Reptomammal

Paleontologists know very little about that critical time, nearly 200 million years ago, when reptiles took the road that turned them into mammals, and eventually into man. They may know more soon. In the Navajo Reservation in northern Arizona, an Indian Service agent found an outcrop of fossil-bearing rock. Driving down from Denver to investigate, Government Geologist G. Edward Lewis found that the fossils were in the Kayenta Formation, a rock stratum that runs through Navajo country for hundreds of miles. For fossil fanciers this was big news: in the Kayenta Formation fossils are almost unknown.

Geologist Lewis chiseled out some blocks of rock and headed back to his laboratory. In one of the chunks he found a paleontological El Dorado: the skull of a part-reptile, part-mammal tritylodontoid, a transitional creature that lived about 165 million years ago when mammals were just evolving from reptiles. Only a few small tritylodontoid fragments have been found in the old world; none at all had been found in the new world.

After three trips to his fossil claim, Lewis has four skulls, two skeletons, and a lot of detached bones. The skulls are about 6 inches long and 2 1/2 inches wide, with both reptile and mammal features. It will take years of finicky work with delicate tools to separate the bones from the rock, but already Lewis can describe the animal roughly. It was about as big as a cocker spaniel, with a long, heavy tail. Lewis does not know yet whether it had hair or scales, whether it laid eggs or bore its young alive, or how it made its living. When he has completed his work, he hopes to know all these things about man's low but aspiring ancestor.

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