BioEd Online

New evidence from 100-million-year-old jawbones found in Australia suggests that egg-laying mammals such as the platypus may have evolved more slowly than other mammals, researchers say.

Egg-laying mammals, called monotremes, are generally divided into two groups: the duck-billed, flat-footed platypuses, and the roly-poly, spiny anteaters called echidnas. Both are native to Australia.

Both are also poorly represented in the fossil record, so researchers wanting to trace the monotreme family tree have relied heavily on molecular data. By counting the number of differences in the sequence of a given gene or protein found in platypuses and echidnas, researchers can estimate how long ago the two groups split. The results have varied depending on which gene or protein was studied, and estimates have placed the split anywhere from 17 million to 80 million years ago.

But some researchers wondered whether an ancient creature called the Teinolophos, which lived in Australia more than 100 million years ago, might be an early platypus. If so, it would push the date of the split much farther back.

From examinations of Teinolophos skeletons, it was unclear whether the creature was a platypus or some general monotreme ancestor. So Timothy Rowe of the University of Texas in Austin and his colleagues put three of the skulls through high-resolution X-ray tomography scans to investigate.

X-rayed snout

The scans revealed an enlarged canal that runs through the jaw, the shape and size of which is indicative of platypuses but not echidnas. In modern animals, this canal supplies a platypus's sensitive bill with nerves and blood. Examinations of the animals' teeth likewise showed features common in platypuses but not echidnas.

That suggests that the split between these two groups happened before this creature came into existence, more than 100 million years ago.

How then to reconcile the two results? The time of the platypus–echidna split was calculated based on the rate of sequence changes observed in other mammals. But if monotremes evolved more slowly than other mammals, as Rowe suggests, researchers could have underestimated the time it took for mutations to accumulate.

“It looks like the monotremes may have had a really slow evolutionary history,” says Rowe.

Slow pokes

What exactly put the breaks on monotreme evolution is unclear, however. Rowe points out that monotremes generally have slow metabolisms and long generation times, both of which could theoretically slow the rate of evolutionary change. And if they were very well suited to their ecological niche, this would also be expected to have reduced the selective pressure to change.

Rowe thinks the creatures probably didn't need to evolve because their hunting abilities were so fine-tuned: the platypus bill is laced with highly sensitive nerve endings for detecting electrical signals emitted by prey.

David Wake, an evolutionary biologist at the University of California, Berkeley, says the find is "really significant". But he has a different solution to the jawbone conundrum: “I’m not convinced it’s a platypus,” he says. “If you have to put it somewhere, you have to put it with a platypus, but we don’t know anything about all the other ancient monotremes that once existed.” Future fossil discoveries may shift its assignment, he cautions.

It is possible, for example, that the canal has evolved twice – once before the split and then again afterwards. If that were the case, the Australian jawbones may yet predate the split between platypuses and echidnas.