Name Meaning: Wondrous Lizard
Geologic Era: Late Cretaceous
Location Discovered: Wyoming (our specimens are from Montana)
Estimated Range: Western United States, possibly southern Canada
Size: Up to 4 meters long
Extinction Date: 65 million years ago (K-T extinction)
This little bone was my introduction to paleontological mysteries. It was sold to me as a theropod tail bone, but I wanted more information. I turned to the paleontologists at the Field Museum, who were able to identify it as coming from Thescelosaurus. The processes are gone, but the underlying cancellous (spongy) bone is more visible as a result. Out specimen would have come from near the base of the tail, based on its shape. Caudal vertebrae become more elongated the further they get from the tail base, and this bone being as short as it is points to it being very close to the base of the tail. The chevrons are also absent, but probably broke away at some point.
Thescelosaurus was a small, herbivorous dinosaur. The amount of of preserved skeletons indicates that it probably preferred streams in terms of habitat, as watery environments aid fossilization. It is generally classified as a hypsilophodontid type dinosaur, based on its skeletal structure and its teeth. The tail would have been relatively rigid, based on the presence of ossified (calcified) tendons in the tail. Thescelosaurus may have been able to move on all four legs, though it could just as easily have moved on two. One possibility is that it stayed in a quadrupedal stance while browsing for low vegetation, but used a bipedal stance when it needed to move quickly.
Thescelosaurus probably ate plants low to the ground such as ferns, though it could have taken advantage of low branches as well. Thescelosaurus’ legs indicate it was not a particularly fast runner, and so may have relied on dense vegetation or trees to evade predators. Predators of the time would have included Tyrannosaurus, Dakotaraptor, and Acheroraptor.
One Thescelosaurus specimen, called Willo, was thought to have been found with a preserved heart. Soft tissue preservation is not unheard of in dinosaurs, but it is fairly rare. The first paper that presented this object as a heart tried to show that it was four chambered, like modern birds. Later analysis showed that this object was unlikely to be a heart, but was more likely a lump of sediment that had aggregated roughly where the heart would have been.
Dinosaurs, being closely related to birds, probably did have 4 chambered hearts and were much more active than previously thought. They were also probably endothermic (warm blooded). Finding a true heart would have really supported this hypothesis, but most of the evidence points in that direction anyway. Aside from the heart controversy, the Willo skeleton is beautifully preserved and is an excellent example of an articulated skeleton.
Life Restoration: “Thescelosaurus BW” by Nobu Tamura (http://spinops.blogspot.com) – Own work, based on skeletal at , pencil drawing, digitally enhanced using PSP. Licensed under CC BY 2.5 via Commons – https://commons.wikimedia.org/wiki/File:Thescelosaurus_BW.jpg#/media/File:Thescelosaurus_BW.jpg
Willo Skeleton: “Willo” by Ryan Somma – Willo: ThescelosaurUploaded by FunkMonk. Licensed under CC BY-SA 2.0 via Commons – https://commons.wikimedia.org/wiki/File:Willo.jpg#/media/File:Willo.jpg