Yay, more involved invertebrate anatomy I don’t really understand but can’t resist writing about!
In other words, Martin Smith, one of the guys who likes to argue that a number of Cambrian weirdos are actually molluscs, published new stuff about, guess what, Cambrian weirdos he considers molluscs.
Above: the delightfully odd Wiwaxia by Nobu Tamura, from Wikipedia.
I don’t think he and JB Caron had much luck with the Nectocaris-is-a-cephalopod idea, but maybe he’s on to something with Odontogriphus and Wiwaxia. I mean, one of the big complaints about Nectocaris being a proto-squid was that none of the 90-odd specimens preserve any mouthparts at all, and mouthparts are normally among the toughest and most recognisable features of cephalopods. This time in the Proceedings of the Royal Society (Smith, 2012), there are mouthparts galore. The paper is about mouthparts.
Above image from the paper features an electron micrograph, closeup and drawing of a specimen belonging to Odontogriphus, showing two kinky rows of hooked teeth that were probably all embedded in one chunk of tissue in life (they tend to stay together in the fossils). They look kinda freaky, but more importantly, Smith argues, they look remarkably like a simple version of a radula. Which is a very mollusc thing to have – nothing else alive today has one. Most living molluscs sport more complicated radulae than this, with many more than the two or three tooth rows found in the Cambrian creatures in question, but hey, no one said modern radulae suddenly popped into existence fully formed. If they are molluscs, the strange Odontogriphus and Wiwaxia are probably among the early branches within the phylum, which may well mean that their ancestors said bye to the rest of the molluscs before the first hopeful monster with more than three rows of teeth hatched from a slimy clutch of proto-mollusc eggs.
Another major interpretation of these creatures, especially Wiwaxia, is as something close to annelid worms (ragworms, earthworms etc.). Their mouthparts have previously been claimed to resemble dorvilleid annelid jaws, a lovely example of which is shown on the right of this image. (Warning: closeups of polychaete faces are not for the faint-hearted ;)) However, Smith argues that the structure of Odontogriphus and Wiwaxia tooth rows is nothing like that of worm jaws. For example, they consist of teeth that can rotate relative to one another whereas the worm jaws in question have teeth sitting in fixed rows, and the teeth are apparently shed and replaced in completely different ways. Not to mention that the living worms have paired jaws, whereas each tooth row of the Cambrian critters appears connected in the middle. Of course, Smith says they aren’t connected in the same way that some other annelid jaws they’ve also been compared to are.
So, Team Mollusc has delivered some toothy punches, but I’m sure this is not the final word on something as old and weird as Odontogriphus and Wiwaxia. Holding my breath for the responses of Team Worm and Team Grandaddy of Molluscs AND Worms…
(BTW, I recall that Wiwaxia, in addition to its mollusc mouth, also has scaly armour that looks suspiciously like a bunch of fused annelid bristles. Dunno how accurate that information is today, since IIRC I read it in a pretty old book that also had an agenda, but that just goes to show that so soon after all the Common Ancestors lived, family resemblances might get a bit muddled… And crazy thought, what if the common ancestor of annelids and molluscs – they are pretty close relatives, after all – had a radula? Annelids today don’t, but you never know. It wouldn’t be the first time an animal lost something complex in the history of evolution.)
Smith MR (2012) Mouthparts of the Burgess Shale fossils Odontogriphus and Wiwaxia: implications for the ancestral molluscan radula. Proceedings of the Royal Society B, advance online publication, doi: 10.1098/rspb.2012.1577