Jurassic Beaver Discovered

Petrel · Feb 24, 2006

Small beaver-like mammal found in Chinese Jurassic deposits.

The original article was published in Science, but I won't be able to get a look at that until Monday at the earliest.

The fossil was found in the Middle Jurassic Jiulongshan Formation, a deposit rich in the remains of dinosaurs, early insects and other organisms.

Like modern beavers, the creature had fur, a broad scaly tail, and webbed feet for swimming. It was about the size of a small female platypus and had seal-like teeth for eating fish.
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Petrel · Mar 4, 2006

Ji, Q.; Luo, Z.-X.; Yuan, C.-X.; Tabrum, A. "A Swimming Mammaliaform from the Middle Jurassic and Ecomorphological Diversification of Early Mammals." Science, 2006, 311, 1123-1127.

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.
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This mammaliaform animal has guard hairs, an inner coat of fur, and molars specialized for catching fish. The tail is somewhat flattened like a modern beaver's, although narrower. The upper portion of the tail is furred on the sides and scaled in the middle, although lower down the fur thins out and it is all scaley. The caudal vertebrae show a characteristic flattened, butterfly shape typical of animals adapted to swimming such as beavers and otters. There is a trace in the fossil that suggests webbing on the hind paws, and the forelegs are thickened and sprawling, probably originally for digging but now used for paddling as with the platypus. They have odd plated, overlapping ribs which were apparently common among mammaliaforms and cynodonts (a type of primitive reptile).

I'm going to quote what they say about the ear structure because I'm not sure what it all means and don't want to get it wrong:

Castorocauda is preserved with intact middle ear bones (Fig. 2) on the mandible, including the articular (malleus), the surangular, and the angular (ectotympanic). The middle ear bones in anatomical association with the mandible corroborate a previous interpretation of the middle ear in docodontans (26). A concavity on the posterior aspect of the mandibular angle accommodates the ectotympanic (angular). The posterior position of the ectotympanic concavity on the mandibular angle in docodontans is different from and more derived than that in the mammaliaforms Sinoconodon and Morganucodon, in which the ectotympanic concavity is on the medial aspect of the mandibular angle (6, 7). The manubrium of the malleus (retroarticular process of the articular) is anteriorly curved and long in comparison with the short manubrium of Morganucodon and Sinoconodon (6, 7, 27). The proportion of the malleus manubrium is similar to that of extant monotremes, although slightly more robust than in the latter. Castorocauda is similar to crown Mammalia and more derived than Sinoconodon, Morganucodon, and all premammaliaform cynodonts (27) in preserved middle ear features.
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Petrel · Mar 4, 2006

Fixed pic:

Jurassic Beaver Discovered

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