A new Paleocene marine crocodile, Guarinisuchus munizi

After the end-Cretaceous extinction, an "empty" world was left to fill up. The non-avian dinosaurs were gone, as were the mosasaurs, ammonites, pterosaurs, and other creatures. Indeed, in marine environments the large Mesozoic predators were eliminated in the extinction event, allowing sharks and crocodiles to evolve and diversify now that they were no longer any mosasaurs patrolling the waters. One such crocodylian that moved into open ecological space was Guarinisuchus munizi, just described in the Proceedings of the Royal Society B.

Although marine crocodiles have been found elsewhere in the world from varying ages, Guarinisuchus is notable because fills in a temporal gap between the late Cretaceous and the early Paleocene. Indeed, its presence in South America after the Cretaceous has important implications for the biogeography and evolution of marine crocodiles, and it seems like relatives of Guarinisuchus evolved in northern Africa during the Cretaceous, going east to swim down an inland sea where the Sahara now spans, or going west down the coast of Africa, both routes eventually leading them to move across the oceanic gap to South America. Which hypothesis is correct will require more study and a better understanding of Mesozoic marine crocodiles from northern Africa.

Although the authors note that 3m long Guarinisuchus was probably one of the top predators of its environment, marine crocodiles did co-exist with large mosasaurs during the Mesozoic. During the Cretaceous, a marine crocodile named Thoracosaurus swam the same waters as Mosasaurus, and I've collected the remains of both from the greensand of southern New Jersey. The placement of the crocodiles can be problematic, though, as the K/T boundary is hard to find at the location I've been at, and sometimes it's difficult to tell whether fossils are of end-Cretaceous or earliest Paleocene age. Still, there's a rather abrupt change in the southern New Jersey stratigraphy, and not far above the level of the latest mosasaurs, sponges and shark teeth are the most common fossils.

Of further note is an unfortunate taxonomic label that will will be maddening to even mildly dyslexic paleontologists. Guarinisuchus belongs to the family Dyrosauridae, which (the first time I read it) looks nearly identical to the Dryosauridae, representing small ornithopod dinosaurs. For a few minutes I went searching the literature, wondering how such a mix-up could have happened in the paper, but it turns out that I simply didn't read the name correctly. Still, such similar names should probably be avoided, although the current rules of nomenclature care more for priority than creativity or ease of use.

Barbosa, J.A., Kellner, A.W., Viana, M.S. (2008). New dyrosaurid crocodylomorph and evidences for faunal turnover at the K-P transition in Brazil. Proceedings of the Royal Society B: Biological Sciences, -1(-1), -1--1. DOI: 10.1098/rspb.2008.0110