Dinosaurs Benefited from Mass Extinction
|New fossil suggests dinosaurs not so fierce after all
The new dinosaur, named Sarahsaurus, was studied by an international team of scientists, including Robert R. Reisz, professor and chair of biology at the University of Toronto Mississauga, Tim Rowe, professor of paleontology at the University of Texas at Austin's Jackson School of Geosciences and Hans-Dieter Sues, curator of vertebrate paleontology at the National Museum of Natural History in Washington, D.C. The results of this research appeared in the online edition of the journal Proceedings of the Royal Society B on Oct. 6.
"Until recently, we've viewed dinosaurs as very successful animals that outcompeted other species wherever they went," says Reisz. "But this study puts dinosaurs in a very different light — that they were more opportunistic creatures that moved into North America only when a mass extinction event made eco-space available to them."
Conventional wisdom says that soon after dinosaurs originated in what is now South America, they rapidly spread out to every corner of the world, overwhelming all the animals in their path. Sarahsaurus challenges that view.
Sarahsaurus lived in what is now the state of Arizona about 190 million years ago, during the Early Jurassic Period. The remains show that it was a 4.3-metre-long bipedal plant-eating animal with a long neck and small head, and weighed about 113 kilograms. Sarahsaurus is a sauropodomorph dinosaur, a relatively small predecessor to the giant sauropods, the largest land animals in history.
A team of researchers and students led by Rowe discovered an articulated skeleton of this creature during a field trip in Arizona in 1997. The team excavated the site over three years, exposed the skeleton in the Austin lab, but was stymied in the research because little of the skull was preserved.
Reisz and Sues had been working on a sauropodomorph skull from the same area in Arizona, and were ready to submit a paper describing and naming this new dinosaur, when they realized that the skull they were examining and the skeleton discovered by Rowe were the remains of the same species. Working together, the three scientists were able to put together their findings from different parts of the skeleton, and discover its evolutionary significance.
Oldest evidence of dinosaurs found in Polish footprints
"We see the closest dinosaur cousins immediately after the worst mass extinction," says Stephen Brusatte, a graduate student affiliated with the Division of Paleontology at the American Museum of Natural History. "The biggest crisis in the history of life also created one of the greatest opportunities in the history of life by emptying the landscape and making it possible for dinosaurs to evolve."
The new paper analyzes three sets of footprints from three different sites in the Holy Cross Mountains of central Poland. The sites, all quarries within a 25-mile radius of each other, are windows into three ecosystems because they represent different times periods. The Stryczowice trackway is the oldest at 250 million years. The Baranów trackway is the most recent at 246 million years of age while the Wióry trackway is sandwiched in time between the others.
Because footprints are only an imprint of a small part of the skeleton, identification of trackmakers is often tricky. Luckily, dinosaurs have a very distinctive gait, especially when compared to their diapsid relatives (the evolutionary group that includes birds, reptiles, and extinct lineages) like crocodiles and lizards. While lizards and crocodiles have a splayed walking style, dinosaurs place their two feet closer together. The footprints at all three Polish sites show this feature as well as indisputable dinosaur-like features, including three prominent central toes and reduced outer two toes, a parallel alignment of these three digits (a bunched foot), and a straight back edge of footprints, additional evidence of a dinosaur-like simple hinged ankle.
These dinosaurs, though, are considered "stem dinosaurs," or the immediate relatives of dinosaurs not part of the slightly more derived clade that technically defines dinosaurs. Also, this animal did walk on all four limbs, an abnormal posture for early dinosaurs and their close relatives, although it appears that its forelimbs were already being reduced to more dinosaur-like proportions since the footprints overstep handprints.
The Baranów and Wióry trackways show changes early in the evolutionary history of dinosaurs. Wióry at 248 million years ago shows slight diversification in the types of tracks, but all tracks remain quadrupedal. Footprints from Baranów at 246 million years ago, however, may be the earliest evidence of moderately large-bodied and bipedal true dinosaurs. These tracks, which are called Sphingopus, are 15 centimeters long.
"Poland is a new frontier for understanding the earliest evolution of dinosaurs," says Grzegorz Niedźwiedzki of the University of Warsaw and the Polish Academy of Sciences, who led the project and has been excavating footprints from the three sites for nearly a decade. "It used to be that most of the important fossils were from Argentina or the southwestern U.S., but in Poland we have several sites that yield footprints and bones from the oldest dinosaurs and their closest cousins, stretching throughout the entire Triassic Period."
Finally, although the dinosaur group emerged soon after the Permian extinction, dinosaur-like tracks are rare in the footprint assemblages, representing only 2 percent of the prints discovered as opposed to 40 percent for crocodile-like archosaurs. Dinosaurs became more abundant tens of millions of years later.
"For the first 20 million years of dinosaur history, dinosaurs and their closest relatives were living in the shadow of their much more diverse, successful, and abundant crocodile-like cousins," says Brusatte. "The oldest dinosaurs were small and rare."