Pushing Back Placental Mammals

Way back. A close relationship between zalambdalestids and rodents and rabbits strengthens the idea of earlier placentals.

When it comes to the origins of placental mammals, fossils and DNA rarely see eye to eye. Some molecular clocks based on genetic differences estimate that major lineages began to split and diversify as far back as 100 million years ago. But the oldest undisputed fossils of placental mammals--those that bear live young rather than lay eggs or gestate them outside the womb like marsupials--date to only 65 million years ago. Now a team of paleontologists argues that an 85-million-year-old extinct mammal called a zalambdalestid is placental, which may help reconcile the molecular data with the fossils.

Five years ago, paleontologist David Archibald at San Diego State University proposed that a different group of 85-million-year-old mammals called the zhelestids was placental, closely related to the lineage that led to the hoofed mammals (Science, 24 May 1996, p. 1150). The claim remains controversial. But zhelestids aren't the only Cretaceous mammals with a resemblance to modern placental groups. In the 1960s, a paleontologist first proposed that the zalambdalestids have placental characteristics based on fossil teeth. But these suspicions were never tested in a rigorous way.

Archibald and his co-workers have now discovered 40 new specimens of a chipmunk-sized zalambdalestid known as Kulbeckia kulbecke in the Uzbekistan desert. In the 1 November issue of Nature they report that when they compared 70 features of the new skeletons with those of other fossil mammals, they found that zalambdalestids are more similar to extinct rabbits and rodents than to fossils of hoofed placentals or extinct nonplacentals. This suggests that by the time Kulbeckia lived, placental mammals had already begun to diversify--just as molecular estimates predict.

"It's a hypothesis that merits widespread attention," says Richard Cifelli of the Oklahoma Museum of Natural History. But he and others caution that more bones are needed before the timing of placental diversification can be pinned down for sure. Says Zhexi Luo of the Carnegie Museum of Natural History: "This is genuine progress, but not the final word."

Related sites

David Archibald's Web site
A primer on mammal evolution

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