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Re: A Jurassic 'beaver'



> They cite three references for the 164 million year date:

...of the Jiulongshan Fm. But it may belong to the Yixian Fm instead...

Interestingly, however, the beastie is a docodont! If it is EK, that makes
it the youngest known docodont in the world by some 20 million years.

Anyway:
Full ref:
Ji Qiang, Luo Zhexi, Yuan Chongxi & Alan R. Tabrum: A Swimming Mammaliaform
from the Middle Jurassic and Ecomorphological Diversification of Early
Mammals, Science 311, 1123 -- 1127 (24 February 2006)

Abstract:
"A docodontan mammaliaform from the Middle Jurassic of China possesses
swimming and burrowing skeletal adaptations and some dental features for
aquatic feeding. It is the most primitive taxon in the mammalian lineage
known to have fur and has a broad, flattened, partly scaly tail analogous to
that of modern beavers. We infer that [all???] docodontans were semiaquatic,
convergent to the modern platypus and many Cenozoic placentals. This fossil
demonstrates that some mammaliaforms, or proximal relatives to modern
mammals, developed diverse locomotory and feeding adaptations and were
ecomorphologically different from the majority of generalized small
terrestrial Mesozoic mammalian insectivores."

Quite interesting dentition. The rear molars (numbers 3 to 6) are like those
of other docodonts, while the first two are like those of triconodonts,
seals or early whales (five cusps in a straight line).

An "extratarsal spur" is present.

"The proximal 25% of the tail is covered by guard hairs, the middle 50%
mostly covered by scales with sparse hairs, and the distal 25% by scales
interspersed with guard hairs."

"The forelimb of Ornithorhynchus is adapted to digging and also used for
rowing during swimming and diving [...]. It has been hypothesized that the
docodontan Haldanodon was semiaquatic [...]. From the additional evidence of
Castorocauda, it appears that many docodontans were burrowing mammals with
sprawling limb posture and gait in terrestrial locomotion. They may have
also used the forelimbs for rowing during swimming, as an exaptation [...],
as in the platypus."

The size is that of a not too large female platypus.

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