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.