STL Science Center

STL Science Center

30 June 2018

Coming Back with A Sun God

First, it is really nice to be able to sit and write something for this page again. The past few months it has been difficult to work on writing all day long and then have any energy at the end of the day to write for the page. However, this week, at least, I can spend some time and brain power on writing for the page. This is a good thing.

This week we are going to go back a little further in time and talk about an animal that has, as I find it, strangely not made its way onto the page before. Named for the Hopi word for the sun god of the (all, not just Hopi) Pueblo people and the founder of the Ghost Ranch Museum of Paleontology, Ruth Hall, Tawa hallae was a small Triassic theropod contemporaneous with Dromomeron, Chindesaurus, and potentially even the well-known dinosaur Coelophysis. Tawa is known from a number of different skeletons, none of which are entirely complete, and the initial fossils are thought to be from a juvenile specimen, meaning that the pieced together full version of the animal, weighing in at 15 kg (33 lb) and measuring 2.5 m (8 ft 2 in), is a rough estimate of adult size. The difference between known juvenile and estimated adult size leaves us with a gracile early theropod dinosaur somewhere in the size vicinity of corgi in weight but a golden retriever or a slightly larger dog in height; lengths do not translate to dogs well so I do not know what to give you as a comparison for that measurement.

©Nobu Tamura
Despite living 215-213 million years ago and being one of the more successful carnivores of its day, the size of Tawa was nothing to some of the chief herbivores of the day, such as Plateosaurus (214 to 204 million years ago). Though these two animals may have never crossed paths (Plateosaurus is known from European fossils and Tawa from North American fossils exclusively), the size difference would have likely made it nearly impossible for Tawa to prey on a healthy individual because it was a full 2.3 m (8 ft) shorter and 585 kg (567 lb) lighter than the smallest known Plateosaurus. Sick and injured individuals are always a different story and perhaps Tawa had some way of dealing with large prey like this that we do not know about yet. Regardless, I bring this up only to state that not all of the animals of the Triassic were small and that early dinosaurs were already attaining rather varied sizes. The diet of Tawa was not reliant on such large animals, however, as other small archosaurs, therapsids, insects, phytosaurs, and many other animals were available as part of the diet of this small (for a theropod) carnivore.