Ceratosaurus

Credit: R.J. Palmer.

*Ceratosaurus* is an extinct genus of ceratosaurian theropod dinosaur that lived in North America during the Late Jurassic.

## Description

*Ceratosaurus* is famous for being the only large theropod with a singular nasal crest. This structure was probably used completely for display. Combined with the large lacrimal crests, these would have made the animal look more dangerous, and accordingly more intimidating. Evidence for the idea they were used for display comes in the form of a juvenile *Ceratosaurus* skull known to have had smaller lacrimal and nasal crests than the adults. This clearly demonstrates the fact that the crests developed as the animal matured, reinforcing the idea they were used for displays. Likewise, the horn was actually weak and thin, contrary to Othniel Charles Marsh's claim that:

—Marsh, 1884
The teeth of *Ceratosaurus*, some 9.3 cm long in the upper jaw, were proportionally among the longest of any theropod. This is an obvious indicator that *Ceratosaurus* would have used its skull, rather than its arms, to dispatch prey. Teeth apparently assigned to *Ceratosaurus* are rounded in cross-section, terminating in a thin triangular shape at each end.

*Ceratosaurus*' skull was relatively weak compared to that of the coexisting *Allosaurus fragilis*, etc (Paul, 1988). There is a notch between the premaxilla and maxilla which, though inconspicuous compared to the likes of *Dilophosaurus* and *Spinosaurus*, is still readily visible. The purpose of this notch is unclear, though that of the aforementioned theropod taxa may have been used for the manipulation of small prey and/or piscivory. Since the premaxillary notch of *Ceratosaurus* is noticeably smaller than that of *Dilophosaurus* and *Spinosaurus*, it is difficult to determine its purpose, though it should be noted that many theropods possessed similar notches.

*Ceratosaurus* had four digits on each hand. Three of these bore long claws, but the length of these arms suggests they may not have had that great a role in prey restraint. The arms may have been in the process of atrophy, in which they were beginning to reduce in size. The same situation took place in basal abelisauroids, but in their kind, it was taken to the extreme; one abelisaurid taxon, *Aucasaurus*, hardly had any arms at all: four metacarpals were present, but metacarpals I and IV lacked carpals altogether. II and III did bear carpals, but these lacked unguals.

Some people have gone as far as proposing that *Ceratosaurus*' forelimbs did not retain their original grasping function, citing the further reduced forelimbs of abelisaurids and the related *Limusaurus* as evidence. However, Carrano and Choiniere (2016) noted close similarities between the hand of *Ceratosaurus* and other more basal theropods, and suggest that a grasping ability would indeed have been present. As in all non-abelisaurid theropods, Digit I would have slightly turned inwards when flexed. This is, in itself, strong evidence that *Ceratosaurus*' manus still retained a strong grasping function. Whether more derived taxa such as the Upper Cretaceous *Genyodectes* still retained this function is unknown, since all that is known from this taxon is a partial premaxilla, maxilla and dentary.

As in almost all theropods, three toes on each foot are used to bear the animal's weight; the big toe, also known as the hallux, was held off the ground. Gregory S Paul once suggested that *Ceratosaurus* could bear its weight on its tail while lashing out with its feet, similar to a kangaroo. However, there are no adaptations for this sort of behaviour present anywhere in the animal's body, so it seems most likely that *Ceratosaurus* did not kick at its enemies. Doing it with a single foot is slightly more plausible than using both (as Paul proposed in 1988), though likely ineffective against a predator of equal size or a far larger one. If anything, it seems more likely that such behaviours would be limited to intraspecific combat, if employed at all. It seems far more likely that any intraspecific combat between two *Ceratosaurus* would have involved the jaws.

## Paleoecology

*Ceratosaurus* lived with *Allosaurus* and *Torvosaurus* as well as similar animals in the Morrison Formation. Inter species conflict between *Ceratosaurus* and *Allosaurus* is apparent, however this is disputed between scientists. Fossils of *Ceratosaurus* teeth marks on *Allosaurus* foot bones show that the two species often attacked each other. *Ceratosaurus* has a large horn on its snout, the use of which is unknown.

Ecologically, *Ceratosaurus* and the more famous *Allosaurus* are quite interesting. The short-snouted species of the former, *Allosaurus fragilis*, concentrated its bite force into a smaller area and likewise specialized in producing stabbing wounds (likened to felids), occupying a different niche. The long-snouted *Allosaurus jimmadseni*, however, probably occupied a similar niche to *Ceratosaurus* (likened to canids). In 1998, Donald Henderson proposed that *A. jimmadseni* outcompeted *Ceratosaurus* in most areas, contributing to its rarity. Areas where *jimmadseni* were rare, such as Garden Park and the Dry Mesa Quarry, were ruled by *Ceratosaurus*.

Henderson also suggested that *Ceratosaurus* evolved to occupy a different niche to its competitor and that its enlarged teeth evolved for this purpose. However, a problem was eventually found with this theory. The *Allosaurus* illustrations that Henderson used as references portrayed the shout as being shorter than it really was. This, unfortunately, means that the very basis of Henderson's idea, one that otherwise appears relatively sound, is entirely invalid.

In 2004, Robert Bakker and Gary Bir suggested that *Ceratosaurus* was a specialist hunter of aquatic prey, such as lungfish, crocodiles and turtles, one of their arguments being that the tail may have been used as a sculling organ. This idea is presumably based on the vaguely-crocodilian cross-section of *Ceratosaurus*' tail, and the presence of *Ceratosaurus* teeth in rock layers where lungfish fossils are abundant. However, there is no good evidence to support this. As stated by Naish and Barrett in a 2018 book:

—Barrett and Naish, 2018
In short, *Ceratosaurus* was maladapted to an aquatic lifestyle in most respects. Indeed, in 2019, it was suggested that *Ceratosaurus* was merely more well-adapted to hunting prey than other Morrison theropods, rather than being fully semiaquatic. Massive specimens are known from the Dry Mesa Dinosaur Quarry.

## Classification

When first named, Ceratosaurus was considered the sole member of the infraorder *Ceratosauria*. In 1892, Edward Drinker Cope reclassified it as a species of *Megalosaurus*. Subsequently classified as carnosaurs, coelurosaurs, deinodontids and megalosaurids, it was eventually found that the Ceratosauria was a valid clade, consisting of abelisaurs, ceratosaurs, elaphrosaurs, and noasaurs. It was once believed that *Proceratosaurus* was related to *Ceratosaurus*, on the basis of their shared nasal horn. Below is a cladogram showing the relationships of *Ceratosaurus*, after Pol & Rauhut (2012).

## Notable Specimens

* "Bakker": A *C. nasicornis* skull, in-which an acquired replica inspired the creation of the Four State Museum of Natural History. It is currently the mascot of said museum.

## Species

* *C. nasicornis*
* ?*C. dentisulcatus*
* ?*C. magnicornis*

## References

* Bakker, RT; Bir, G; 2004 "Dinosaur crime scene investigations: theropod behavior at Como Bluff, Wyoming, and the evolution of birdiness". In Currie, PJ; Koppelhus, EB; Shugar, MA; Wright, JL (eds.). Feathered Dragons: Studies on the Transition from Dinosaurs to Birds. Bloomington and Indianapolis: Indiana University Press. 301–342.
* Carrano, MT; Choiniere, J; 2016 "New information on the forearm and manus of Ceratosaurus nasicornis Marsh, 1884 (Dinosauria, Theropoda), with implications for theropod forelimb evolution". Journal of Vertebrate Paleontology. '36' (2).
* Henderson, DM; 1998 "Skull and tooth morphology as indicators of niche partitioning in sympatric Morrison Formation theropods". Gaia.
* Paul, GS; 1988 "Predatory Dinosaurs of the World: A Complete Illustrated Guide". Simon and Schuster Inc. New York. 274–279.
* Pol, D; Rauhut, OWM; 2012 "A Middle Jurassic abelisaurid from Patagonia and the early diversification of theropod dinosaurs." Proceedings of the Royal Society B: Biological Sciences. 279 (1804): 3170–5.
* Sales, M A F; Schultz, CL; 2017 "Spinosaur taxonomy and evolution of craniodental features: Evidence from Brazil". PLoS ONE. '12' (11).
* Thierren, F; Henderson, DM; Ruff, CB; 2005 "Bite me: Biomechanical models of theropod mandibles and implications for feeding behaviour". Carnivorous Dinosaurs—Indiana University Press. pp. 179–237.
* Yun, C; 2019 "Comments on the ecology of Jurassic theropod dinosaur Ceratosaurus (Dinosauria: Theropoda) with critical reevaluation for supposed semiaquatic lifestyle." Volumina Jurassica.
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