The fossils of two new mammals discovered in China, estimated to be 160 million years old, are found to exhibit an unusual mosaic of highly specialized characteristics, including adaptations for gliding. The findings, reported in two papers published online this week in Nature, indicate distinct character combinations previously unknown in ancient mammals.
Mesozoic ancestors to mammals, which lived around the same time as dinosaurs, provide critical evidence regarding anatomical evolution and ecological diversification during the earliest mammalian history. These early mammals nonetheless developed many features that are commonly found among mammals today. The progression of gliding behaviour is an important evolutionary transition between divergent land-based and aerial habitats.
Zhe-Xi Luo, Qing-Jin Meng and colleagues describe the fossilized skeletons and skin membranes from two new gliding mammals, named Maiopatagium furculiferum and Vilevolodon diplomylos, from the Jurassic Tiaojishan Formation of China. Maiopatagium and Vilevolodon are haramiyids - the earliest known herbivore mammals and the most primitive known gliders in mammalian evolution - and both evolved approximately 100 million years before the earliest known mammalian gliders. The fossilized wing membrane and fused wishbones of Maiopatagium are reminiscent of those found in birds, whereas the shoulder girdles are more consistent with the modern egg-laying platypus than mammals or marsupials. Most similar in appearance to the modern flying squirrel, Maiopatagium furculiferum exhibits evolutionary adaptations similar to that of certain tree-dwelling marsupials and mammals that give birth to live young.
Vilevolodon diplomylos is described in detail in a second paper. Its tooth replacement patterns are unique to most other early mammals, and its molars, with a shape reminiscent of dual mortar and pestles, were probably used for dry crushing and grinding soft plant tissue and seeds. These fossils are therefore the first known gliding, herbivorous stem mammaliaforms associated with pre-angiosperm (that is, non-flowering) plants.