Adorable Alligatorellus

Today’s alligators, crocodiles, and gharials are gorgeous animals, but you’d be hard pressed to call them cute. Chirpy little infants, maybe, but the exposed grins of the adults inspire more fear than adoring sighs. In the prehistoric past, however, there were crocs worth squeeing over. Alligatorellus was one of them.

I must admit that I hadn’t heard of Alligatorellus until last week. The two-foot-long critter belonged to a relatively obscure group of crocodyliforms called atoposaurids, and is known from several skeletons found in the 152 million year old limestone of France and Germany. They’re very pretty fossils, but they haven’t exactly gotten the press that Archaeopteryx has enjoyed. Thankfully, though, Imperial College London paleontologists Jonathan Tennant and Philip Mannion have brought the delightful little croc back to our attention through a new PeerJ paper.

The aim of Tennant and Mannion’s study is a revision of Alligatorellus to determine how many species there really were. The first species, Alligatorellus beaumonti, was found in France and named by paleontologist Paul Gervais in 1871. A century later, paleontologist Peter Wellnhofer described additional Alligatorellus found in rocks of the same age in Germany, and these were subtly different enough that Wellnhofer proposed that the German Alligatorellus belonged to a different subspecies.

But, as Tennant and Mannion conclude, the Alligatorellus from France and Germany are different enough to be split into separate species. The ones from France are still called Alligatorellus beaumonti, and, bumping up Wellnhofer’s suggestion to a species epithet,  those from Germany are Alligatorellus bavaricus. The skeletons of both species have short, triangle-shaped skulls and delicate proportions, but the fine details of their armor and bones mark them as distinct.

Maps showing the occurrence of atoposaurids in western Europe. From Tennant and Mannion, 2014.
Maps showing the occurrence of atoposaurids in western Europe. From Tennant and Mannion, 2014.

What’s odd is that Alligatorellus weren’t the only cute little crocs running around in Late Jurassic Europe. The same deposits have yielded other, closely-related animals named Atoposaurus and Alligatorium. Stranger still, the species of Atoposaurus and Alligatorium also differ between the sites in France and Germany. The croc communities are different at the species level.

These groupings of closely-related, but distinct, species is an evolutionary signal. During the Late Jurassic, Tennant and Mannion point out, western Europe was an archipelago, with shifting sea levels changing connections between islands. The differences in Alligatorellus and its neighbors likely reflect populations that were cut off from each other, diverging in form as they evolved in isolation from each other. This could be a case of allopatric speciation – a phenomena still in action today but often difficult to discern in the vertebrate fossil record. In other words, Alligatorellus isn’t just a pretty face.

Reference:

Tennant, J., Mannion P. 2014. Revision of the Late Jurassic crocodyliform Alligatorellus, and evidence for allopatric speciation driving high diversity in western European atoposaurids. PeerJ 2:e599

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