Sunday, May 24, 2009

Palaeoporn 13

Look up there! Is it a brachiopod? Is it an annelid? Is it a mollusc?

Umm  . . . actually, its a bit of each really.

This is a wiwaxiid from the Emu Bay Shale. Unfortunately undescribed, but closely linked to Wiwaxia and the halkieriids. It would be one of the oldest examples of a wiwaxiid, as the others are from the Middle Cambrian Burgess Shale and comparable deposits.

The beast was bilaterally symmetrical, oval in shape, and covered with short scales - called sclerites. Also present were a number of rows of larger spines which protruded upwards in a probable defensive array (see reconstruction at right)

Wiwaxiids had a flat foot-like underside. Little is known of the internal anatomy.

Wiwaxiids have been classified as molluscs, annelids and stem group annelids (a group closely related to annelids). The spines have been compared to the eltrya, or scales, of polychaete worms, and even bristles of molluscs, annelids and brachiopods, and halkieriids, of course, have little brachiopod shell caps!

One feature found in wiwaxiids is a radula-like feeding bar. So I looked to see if my specimens had a radula-like feeding bar. I've switched to black and white photos for higher resolution. A bar structure, formed of calcium phosphate, was found towards the front of the specimen. After preparing out (lower image) half the bar remained and the other half (the distal, or outer, part of the bar is removed, leaving a mold of the lower surface of the bar. in this mold can be seen several depressions along the 'upper' margin, represented by shadow (the light is coming from the top right of the image). These are 'teeth' which would have protruded from the lower margin of the bar (the bar is approx. 5 mm long.)

So, spines, radula-like feeding bar, seems like a Lower Cambrian wiwaxiid!

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