If you asked paleontologists what the earliest ancestors of dinosaurs were like, most would put their money on the animals resembling miniature dinosaurs: small, meat-eating animals that walked on two legs. But in a paper published in Nature scientists describe the earliest known dinosaur relative: a six-foot-long lizard-like carnivore called Teleocrater rhadinus.
"Teleocrater has unexpectedly crocodile-like features that are causing us to completely reassess what we thought about the earliest stages of dinosaur evolution," says Ken Angielczyk, The Field Museum's associate curator of fossil mammals and one of the paper's authors. "Surprisingly, early dinosaur relatives were pretty profoundly not dinosaur-like."
This is the new species Teleocrater rhadinus hunting a cynodont, a close relative of mammals.
The finding, published in the journal Nature April 12, fills a critical gap in the fossil record. Teleocrater, living more than 245 million years ago during the Triassic Period, pre-dated dinosaurs. It shows up in the fossil record right after a large group of reptiles known as archosaurs split into a bird branch (leading to dinosaurs and eventually birds) and a crocodile branch (eventually leading to today's alligators and crocodiles). Teleocrater and its kin are the earliest known members of the bird branch of the archosaurs.
"The discovery of such an important new species is a once-in-a-lifetime experience," said Sterling Nesbitt, an assistant professor of geosciences in the College of Science.
Life model of the new species Teleocrater rhadinus, a close relative of dinosaurs, preying upona juvenile cynodont, a distant relative of mammals.
Credit: Museo Argentino de Ciencias Naturales
Teleocrater fossils were first discovered in Tanzania in 1933 by paleontologist F. Rex Parrington, and the specimens were first studied by Alan J. Charig, former Curator of Fossil Reptiles, Amphibians and Birds at the Natural History Museum of London, in the 1950s.
Largely because the first specimen lacked crucial bones, such as the ankle bones, Charig could not determine whether Teleocrater was more closely related to crocodylians or to dinosaurs. Unfortunately, he died before he was able to complete his studies. The new specimens of Teleocrater, found in 2015, clear those questions up. The intact ankle bones and other parts of the skeleton helped scientists determine that the species is one of the oldest members of the archosaur tree and had a crocodylian look.
Nesbitt and co-authors chose to honor Charig's original work by using the name he picked out for the animal, Teleocrater rhadinus, which means "slender complete basin" and refers to the animal's lean build and closed hip socket.
Scientists excavate the remains of Teleocrater rhadinus and other animals in southern Tanzania in 2015. Christian Sidor (left), Sterling Nesbitt (middle left), Kenneth Angielczyk (upper right), Michelle Stocker (lower right).
Credit: Roger Smith
"This research sheds light on the distribution and diversity of the ancestors of crocodiles, birds, and dinosaurs," says Judy Skog, program director in the National Science Foundation's Division of Earth Sciences, "and indicates that dinosaur origins should be re-examined now that we know more about the complex history and traits of these early ancestors."
cousins existed and went extinct before dinosaurs even appeared in the fossil record.
The team's next steps are to go back to southern Tanzania this May to find more remains and missing parts of the Teleocrater skeleton. They will also continue to clean the bones of Teleocrater and other animals from the dig site in the paleontology preparation lab in Derring Hall.
Life reconstruction of the new species Teleocrater rhadinus, a close relative of dinosaurs.
Credit: Museo Argentino de Ciencias Naturales
"It's so exciting to solve puzzles like Teleocrater, where we can finally tease apart some of these tricky mixed assemblages of fossils and shed some light on broader anatomical and biogeographic trends in an iconic group of animals," said Stocker.
The fact that Teleocrater has lots of features in common with crocodilians gives us a new picture about how dinosaur-like features evolved. "We used to think that many of the distinctive features of bird-line archosaurs evolved very quickly after they diverged from the crocodile line because early bird-line archosaurs like Marasuchus, Dromoeron, and Lagerpeton were small and very dinosaur-like," explains Angielczyk. "However, Teleocrater shows us that bird-line archosaurs initially inherited many crocodile-like features from the common ancestor of all archosaurs, and that the 'typical' bird-line features evolved in a step-wise fashion over a longer period of time. Scientists generally don't love the term 'missing link,' but that's kind of what Teleocrater is: a missing link between dinosaurs and the common ancestor they share with crocodiles."
Just as Teleocrater fills in a gap in the fossil history, its official description in this paper represents the missing piece of a long academic investigation. "While Teleocrater is being formally named for the first time in this paper, it was first identified by a British paleontologist named Alan Charig in the 1950s, using fossils that were collected in the 1930s," says Angielczyk. "In his thesis, Charig informally identified fossils of an early lizard-like animal that he called Teleocrater, but the understanding of dinosaur evolution at the time was such that he didn't recognize the connection with dinosaurs." So Teleocrater remained largely a mystery until Angielczyk and his colleagues discovered more specimens on a recent dig in Tanzania.
"We found fossils that we thought might be from Teleocrater, but it wasn't until we were back in the lab that we realized we'd found something really amazing," says Angielczyk. The team's paper officially gave the animal Charig's name, Teleocrater rhadinus, meaning "slender complete basin," a reference to the animal's lean build and closed hip socket.
"The discovery of such an important new species is a once-in-a-lifetime discovery," says Sterling Nesbitt of Virginia Polytechnic Institute and State University, the paper's lead author. "Teleocrater fundamentally changes our ideas about the earliest history of dinosaur relatives."
Lindsay Key, Virginia Tech