Castorocauda is the largest known Jurassic mammaliaform (including mammals). By its preserved skull length of 60 mm and the well-established scaling relation of skull and body mass (8, 41), we estimate that the body mass of the holotype specimen was at least 500 g. The preserved length from rostrum to tail is 425 mm, but the actual body length is certainly greater. The length of female platypuses with similar fossorial and semiaquatic habits ranges from 390 to 550 mm, corresponding to a body mass range of 700 to 2400 g. We estimate the upper limit of body mass to be approximately 800 g for Castorocauda. All other Jurassic mammals are small (1). Constrained by their small size, most were generalized terrestrial insectivores or omnivores. Previously, the largest taxon was Sinoconodon rigneyi (7); its largest individuals reached an estimated body mass of 500 g (8). Based on its relatively large size, swimming body structure, and anterior molars specialized for piscivorous feeding, Castorocauda was a semiaquatic carnivore, similar to the modern river otter. This fossil shows that basal mammals occupied more diverse niches than just those of small insectivorous or omnivorous mammals with generalized terrestrial locomotory features. Castorocauda also suggests that mammaliaforms developed physiological adaptations associated with pelage, well before the rise of modern Mammalia, and had more diverse ecomorphological adaptations than previously thought, with at least some lineages occupying semiaquatic niches.
