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This is Hallopus victor, a 3-foot long reptile that lived in the Morrison Formation of western North America during the late Jurassic Period approximately 150 million years ago. Hallopus belonged to a group of reptiles called the “sphenosuchians” a group that was closely related to crocodiles. Another example of a Jurassic sphenosuchian is the 5-foot-long Macelognathus, which you can read about here.

Hallopus is only known from fragmentary remains. However, we know that it had a thin build and it also had unusually long back legs. This suggests that Hallopus was a physically active predator that ran after its prey, and would have been capable of running on only their hind legs like a theropod dinosaur. Hallopus had a narrower pointier skull compared to its much larger relative Macelognathus, and would have looked similar in many respects to Terrestrisuchus, a 3-foot-long sphenosuchian from Europe. However, like Macelognathus, Hallopus had no teeth in the front end of its lower jaw; Terrestrisuchus, by contrast, had a full set of teeth. Thus suggests that Hallopus may have been an egg-eater.

Sphenosuchians like Hallopus, Macelognathus, and Terrestrisuchus have traditionally been depicted in paleo-art as being quadrupedal. However, many of the renditions of these animals walking quadrupedally look downright awkward. The back is shown as being strongly arched to the point where it looks as if its spine is being curved beyond its natural state, the joint between the cervical vertebrae and the dorsal vertebrae is cranked into a very uncomfortable position, and due to its long back legs, its rear end is so high up in the air that it almost looks as if its “presenting” to its mate.

Below is a drawing depicting Hallopus in the traditional quadrupedal manner. You can see some of the inherent problems in reconstructing an animal like this.

It’s more likely that these long-legged crocodile relatives were bipedal. As to how often they were bipedal, that’s a subject for debate. An “obligate biped” means that it was bipedal all or most of the time; moving on two legs was its default setting. A “facultative biped” means that it was usually quadrupedal, but it could rear up on its back legs if it wanted to. So, was Hallopus an obligate biped or a facultative biped? With its unusually long back legs, I am more than inclined to believe that Hallopus spent most of its time on two legs.

Below is a revised drawing showing Hallopus standing upright on two legs. This is probably the more accurate way of depicting this animal.

I hope that you found this interesting, and I also hope that it leads you to challenge the common traditional images that have dominated aspects of paleo-art for so long. Keep your pencils sharp, everyone.