Early Aquatic Mammal

doi:10.1126/science.311.5764.1068b

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Mesozoic mammals have been thought to have been small, nocturnal, and confined to a few niches on land until the demise of the dinosaurs 65 million years ago. Most are recorded by isolated jaw fragments or teeth. Ji et al. (p. 1123; see the cover and the Perspective by Martin) now describe a Jurassic mammal from China that breaks this mold. The fossil is well preserved, and impressions of fur can be seen on its body and scales on a broad tail (similar to a beaver overall). The animal was fairly large, approaching not quite half a meter in length, and the shape of its limbs suggest that it was adapted for swimming and burrowing. The combination of both primitive and derived features in this early mammal, and the demonstration that mammals had occupied aquatic habitats by this time, expands the evolutionary innovations of early mammals.