July 29, 2011
If you haven’t heard of Camposaurus, you’re not alone. This is one obscure dinosaur (and not to be confused with the better-known and very different Camptosaurus). First described in 1998, this animal may hold a critical place in the evolutionary tree of theropod dinosaurs, although, then again, it might not.
Very little is known about Camposaurus. The only parts that have been found and definitively referred to this dinosaur, recovered from the Late Triassic rock of Arizona, are a few parts of the dinosaur’s lower limb bones. (The original description mentioned bones from other individuals, but it is unclear whether these really belong to Camposaurus.) Still, the anatomy of these parts identified the dinosaur as a neotheropod dinosaur, and its geologic context made it potentially the oldest known representative of the huge, diverse group of dinosaurs which contained genera such as Ceratosaurus, Allosaurus, Tyrannosaurus, Spinosaurus and many, many more. The Camposaurus fossils, as a consequence, could be important for calibrating the early evolutionary history of theropod dinosaurs.
Naturally, the fact that so little is known about Camposaurus has made it a controversial dinosaur. Paleontologists have been trying to figure out where it fits in the theropod family tree—and whether the dinosaur even deserves a distinct name—for over a decade. The known bones are so hard to properly diagnose that they seem more likely to confuse than enlighten. Now paleontologists Martin Ezcurra and Stephen Brusatte have published a reexamination of the paltry Camposaurus bones, and they affirm that the dinosaur will remain important to questions about the early days of theropod dinosaurs.
According to Ezcurra and Brusatte, there are two subtle features which set Camposaurus apart from other early theropods, such as the well-known Coelophysis. The first is a distinctive ridge on one of the lower leg bones—the tibia—where it articulates with the fibula, and the second is the absence of a knob of bone on part of the ankle. Such subtle differences can make all the difference between whether a dinosaur genus or species is kept as distinct, ends up being lumped into another taxon, or remains a problematic mystery.
Ezcurra and Brusatte also attempted to figure out where Camposaurus fit among other theropod dinosaurs. As had been previously suspected, the dinosaur turned out to be most closely related to Coelophysis—so close, in fact, that Camposaurus might turn out to be a species of Coelophysis itself. Additional fossils will be needed to be sure, and, at Chinleana, paleontologist Bill Parker brings up an important point about the significance of the specimen in terms of its age.
Camposaurus has been thought to be the oldest known neotheropod dinosaur based upon the geologic details of the place it was found, known as the Placerias quarry. This site was thought to correspond to a certain part of Triassic rock called the Mesa Redondo Member of the Chinle Formation, but Parker reports that he has found this to be in error. The quarry is actually in slightly younger rock than has been proposed, meaning that Camposaurus is not as old as had been assumed. It’s still a very old theropod, but how old it really is and its relationship to other theropods remains tentative.
The takeaway from all these paleontological jots and tittles is that our knowledge of early dinosaurs is still in a state of flux. Determining the identities, relationships and ages of Triassic dinosaurs is an ongoing task, and our understanding will continue to change as new fossils are found. At the moment, the Camposaurus fossils play an important role in providing some of the only context we have for the early evolution of the neotheropod dinosaurs, and hopefully paleontologists will soon find the fossil clues that will allow us to understand how this great lineage got its start.
EZCURRA, M., & BRUSATTE, S. (2011). Taxonomic and phylogenetic reassessment of the early neotheropod dinosaur Camposaurus arizonensis from the Late Triassic of North America Palaeontology, 54 (4), 763-772 DOI: 10.1111/j.1475-4983.2011.01069.x
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