An Ancient Pack Hunter or Evidence of Juvenile Gregarious Behavior

 

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. 

 

Who Am I?

 

 Hello!

 

 

 Majungasaurus and I at Dino Shindig 2025.

I'm an early-career researcher with experience in a few areas of archosaur paleontology. I like to talk about weird guys from the Triassic and birds occasionally.  

This is a place for me to publish my work, be it opinion, fun facts, or art. Nothing here is peer reviewed. Please do not cite it for research. This is a place for my opinions and ideas, as well as sharing facts about the natural world. I cite my sources to try keeping everything accurate, but there's nobody besides me checking for that and so of course, this work is subject to bias. Rather than a serious scientific repository, this is my little space to talk about things I find interesting. 

With that, I hope you enjoy. 

The Triassic Bigfoot

Ctenosauriscus koeneni

One of the most bizarre archosaurs of the Early Triassic is Ctenosauriscus. It's a strange looking predatory animal with few body fossils, a complicated taxonomic history, and a sail along its back (sound familiar, Spinosaurus..?) Ctenosauriscus is, despite all that, one of the most well known members of the group ctenosauriscidae. The others are extraordinarily fragmentary bar Arizonasaurus, and not a single complete specimen has ever been found. They are some of the oldest archosaurs out there, with Xilousuchus (pictured below) being potentially the oldest archosaur, full stop. It's from a fossil formation in China, dated to ~247 million years old.

Xilousuchus saipingensis

 Ctenosauriscus is the largest member of the group, with the only specimen known hitting about ten feet in length and the tip of the sail being around three and a half feet tall. Not exactly terrifying, Jurassic-Park-Monster sized, so our potential for a ctenosauriscid horror movie antagonist is unfortunately slim... That is, if you stick to body fossils. C. Deidrich described a series of footprints in Northern Germany that he suggested belonged to a relative of Ctenosauriscus, associated with the ichnospecies Isochirotherium herculis. (Ichnospecies are specific kinds of ichnofossils, like how a species is a kind of living thing. An ichnofossil is a 'trace fossil,' like a footprint, burrow, or poop left by an extinct organism.) In this case, the animal that left the trace fossils, called their Trackmaker, was gigantic, especially for the time. It moved in a big straight line across a dried up seabed, where there were several tracks left by meandering, smaller animals. Due to the clearly meat-focused dentition of the Ctenosauriscids, it's reasonable to suggest that the trackmaker might have preyed upon the small archosaurs in the area. Another fun detail (with little relevance, I'll admit) is that there are bits of a nothosaur, a marine reptile, stuck inside one of the footprints, which is just a fantastic mental image. Either the little guy was stepped on, or more likely, washed into the footprint by the tide when it returned. 

So how big really was our giant Triassic trackmaker? Using Arizonasaurus as a model, the author of the paper suggested a maximum size of 5-6 meters for the giant poposauroid. For my American Friends, that's about 17-20 feet long. It's no T. rex or Allosaurus, but it's certainly an animal large enough to be scary. The implied presence of a dorsal sail also makes it look a lot bigger in size estimation charts. Obviously we can't know if it was there, but since all members of the ctenosauriscidae have it, there's no reason I. herculis's trackmaker would lack the distinctive feature. Below are Ctenosauriscus and the giant mystery beast (in dark and light grey respectively) compared to an average-height human. The grid squares are 1 Meter each.

 Ctenosauriscus, the I. herculis trackmaker, and Homo.

 

Now that looks movie-monster sized, and it's plausible without having to drastically increase the size of a real animal like in Jurassic Park. Though I can't say it's particularly plausible for one of these guys to be running around eating people in a theme park. You'll need to come up with a scientific justification for that one yourself.