Thursday, 30 August 2012

New Research Sheds Light On The True Nature Of Early Mollusks And Their Teeth

Go into any garden and turn over a log or flower pot and underneath, in the damp soil, you will find a multitude of small arthropods: woodlice, ants and beetles. Which scuttle away into the undergrowth as their cool dark home is disturbed. Hard shelled arthropods, the descendants of some of the first ancient predators to cruise the seas of the primordial Earth, are the most common, but occasionally creatures with a far more ancient heritage can be found, namely the molluscs.

The 510 million year old fossil of Wiwaxia. The dark
mouth parts are just visible at the top o0f the fossil
Slugs and snails are usually relegated to disgusting ground feeders sheathed in a cocoon of mucus. Yet they carry a fascinating evolutionary history. A history which is understood for the first time, owing to incredible new studies of fossils which have been present in our collections for over 100 years.

A defining feature of nearly all molluscs is a structure called a radula. Homologous to a tongue, a radula consists of hundreds of sharp, interlocking teeth used to feed. In slugs and snails, its purpose is to simply grind its way through vegetable matter. In larger creatures such as the squid, its purpose is a little more gruesome: scraping the flesh and muscle from prey. Such a distinctive organ must have been present in the earliest molluscs.

It is likely that Kimberella, one of those strange organisms which are part of the Ediacara biota, was the first of its kind. Scratch marks found close to the head in some fossils suggest that it may have had a scraping structure similar or possibly even a precursor to a radula. Yet no actual organ has been preserved on account of the owner's soft-bodied nature. As a result, the origin of the radula remained quite mysterious until recently.

The electron micrograph image of the radula of Wiwaxia
Palaeontologists theorized that two Cambrian creatures, Odontogriphus and Wiwaxia, were ancestors of snails and slugs. Now analysis of old fossils recovered from the world famous, 110 million year old, Burgess Shale has changed this idea. 'I put the fossils in the microscope, and the mouth parts just leaped out,' said Martin Smith, a PhD candidate in University of Toronto's Department of Ecology and Evolutionary Biology.

'You could see details you'd never guess were there if you just had a normal microscope.' Previously, it was impossible to analyse complete fossils, only small samples of a material. Despite the fact that the fossils of Wiwaxia, a gumdrop shaped creature covered in armor plates and spines, and Odontogriphus, lozenge shaped with a shell on its back and a single muscular foot, were tiny, they were still too large to analyse as a whole.

The final model of the radula of Wiwaxia
Using a new form of non-destructive electron microscopy, Smith was able to easily identify the radula on both primitive creatures. 300 different fossils later, multiple micrographs for each fossil and lumps of plasticine, he ended up not just with an image of the complete radula, but how they grew. The final model consists of 2 to 3 rows of symmetrical teeth, an asymmetrical central tooth and smaller teeth around the edge of the entire structure.

To feed, the teeth moved back and forth along to scrape algae and detritus from rocks and the sea floor. 'When I set out, I just hoped to be a bit closer to knowing what these mysterious fossils were,' said Smith. 'Now, with this picture of the earliest radula, we are one step closer to understanding where the molluscs came from.'