined multiple times with different content (see the help page).
  22. ^ p. 186 and 190 in Mazzetta et al. (1998)[6]

References

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  17. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1111/j.1469-7998.2009.00640.x, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1111/j.1469-7998.2009.00640.x instead.
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  21. ^ a b Coria, Rodolfo A. (2002). "A new close relative of Carnotaurus sastrei Bonaparte 1985 (Theropoda: Abelisauridae) from the Late Cretaceous of Patagonia". Journal of Vertebrate Paleontology. 22 (2): 460. doi:10.1671/0272-4634(2002)022[0460:ANCROC]2.0.CO;2. ISSN 0272-4634. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: date and year (link)
  22. ^ Ezcurra, M. D. (2010). "An abelisauroid dinosaur with a non-atrophied manus from the Late Cretaceous Pari Aike Formation of southern Patagonia" (PDF). Zootaxa. 2450: 14. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  23. ^ Sereno, P. C. (2004-07-07). "New dinosaurs link southern landmasses in the Mid-Cretaceous". Proceedings of the Royal Society B: Biological Sciences. 271 (1546): 1325–1330. doi:10.1098/rspb.2004.2692. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  24. ^ Tykoski, Ronald B. (2004). "Ceratosauria". In Weishampel, David B.; Dodson, Peter; & Osmólska, Halszka (eds.) (ed.). The Dinosauria (Second ed.). Berkeley: University of California Press. p. 65. ISBN 0-520-24209-2. {{cite book}}: |editor= has generic name (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: multiple names: editors list (link)
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