O is for Ojoceratops

Paleontologists have  named both too many and too few dinosaurs. The tricky business of taxonomy is to blame. While hundreds of dinosaur species rest as-yet-undiscovered in geologic formations and museum collections, researchers also tussle over the identities of specimens given new names. The continuing academic struggle over whether Torosaurus and Nedoceratops really represent growth stages of Triceratopsreally represent growth stages of Triceratops is one such struggle, and, even among ceratopsid dinosaurs, isn’t the only debate. Paleontologists are also unsure about the identity of Ojoceratops fowleri, a poorly known horned dinosaur found in New Mexico.

Robert Sullivan and Spencer Lucas named Ojoceratops from a smattering of roughly 69 million year old fossils found with in new Mexico’s Ojo Alamo Formation. The singular fossil that carries the name is a left squamosal – the wing-shaped bone that made up the side of the herbivore’s frill – although Sullivan and Lucas tentatively referred other skull parts and two other bones from additional specimens that may or may not be from the same species. The proportions and angles of the Ojoceratops squamosal, the researchers proposed, distinguished the dinosaur from all other chasmosaurines (the subgroup of horned dinosaurs that includes Triceratops, Chasmosaurus, and their close relatives).

If Ojoceratops really is a distinct dinosaur, the ceratopsid fits into a geographical pattern of dinosaur evolution that is starting to come into focus. Paleontologists have begun to recognize that dinosaurs in older, roughly 75 million year old rock  varied across the latitudes – similar communities of dinosaurs were present in Alberta, Canada and southern Utah at about the same time, but those assemblages were made up of different genera and species. While no one has yet identified the barrier(s), it seems that there was some sort of split that caused dinosaurs in the north and in the south to evolve into disparate forms. Agujaceratops, a ceratopsid found in Texas, was one of these dinosaurs, and had previously been considered to be another species of the northern Chasmosaurus.

So far, these discussions of dinosaur provincialism have focused on the Campanian, about six million years before the time of Ojoceratops. The earliest part of the Maastrichtian, when Ojoceratops lived, isn’t as well-known, and so any potential signature of north-south separation isn’t yet apparent. Ojoceratops could continue the pattern of a distinct, southern radiation of horned dinosaurs, or, as some researchers have argued, the dinosaur might embody a southern population of previously-known dinosaurs. In his running appendix of dinosaur genera, Thomas Holtz noted that Ojoceratops could be a southern representative of Eotriceratops – a large horned dinosaur thought to be a close predecessor of Triceratops and Torosaurus.  And when Nicholas Longrich named Titanoceratops – itself probably a synonym of Pentaceratops – he suggested that Ojoceratops was actually Triceratops itself.

For now, there simply isn’t enough of Ojoceratops to know whether it’s a distinct dinosaur or an early, southern population of dinosaurs better known from later, northern deposits. It’s part of the dinosaur story caught between the exceptional sites of the Late Campanian and Late Maastrichtian, faunas separated by about 10 million years. The dinosaurs in this gap are critical to figuring out dinosaur evolution and diversity leading up to the Cretaceous mass extinction, but, as yet, are little known. The ultimate fate of Ojoceratops will not only rest on scientific debate and taxonomic philosophy, but on efforts to fill out the penultimate chapter in non-avian dinosaur history.

Previous entries in the Dinosaur Alphabet series:

N is for Nqwebasaurus

M is for Montanoceratops

L is for Leaellynasaura

K is for Kileskus

J is for Juravenator

A-I at Dinosaur Tracking.

References:

Holtz, T. 2011. Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages, Winter 2010 Appendix.

Longrich, N. 2011. Titanoceratops ouranos, a giant horned dinosaur from the Late Campanian of New Mexico”Titanoceratops ouranos, a giant horned dinosaur from the Late Campanian of New Mexico”. Cretaceous Research. 32, 3: 264–276.

Sullivan, R., Lucas, S. 2010. A New Chasmosaurine (Ceratopsidae, Dinosauria) from the Upper Cretaceous Ojo Alamo Formation (Naashoibito Member), San Juan Basin, New Mexico, pp. 169-180, in: Ryan, M., Chinnery-Allgeier, B., Eberth, D. (eds.) New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium, Bloomington, Indiana University Press.