Geologist Gives New Perspective on the Origin of Dinosaurs
Valley of the Moon, Ischigualasto, Argentina- For years, paleontologists and the public have focused on the question of what caused the dinosaurs to disappear, but the real question, according to Dr. William Sill, is how they arose in the first place, and he thinks he has found the answer here in the bright red lunar landscape of Ischigualasto. Sill, who conducted his research with the help of a grant and volunteers from the Earthwatch Institute, says that in the period just before the dinosaurs first showed up, the early Triassic, the world was dominated by warm-blooded, furry therapsids, which looked poised to become mammals. The dinosaurs progenitors were marginal, apparently inferior crocodile-like creatures. But then, by the late Triassic, everything changed. "Suddenly, Sill says of the late Triassic, "there are no more therapsids, and the world is full of dinosaurs. To paraphrase General Custer, we might ask, Where did all those dinosaurs come from?"
There have been many theories to explain this abrupt and meteoric rise of dinosaurs, but none has fully addressed all the questions. Sill believes that his theory does, and that it may offer a mechanism for evolution generally. The retired Curator of Palaeontology at the Museum of Natural Sciences in San Juan, Argentina, and professor at the university there, set forth his theory in an article published August 15th on the Earthwatch Institute website. Click here to read it.
Sill argues that the dinosaurs ancestors did not succeed in spite of being marginal, but because of it. Sill's theory applies Harvard biogeographer E.O. Wilson's theory of marginal habitats to the evolution of dinosaurs and mammals. Marginal environments, like arid scrublands and mountaintops above treeline, Sill says, offer inhabitants few resources, so survival is a challenge and natural selection operates at its most ruthless efficiency. The stress of life in marginal habitats accelerates the process of evolution, leading to the rapid development and diversification of new life forms admirably adapted to less-than-optimal environments. And, of course, animals that have developed to thrive in marginal habitats are more than capable of thriving in prime locations, so any disturbance in the status quo of the dominant lifeforms creates an opening for the new species. This, Sill suggests, is what happened with therapsids. The crocodile-like ancestors of dinosaurs had been honed by adversity, when some problem perhaps a drought or a major disease outbreak weakened the therapsids dominant position, and the dinosaurs took over. The therapsids, in turn, were relegated to the marginal habitats. In these scenarios, Sill proposes the new outcasts may repeat the cycle of adaptation and eventually return, leaner and meaner, to take over the dominant position. Or, like the therapsids, they may remain outcasts or die out.
Sill bases his theory on three decades of palaeontological exploration and excavation in the famed badlands of Ischigualasto or "Valley of the Moon" in north-central Argentina. This is the only known spot on Earth that harbors a complete sequence of Triassic-era sediments (from roughly 235 to 208 million years ago), when dinosaurs and mammals first appear in the fossil record.
Earthwatch teams first joined Sill at the site in 1994 and have helped unearth a host of early dinosaurs, like Eoraptor and Herreresaurus; ancient but anatomically modern, mammal-like reptiles, called therapsids; and crocodile-like protodinosaurs. Significantly, many of the protodinosaurs have turned up in what during the mid-Triassic period were marginal habitats. Sill campaigned for Ischigualasto to be included in UNESCO's list of World Heritage Sites, and in November of 2000, the World Heritage Committee granted it that status.
A native of Las Vegas, Nevada, Sill received his undergraduate degree from Brigham Young University and doctorate in geology from Harvard University in 1969. He was a curatorial associate at the Yale Peabody Museum from 1969 to 1971, when he moved to Argentina to become professor of palaeontology at the Universidad Nacional. He left Argentina from 1980 to 1988 to serve as senior lecturer at the University of Texas. Since then, Sill has been based in Argentina.
To view the popularized article about Sill's theory in the August issue of the Earthwatch Journal in text, click here.
For the graphical pdf version, which requires adobe acrobat, click here.
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