D. quartacolonia is a relatively cute little archosaur; but is that because it was still a baby?
In the Middle Triassic, around 240 million years ago, a few lovely animals were in the same place at the wrong time. Swept up by a flood event or something similar, they were transported a short distance and rapidly buried before they could attempt to escape. Their remains were not scavenged and are wonderfully preserved, providing the basis for a series of studies on the frankly very underrated specimen! We hear often about the Fighting Dinosaurs and the Dueling Dinosaurs and the Babysitting Dinosaurs... a lot of dinosaurs. But there is surprisingly little attention, from the press or otherwise, on this bonebed, a single fossil unit that preserves ten Decuriasuchus quartacolonia in three dimensions, and in various degrees of articulation (articulated specimens are those where the bones are connected, rather than scattered randomly, like you would find in a flowing riverbed.)
Another interesting, but not so surprising, feature of the animals in the bonebed is their apparent age. The Decuriasuchus preserved here are all relatively young, and far from their maximum size. An unfortunate consequence of this is that there is a serious dearth of reconstructions of adult Decuriasuchus; and mine is no exception. I based it solely on the material present in the paper, so it represents a juvenile. This means the adult appearance of Decuriasuchus is a potential mystery. I say potential because of certain ideas I will discuss later.
Anatomy and a Brief Rant about Osteoderms
Regardless, the anatomy of the animal is interesting; like many archosaurs today, it's got a really long tail compared to its overall body length. In the extant croc-line archosaurs, the tail is a propulsor for aquatic movement, but that does not seem to be the case with Decuriasuchus given the construction of its skull: Eyes on the side and forward-oriented nostrils coupled with a tall cranium and recurved teeth do not a good aquatic hunter make. So it's bizarre to see an ancient, terrestrial archosaur that has such a long tail. It shares some other similarities with our modern crocs, too, despite its age. It's got dorsal osteoderms that form a nice plate of armor atop its body, but they are generally simple in structure and not as ornamented as the living crocs or some of their extinct relatives like the aetosaurs. They are simple plates and cover the back and stomach, probably offering protection from predators and rival Decuriasuchus. They are found in sets of four along the back, where they are well preserved. On the lower side of the tail the other set of osteoderms, which are comparatively less nicely fossilized. Frustratingly, the description paper glosses over the preserved osteoderms, which I do not appreciate.[1] There is plentiful information to be found in an osteoderm, even in a cursory description; are they smooth? Rough? Do they vary in size up or down the tail? They are abundant, why not cut one or two in half and do some histology. Some data suggests that the structure, internal and external, of osteoderms could be a proxy for lifestyle... with good analysis we could 'confirm' that Decuriasuchus was likely terrestrial.[2] While the paper that reports on the trend of osteoderm ornamentation loss in terrestrial pseudosuchians suggests that ornamented osteoderms help swimming pseudosuchians heat themselves up faster, I am somewhat skeptical of this idea. I do agree that the osteoderms play an important role in thermoregulation; they are demonstrated to do so. However, I think that this correlation may have less to do with lifestyle in relation to aquatic activity and more to do with endo/ectothermy. But that's a conversation for another day.
Juvenile Gregarious Behavior and the Identity of Decuriasuchus
Many species of archosaurs group together when young but live more solitary lives as adults. Aetosaurus ferratus individuals are known to group together for a time after hatching,[3] and Diplodocus from the Mothers' Day site in the Morrison Formation represent a sauropod herd comprised exclusively of subadult individuals. This is not abnormal for reptiles, who grow slowly and consistently rather than rapidly like mammals. There would be little survival advantage to staying with your parents when you are large enough to sustain yourself by foraging but not large enough to eat the same material. This is called ontogenetic niche shift, a phenomenon often talked about in Tyrannosaurus rex but far from unique to it. In some species, this niche shift means that parents and offspring barely resemble each other morphologically...
...so how do we know we haven't already found an adult Decuriasuchus? We don't. It would be reckless to try matching it willy-nilly to any large predatory archosaur from the area, but there's a surprisingly compelling argument that it's a juvenile Prestosuchus. Firstly, a good amount of the percieved difference in skull shape is due to taphonomic deformation. This seems to be the case mainly because all of the preserved skulls are quite strikingly different in shape from one another, but only the best-preserved one is present in most skeletals. This one is quite compressed dorsoventrally and probably does not quite represent the animal's skull shape in vivo. The skull of Decuriasuchus is still markedly similar to that of the other prestosuchids, though. Perhaps the most compelling evidence for the idea that Decuriasuchus represents a juvenile Prestosuchus is the presence of adult Prestosuchus in the same outcrop and the similarity of their bone development. [4] This issue, however, is far from resolved. Prestosuchus chiniquensis is an animal with diagnostic traits associated mainly with the postcrania, while Decuriasuchus's diagnostic traits are only associated with the skull. This is another issue I have with the description paper. The best way to resolve this issue, in my opinion, is an updated description of D. quartacolonia that takes good care to comment on the abundance of postcranial material from this species.
There are a lot of questions to be asked about this animal, and in a situation rare in paleontology, the specimens are available to answer a few of them! Decuriasuchus deserves a lot more attention than it's getting, especially compared to its dinosaur relatives. So my message is, as always, to give some more love to the Pseudosuchians.
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