While it may seem that every paper on dinosaur paleobiology is about Tyrannosaurus rex, or at least some kind of theropod, this is not true; occasionally one slips out on a sauropodomorph or ornithischian. In the past few weeks, in fact, two have come out on aspects of our old favorite Thescelosaurus. Both feature North Carolina Museum of Natural Sciences (NCSM) 15728, also known as "Willo" (the one formerly thought to have a fossil heart). The earlier of the two, Senter and Mackey (2023), considers what Thescelosaurus could do with its arms, and the more recent, Button and Zanno (2023), sheds light on what may have been going on inside of its sharply pointed skull.
Thescelosaurus forelimbs
Between you and me, I've long been of the opinion that Thescelosaurus, while aesthetically pleasing in many ways, had kind of funny-looking arms. In particular, the humeri look kind of long and slender for something that's so overbuilt elsewhere. Then, of course, although the hands have five fingers, they're all pretty stubby; this animal was certainly not doing needlepoint or picking locks. Occasionally the proposal that Thescelosaurus was quadrupedal has surfaced (e.g., Galton 1974), but it has never been especially popular. Alternatively, one reconstruction, featured in Sternberg (1940), stuck the arm out almost perpendicular to the body.
Senter and Mackey (2003) looked at the range of motion present at the shoulder joint. Thescelosaurus, as it turns out, was fairly limited in what it could do. In the parasagittal plane, the authors could not bring the humerus farther forward than a bit short of vertical, so the animal could not reach very far forward. It could, though, pull the humerus back to a bit above horizontal, so the arm could be pulled back along the body. Laterally, the humerus could be made to stick out, but this was not described anywhere as the natural position. With the natural curvature of the anterior part of the torso into the neck, the arms are long enough to touch the ground, but this would not have been useful for mobility because the hands cannot be rotated correctly: instead of a solid pull by a hand placed flat on the ground and retracted, with fingers pointing in the direction of travel, the animal would instead have been delivering a karate chop to the ground at best.
|
| Or perhaps this would make more sense than my secondhand ramblings? This is Figure 2 from Senter and Mackey (2023), which see for the extensive caption. CC BY 4.0. |
The brain of Thescelosaurus
A farewell to arms? We'll see...
NCSM 15728 also happens to have a pretty good head on its shoulders, which is analyzed in Button and Zanno (2023). The authors used CT-scanning of the skull to create a digital endocast of the brain. This turned out to be in the average to below-average range for NCSM's mass, more comparable to ceratopsids and thyreophorans than other ornithopod-types. Of course, one can have a proportionally or absolutely huge brain and not be doing much with it, as many humans have proved over the years, although we can be comfortable in assuming that Thescelosaurus neglectus did not produce, for example, many chess masters (despite the cerebral hemispheres occupying about 30% of the endocast). What was it doing?
|
| Figure 1 in Button and Zanno (2023), which see for caption. CC BY 4.0. Was there a thescelosaur Descartes, and a thescelosaur "Cogito, ergo sum"? |
One thing that it was probably quite good at was smelling things. The olfactory bulbs make up about 3% of the endocast volume, which is something you might find in rodents and rabbits. On the other hand, it was not enjoying an especially rich world of sounds. The best hearing range is calculated at about 296 to 2150 Hz, topping out at 3051 Hz, and mean best hearing around 1100 to 1200 Hz. Button and Zanno (2023) note that this is less than other ornithopod-types, but very similar to some crocs, such as the spectacled caiman, and some lizards, such as the ocellated skink. (This is also rather similar to Struthiosaurus austriacus per Schade et al. 2022, as we saw a couple of years ago.) The short cochlear duct also indicates relatively poor ability to discriminate low- and high-frequency sounds. The semicircular canals indicate the skull was routinely held at a slight upward tilt. The anterior semicircular canal is unusually long and slender; its length may imply a great sensitivity to balance.
Button and Zanno draw some inferences about the possible lifestyle of T. neglectus from these data. The relatively small brain indicates it may not have had as complex a social life as, say, a hadrosaur, with smaller group sizes. This would also fit with the relatively limited hearing: it wouldn't have had as great a need to communicate with others of the same species. (Borrowing from Wikipedia, the spectacled caiman apparently has a repertoire of nine different calls, so there's no need to think of Thescelosaurus as entirely solitary and non-vocal.) The enhanced sense of smell is comparable to ankylosaurs, which are sometimes interpreted as scratch-diggers for food like roots and tubers, so we have come back around to arms. Senter and Mackey (2023) show the arms could be pulled back farther than they could reach forward, but we still have the issue of the non-pronating wrists. Some features of the brain suggest the animal was not especially agile, but the sensitivity for balance may be a feature for living in swampy or marshy areas. Button and Zanno comment on the possibility that some of the brain anatomy may be adaptations for burrowing, a la Oryctodromeus, or at least represent a past evolutionary history of burrowing. This in turn has implications for some of our common narratives about surviving the end-Cretaceous extinction: If Thescelosaurus could burrow, then burrowing was not a "get-out-of-extinction" card.
References
Button, D. J., and L. E. Zanno. 2023. Neuroanatomy of the Late Cretaceous Thescelosaurus neglectus (Neornithischia: Thescelosauridae) reveals novel ecological specialisations within Dinosauria. Scientific Reports 13(1):19224. doi: https://doi.org/10.1038/s41598-023-45658-3.
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.
Schade, M., S. Stumpf, J. Kriwet, C. Kettler, and C. Pfaff. 2022. Neuroanatomy of the nodosaurid Struthiosaurus austriacus (Dinosauria: Thyreophora) supports potential ecological differentiations within Ankylosauria. Scientific Reports 12:article 144. doi: https://doi.org/10.1038/s41598-021-03599-9.
Senter, P. J., and J. J. Mackey. 2023. Forelimb motion and orientation in the ornithischian dinosaurs Styracosaurus and Thescelosaurus, and its implications for locomotion and other behavior. Palaeontologia Electronica 26(3):a41. doi: https://doi.org/10.26879/1289.
Sternberg, C. M. 1940. Thescelosaurus edmontonensis, n. sp., and classification of the Hypsilophodontidae. Journal of Paleontology 14(5):481–494.
No comments:
Post a Comment