Friday, 14 July 2006

Shell-less shale snail

Now this is way cool. (Oh dear. Did I just say that aloud?) It’s not often that molluscs make it into Nature but they slid into the latest journal. Why? Because Odontogriphus, that enigmatic beast of the Burgess Shale, turns out to be a mollusc.

I wouldn’t be at all surprised if you hadn’t heard of Odontogriphus. It’s played not so much second fiddle as seventh bassoon to its more famous associates in the Cambrian fauna. (Hallucigenia, anyone?) Many of the blobby fossils show few characteristics that could place them in any known group of animals. When studied, only the toothed, looped structure at the front end provide any clues to their relationships. It looked a bit like a lophophore—the tentacled feeding apparatus found in bryozoans, brachiopods and phoronids. But it also looked a bit like the dentate elements of conodonts, an odd bunch that belong with us in the phylum Chordata. But it didn’t fit well with either gang of animals.

After examining almost two hundred specimens, the international team of palaeontologists and malacologists realised that the toothy band was a radula, the rasp-like ‘tongue’ characteristic of molluscs. It also had a broad foot and multiple paired gills in a mantle groove. They’d finally cracked the identity of one of the more mysterious animals of the Cambrian explosion.

(And yes, Australia has its own odontogriphoid. Late Permian lake deposits near Blackwater in eastern Queensland contain Bowengriphus They’re lying in a shale deposit that has been baked into clinker by intense heat, either from a fire beneath the coal seam or by a local igneous intrusion. Nothing’s straightforward about these things!)

Read more

Caron, J-B, Scheltema, A., Schander, C. & Rudkin, D. (2006). A soft-bodied mollusc with a radula from the Middle Cambrian Burgess Shale. Nature 442: 159–163.

Ritchie, A. & Edgecombe, G.D. (2001). An odontogriphid from the Upper Permian of Australia. Palaeontology 44: 861–874.

No comments: