Leave It To “Beaver” February 28th, 2006 by Mike Keesey :: see related comic

(Yes, I live in Hollywood.)

Say hello (and goodbye) to Castorocauda lutrasimilis Ji et al. 2006, a docodont mammaliaform from China, and the earliest example of a semi-aquatic synapsid. The holotype specimen is exquisitely preserved, including remains of fur all over the body and scales on the paddle-like tail. The name, which is pretty apt, means “otter-like beaver-tail”. Very cool discovery.

I’ve been disappointed with much of the press’ coverage of this animal, though. Some examples of headlines:

• Fossil Find Alters Evolutionary Thinking (Pittsburgh Post Gazette)
• Jurassic Beaver Swam With the Dinosaurs (ABS Science Online, Australia)
• Scientists Discover Early Mammalian Fossil (Pravda)

First of all, this animal does not really alter evolutionary thinking. That’s sensationalist hyperbole. It fits in quite comfortably with other (less well-preserved) docodonts, and does not alter our understanding of the phylogenetic tree. What it does alter is our ecological understanding of docodonts and Mesozoic mammaliaforms in general—we can no longer assume that every Mesozoic mammaliaform is scansorial (tree-climbing) or fossorial (burrowing).

Second of all, it’s not a mammal. Docodonts are very, very, very close (and have historically been considered mammals), but they lie just outside the monotromes + marsupials + placentals node. (Calling it a “beaver” sans quotation marks is, of course, much worse.)

A final problem (which is actually not the press’ fault) is that the Daohugou Beds may not be Jurassic. A dissenting opinion was put forward by He et al. (2004), positing that they are middle Early Cretaceous (Valanginian–Barremian). So it may not be the largest mammaliaform in the Jurassic, as reported by Ji et al. (The largest mammaliaform from the Cretaceous is Repenomamus.)

Nearly lost in all of this are two really cool things about Castorocauda. First of all, it’s the first direct evidence of fur (and lots of it) in a non-mammal. It had already been thought that close mammal relatives were hairy due to the existence of “whisker pits” in some skulls, and now it’s confirmed. Any time a scientific prediction is confirmed by direct evidence, that’s pretty cool in my book (especially if it’s confirmed by a spectacular fossil).

Second of all, as reported previously by Hurum et al. (2006), this animal has a spur on its hindlimbs suspicously similar to that of extant monotremes, the platypus (Ornithorhynchus anatinus) and the echidnas (Tachyglossidae). As Hurum et al. note, certain other non-therian mammaliaforms had this structure, the os calcaris, as well. In platypuses, this spur is associated with a venom gland. Hurum et al. conjecture that a poisonous spur may have been an ancestral character of mammals and their closest relatives, later lost in therians (marsupials + placentals). We may have had tiny, venomous ancestors.

(Don’t worry, Carney and Carnita are too big to be affected.)

References:

• Hurum, J. H., Z.-X. Luo, and Z. Kielan-Jaworowska. 2006. Were mammals originally venomous? Acta Palaeontologica Polonica 51(1):1–11.
• Ji, Q., Z.-X. Luo, C.-X. Yuan, and A. R. Tabrum. 2006. A swimming mammaliaform from the Middle Jurassic and ecomorphological diversification of early mammals. Science 311: 1123–1127. (PDF online)
• He H., Wang X., Zhou Z., Zhu R., Jin F., Wang F., Ding X., and A. Boven. 2004. ⁴⁰Ar/³⁹Ar dating of ignimbrite from Inner Mongolia, northeastern China, indicates a post-Middle Jurassic age for the overlying Daohugou Bed. Geophysical Research Letters 31, L20609.

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