Most of us have grown up with the idea that the Mesozoic Era was, excepting the Early Triassic, a time when dinosaurs dominated life on land. Or, put another way, a time when dinosaurs were the most ecologically significant and most obvious of all land animals. The familiar generalization, recounted in every book on Mesozoic life, is that dinosaurs were the only diverse big-bodied land vertebrates during the Jurassic and Cretaceous and, for as long as this was the case, other tetrapod groups were unable to achieve big body size. But in the same way that the modern world isn’t really ‘dominated’ by any single taxonomic group, it turns out that this wasn’t the case in the Mesozoic either. In fact dinosaurs weren’t the only tetrapod group that evolved big, scary, terrestrial species during the Mesozoic. Furthermore, non-dinosaurian terrestrial reptiles may have been significant apex predators in varied Cretaceous ecosystems….
In characteristically childish fashion, I enjoy comparing this with the way the role of The Galactic Empire is portrayed in the Star Wars films and their spin-off stories. Star Wars novels and computer games sometimes state or imply that The Empire rules the entire galaxy with an iron fist, suppressing all resistance and imposing a consistent Imperial presence across all galactic systems. In fact, as evidenced by the role of the Galactic Senate, by Bespin, by what occurs on Tatooine and elsewhere in the Outer Rim, and by the fact that the galaxy is still crawling with such things as bounty hunters, mon calamari, dresselians, ugnaughts and even (shudder) ewoks, there is still plenty of room in the Star Wars universe for all manner of diversity to survive and thrive alongside the significant presence of The Empire. Substitute ‘The Empire’ for ‘dinosaurs’ and you might see what I’m getting at. Dinosaurs were big during the Mesozoic in every sense of the word, but it turns out that their presence did not prevent or suppress the evolution of everything else, even among terrestrial non-dinosaurian reptiles [excellent restoration of Baurusuchus above by Deverson da Silva, from Carvalho et al. (2005)].
Crocodilians (I use this word as a vernacular term for the clade Crocodyliformes) have an amazingly rich and diverse Mesozoic evolutionary history, involving small terrestrial generalist predators, long-skulled amphibious and fully aquatic forms, box-headed herbivores and omnivores, as well as the ancestors of modern alligators, crocodiles and gharials (Naish 2001: pdf available, please ask). Most Mesozoic crocodilians were ‘small’ – less than 3 m long – and giants were rare. The existence during the Cretaceous of gigantic amphibious crocodilians [namely the alligatoroid Deinosuchus from Campanian North America, the bizarre suspension-feeding giant Stomatosuchus from Egypt, and the immense pholidosaurid Sarcosuchus from the Saharan Cretaceous], is well known, but it’s usually argued that, because these animals were amphibious, they were clearly occupying a quite different ecological niche from predatory dinosaurs, and hence weren’t competing with them [adjacent Sarcosuchus image from wikipedia].
But what if there were big terrestrial predatory crocodilians during the Mesozoic? Surely such animals would have been interacting with predatory dinosaurs: living in the same places, preying on the same prey, and scavenging from the same carcasses. Big, deep-skulled terrestrial crocodilians of this sort are well known from the Cenozoic, mostly thanks to Sebecus icaeorhinus, a South American Eocene crocodilian first described by the famous evolutionary scientist George Simpson (1902-1984) in 1937 [more on Sebecus and other Cenozoic sebecosuchians in a later post].
Sebecus had a laterally compressed, tall snout, and a concave palate: in cross-section its snout was virtually parallel-sided, this forming the skull shape termed either oreinisrostral or altirostral. Skulls of this sort – present widely among predatory terrestrial archosaurs – are best for dealing with compressive vertical loading of the sort encountered during the slashing and slicing of prey (Busbey 1986, 1995), and are quite different from the dorsoventrally compressed platyrostral skulls typical of modern crocodilians (though – note – not universally present even in these animals).
While there is no doubt today that Sebecus and its relatives are crocodilians, little known is that Simpson (1937) initially regarded Sebecus as an unusual crocodilian-like archosaur of uncertain and ‘doubtfully crocodilian’ affinities. His resultant naming of the group Sebecosuchia* was not, therefore, the recognition of a new and distinctive group of crocodilians, but instead the result of a frustrated effort to provide this Eocene reptile with its own group. The discovery of good Sebecus material allowed Simpson to realize that the post-Cretaceous ‘theropod’ teeth regarded by Florentino Ameghino as representing late-surviving dinosaurs actually belonged to sebecosuchians: Simpson had discussed the teeth earlier (Simpson 1932) and, though unable to show that they were non-dinosaurian, had concluded that they belonged to some other taxon that possessed theropod-like teeth. These teeth are of the sort known as ziphodont: laterally compressed, gently recurved, and with serrated keels along their edges [adjacent image shows a typical ziphodont tooth, though from a theropod and not a crocodilian]. Like the oreinirostral skull shape, they occur throughout predatory archosaurs and aren’t unique to either theropods or sebecosuchians.
* It was not coined by Colbert (1946) as stated in one recent paper (Turner & Calvo 2005).
Though mostly associated with the Cenozoic, we’ve known since the 1890s that sebecosuchians were also present in the Cretaceous as 1896 saw the description of the Argentinean taxon Cynodontosuchus rothi Woodward, 1896. It’s from the Coniacian-Santonian Pichi Picun Leufu Formation. Known only from an incomplete snout and articulated lower jaw, Cynodontosuchus exhibited a tooth pattern common to a number of sebecosuchian taxa: the premaxillary teeth were small and subcircular in cross-section, the maxillary teeth were few in number (there are just five) and the first maxillary tooth was small while the second was particularly large, and a diastema between the premaxilla and maxilla housed an enlarged mandibular tooth when the jaws were closed. The big second maxillary tooth could truly be described as a ‘saber tooth’ as its tip extended to the ventral edge of the lower jaw. This strongly differentiated dentition is rather different from that of Sebecus and its relatives, and consequently Cynodontosuchus and its relatives are grouped together in the sebecosuchian group Baurusuchidae.
Highly similar to Cynodontosuchus was another Cretaceous form, Baurusuchus. Baurusuchus was notably robust-skulled and scary-looking. Two species are known: B. pachecoi Price, 1955 and B. salgadoensis Carvalho et al., 2005 [adjacent image, showing bones and model of B. salgadoensis, from here]. Incidentally, the specific name of the latter is nothing to do with Leonardo Salgado*, but is a reference to General Salgado County in Brazil’s Sao Paulo State. Authors have disagreed on how Baurusuchus compares with the earlier-named Cynodontosuchus. Gasparini (1972), Gasprini et al. (1993) and Carvalho et al. (2005) argued that the two are so similar that they might turn out to be synonymous while Bonaparte (1996) argued that Cynodontosuchus had a rather ‘weaker’ dentition than Baurusuchus, and thereby implied that it was quite different. Regardless, Cynodontosuchus is much smaller than either of the Baurusuchus species, and hence probably quite different in ecology (Bonaparte 1996).
* Just as well given that it means ‘from Salgado’.
Similar in some details to Baurusuchus and Cynodontosuchus was Stratiotosuchus maxhechti Campos et al., 2001 from the Turonian-Santonian Adamantina Formation of Brazil, another baurusuchid. Its skull bones are reported to be particularly thick and its snout was shallower in side view than that of the other baurusuchids. It was also alarmingly big, with a skull over 50 cm long. The two Baurusuchus species were similar in size, with skulls exceeding 40 cm in length.
All the sebecosuchians we’ve seen so far have been South American. But we now know that these animals weren’t restricted to this continent, as once thought. Pabwehshi pakistanensis Wilson et al., 2001 is from the Maastrichtian Pab Formation of Pakistan. Though known only from an incomplete snout and lower jaw tip, enough is preserved to show us that Pabwehshi shares with Baurusuchus a reduced premaxillary dentition, a third premaxillary tooth that overhangs the dentary, a reduced first maxillary tooth, and an enlarged caniniform second maxillary tooth (Wilson et al. 2001). Accordingly, it again seems to be a member of Baurusuchidae, showing us that sebecosuchians of this group were present in Upper Cretaceous India as well.
How did these animals live? We can’t be sure of course, but their anatomy indicates that they were predominantly, if not entirely, terrestrial. Several palaeoherpetologists have argued at times that sebecosuchians and similar crocodilians were amphibious and in the habit of ambushing prey from the water’s edge, but there doesn’t seem to be any good reason to assume this. Their skull and tooth morphology suggests that they behaved in a similar manner to theropods and big monitor lizards, though the recent discovery of the ziphodont thalattosuchian Dakosaurus andiniensis shows that such features weren’t restricted to terrestrial archosaurs (go here for more on metriorhynchids). Unfortunately the post-cranial anatomy of these animals remains poorly known. Near-complete, articulated baurusuchid skeletons were reported in 2004 [free pdf here] but as yet very little information has appeared on them. In the absence of further information, what we know of related forms (including the notosuchians and peirosaurids [image below shows skull of peirosaurid Uberabasuchus, from here) is consistent with a mostly terrestrial lifestyle [life restoration of two baurusuchids from here].
The remains we have of such sebecosuchians as Baurusuchus shows that they were large animals, with body lengths exceeding 3.5 m and approaching 4 m in Stratiotosuchus. If the idea of a 4-m-long terrestrial predatory Cretaceous non-dinosaurian reptile doesn’t grab your imagination, shame on you. It is veeery tempting to wonder whether, and how, sebecosuchians and theropods interacted: surely they must have. I wanted to mention this idea in my 2001 review article on crocodilians, but my reviewer urged me to remove it. Candeiro et al. (2006) however, have hinted at this idea, noting that, in some Late Cretaceous South American faunas, terrestrial crocodilians ‘had a more important ecological role to play as the main carnivorous group … than did theropods’ (p. 937).
One last thing. Baurusuchids weren’t unique to the Southern Hemisphere, as Iberosuchus macrodon Antunes, 1975 from Spain and Bergisuchus dietrichbergi Kuhn, 1968 from Messel in Germany are also thought to be members of the group. Atacisaurus crassiproratus Astre, 1931 from France has been reidentified as cf. Iberosuchus and Eremosuchus elkoholicus* Buffetaut, 1989 from Algeria has also recently been identified as a baurusuchid. But here we come to something else really interesting: Iberosuchus, Bergisuchus and Eremosuchus are all Eocene in age, which means that baurusuchids survived the end-Cretaceous event and were apparently doing fine many millions of years after the Cretaceous.
* Its specific name refers to the locality of El Kohol and is nothing to do with beers, wines or spirits.
What was it about these terrestrial predators that made them different from the Cretaceous dinosaurs that they were formerly contemporaneous with? Was it something to do with their metabolism, or maybe their habits? We don’t know, but their survival suggests that they were significantly different from dinosaurs in some way.
The Triassic might be regarded as the heyday of crurotarsan archosaurs, but it is simply unfair and incorrect to imply that these non-dinosaurian ruling reptiles failed to continue ruling throughout the Age of Dinosaurs and beyond. And you don’t need me to remind you that crocodilians remain big, ecologically important predators today.
More on sebecosuchians in future. Still to come soon (honest): vampire pterosaurs (or not), beluwhals and proto-narwhals, amazing social life of the Green iguana, and everything else I’ve been promising for the past year and two months. And I just saw a movie trailer for a certain film, which reminds me: war rhinos. And rhinogradentians.
Refs – –
Bonaparte, J. F. 1996. Cretaceous tetrapods of Argentina. Muncher Geowissenschaften, Abhandlungen 30, 73-130.
Busbey, A. B. 1986. New material of Sebecus cf. huilensis (Crocodilia: Sebecosuchidae) from the Miocene La Venta Formation of Colombia. Journal of Vertebrate Paleontology 6, 20-27.
– . 1995. The structural consequences of skull flattening in crocodilians. In Thomason, J. J. (ed) Functional Morphology in Vertebrate Paleontology. Cambridge University Press (Cambridge), pp. 173-192.
Candeiro, C. R. A., Martinelli, A. G., Avilla, L. S. & Rich, T. H. 2006. Tetrapods from the Upper Cretaceous (Turonian-Maastrichtian) Bauru Group of Brazil: a reappraisal. Cretaceous Research 27, 923-946.
Carvalho, I. de S., Ribeiro, L. C. B. & Avilla, L. dos S. 2004. Uberabasuchus terrificus sp. nov., a new Crocodylomorpha from the Bauru Basin (Upper Cretaceous), Brazil. Gondwana Research 7, 975-1002.
Colbert, E. H. 1946. Sebecus, representative of a peculiar suborder of fossil Crocodilia from Patagonia. Bulletin of the American Museum of Natural History 87, 217-270.
Gasparini, Z. 1972. Los Sebecosuchia (Crocodilia) del territorio Argentino. Consideracones sobre su “status” taxonomico. Ameghiniana 9, 23-34.
– ., Fernandez, M. & Powell, J. 1993. New Tertiary sebecosuchians (Crocodylomorpha) from South America: phylogenetic implications. Historical Biology 7, 1-19.
Naish, D. 2001. Fossils explained 34: Crocodilians. Geology Today 17 (2), 71-77.
Simpson, G. G. 1932. The supposed association of dinosaurs with mammals of Tertiary type in Patagonia. American Museum Novitates 566, 1-21.
– . 1937. New reptiles from the Eocene of South America. American Museum Novitates 927, 1-3.
Turner, A. H. & Calvo, J. O. 2005. A new sebecosuchian crocodyliform from the Late Cretaceous of Patagonia. Journal of Vertebrate Paleontology 25, 87-98.
Wilson, J. A., Malkani, M. S. & Gingerich, P. D. 2001. New crocodyliform (Reptilia, Mesoeucrocodylia) from the Upper Cretaceous Pab Formation of Vitakri, Balochistan (Pakistan). Contributions from the Museum of Paleontology, The University of Michigan 30, 321-336.