This is Macelognathus vagans. This animal lived in the Morrison Formation of western North America during the late Jurassic Period approximately 150 million years ago.
Macelognathus belongs to a group of animals called the “sphenosuchians”, which is a group that’s very closely related to crocodiles. They even possessed a double-row of crocodilian-like scutes running down the middle of their back. However, unlike crocodiles, sphenosuchians were 100% terrestrial animals, and likely occupied the same ecological niche that foxes and wildcats do today. Most sphenosuchians were small, measuring 3 feet long or less, but Macelognathus was unusually large, measuring 5 feet long (this size measurement is only an estimate, since a complete skeleton has never been found). The anatomy of sphenosuchians strongly suggests that they were physically active animals that were built for the chase. Many species possessed a long lithe body, short front legs and long back legs, and a long thin tail. Sphenosuchians were possibly “obligate bipeds”, meaning that they always walked around only on two legs, but they were almost certainly “facultative bipeds”, meaning that they could sometimes move on two legs if they wanted to.
Macelognathus is known from only fragmentary remains, including the front half of its lower jaw. What is most distinctive about this animal is that the front of its lower jaw was toothless, forming a flat palate. So far, we have not found an upper jaw, so we don’t know if the front of both the upper and lower jaws were toothless, but it seems highly probable. Based upon its jaw structure, it’s likely that Macelognathus was a creature that had a particular preference for eating eggs. The image that comes to mind of this animal, therefore, is a nest-raider, a scavenger, or an ambush predator. I liken it as a Jurassic analog of a large monitor lizard.
There is some evidence which suggests that Macelognathus and another sphenosuchian named Hallopus are in fact the same animal. However, since both Macelognathus and Hallopus are known from only fragmentary remains, a definite answer cannot be given until more remains of both species are discovered and can be compared.
The drawing below was made with No. 2 pencil on printer paper.
Disclaimer: Even though the back legs are noticeably longer than the front legs, it’s still possible that I made the back legs too short in this drawing.
Keep your pencils sharp, everyone.