Potential Coevolution of Theropods in Cretaceous Gondwana

When I wrote about the new species of predatory dinosaur, Carcharodontosaurus iguidensis, this past December, I made a note of how interesting it was that in Cretaceous Gondwana there seems to be a certain triumvirate of predatory dinosaur groups. According to the data presented in Brusatte and Sereno (2007), remains of spinosaurids, carcharodontosaurids, and abelisauroids have been found near each other in various locations in a range of Cretaceous-aged strata on the African continent, perhaps reflecting a guild structure like that of extant mammalian African carnivores.

Approximately 95 million years ago, Spinosaurus, Carcharodontosaurus saharicus, and Deltadromeus may have lived in close proximity to each other in what is now northern Africa. In Niger, the remains of Carcharodontosaurus iguidensis were found near those of the abelisaurid Rugops and the teeth of an unidentified spinosaurid, although the association is still tentative. This sort of system was also in place in Niger at an even earlier time (about 122 million years ago) according to the new paper describing the abelisaurid Kryptops palaios and the charcharodontosaurid Eocarcharia dinops, as it appears that the spinosaurid Suchomimus lived in the same place during the same time. Could such an arrangement be evidence of coevolution among large Gondwanan theropods during the Cretaceous?

In order to understand the feeding habits of the large theropod dinosaurs we need to have a good grasp on skull morphology. This is easier said than done, and the material found for both Kryptops and Eocarcharia is unfortunately incomplete. The available material does allow the specimens to be recognized as new genera and tentatively placed within phylogenetic trees, but skull & jaw mechanics will have to remain somewhat obscure for now. What I found intruiging while looking over the new paper, however, were the differences between the maxilla of Eocarchia and those of the two known species of Carcharodontosaurus.

Comparing the maxillae can be somewhat difficult given that they differ in their levels of completeness, but a few things stand out. The maxilla of Eocarcharia doesn't seem as stout as those of the Carcharodontosaurus species, and it lacks the heavy furrows that mark the maxillae of the latter genus. Likewise, Eocarcharia exhibits three fenestrae in front of the antorbital fenestrae, something not seen in Carcharodontosaurus. Indeed, the maxilla of Eocarcharia seems lighter and less robust than in Carcharodontosaurus, perhaps indicating a change in feeding strategies and bite force among carcharodontosaurids over time in Africa.

At this point it might be useful to mention what material was recovered for each new genus. Kryptops is known from the maxilla, pelvic girdle, vertebrae and ribs of a single individual, and Eocarcharia was diagnosed based upon several teeth and skull bones. This was enough to allow the researchers to realize that both dinosaurs are very significant as they are both among the most basal known members of their respective groups. Indeed, as the authors note in the abstract, the presence of large body size, some derived traits among the skull elements, and the existence of Eocarcharia in Africa infer that many of the identifying features of these groups of predatory dinosaurs evolved by the Mid-Cretaceous, and the correlation of these new dinosaurs with close relatives suggests that Africa and South America separated about 100 million years ago rather than 120 million years ago (although more work is required to narrow down the timing of the split).

My primary interest in this paper primarily has to do with something not explicitly noted by the authors, triggered by their last remark in their conclusion. They stated that research into whether abelisaurids, carcharodontosaurids, and spinosaurids made it to the latest Cretaceous would help researchers understand how the African theropods compared to those from South America and elsewhere, and I thought this would be especially interesting if the makeup of the predatory dinosaur niche changed. Given that such finds have yet to be made, though, I'll stick to what was mentioned in this particular paper as a brief faunal list is included.

In what is now Niger during the time of Eocarcharia, there would have been at least three large predatory dinosaurs (Eocarcharia, Kryptops, and Suchomimus) in the area with one huge crocodylian (Sarcosuchus). These four large predators fed on a slightly greater diversity of herbivores featuring such strange dinosaurs as Nigersaurus and Ouranosaurus. In a ScienceDaily report on the new dinosaurs, Sereno and Brusatte divvied up the territory a bit given the diversity of carnivores during this one point in time. Being that Kryptops had a short, stout maxilla, they inferred that it was the dinosaurian-equivalent of a spotted hyena, crunching bones and gorging on carcasses. Eocarcharia, then, was a predator that took down live prey and Suchomimus was a fish-eater. If Suchomimus was a fish-eater, it may have put it into some competition with Sarcosuchus, and perhaps was sometimes a meal for the giant crocodylian. (It should also be noted that an earlier ScienceDaily article about another Sereno discovery, Rugops primus, also alludes to an abelisaurid scavenger, although not a bone-crusher like Kryptops).

These hypotheses are reasonable, but for the moment they remain to be supported by more conclusive evidence; if more material from the skulls and necks of these dinosaurs is found biomechanical studies might shed some light on bite forces, how much stress the skulls could take, etc. Likewise, a predisposition to certain prey-utilization strategies does not make a predator entirely a hunter or a scavenger. Even if Kryptops was capable of crushing bone and making the most of carcasses, for example, it shouldn't be inferred that it was not capable of catching live prey (I'm not saying that Sereno & Brusatte are proposing this view, only that it might be easy to misconstrue the potential habits of these dinosaurs from the popular article). The hyena comparison should remind us that for some time spotted hyenas were regarded as scavengers, essentially taking advantage of the more "noble" lions, but research by George Schaller and others showed that hyenas hunt and kill a considerable amount of prey. They can successfully hunt alone and in groups, although their bone-crunching ability allows them to take advantage of more opportunities than are afforded to carnivores that gain most of their nutrition from flesh.

The reconstruction of spinosaurids as fish-eaters is also popular, particularly because their skulls are somewhat reminiscent of those of crocodylians. As I mentioned above, if Suchomimus was primarily a fish-eater it would have had some competition at the water's edge with Sarcosuchus (individuals perhaps putting themselves in danger of being killed and eaten while they were on the hunt themselves), although I doubt that Suchomimus would have been an exclusive piscivore. I would also have to wonder, then, why spinosaurids kept getting larger if they were not in competition with other predatory dinosaurs for food. What could explain the difference in body size between Suchomimus and the later Spinosaurus? I'm not asking these questions to mock the idea that these dinosaurs ate fish (we know that definitively know that Baryonyx did, at least), but the occurrence of the same three groups of theropods makes me wonder about coevolution. How could three large, predatory dinosaurs differently utilize the landscape and not be in constant conflict with each other? There would have been conflict, certainly, but the same pattern emerging among the Gondwana communities would suggest that there was some sort of continuing balance in place.

I should admit that most of what I wrote here is not mentioned anywhere in the paper, but it's difficult not to notice the trend and wonder about its implications. Hopefully some detailed studies of paleoecology (and keeping an eye out for tooth marks/teeth on the bones of prey animals) will help resolve the niches of the three groups of predatory dinosaurs. If such determinations could be made, it may be possible to start figuring out how the predators may have shaped each others evolution, particularly in terms of prey specialization, hunting behavior, and body size. The taxonomic position of the new theropod genera described in the new paper will likely cause a good amount of discussion and debate, but as more fossils are discovered from Cretaceous Africa I hope that paleoecology will be an equal concern.

References;

Brusatte, S.L., and Sereno, P.C. (2007) "A New Species of Carcharodontosaurus (Dinosauria: Theropoda) From the Cenomanian of Niger and a Revision of the Genus." Journal of Vertebrate Paleontology 27(4): 902-916

Sereno, P.C., Brusatte, S.L. (2008). Basal abelisaurid and carcharodontosaurid theropods from the Lower Cretaceous Elrhaz Formation of Niger. Acta Palaeontologica Polonica, 53(1), 15-46.