#cretaeous

We are two weeks into the year and we already had a bunch of big croc papers, so today I'll cover the first of the two new genera named so far. Varanosuchus sakonnakhonensis (Monitor lizard crocodile from Sakon Nakhon) is a small atoposaurid neosuchian from the Early Cretaceous of Thailand, a country that has seen a virtual boom in croc papers this past year between the description of Alligator munensis and Antecrocodylus.

Varanosuchus was a small animal, maybe a meter in length if a little longer with a notably short and deep skull and long slender limbs revealing it to have been at least somewhat terrestrial. We actually have a decent amount of material of this guy. The holotype consists of a 3 dimensionally preserved skull as well as assorted postcranial remains (vertebrae, ribs, osteoderms and limbs), there is a second skull of whats likely to be a differently aged individual also showing a 3D skull and well the third ones just a skull table but 2/3 is still great.

Now this guy was an atoposaurid, which is a group of crocodylomorphs that lived from the Jurassic to the end of the Cretaceous, their last members existing on the island of Hateg some 66 million years ago. Atopsaurids were generally small animals with short snouts and longish legs. Some examples of atoposaurids include Knoetschkesuchus from Germany, Aprosuchus from Romania and Alligatorellus from France and Germany, all three pictured below, art by @knuppitalism-with-ue

Now the matter of ecology for atoposaurids in general and Varanosuchus in particular is not clear. Altirostral skulls such as that of Varanosuchus are generally associated with terrestrial crocodylomorphs as best examplified by notosuchians. Their teeth and size both obviously speak against being shoreline ambush predators like modern crocs and their legs are straight and slender, suggesting they had an erect posture and not the more sprawling one seen in semi-aquatic forms. Though they could have still had some aquatic affinities. The authors for instance argue that the osteoderms, having plenty of pits, are more like those of an animal that spends time in the water and would thus use them in thermoregulation. So maybe they did enter water from time to time, somewhat like some modern lizards, tho I think its fairly certain that they spend a decent amount of time on land. The artwork below is the reconstruction from the paper itself.

Another matter discussed in the paper is phylogeny, more precisely the relationship of Neosuchians and how Eusuchia is defined. On the first front, its worth noting that the paper recovered both atoposaurids and paralligatorids as monophyletic groups and had them be each others closest relatives, a notion that has been recovered before. More interesting perhaps is the fact that the next closest relatives to these two were hylaeochampsids and Bernissartia, which are typically recovered closer to modern crocs. Which in fact form a separate branch that is the sister group to all the afforementioned clades and taxa. And then you got goniopholids, dyrosaurs and pholidosaurs which are all more basal than the paralligatorid+atoposaurid+crocodilian group, which is back to the ordinary really. The second thing is the definition of Eusuchia. So for the longest time Eusuchia has been defined to include those Neosuchians that have choanae that are fully enclosed by the pterygoid bones (I know I know a bunch of anatomy stuff bear with me). So if the choanae was surrounded by the pterygoid, its an Eusuchian, if not, its more basal. Well, atoposaurids don't have that....BUT VARANOSUCHUS DOES. This, coupled with hylaeochampsids also having this feature and being recovered closer to atoposaurids than to Crocodilians basically suggests that the feature is not diagnostic for Eusuchia and instead appeared multiple times independently.

Moving away from anatomy and phylogeny and all that stuff, I think its very cool that croc research in Thailand has kinda picked up this last year. And fittingly enough some people have even worked on a short documentary covering the known diversity of pseudosuchians from Thailand, giving an overview over the named forms from the Jurassic to today, from titans like Chalawan to even these newest dwarf forms. While the narration is obviously in Thai, there are English subs and I highly recommend looking into it (even if I disagree with their depiction of Varanosuchus as arboreal, its perhaps overshooting the goal a little bit).

Finally here's the paper itself (tho paywalled) New Cretaceous neosuchians (Crocodylomorpha) from Thailand bridge the evolutionary history of atoposaurids and paralligatorids | Zoological Journal of the Linnean Society | Oxford Academic (oup.com) and the wikipedia page I've been working on Varanosuchus - Wikipedia