Thescelosaurus: hello old friend

Thescelosaurus is near and dear to my heart, and Clint Boyd's recent publication on the skull of T. neglectus (Boyd 2014) makes it a good time to ruminate on this animal. Thescelosaurus is one of the most common dinosaur genera from the end of the Cretaceous in North America, and close to a dozen good specimens have at least been mentioned in the literature, to say nothing of the plethora of stray vertebrae, teeth, and limb elements littering the Hell Creek, Lance, Scollard, Frenchman, and other formations. This set of end-Cretaceous formations in western North America is best known for Tyrannosaurus, Triceratops, and the Duckbill Formerly Known As Trachodon/Anatosaurus/Anatotitan (Edmontosaurus annectens), but your standard End-K toybox also comes with Ankylosaurus, Pachycephalosaurus, Torosaurus (depending on where you buy your toys), the original Ornithomimus, this spring's darling Anzu, various nameless dromaeosaurids (perhaps mostly Acheroraptor) and troodontids, and of course Thescelosaurus. Oddly, many of these are among the largest examples of their lineages.

USNM 7757, the type specimen of Thescelosaurus neglectus, as it had been exhibited at the National Museum of National History; we'll see how it looks after the renovation. The neck and skull are reconstructions.

This is more or less how it was found as well, with the neck and skull absent. The right leg was also repositioned (it had been disarticulated from the hip and forming a right angle with the long axis of the body). From Gilmore (1915).

Not many people claim Thescelosaurus as their favorite dinosaur. Maybe it was the down-to-earth size, maybe it was underdog appeal next to flashier beasts, maybe it was the fact that the Science Museum's life model is red-orange and orange is my favorite color. Who knows? It just stuck with me at some point in the single-digit years. As a high school student, I was a member of Mentor Connection, a sort of post-secondary enrollment program where students would be paired with mentors active in the students' field of interest. I got to work with Bruce Erickson and Lee Hallgren at the Science Museum, and one of my projects was getting together a cast of Thescelosaurus for a trade.

You! This is all your fault!

Something that impressed me about this dinosaur when I got the chance to work with it was its robustness. Many dinosaur enthusiasts who came of age in the 1980s and 1990s probably have the folowing Pavlovian response to "hypsilophodont" and various derivations thereof: "gazelles of the Mesozoic". This analogy fails when confronted with Thescelosaurus. If you ever get the chance to play around with "hypsilophodont" long bones, you'll notice that in general they are slim structures befitting what are interpreted as swift and agile herbivores. Thescelosaur long bones are more like clubs. The femur (thigh bone) is thick, the tibia (shin bone) is thick, the arms are stocky, even the ribs are thick. Thescelosaurs would have been hell on modern roads; they would have been the kind of animal that walked away from automobile strikes that disabled the vehicle. The things were built like tubby mobile bomb shelters, at least from the shoulder back. Do not think "gazelle", think more along the lines of "bipedal pig with a long neck and skinny head". Coincidentally, thescelosaur bones can be easily confused with pachycephalosaur bones, Pachycephalosaurus being a similarly tubby and robust animal of comparable size. (I suppose we then credit Tyrannosaurus with promoting the innovation of ornithischians that cannot be cracked open?)

Behold your gazelle. From Gilmore (1915).

Something like this, fluffy or not fluffy—you get the idea. Go art.

The skull, as described profusely in Boyd's paper, is somewhat unusual compared to the rest of the skeleton, being elongate and frankly fragile-looking in the thin pointed beak. The obvious implication is that thescelosaurs were selective feeders, poking their long pointed snouts to pick out only what they wanted. Tubby gourmands, if you will. The hands probably were not much help for feeding. Although there were five fingers, none of them were especially long, nor were they heavily clawed. The broad stubby hand is actually vaguely like a baby's. This makes it all the more interesting that the arm bones are robust, like the rest of the postcrania, and the shoulder blades are broad. Some of the other North American thescelosaurids, particularly Oryctodromeus, are interpreted as burrowers, but this has yet to be put forward for Thescelosaurus itself (although occasionally someone tries to put it on all fours, for which it would have had to bend over a fair bit). It was probably not particularly swift; the femur is longer than the tibia, and the metatarsals are short and stocky. Based on taphonomy (the study of how dead things become fossils, broadly), it appears that thescelosaurs may have preferred to live near rivers and streams (Lyson and Longrich 2011). Outside of the robust skeleton and elongate skull, there are no particular bony anomalies, no elongate vertebral spines making a sail back, no crest or horns, no large claws, just 3 to 4 meters of long, low, rotund biped. This combination does not bode well for defensive purposes, but perhaps they relied on "ornery and overbuilt" when confronted with danger (it's not as flashy as horns, spikes, or speed, but being ill-tempered and strong can be an effective defensive strategy).

The skull of "Willo" (North Carolina State Museum of Natural Sciences 15728), which you may have encountered elsewhere as "the dinosaur with a heart". From Boyd (2014), via compositing by FunkMunk at Wikipedia.

The forearm and hand of USNM 7757 (Gilmore 1915), perfectly adapted for an open-hand slap.

As you may know, Thescelosaurus neglectus means roughly the "neglected wonderful or marvelous lizard", Charles W. Gilmore's way of describing his surprise at finding such a specimen had been forgotten in a shipping crate for about 20 years. It is quite a fine specimen, even featuring sternal elements and possible carbonized remnants of skin. This latter material was found in the shoulder and torso regions and was described by Gilmore as "punctured", which is intriguing in light of quill-like and filament-like structures being found on other ornithischians. After its description in 1913, the genus generally did its best to live up to Gilmore's species name, popping up about once every decade or two as the primary topic of a paper (Gilmore 1915; Parks 1926; Sternberg 1940; Galton 1974; Morris 1976). This is actually not bad considering the general lack of attention given to "hypsilophodonts". Things picked up in the 1990s (Galton 1995, 1997, 1999), and "the dinosaur with a heart" made its appearance in 2000 (Fisher et al. 2000) (only to be shot down pretty well in Cleland et al. 2011). A modern renaissance in study of this genus has resulted in several papers (Boyd et al. 2009; Brown et al. 2011; Boyd 2014). For what it is, there are a fair number of species mixed in. In order of appearance, they are:

• T. neglectus (the type), Gilmore 1913
• T. warreni, Parks 1926, transferred to the new genus Parksosaurus in Sternberg 1937 (a GSA abstract; you could get away with that then)
• T. edmontonensis, Sternberg 1940, based on a good skeleton but currently considered indeterminate within Thescelosaurus
• T. garbanii, Morris 1976, based on scrappy material but currently accepted as valid
• Bugenasaura infernalis Galton 1995, thought for a while to be a larger, more robust relative, but now considered indeterminate within Thescelosaurus
• T. assiniboiensis, Brown et al. 2011, which in hindsight is the source for the Science Museum's cast

For years Thescelosaurus had been tucked away in Hypsilophodontidae, which was similar to the versions of Coelurosauria and Carnosauria that get giggles now ("little theropods and big theropods"). Hypsilophodonts were small ornithopods, more or less. After the cladistic revolution in the 1980s, Hypsilophodontidae was maintained for a time, but it collapsed during the first decade of the 2000s. Thescelosaurus had a handy pre-made place to go in Thescelosauridae. Thescelosauridae presently includes many of the traditional hypsilophodonts, except for Othnielia (which changed its name to Othnielosaurus and moved), the dryosaurs, and Hypsilophodon itself. As the chart shows, the membership is divided between orodromines (small, all from the Cretaceous of North America, and possibly all adapted for burrowing), jeholosaurs (East Asian ornithopods with strikingly Thescelosaurus-like skulls), and North American thescelosaurines (Thescelosaurus and Parksosaurus, its svelter, swifter cousin).

Membership largely after Brown et al. (2013), with additional jeholosaurs after Han et al. (2012).

As a bonus, here is a table of the best-known Thescelosaurus specimens in the literature (there are also several undescribed skeletons).

(click to embiggen)

(P.S.: in the time it took me to write this, I got pipped at the thescelosaur post by Brian Switek at Dinologue; if you're feeling like more news of wonderful lizards, you ought to stop by.)

References:

Boyd, C. A. 2014. The cranial anatomy of the neornithischian dinosaur Thescelosaurus neglectus. PeerJ 2:e669.

Boyd, C. A., C. M. Brown, R. D. Scheetz, and J. A. Clarke. 2009. Taxonomic revision of the basal neornithischian taxa Thescelosaurus and Bugenasaura. Journal of Vertebrate Paleontology 29(3):758–770.

Brown, C. M., C. A. Boyd, and A. P. Russell. 2011. A new basal ornithopod dinosaur (Frenchman Formation, Saskatchewan, Canada), and implications for late Maastrichtian ornithischian diversity in North America. Zoological Journal of the Linnean Society 163(4):1157–1198.

Brown, C. M., D. C. Evans, M. J. Ryan, and A. P. Russell. 2013. New data on the diversity and abundance of small-bodied ornithopods (Dinosauria, Ornithischia) from the Belly River Group (Campanian) of Alberta. Journal of Vertebrate Paleontology 33(3):495–520.

Cleland, T. P., M. K. Stoskopf, and M. H. Schweitzer. 2011. Histological, chemical, and morphological reexamination of the "heart" of a small Late Cretaceous Thescelosaurus. Naturwissenschaften 98(3):203–211.

Fisher, P. E., D. A. Russell, M. K. Stoskopf, R. E. Barrick, M. Hammer, and A. A. Kuzmitz. 2000. Cardiovascular evidence for an intermediate or higher metabolic rate in an ornithischian dinosaur. Science 288(5465):503–505.

Galton, P. M. 1974. Notes on Thescelosaurus, a conservative ornithopod dinosaur from the Upper Cretaceous of North America, with comments on ornithopod classification. Journal of Paleontology 48(5):1048–1067.

Galton, P. M. 1995. The species of the basal hypsilophodontid dinosaur Thescelosaurus Gilmore (Ornithischia: Ornithopoda) from the Late Cretaceous of North America. Neues Jahrbuch fèur Geologie und Palèaontologie Abhandlungen 198(3):297–311.

Galton, P. M. 1997. Cranial anatomy of the basal hypsilophodontid dinosaur Thescelosaurus neglectus Gilmore (Ornithischia: Ornithopoda) from the Upper Cretaceous of North America. Revue de Paleobiologie 16:231–258.

Galton, P. M. 1999. Cranial anatomy of the hypsilophodontid dinosaur Bugenasaura infernalis (Ornithischia; Ornithopoda) from the Upper Cretaceous of North America. Revue de Paleobiologie 18:517–534.

Gilmore, C. M. 1913. A new dinosaur from the Lance Formation of Wyoming. Smithsonian Miscellaneous Collections 61(5):1–5.

Gilmore, C. M. 1915. Osteology of Thescelosaurus, an orthopodus dinosaur from the Lance Formation of Wyoming. Proceedings of the U.S. National Museum 49(2127):591–616 (plus plates).

Han F.-L., P. M. Barrett, R. J. Butler, and Xu X. 2012. Postcranial anatomy of Jeholosaurus shangyuanensis (Dinosauria, Ornithischia) from the Lower Cretaceous Yixian Formation of China. Journal of Vertebrate Paleontology 32(6):1370–1395.

Lyson, T. R., and N. R. Longrich. 2011. Spatial niche partitioning in dinosaurs from the latest Cretaceous (Maastrichtian) of North America. Proceedings of the Royal Society B 278(1709):1158–1164.

Morris, W. J. 1976. Hypsilophodont dinosaurs: a new species and comments on their systematics. Pages 93–113 in C. S. Churcher, editor. Athlon: essays in palaeontology in honour of Loris Shano Russell. Royal Ontario Museum, Toronto, Ontario, Canada.

Parks, W. A. 1926. Thescelosaurus warreni, a new species of orthopodous dinosaur from the Edmonton Formation of Alberta. University of Toronto Studies (Geological Series) 21:1–42.

Sternberg, C.M. 1937. Classification of Thescelosaurus, with a description of a new species. Geological Society of America Proceedings for 1936:365.

Sternberg, C. M. 1940. Thescelosaurus edmontonensis, n. sp., and classification of the Hypsilophodontidae. Journal of Paleontology 14(5):481–494.