Wednesday, May 19, 2010

Burgess Shale animals live(d) on...

The Burgess Shale is perhaps the iconic illustration of the Cambrian Explosion, which began 543 million years ago and saw the appearance of most modern animal body plans in the fossil record. What makes the Burgess Shale so important is its preservation of impressions of soft tissue, unlike most fossils which (unsurprisingly) represent only hard parts like shells and bones. After the Middle Cambrian, though, much of the soft-bodied Burgess fauna disappears.

New fossil finds from Morocco's Fezouta Formations, however, shows that much of the Burgess Fauna in fact survived into the Lower Ordovician, which is the period that followed the Cambrian. This means that the apparent disappearance of the Burgess Fauna was mostly due to the difficulty of preserving soft tissue, and not their actual extinction. The picture above (which made the cover of Nature) shows an arthropod from Fezouta that resembles the Burgess arthropod Marrella which was among the fossils popularized by Stephen Jay Gould in his book about the Burgess Shale, Wonderful Life.

This find is significant because many of the fossils represent so-called 'stem groups'. Stem-group fossils are related to known groups of modern organisms but diverged from the lineage before the common ancestor of those modern species. Hence they can tell us facts about the early evolution of those groups that cannot be inferred from studying modern members alone. The persistence of Cambrian stem groups into the Ordovician may hence redraw our estimates for when some of the major branches of the animal tree of life diverged, and how animal life diversified during Earth's history.

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