Dawn of the Dinosaurs: Paleontologists Probe the Majestic Reptiles' Origin and Rise

Article excerpt

Any 10-year-old knows how the dinosaurs met their end: A huge meteorite slammed into Mexico's Yucatan Peninsula 65 million years ago, blasting the planet beyond anything imagined by Bruce Willis in Armageddon.

But neither kids nor Hollywood have spent much time thinking about how dinosaurs appeared in the first place. "We know a heck of a lot more about the extinction of dinosaurs than their origins," says Randall Irmis, a paleontologist at the Utah Museum of Natural History and the University of Utah in Salt Lake City.

Lately, though, new discoveries have begun to flesh out the script of dinosaurs' earliest days. From ghostly footprints in a Polish quarry to the bones of a pint-sized predator in Argentina, these findings tell a more complete and nuanced tale of dinosaur genesis. Dinosaurs, it turns out, were not predestined to rule the planet for more than 130 million years.

Instead, dinosaurs appeared on the scene tentatively, perhaps as early as 250 million years ago. Paleontologists are now unearthing fossils so primitive that they may be close to the common ancestor at the root of the dinosaur family tree--the "ur-dinosaur," as Paul Sereno of the University of Chicago puts it. Ingloriously, that creature probably looked something like a chicken: two-legged, scurrying around in the corners, snapping up plants, insects, small animals and whatever else passed its way.

Other new findings come not from dinosaurs themselves, but from some of their closest relatives. Just as a long talk with an estranged cousin can help fill in family history, these discoveries are illuminating how dinosaurs evolved alongside other animals.

"We're really in a renaissance in our understanding of early dinosaur history," says Stephen Brusatte, a paleontologist at the American Museum of Natural History in New York City.

In the beginning

A famous extinction event may have wiped most dinosaurs out, but another extinction, just as spectacular, made way for their inauspicious rise. It occurred 252 million years ago, at the boundary between the Permian and Triassic periods of geologic time.

Scientists aren't sure what caused the Permo-Triassic extinction; there's no impact crater to serve as a "smoking gun" like there is for the close of the dinosaur era. But Earth was roiled by huge environmental changes, from massive volcanic eruptions in Siberia to radical shifts in ocean chemistry. For whatever reason, 90 percent of marine species and 70 percent of land species went extinct.

That left the slate clean for dinosaurs to arise in the early part of the Triassic. "You had your status quo basically wiped out, and new groups had the opportunity to originate and flourish in this post-apocalyptic world," Brusatte says.

Beyond dinosaurs, those new groups included the ancestors of creatures that would look familiar today, such as lizards, frogs and salamanders. These animals--in particular, the crocodile-like creatures known as crurotarsans- were far more abundant than the first dino pip-squeaks. "If you were standing in the Triassic, you would say these crocodile-like animals, not dinosaurs, would expand and be dominant" in the eons to come, says Brusatte.

Triassic Park was not exactly Jurassic Park. But dinosaurs were around, and new research hints at when they first appeared and what they looked like.

Fossil footprints from Poland's Holy Cross Mountains provide the oldest clue. Last fall Brusatte and his colleagues, including University of Warsaw paleontologist Grzegorz Niedzwiedzki, reported finding three sets of tracks, the oldest dating back 250 million years. That's right after the Permo-Triassic extinction.

Only several centimeters long, these tracks were made by a four-legged creature no bigger than a house cat. But certain characteristics of the footprints are distinctly dinosaur-like, Brusatte says.

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Among other traits, the outer digits-the first and fifth toe--are smaller than the others, and the long bones of the foot bunch together more closely than in non-dinosaurs. …