Ancient crustaceans: acid bath reveals fossils in the early Cambrian

Image from Nature

It may not be obvious, but the creature on the right (Yicaris dianensis) is a big deal. Not in size; at its largest, it appears that Yicaris was only about a millimeter long. Rather, its location makes it noteworthy: it was found in rocks in the early Cambrian, suggesting that true crustaceans are quite a bit older than previous evidence had indicated. Its discovery was reported in this week's issue of Nature.

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Crustaceans are well represented in fossils younger than 500 million years, but are largely absent from finds that date to the early Cambrian. But there was evidence to suggest that they should be there; a closely related group, the Phosphatocopina, was present at the time, and some embryos with features that suggested a crustacean identity had also been found. The discovery of Yicaris, however, had to wait for someone willing to risk dissolving Cambrian rocks in acid and sift through the results with a microscope sufficient to detect the tiny creatures. The effort was well rewarded; the search picked up two additional arthropod species, and a number of Yicaris samples at various stages of development.

The early date of Yicaris' presence has a number of implications. For one, it suggests that crustaceans were already around before the fuse of the Cambrian Explosion had even been lit, as it were. While the Cambrian was once thought to mark the sudden emergence of many creatures, fossils like Yicaris and Halkieria have been hammering two points home: many of the phylogenetic groups once thought to have exploded onto the scene in the Cambrian actually existed before it, and the physical features of new fossils can establish connections (via common descent) between previously known groups.

One key feature in Yicaris appears to be an early version of the epipodite. In modern crustaceans, such as lobsters, the equivalents of these structures apparently form the gills that are attached to the base of the legs. In Yicaris, the epipodites appear as a bit of a hybrid structure that helps make it clear that they originated by the modification of an existing spiny structure. Epipodites are especially interesting from an evolutionary standpoint because it has been proposed that they also gave rise to the wings of insects.

Nature, 2007. DOI: 10.1038/nature06138

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