Images: Monash Science Centre, fossil; DRAGA GELT, reconstruction TINY FOSSILIZED JAWBONE of a diminutive shrew-like animal a mere 8 centimeters long (right; jawbone area in white) is shaking the established family tree of mammalian evolution. This possibly placental creature, named Ausktribosphenos nyktos, apparently scurried out of the way of dinosaurs in what is now Australia during the Cretaceous--some 110 million years before the accepted appearance of such mammals there.

In the 23 years that paleontologists Thomas H. Rich of the Museum of Victoria and Patricia Vickers-Rich of Monash University have headed an effort to find the origins of Australia's unique mammals and birds, they have learned to expect the unexpected. But when excited field workers at a dig near Melbourne showed Tom Rich a fossil of a tiny mammalian jawbone on March 8, 1997, it was his jaw that dropped. As the implications of the discovery sank in, Rich looked up from the microscope with a smile breaking across his face. All he could say was, "My God."

Image: ANDRE COFFA CAUTIOUS ELATION. Tom Rich, lead author of the paper, looks up from his microscope after examining the A. nyktos jaw for the first time.

Rich's astonishment confirmed what the digging team had suspected since the fossil was discovered earlier that day: this bone simply should not be in Australia. The jawbone, barely 16 millimeters long, did not bear the characteristics of an egg-laying monotreme, such as the platypus; nor did it resemble the jaws of pouched marsupials, such as kangaroos and wombats. Instead, it seemed to be the jaw of a placental mammal--the group that includes horses, cats, bats, whales and people.

The trouble is that the fossil was wrested from Cretaceous rock that was 115 million years old while well-established theory holds that the first placental mammals in Australia were island-hopping rodents that arrived a mere 5 million years ago. "The case that terrestrial placentals had not reached Australia until 5 million years ago was so strong," Rich recalls, "that the thought of them being in Australia this early never crossed my mind."

The researchers' conclusion that terrestrial placental mammals may have lived down under 110 million years earlier than expected, as reported in the November 21, 1997 issue of Science, could all but uproot the mammalian family tree. Based on the fossil record, it is generally assumed that mammals arose some 200 million years ago, when the continents were still joined together in a single landmass called Pangaea. By the Cretaceous period (146 to 65 million years ago), the continental configuration had changed, and two supercontinents arose: Laurasia in the north and Gondwana in the south.

Current theory holds that the southern hemisphere saw the emergence of the egg-laying monotremes, which then crossed into South America, where they eventually died out. Marsupials, whose young continue their growth in the mother's pouch, seem to have originated in North America. And placentals, whose young form entirely inside the mother, arose in Asia.

DISCOVERY SITE

But the fossil unearthed at the Flat Rocks site near Inverloch apparently lived in Gondwana--unexpectedly far south of the Asian placentals. Rich suspects that the animal would have been only about 8.5 centimeters long and a "generalized insectivore." He cites the spineless hedgehog as the closest modern analog to this ancient creature. It lived in a temperate valley among the polar dinosaurs." Because these warm-blooded creatures had to endure the darkness of winter in the polar regions, Rich's group gave it the official name of Ausktribosphenos nyktos, the "Australian Cretaceous tribosphenic mammal that lived by night."

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