A polyphyodont, semiaquatic eutriconodontan mammal from the Early Cretaceous of China.

Diphyodonty (the presence of two generations of teeth) characterizes extant mammals and has long been regarded as pivotal to the evolution of other mammalian traits, including precise occlusion and determinate growth. Nevertheless, our understanding of the pattern of dental replacement in Mesozoic mammals is incomplete. Here, we report a nearly complete skeleton of an Early Cretaceous eutriconodontan that is the only mammal to our knowledge that appears to have multiple generations (polyphyodonty) at some loci in the four tooth types. In the auditory apparatus, its ossified Meckel's cartilage has slight contact with the mandible, supporting the interpretation that Meckel's separated from the mandible before it separated from the middle ear ossicles. The new fossil shows adaptations for semiaquatic life, with hands and feet like the platypus that were similarly webbed. Its unusual pattern of dorsoventrally flattened tail bones resembles the extant nutria (coypu), a rodent native to South America. Reappearance of polyphyodonty and molariform replacement in the new fossil and related gobiconodontids is probably associated with their ecomorphological niche and carnivorous diet.