Whatever else it ate, it may have consumed a whole school of thought about where and how dinosaurs evolved, say Berkeley integrative biology Ph.D. student Randall Irmis and co-researchers of their find in Arizona’s Petrified Forest National Park.
Irmis and three others from separate institutions excavated a complete skeleton of a yard-long relative of modern crocodiles — with park paleontologist William Parker, who found the first fossil fragments of the creature on a promising outcrop in a previously unscoured area of the park. They measured, compared, and thought about the specimen, then published their conclusions in the journal Proceedings of the Royal Society, Biological Sciences. And thereby rearranged the dinosaur family tree.
Their critter, Revueltosaurus callenderi, has a Berkeley pedigree of sorts (which we’ll get back to) and was long thought to be a small dinosaur — an ancestor of larger plant-eaters like Triceratops and Stegosaurus of the dino-dominant Jurassic and Cretaceous periods.
Like many of the herbivores, Revueltosaurus had been identified first by its teeth. But with the rest of the beast in hand, it became clear to Irmis and company that the teeth were misleading. They were of a plant-eater, all right, but the other body parts had no dinosaur characteristics.
Scratch the dino connection. Not an ancestor to those guys. Merely a branch on the line to crocodilians, one that died out a long time ago.
But as a presumed dinosaur, Revueltosaurus had been a vital link on the direct line of evolution to the later big herbivores. Without that link, the ancestry of many high-profile dinosaurs, and quite a few of their lesser-known predecessors, is suddenly a big question mark.
Because the teeth of Revueltosaurus “look like those we know from herbivorous dinosaurs,” says Irmis, “people assigned them to the dinosaurs. We think we’ve shown that you can’t rely on the dentition to determine what is an early dinosaur, which casts doubt on all the ornithischians from the Triassic of North America.”
This means the big meat-eaters, like Tyrannosaurus rex, which belong to the other major group of dinosaurs, the saurischians, didn’t evolve essentially side by side with the “bird-hipped” ornithiscians, but rather had a large part of the world more or less to themselves for 25 million years longer than most people thought, until the beginning of the Jurassic, when the ornithiscians finally began to catch up, diversifying and getting big.
During that time, the carnivores might have dined on Revueltosaurus and its relatives. As riveting as their caloric value might have been back then, there are very different aspects of interest now. In addition to not being dinosaurs, they’re a “totally unrecognized group of possibly herbivorous crocodilians,” says fossil-finder Parker. He, Irmis, and their team unearthed a second relatively complete skeleton and bones from a dozen or more other individuals over two digging seasons at the same site.
Revueltosaurus’s other ties to Berkeley go back to the 1930s, when paleontology professor Charles Camp found teeth from the creatures before they had been identified. The species was named in 1989, when Adrian Hunt, now director of the New Mexico Museum of Natural History and Science found teeth in Revuelto Creek in that state. The following year, Kevin Padian, a professor of integrative biology here, found and identified Revueltosaurus teeth from Petrified Forest National Park, a dozen years before Randall Irmis started working the terrain as an undergrad at Northern Arizona University.
Padian’s fossils, and the teeth that Camp found earlier, are housed in the UC Museum of Paleontology on this campus. The specimens that Irmis and crew excavated are now part of the Petrified Forest National Park collections.
This summer, Irmis will be off to a new dig, this one in northern New Mexico, in an area he expects will yield “several early dinosaur species — probably new — as well as many other vertebrates.” Which will fit just fine with the focus of his dissertation: the early evolution of dinosaurs.
His site is near to, and several million years older than, the Coelophysis quarry of Ghost Ranch, one of the better-known landmarks in paleontology. Dinosaur bones turned up there in the 1880s, (during the great race to name species, dubbed the Bone Wars), and in 1947 hundreds of fossilized skeletons of the small carnivorous late Triassic bipedal dinosaur Ceolophysis were found in one astonishing natural graveyard. Ghost Ranch is more widely famed as the longtime home of the late Georgia O’Keeffe, who often painted the nearby landscape and was fond of including bones in her work. Earlier this year, coincidentally, a new crocodilian relative was found lingering anonymously in storage at New York’s American Museum of Natural History. Museum curator Mark Norell and Columbia University graduate student Sterling Nesbitt were looking for Coelophysis fossils in the collection, excavated in the late 1940s from Ghost Ranch. A plaster casing contained a previously unknown archosaur, which Norell and Nesbitt named Effigia okeeffeae, honoring Ghost Ranch, the fossil’s years hidden away, and Georgia O’Keeffe.
Sterling Nesbitt, before heading off to Columbia, was an undergrad majoring in integrative biology and minoring in geology — at Berkeley. His faculty mentor was Kevin Padian. And what is Nesbitt doing this summer? Excavating. At Ghost Ranch. With Randall Irmis.
Few stories these days are without multiple Berkeley connections.