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The evolution of dwarf crocodiles!

July 31 , 2016

by protohedgehog

For the last 3-4 years of my PhD, I’ve been doing first-hand research on a group of extinct crocodile-ancestors (Crocodyliformes) called atoposaurids. I’ve published a couple of papers on these already as part of an ongoing study into their morphology, taxonomy and evolutionary relationshps.

So I’m pleased to announce a monstrous new paper that looks in great detail at the evolutionary relationships of these little crocs. It’s open access, obviously, and here’s the abstract in all it’s glory! (feel free to skip to below)

Atoposaurids are a group of small-bodied, extinct crocodyliforms, regarded as an important component of Jurassic and Cretaceous Laurasian semi-aquatic ecosystems. Despite the group being known for over 150 years, the taxonomic composition of Atoposauridae and its position within Crocodyliformes are unresolved. Uncertainty revolves around their placement within Neosuchia, in which they have been found to occupy a range of positions from the most basal neosuchian clade to more crownward eusuchians. This problem stems from a lack of adequate taxonomic treatment of specimens assigned to Atoposauridae, and key taxa such as Theriosuchus have become taxonomic ‘waste baskets’. Here, we incorporate all putative atoposaurid species into a new phylogenetic data matrix comprising 24 taxa scored for 329 characters. Many of our characters are heavily revised or novel to this study, and several ingroup taxa have never previously been included in a phylogenetic analysis. Parsimony and Bayesian approaches both recover Atoposauridae as a basal clade within Neosuchia, more stemward than coelognathosuchians, bernissartiids, and paralligatorids. Atoposauridae is a much more exclusive clade than previously recognized, comprising just three genera (Alligatorellus, Alligatorium, and Atoposaurus) that were restricted to the Late Jurassic of western Europe, and went extinct at the Jurassic/Cretaceous boundary. A putative Gondwanan atoposaurid (Brillanceausuchus) is recovered as a paralligatorid. Our results exclude both Montsecosuchus and Theriosuchus from Atoposauridae. Theriosuchus is polyphyletic, forming two groupings of advanced neosuchians. Theriosuchus (restricted to Theriosuchus pusillus, Theriosuchus guimarotae, and Theriosuchus grandinaris) spanned the Middle Jurassic to early Late Cretaceous, and is known from Eurasia and North Africa. Two Cretaceous species previously assigned to Theriosuchus (‘Theriosuchus’ ibericus and ‘Theriosuchus’ sympiestodon) are shown to be nested within Paralligatoridae, and we assign them to the new genus Sabresuchus. The revised phylogenetic placement ofTheriosuchus has several implications for our understanding of eusuchian evolution. Firstly, the presence of fully pterygoidean choanae, previously regarded as a defining characteristic of Eusuchia, is not found in some basal members of Eusuchia. However, eusuchians can be distinguished from Theriosuchus and other basal neosuchians in that their choanae are posteriorly positioned, with an anterior margin medial to the posterior edge of the suborbital fenestra. This feature distinguishes eusuchians from Theriosuchus and more basal neosuchians. Secondly, our refined understanding of Theriosuchus implies that this taxon possessed only amphicoelous presacral vertebrae, and therefore fully developed vertebral procoely is likely to have evolved only once in Crocodylomorpha, on the lineage leading to Eusuchia. These and other findings presented herein will provide an important framework for understanding the neosuchian–eusuchian transition.

It’s actually pretty complicated with a lot of taxonomic buzzwords, as befits our field. Here are the highlights though:
Atoposaurids are ultra cute little crocs, but membership of this group is much more restricted compared to what we previously thought.
Animals such as Theriosuchus are most likely not atoposaurids, and much more advanced species.
Some species of Theriosuchus aren’t Theriosuchus at all! We had to name some a whole new genus, Sabresuchus, which probably belongs to a group called Paralligatoridae.
This means that atoposaurids were restricted to the Late Jurassic shallow seas that occupied Europe around the time, and went extinct around the Jurassic/Cretaceous boundary as the environment began to change.

So it’s a highly specialised topic, but I’m hoping that I can use this as a basis to explore more about the origins of modern crocodiles, and what their ancestors were doing during their Jurassic/Cretaceous experimental phase. Any questions about it for now, shoot me a comment or a message!



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