Earliest tree-dweller, burrower join mammal tree of life

Meet two newly discovered ancestral mammals:  the oldest known subterranean specialist from the depths of mammalian history and the group’s oldest known tree-dweller.

Fossils of both, found in northeastern China, belong to an extinct group of mostly small creatures called docodonts, researchers report in a pair of papers in the Feb. 13 Science.  Docodonts, sometimes not considered strictly mammals, branched off early from the ancient lineage that eventually gave rise to modern mammals.

The fossil of what looks like a shrew-sized specialized burrower, now named Docofossor brachydactylus, is estimated to be about 160 million years old. It “had a supercapacity to dig but was not very good at much else,” says Zhe-Xi Luo of the University of Chicago, an author on both papers.

What he and his colleagues think was an arboreal creature, Agilodocodon scansorius, dates from about 165 million years ago. Its skull is not much longer than a penny. “These are spectacular fossils,” Luo says. They preserve fine details such as the curved claws that helped make the case that Agilodocodon climbed and scurried deftly among branches, Luo says.

These unusual docodonts join other finds from recent years that, taken together, have upset long-standing prejudices about mammal ancestors, Luo says. “There was this stereotype that they did not diversify much because dinosaurs were supposed to be overpowering,” he says. Paleobiologists had long had low expectations for ancient mammal precursors: envisioning them as mostly generalists chasing whatever insects they could catch at night and staying out of dinosaurs’ way. Yet fossils have been turning up — such as a beaverlike, semiaquatic docodont described in 2006 — that suggest mammal ancestors were moving into and adapting to a wide range of habitats while dinosaurs were still alive.

If so, then these early members of a lineage that sprouted modern mammals may already have evolved some of the specializations and oddities that underlie today’s mind-boggling shrew-to-elephant-to-whale-to-giraffe array of mammal shapes and habits. The tree-dwelling docodont, for instance, inspired comparisons with modern arboreal mammals. Agilodocodon’s lower inscisors resemble the spade-shaped or spatula-like ones of some modern New World monkeys that gnaw on tree bark and eat sap, the researchers suggest. The oldest ancestral mammal to frequent trees may have eaten them too.

The newly discovered burrowing docodont has fewer segments in the digit bones in its almost shovel-like digger paws than standard hands and paws do. Docofossor’s specialized paws anticipate adaptations that later arose in modern burrowers called golden moles.

The fossils hint that Docofossor may already have had some of the same kind of genetic controls shaping its body that modern mammals have. Docofossor paws look much like developmental quirks in modern mammals attributed to variations in GDF and BMP genes, the reseachers say. And the way the rib cage ends around the waist looks like transitions sculpted in modern mammals with Hox 9-10 and Myf 5-6 genes.

These papers make “a compelling link” between modern studies of the evolutionary genetics of developing embryos and fossil interpretation, says Robert Asher, a paleobiologist at the University Museum of Zoology in Cambridge, England, who studies mammal evolution.