When a paleontologist wishes to define a new species ‘dem bones is all ‘dere is. Unlike with animals living today, paleontologists can’t look into the past to document an ancient beast’s physiology or mating habits. Using all of the information available, paleontologists must confront the fossil world reality that the classification of a new fossil species is subjective and varies among taxonomists.
But how much different do the bones of similar animals have to be for the classification of a new species?
That is the question that drove Stephanie Novak, a new doctoral candidate in the Department of Earth and Planetary Sciences in Arts & Sciences at Washington University in St. Louis, to develop a novel model to determine classification of a new species. Novak presented details of her model at the annual meeting of the Geological Society of America, held Nov.2-5 in Seattle.
Her discovery began in 2002 while Novak pursued her master’s degree at the University of North Carolina at Chapel Hill. She was studying a fascinating critter that competed with early dinosaurs 220 million years ago but went extinct at the boundary of the Triassic and Jurassic eras. Postosuchus (post-o-SOOK-us), though appearing superficially to resemble a classic meat-eating dinosaur with a huge skull and powerful flesh-tearing jaws, was actually a member of the Rauisuchia (raw-ih-SOO-kee-a), the dominant terrestrial predators during the Middle and Late Triassic, and the “Porches” of their time. The beast moved mainly on four legs and looked like, as Novak refers to it, “an alligator on stilts.” It thrived at a time when there were no hardwood trees, grass, or flowers and dinosaurs were just coming onto the scene. Postosuchus is estimated to have reached a length of 25 feet, and was distinguished from dinosaurs by hip structure and a special ankle structure enabling it to walk heel-toe as alligators and crocodiles do today, as opposed to the on-the-tips-of-the-toes-walking observed in dinosaurs.
Novak investigated a Postosuchus specimen excavated from the Coelophysis Quarry of Ghost Ranch, New Mexico, a famous locality teeming with dinosaur fossils (mainly Coelophysis bauri) as a result of a mass death. While comparing the Ghost Ranch specimen with the two specimens of Postosuchus kirkpatricki from Texas, she noticed some differences in the bones. Not sure whether these differences were numerous enough or skeletally important enough to make the Ghost Ranch Postosuchus a new species, she decided to do a little more research before making a final decision.
Because the rauisuchian fossil record is generally sparse, Novak instead dove into the dinosaurian fossil record in attempts to quantify the amount of skeletal difference historically regarded as valid to erect a new species within the same genera. She analyzed 28 genera containing 68 species from both the saurischian (lizard-hipped) and ornithischian (bird-hipped) orders. Using the fact that the skeleton of a dinosaur generally contains approximately 338 different bones, she catalogued the number of differences as well as where the differences were found on the skeleton. Calculations indicated that, on average, two species of dinosaur that are members of the same genera varied from each other by just 2.2 percent. Translation of the percentage into an actual number results in an average of just three skeletal differences out of the total 338 bones in the body. Amazingly, 58 percent of these differences occurred in the skull alone.
“This is a lot less variation than I’d expected,” said Novak, whose advisor is Josh Smith, Ph.D., Washington University assistant professor of earth and planetary sciences. “As a concept, this is not statistically perfect. But I think it’s something taxonomists can consider if they are in doubt over classifying something. It’s a kind of benchmark with historical validity.”
Novak was able to determine, using her Archosaurian Morphospecies Concept, that the Ghost Ranch Postosuchus was indeed the same species, Postosuchus kirkpatricki, as the two specimens from Texas.