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Hot Topic Origins Origin & Evolution of Life Dinosaurs Benefited from Mass Extinction
 
Dinosaurs Benefited from Mass Extinction
Based on a University of Torontoand American Museum of Natural History news releases
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Origin & Evolution of Life
Posted:   10/08/10

Summary: A newly discovered species of dinosaur may show that dinosaurs spread throughout the world by taking advantage of a natural catastrophe that wiped out their competitors. The study provides an example of how extinction events have shaped the evolution of Earth's biosphere.
New fossil suggests dinosaurs not so fierce after all

An artists' illustration of the Sarahsaurus -- a newly discovered dinosaur species. Image Credit: Nicola Wong Ken, University of Toronto
A new species of dinosaur discovered in Arizona suggests dinosaurs did not spread throughout the world by overpowering other species, but by taking advantage of a natural catastrophe that wiped out their competitors.

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.

This is a reconstruction of cat-sized stem dinosaur Prorotodactylus isp. found in Stryczowice, Poland that was a quadruped with a dinosaur-like gait and orientation of the toes. Credit: Grzegorz Niedźwiedzki
One of the five great mass extinction events in Earth's history happened at the end of the Triassic Period — about 200 million years ago — wiping out many of the potential competitors to dinosaurs. Evidence from Sarahsaurus and two other early sauropodomorphs suggests that each migrated into North America in separate waves long after the extinction and that no such dinosaurs migrated there before the extinction.

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

The 246 million year old footprints of Sphingopus isp. from the Early Anisian of Baranów, Poland are associated with a trackway that is even more dinosaur-like in that the gait was bipedal.These tracks are the oldest record of a large-bodied (track length 15 cm) and bipedal member of the dinosaur lineage. Credit: Grzegorz Niedźwiedzki
The oldest evidence of the dinosaur lineage — fossilized tracks — is described this week in Proceedings of the Royal Society B. Just one or two million years after the massive Permian-Triassic extinction, an animal smaller than a house cat walked across fine mud in what is now Poland. This fossilized trackway places the very closest relatives of dinosaurs on Earth about 250 million years ago — 5 to 9 million years earlier than previously described fossilized skeletal material has indicated. The paper also described the 246-million-year-old Sphingopus footprints, the oldest evidence of a bipedal and large-bodied dinosaur.

"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.

The 250 million year old footprints of Prorotodactylus isp. from the Early Olenekian of Stryczowice, Poland show reduced digits I and V and parallel three middle digits, traits of the dinosaur-lineage. The gait, though, was quadrapedal. These are the oldest known fossils of the dinosaur lineage. Credit: Grzegorz Niedźwiedzki
Because all of these features are seen in footprints at the oldest site, Brusatte and colleagues conclude that the Stryczowice prints — which are only a few centimeters in length — are the oldest evidence of the dinosaur lineage.

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."

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