Geologists say this valley may once have been as far south as 75 degrees south latitude—only 15 degrees from the South Pole. The average annual temperature would have been about freezing—possibly rising as high as 70 degrees Fahrenheit in summer and plummeting to less than –10°F in the winter.
Despite cold, snow, and three months of darkness every winter, life thrived here, as it does today in parts of Alaska with a similar climate. Forests of tree ferns, conifers, cycads, and ginkgos with trunks three feet across covered the valley floor. Hundreds of bones found here reveal that wallaby-size hypsilophodontid dinosaurs nibbled on undergrowth, as did club-tailed ankylosaurs, which reached the size of small cars. Large, two-legged meat eaters stalked through snow looking for small prey, or they hibernated. Egg-laying monotreme mammals related to modern-day platypuses scurried through the bushes. Pterosaurs and primitive birds glided above, and rivers teemed with turtles, fish, and fish-eating plesiosaurs.
Tom Rich and his colleagues believe the half-inch-long jawbone belonged to a 115-million-year-old placental mammal that lived in what is now Australia. Teeth are crucial for fossil identification because they are both durable and closely regulated by genes. Reptilian teeth, for example, are simple pointed cones; mammalian teeth are distinctive for the basins and cusps that form complex interlocking upper and lower molars. The inset at right shows 15 features on the jawbone’s molars. One reason Rich believes the jawbone belonged to a placental mammal is that it has three molars and one smaller, somewhat similar tooth called a premolar. Marsupial mammals have four molars. |
Kool stops to point out black-flecked sandstone at the base of the cliff. The flecks are bits of carbonized wood that accumulated in a riverbed. “It looks like some annual flooding event,” she says, noting five layers in the speckled rock. “Each layer is deposited directly above the previous one. It’s quite possible they were laid down in five consecutive years.”
The layers slope downward into the waves. Farther into the waves lies a spot where hundreds of bones settled, perhaps because a sandbar slowed the ancient river’s current. Since the first jawbone was found here in 1997, Rich’s team has chipped at least 28 more from the rock. Many of the jaws come from two creatures Rich has deemed placental mammals—Ausktribosphenos nyktos and Bishops whitmorei.
No other type of bone from a mammal has turned up here. Rich says his hunch is that “we have a Samson effect. Samson slew a thousand Philistines with the jawbone of an ass. It’s the toughest bone in the body.” For mammals, jawbones are the remains most likely to last millennia.
Rich’s trove of jawbones are not the only clues that placental mammals could have originated in Gondwanaland. In 1999 a team from the Field Museum of Natural History in Chicago and the University of California at Santa Barbara found a 170-million-year-old jaw in Madagascar that looked as if it belonged to a predecessor of both placental and marsupial mammals. And in 2001, a team from the Museo Paleontológico Egidio Feruglio in Argentina unearthed an intriguing jaw from 160-million-year-old rocks in Chubut, Argentina. Although the Argentine team suspects that the bone came from an egg-laying mammal, other paleontologists believe it shows traits of a placental mammal. These fossils are every bit as assumption-shattering as Rich’s. They turned up at sites on landmasses that once belonged to Gondwanaland—sites where they should not be if placental mammals arose in the northern landmass of Laurasia.
Molecular studies also turn up results that don’t fit with a Laurasian origin for placental mammals. When geneticists compare DNA sequences of the world’s placental mammals, the results cluster into a family tree with four major groups. By studying variations within the DNA of each group, researchers approximate how long it took the groups to diverge and develop as distinct entities. Their calculations suggest that the oldest two groups emerged around 100 million years ago, predating the other two groups by 10 million years. Surprisingly, these two older groups are made up of mammals that are found in South America and Africa. They include animals such as elephants and sloths.
No clear-cut fossil evidence of these older South American and African groups has been found in Laurasia. And even if these older groups once had predecessors on Laurasia that have not yet been found, it would have been impossible for the creatures to migrate to the southern landmasses that long ago because the continents of Gondwanaland and Laurasia made contact only about 80 million years ago.
These inconsistencies can be resolved by the simple hypothesis that placental mammals originated in Gondwanaland, not Laurasia, says mammalogist Tim Flannery, director of the South Australia Museum in Adelaide.
“The earliest branches are in the Southern Hemisphere,” he says, “so that’s where you’d start looking for the origin of placentals. And just coincidentally, here we have these fossils in the south that look like placental mammals.”
