A is for Agujaceratops

Out of the scores of non-avian dinosaurs discovered, some get all the love. Almost everyone can rattle off a few of the most famous–Triceratops, Stegosaurus and, of course, Tyrannosaurus rex (the only one we ever feel compelled to call by its whole name). But the Age of Dinosaurs was a 160-million-year reign filled with a startling variety of species that we’re only just beginning to become acquainted with. It’s truly a shame that we continually focus on the same handful when there were so many wonderful forms. Among the unsung dinosaurs is Agujaceratops, a horned herbivore that was only recently recognized for what it truly was.

The story of Agujaceratops goes back the better part of a century. During excavations in 1938 and 1939, a Works Progress Administration crew picked away at a dense dinosaur bonebed in what is now southwestern Texas’ Big Bend National Park. The team pulled more than 340 bones out of the roughly 75-million-year-old Late Cretaceous rock. Although they didn’t know it at the time, most of these bones belonged to a single species of dinosaur that no one had seen before.

Five decades later, Texas Tech University paleontologist Thomas Lehman returned to the skeletal collection. The various pieces came from at least ten individual dinosaurs–from juveniles to adults–that were entombed in the same place. There was no single articulated skeleton, or even a complete skull, but by sifting through the remains Lehman reconstructed several skulls from the new horned dinosaur species. Drawing a comparison with Chasmosaurus, a previously known horned dinosaur found in Canada with similar anatomical motifs among the horns and frill, Lehman called his animal Chasmosaurus mariscalensis.

Not long after Lehman’s paper, other researchers happened upon a lovely specimen that confirmed the southern ceratopsid as a distinct dinosaur. In 1993, ceratopsian expert Catherine Forster and coauthors described a complete Chasmosaurus mariscalensis skull, showing that this dinosaur had much longer brow horns and a more saddle-shaped frill than other Chasmosaurus species to the north.

Yet, even though this study found that Chasmosaurus mariscalensis was more closely related to other Chasmosaurus species than to Pentaceratops–another southern ceratopsid that was a possible candidate for a Chasmosaurus descendant–the southern species didn’t look quite like the northern ones. The northern Chasmosaurus species had shorter brow horns and expanded, V-shaped frills that didn’t curve upwards in the same way. Why was the southern species so different? Perhaps, Forster and colleagues hypothesized, the southern species retained some archaic characteristics while the northern Chasmosaurus underwent greater modifications.

As paleontologists continued to scrutinize ceratopsids, however, the less the southern species looked like a Chasmosaurus. In a 2006 reevaluation of Chasmosaurus and Pentaceratops,  New Mexico Museum of Natural History and Science paleontologist Spencer Lucas and collaborators placed “Chasmosaurus” mariscalensis in a new genus–Agujaceratops, named in honor of the Aguja Formation in which the dinosaur is found.

Along with other new discoveries–such as Kosmoceratops and Utahceratops from southern Utah–Agujaceratops changed the big picture of ceratopsid biogeography. As Lehman’s paper hints, some paleontologists used to think there was a kind of faunal continuum between northern and southern swaths of North America. In formations laid down at the same time (about 75 million years ago in this case), you’d expect there to be continuity between the dinosaur genera found down the latitudes. Bits and pieces of dinosaurs found in Utah, New Mexico, Texas and elsewhere were attributed to dinosaur genera discovered about 2,000 miles away in Canada. This didn’t only affect horned dinosaurs. Remains of southern tyrannosaurs, previously attributed to the northern predators Albertosaurus and Daspletosaurus, were recently found to be a previously unknown tyrant called Bistahieversor.

By way of new finds and reexaminations of old material, paleontologists have only just started to become acquainted with Agujaceratops, Bistahieversor and other dinosaurs of the southwest’s Late Cretaceous. At the species and genus levels, the southern dinosaurs are different. The big question is, why? Paleontologists know that a shallow, vanished seaway separated dinosaurs on eastern and western subcontinents for millions of years, but on that western subcontinent called Laramidia, there was apparently some other kind of barrier that isolated northern and southern dinosaur populations.

The hypothesis relies on basic evolutionary theory. Isolate populations of an ancestor species in different regions, and through factors such as natural selection and genetic drift, those populations will evolve in different ways. The fact that Agujaceratops, Kosmoceratops and Utahceratops are so different from Chasmosaurus and other northern cousins are a sign that such a barrier was in place. No one has found it yet, though, and a great deal of work remains to be done on whether all these dinosaurs were really contemporaries or reveal a much more complex evolutionary pattern. As these investigations continue, though, Agujaceratops will continue to play an important role as a symbol of isolation and evolution.

Author’s note: This is the first entry in a new series of posts, highlighting fantastic dinosaurs that are little known by the public. You won’t find Archaeopteryx, Brachiosaurus, Tyrannosaurus or other classics on this list. Those dinosaurs are famous enough already. Now it’s time to highlight some of their lesser-known cousins and contemporaries, from Agujaceratops to Zalmoxes.

References:

Forster, C., Sereno, P., Evans, T., Rowe, T. 1993. A complete skull of Chasmosaurus mariscalensis (Dinosauria: Ceratopsidae) from the Aguja Formation (late Campanian) of West Texas, Journal of Vertebrate Paleontology, 13:2, 161-170. doi: 10.1080/02724634.1993.10011498

Lehman, T.1989. Chasmosaurus mariscalensis, sp. nov., a new ceratopsian dinosaur from Texas, Journal of Vertebrate Paleontology, 9:2, 137-162 doi: 10.1080/02724634.1989.10011749

Lucas, S., Sullivan, R., Hunt, A. 2006. Re-evaluation of Pentaceratops and Chasmosaurus (Ornithischia: Ceratopsidae) in the Upper Cretaceous of the Western Interior, in Lucas, S. G. and Sullivan, R.M., eds., 2006, Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin 35.

Sampson, S., Loewen, M., Farke, A., Roberts,E., Forster, C., et al. 2010. New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism. PLOS ONE 5(9): e12292. doi:10.1371/journal.pone.0012292

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.