Today's placental and marsupial mammals range in form from mice and whales to koalas and kangaroos. Yet despite their considerable differences, these creatures share a striking dental similarity: they all have so-called tribosphenic teeth, specialized mortar-and-pestle-like molars that enable them to slice and grind their food efficiently. The third group of living mammals, the monotremes (represented today by the platypus and the echidna), lack teeth, but their ancestors, it seems, had the tribosphenic variety. The morphology of tribosphenic molars is so complex that researchers had largely assumed that they emerged only once in evolutionary history. A study published today in the journal Nature, however, suggests that in fact it happened twice on different continents, painting a radically different picture of mammal origins.

Paleontologist Zhexi Luo of the Carnegie Museum of Natural History and his colleagues analyzed the teeth, jaws and skeletal remains of more than 20 mammal species, both extinct and extant. Whereas conventional wisdom holds that tribosphenic mammals arose on the northern supercontinent, Laurasia, the new results reveal two distinct tribosphenic lineages. One, the team reports, evolved on Laurasia and subsequently gave rise to the placentals and marsupials. The other arose on the southern landmass, Gondwana, and led to the monotremes.

As to the likelihood of the shrew-like ancestors of today's mammals evolving tribosphenic teeth in parallel, Duke University researcher Anne Weil writes in a commentary accompanying the Nature report, "This view of events does not require as unlikely a convergence as it might seem. Early mammals were small and endothermic (loosely speaking, warm-blooded), with high surface-to-volume ratios. They probably had high metabolic rates and correspondingly high nutritional requirements. Living shrews, which face the same constraints, have prodigious appetites. So survival probably depended on efficient food processing, and the tribosphenic dentition provides just that."