Coelophysis: Beast of the Week

This week we will be taking a look at another very well-studied dinosaur.  Check out Coelophyis bauri!  Coelophysis was a relatively small, meat-eating dinosaur that lived during the late Triassic period, about 200 million years ago, in what is now the Western United States.  Fully grown, adults measured about ten feet long from snout to tail.  Coelophysis bones are mostly known from New Mexico (where it is the official state fossil), but evidence of this dinosaur has been found as far as the east coast, as well, in the form of tracks.  The genus name, Coelophysis, translates to "Hollow Form" in reference to the dinosaur's hollow bones, a trait actually common to many dinosaurs, not just Coelophysis.  When alive Coelophysis would have coexisted with creatures like crocodilian relatives, Typothorax, Shuvosaurus, and Postosuchus.

Group of Coelophysis bauri of varying ages gathering in a dried river bed.  Fossils suggest large groups of Coelophysis died at about the same time relatively quickly due to harsh environments.  Watercolor reconstruction by Christopher DiPiazza.

Coelophysis lived during a time in history when dinosaurs were obviously around, but had not become as diverse and large as most people know them to be just yet.  During the Triassic, the biggest, most formidable animals were actually other kinds of archosaurs like pseudosuchians (group that includes modern crocodilians) like Typothorax, Postosuchus, and Shuvosaurus, as well as the crocodile-looking-but-not-really-close-to-crocodiles, the phytosaurs, like Rutiodon.  In fact, actual dinosaurs were somewhat of a rarity during the Triassic. If you would like the full story on how reptiles were evolving and going extinct during this time, please go read my post about my work in New Mexico excavating the fossils of these creatures.  It was a fascinating time in earth's history.

This is a fossil I found that has been split right down the middle, showcasing how hollow it is.  It is most likely from a Coelophysis. 

Coelophysis likely specialized in hunting small prey by using speed and agility.  It was very lightly built, most of its ten-foot length consisting of its neck and tail, and would have been able to run quickly on its hind legs, which were slender, but strong.  It had a long, narrow snout with a slight notch at the tip of the upper jaw, which could have helped it hold on to struggling prey.  Inside the mouth were many small, serrated teeth, for slicing meat.  Originally it was believed that Coelophysis was cannibalistic, since one specimen in New Mexico was discovered with what looked like baby Coelophysis bones inside where its stomach used to be.  Under further inspection, however, it was decided that these bones were in fact belonging to one of the small species of land crocodilians that would have been common back then.  However, small broken bones and teeth that were confirmed to be from baby Coelophysis were found in the stomach cavities and around the mouths of other adult specimens, showing that cannibalism of smaller individuals still was probably taking place.  In turn, Coelophysis would have needed to be weary of becoming prey to the bigger predators alive back then, like Postosuchus or Redondasaurus.

Coelophysis cast on display at the Mesalands Dinosaur Museum, in Tucumcari, New Mexico.  You can see the bones of what was originally believed to be a baby in the stomach cavity.

Coelophyisis is one of the few prehistoric dinosaurs that paleontologists can pretty much completely map out as far as its skeleton and growth is concerned.  This is because of a fossil site in New Mexico, called the Ghost Ranch Formation, which was discovered with literally hundreds of individual specimens of Coelophysis, including fully-grown adults down to small babies.  Nobody knows for certain why so many of these dinosaurs died at this location, but it is likely that the Ghost Ranch area used to be a river during the Triassic.  Animals, like Coelophysis would have come to the river bed to drink during the dry season, died from thirst, then their bodies would have been buried under mud to be fossilized when the water did come through.  Another possibility is that they all gathered to hunt a common food source, like spawning fish (which Coelophysis' long snout and neck could have been adaptations for) and were wiped out by a flash flood.

A team of paleontologists, lead by Dr. Daniel Barta, recently investigated how Coelophysis grew by making thin slices in Coelophysis bones and looking at ring-like patterns on the inside, each ring indicating a year of the animal's life. (much like the growth rings of a tree) They determined that not all individual Coelophysis grew at the same rate.  Some very large individuals turned out to actually be very young when they died, while other relatively small individuals were actually much older.  This implies that Coelophysis' growth was probably more based on how much an animal was eating, which is similar to how many modern reptiles grow today.  This may have been a helpful trait for animals that lived in harsh environments where food wasn't consistently available over time where animals with more consistent growing patterns might starve.

There is evidence that Coelophysis may have also lived on America's east coast of North America, including parts of my home state, New Jersey!  Although no bones were ever found there, we do have lots of theropod dinosaur tracks that are from the exact time that Coelophysis was alive, and match the size of its feet.  Because it cannot be proven 100% (no bones) that these dinosaurs were actually Coelophysis, however, they were given their own name, Grallator.  

Grallator (most likely Coelophysis) tracks that were unearthed in New Jersey.  These slabs are on display at the Rutgers Geology Museum in New Brunswick, New Jersey.

Coelophysis is an important dinosaur, not only because scientists are able to study it so extensively, but also because it represents a turning point in dinosaur evolution.  Coelophysis lived during a time where many other kinds of reptiles, like the phytosaurs and most pseudosuchians, were about to go extinct, but dinosaurs were not.  Coelophysis' lineage would later flourish even more and radiate into the many theropod dinosaurs everyone knows and loves and eventually the birds, which are still successful today!  

That is all for this week!  As always feel free to comment below or on our facebook page.

References

Barta, D.E., Griffin, C.T. & Norell, M.A. Osteohistology of a Triassic dinosaur population reveals highly variable growth trajectories typified early dinosaur ontogeny. Sci Rep 12, 17321 (2022). https://doi.org/10.1038/s41598-022-22216-x

Gay, R.J. (2002). "The myth of cannibalism in Coelophysis bauri". Journal of Vertebrate Paleontology 22 (3): 57A.

Nesbitt, S. J.; Turner, A. H; Erickson, G. M; Norell, M. A (2006). "Prey choice and cannibalistic behaviour in the theropod Coelophysis". Biology Letters. 22 2 (4): 611–614. doi:10.1098/rsbl.2006.0524. PMC 1834007. PMID 17148302.

Rinehart, L.F.; Lucas, S.G.; Heckert, A.B.; Spielmann, J.A. & Celesky, M.D. (2009). "The paleobiology of Coelophysis bauri (Cope) from the Upper Triassic (Apachean) Whitaker quarry, New Mexico, with detailed analysis of a single quarry block". New Mexico Museum of Natural History & Science, a division of the Department of Cultural Affairs Bulletin 45: 260.

Schwartz, Hilde L.; Gillette, David D. (1994). "Geology and taphonomy of the Coelophysis quarry, Upper Triassic Chinle Formation, Ghost Ranch, New Mexico". Journal of Paleontology 68 (5): 1118–1130. JSTOR 1306181.