Toothless Crocodile Relative, 210 Million Years Old, Walked On Two Feet
Credit: Sterling Nesbitt
Two American Museum of Natural History paleontologists who recently opened the plaster jacket encasing a fossil collected in the 1940s in a well-known quarry full of dinosaurs found the remains of an animal that strongly resembles bird-like dinosaurs called ornithomimids, or ostrich dinosaurs. But upon further examination, they were surprised to discover that it is actually a much older crocodile relative that lived at least 80 million years before ostrich dinosaurs even emerged. The new animal is called Effigia okeeffeae, with the generic name meaning "ghost" and referring in part to the fact that it was effectively invisible to science for so long, and the specific name honoring the artist Georgia O'Keeffe, who spent a great deal of time in the region of New Mexico where the fossil was initially unearthed. The finding of this toothless, dinosaur-like crocodylian relative reveals new information about the evolution and diversity of reptiles on Earth. The finding also illustrates the principle of convergence in evolution, whereby organisms or animals that are only distantly related adapt similar body forms and types. In this case, the similarity between the new crocodylian relative and ostrich dinosaurs is quite strong despite the fact that the animals are only distantly related.
The new finding is described in the journal Proceedings of the Royal Society B by Sterling J. Nesbitt, a graduate student enrolled at Columbia University who studies at the American Museum of Natural History and also is affiliated with Columbia's Lamont-Doherty Earth Observatory, and Mark A. Norell, Curator in the Museum's Division of Paleontology.
The six-foot-long specimen (and three other partial skeletons of the same animal also described in the new research paper) is from the Triassic Period and is about 210 million years old. It was discovered at the famous Ghost Ranch Quarry, which was originally excavated in 1947 and 1948 by an American Museum of Natural History team led by the famous Museum paleontologist Edwin Colbert. Until recently, scientists thought that specimens of Coelophysissmall, carnivorous dinosaurs with a relatively simple body planwere the only vertebrate fossils in the quarry. However, scientists lately have reported evidence of many different kinds of reptiles at the site.
Credit: Sterling Nesbitt
E. okeeffeae is closely related to the crocodylians, an ancient group of reptiles that includes today's crocodiles and alligators. Crocodylians generally conserve their body forms across evolutionary time. This new crocodylian relative proves an exception, with a skull and skeleton that is identical in many ways to those found in ostrich dinosaurs. Some of the traits shared with the ostrich dinosaurs include large eyes, a beak, a long tail, and no teeth. Also, both walked on two feet, not four as living crocodiles do. (A crocodile-like ankle is the diagnostic feature that Effigia shares with crocodilians.) Along with other similarities in their bodies, this convergence suggests that E. okeeffeae and ostrich dinosaurs, which appeared at least 80 million years later, lived in the same type of habitat and occupied very similar ecological resource zones.
E. okeeffeae also solves a mystery. In 1993, Sankar Chatterjee of Texas Tech University named Shuvosaurus from an isolated skull found in Texas that was about 210 million years old. Chatterjee recognized the animal's strange features and hypothesized that the animal was a very early ostrich dinosaur. However, many paleontologists questioned this idea and pointed out features of ostrich dinosaurs that Shuvosaurus did not share with them. Now, Mr. Nesbitt and Dr. Norell have compared Effigia and Shuvosaurus and found that they share many features. It appears that Shuvosaurus, like Effigia, is more closely related to crocodylians than it is to dinosaurs. A cladistic analysis of the evolutionary relationships among Effigia, other crocodylians, and dinosaurs, including ostrich dinosaurs, also supports this new understanding of Shuvosaurus as a crocodylian relative.
Noticing that Effigia also resembles early theropod dinosaurs, Mr. Nesbitt and Dr. Norell also reexamined some isolated Triassic reptile specimens and found that Effigia-like animals were common in most of the Triassic sediments of western North America. This finding, along with the discovery of an Effigia relative from the Triassic in Argentina, suggests that dinosaurs were even more rare than thought during the Late Triassic, that Effigia-like animals were widespread in the Americas, and that dinosaur evolution only took off after Effigia went extinct, leaving a particular ecological resource zone available for later and larger dinosaurs to exploit.
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