Twice upon a time: jaw fossils point to multiple origins of the most mammalian of features

Science News,  March 8, 2008  by Amy Maxmen

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Tom Rich has an eye for finding bits of skulls in unlikely places. In January, he and his team reported finding a slight groove in a half inch-long jaw. Using a modified CT scanner, the researchers scrutinized the fossil they had unearthed in Australia a few years earlier. Reviewing the images of the jaw's structure, Rich and collaborators saw the groove and realized they held what remained of a duck-billed platypus out of place in the age of dinosaurs.

The jawbone's groove gave away its owner's identity because living platypus bills bear notoriously wide grooves equipped with nerves to sense their prey in fresh water. But this grooved jaw belonged to a platypus from a time when mammals supposedly were all simple, shrewlike creatures that scurried around the shadows of T. rex,. Platypuses, however--mammals with rubbery duckbills, water-repellent fur, beaver-like tails, and webbed feet--certainly aren't plain.

In the past decade, new fossil finds have contradicted long-held views of the simplicity, of primitive mammals. They seem to have been just as motley and specialized for life on the land or in water as today's mammals are. This emerging view of mammal history suggests not only that complex features evolved millions of years earlier than previously thought, but also that they might have evolved independently in different groups.

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For centuries, related animals have been defined by "key innovations" that presumably allowed lineages thereafter to diversify. But some researchers now believe that these defining characteristics might actually have been commonplace and thus relatively easy to achieve--challenging the long-held notion that, as Yale paleontologist Jacques Gauthier asserts, "Complicated systems do not evolve willy-nilly."

Reports of advanced early mammals, such as Rich's platypus, suggest otherwise. The most contentious recent findings even downgrade the refined mammalian middle ear and ridged molars to less than novel. Though no reptile or other vertebrate has ever evolved these intricate hearing and chewing apparatuses, some rabble-rousing paleontologists allege that they popped up multiple times within the mammals.

It's a controversial idea: Evolutionary biologists invoke parsimony when they assume the fewest number of changes occurred during animal history, unless fossil evidence indicates otherwise. Kangaroos, wallabies, and wombats--all marsupials--bear a pouch, and so do their fossil relatives. So biologists infer that a marsupial ancestor acquired the pouch once, not three separate times. Since the fossil record is incomplete, there has been every reason to think for nearly a century that complicated structures, like the molar and middle ear, evolved one time. Not many.

Tremors shaking the old consensus began ten years ago with digs in the Southern Hemisphere, in hard-to-mine, remote lands in Antarctica and Australia. Before then, fossils of the earliest ancestors of two of the three surviving mammalian lineages had been found in Northern Hemisphere fossil beds. Dating to 144 to 119 million years ago (early in the Cretaceous period) these two lineages include the placental group, who nourish their young in a uterus through a placenta (dogs, whales, and humans are examples), and the pouch-bearing marsupials. Since members of both groups birth live young, paleontologists inferred that the groups descended from a common ancestor on the northern supercontinent Laurasia, which broke apart to form Asia, Europe, North America, and Greenland. And, millions of years later, some individuals from either line made their way south.

That was the accepted story until 1997, when Rich and his colleagues, working in southeastern Australia, dug up their first contentious jaw--a 120 million-year-old fossil belonging to a mammal named Ausktribosphenos nyktos. Though the jaw measured barely over half an inch, its features led the team to describe Ausktribosphenos as an early placental mammal. Finding a fossil of a placental mammal in the Southern Hemisphere dating back to the early Cretaceous, cast doubt on the Laurasian northern-origin hypothesis, Rich, of the Museum Victoria in Australia, and his colleagues reported in a 1997 Science paper.

BIG BITE What really challenged existing thinking, however, was a hefty molar embedded in the lower jaw of Ausktribosphenos. It was the type of tooth that distinguishes modern marsupials and placental mammals from other mammals. Yet 120 million years ago, most mammals didn't have molars. The teeth on most fossil jaws from this time were pointed. Crocodilelike, the teeth swiped past one another like blades on a pair of scissors, slicing crunchy bugs.

But Ausktribosphenos' tooth was clearly--and problematically-the modern crushing molar, Rich reported. Until this find, it was thought that molars developed later. Over time, the pointy back teeth would fuse in a triangular pattern, forming thicker teeth with raised projections where the apex of points used to be. The projections ill upper teeth interlocked with the lower teeth, like pestles into mortars. Once formed, the elaborate teeth, called tribosphenic molars, could crush, pulverize, and grind. Mammals with this molar could munch on plants or animals.