April 13, 2011
Evolution is not a constant march of onward-and-upward progress. Any organism is a mosaic of the ancient and the modern—old features can be modified and put to new uses over time—and the mechanism of natural selection accounts for both an apparent lack of change and dramatic evolutionary transformations. There is no driving force towards perfection, only the persistence of what works as some forms fall into extinction. This facet of evolution is beautifully demonstrated by the newly-described dinosaur Daemonosaurus.
I was glad to see Daemonosaurus get published yesterday evening. I had first seen a presentation about the dinosaur by the Smithsonian National Museum of Natural History’s Hans-Dieter Sues at the 2010 Society for Vertebrate Paleontology meeting last October. It certainly had a unique look. A theropod dinosaur, Daemonosaurus had a remarkably large eye socket and a mouth full of prominent, curved teeth that gave it a buck-toothed appearance.
Described by Sues, Sterling Nesbitt, David Berman and Amy Henrici, Daemonosaurus had a strange mix of skull features that placed it in a position between the earliest known theropods, such as Eodromaeus from South America, and later theropod dinosaurs like Tawa from Triassic North America. This was very strange. Although roughly intermediate in anatomy between the first theropods and the later forms found in New Mexico, Daemonosaurus was not intermediate in age or geography. Something more peculiar had happened.
The single skull of Daemonosaurus was found in the approximately 200-million-year-old rock of Ghost Ranch, New Mexico. In terms of age, it came after Tawa and was found in the quarry that has yielded multiple Coelophysis skeletons. Even though Tawa was more closely related to Coelophysis, Tawa had lived and died during an earlier time. In evolutionary terms, this means that Daemonosaurus belonged to a relatively archaic lineage of theropod dinosaurs that had arrived in North America and persisted alongside early members of newly evolved theropod groups.
If this is correct, numerous other early theropods remain to be found. Daemonosaurus had a short-snouted skull with differently shaped teeth throughout the jaw, which differs from the boxy skull of the earlier Herrerasaurus as well as the narrow, elongated skulls of theropods like Coelophysis. The discovery of Daemonosaurus not only adds to the diversity of early theropods, but it increases the disparity seen between the forms of these dinosaurs. An entire suite of unique, early theropods may await lucky paleontologists in strata between 230 and 200 million years old.
But as with almost any early dinosaur, the relationships of Daemonosaurus will be subject to revision as more fossils are found. For one thing, it is not entirely certain whether the skull represents a juvenile or adult individual. The large eye socket and lack of fusion between bones of the braincase might indicate that this dinosaur was young and therefore different from the adult form, but the sutures on two vertebrae associated with the skull are closed and consistent with the individual being an adult. Untangling this mystery will be important to understanding the evolution of this dinosaur. If the Daemonosaurus skull is from a juvenile, some of the “archaic” characters might be attributed to young age, but if it was an adult then we can be more confident that it really did take up an intermediate position among early dinosaurs. Only additional specimens of Daemonosaurus will solve this puzzle.
Sues, H.; Nesbitt, S.; Berman, D.; Henrici, A. (2011). A late-surviving basal theropod dinosaur from the latest Triassic of North America Proceedings of the Royal Society B, 1-6 : 10.1098/rspb.2011.0410
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