Cambrian survivors - weird critters which (temporarily) cheated extinction

When the Cambrian period comes up in conversation, it is usually in reference to the evolutionary "explosion" which occurred around 530 million years ago. Animal fossils from before that time are typically small or are only traces, but in the latter half of the period (spanning ~488 to 542 million years ago) there is a dramatic increase in the diversity and disparity of organisms. The "small shelly fauna" of earlier times is replaced by a riot of organisms interacting with each other in complex ecosystems - it is one of the most dramatic changes seen in the entire fossil record. What is often forgotten, however, is that many of these weird critters disappeared by the end of the Cambrian in an extinction which appears to have swept away much of what had so recently (from a geological perspective) evolved.

That seemed like the best explanation, anyway, but there was one problem. We know of the existence of many strange Cambrian creatures because they have been recovered from sites of exceptional preservation which fossilized them with soft parts intact. Such sites are virtually unknown from the time right after the Cambrian, during the early days of the Ordovician period (~488-432 million years ago), making it difficult to know precisely when some of the extinct lineages disappeared. Now, as luck would have it, such an Ordovician site has been found in Morocco, and among the exceptionally-preserved fossils within it are creatures thought to have vanished millions of years before.

The details of this Ordovician site are described in this week's issue of Nature by paleontologists Peter Van Roy, Patrick Orr, Joseph Botting, Lucy Muir, Jakob Vinther, Bertrand Lefebvre, Khadija el Hariri, and Derek Briggs. Located in southeast Morocco, the 488-472 million year old deposits represent what was once a deep ocean bottom which was only disturbed by the most severe storms. These storms rapidly covered the organisms which lived on or accumulated on the sandy bottom, preserving them in a similar manner to what is seen in the famous Early Cambrian Chengjiang site in China.

Just what those creatures were is not yet entirely clear. The preliminary estimate provided by the authors states that about 1,500 specimens representing at least 50 different taxa have been collected so far, and while the scientists have recovered a number of organisms previously known from the Ordovician the most surprising discoveries have been those of Cambrian "holdovers" which share affinities with the wonderful fauna of the Burgess Shale. The enigmatic halkieriids, once thought to be constrained to the Cambrian, now appear to have an Ordovician representative, and another fossil is very similar to the strange Burgess Shale arthropod Marrella. Other "Cambrian type" creatures just as strange as their names - wapkiids, demosponges, palaeoscolecids, naraoiids, and others - also lived alongside the more typical Ordovician organisms, meaning that the vestiges of this deepwater habitat combined Cambrian stragglers with new lineages which were undergoing their own diversification.

No doubt the new Moroccan fossils will be closely scrutinized and compared to what is already known, but if the assessment of the authors is correct then we have to rethink what was previously called the Cambrian mass extinction. The absence of species into the Ordovician may have more to do with a lack of suitable sites of preservation than true extinction. The pattern of evolution and extinction during this time was more complex than previously appreciated, and the discovery of this Fezouata fauna will do much to change what we thought we knew about this mysterious time in earth's history.

Van Roy, P., Orr, P., Botting, J., Muir, L., Vinther, J., Lefebvre, B., Hariri, K., & Briggs, D. (2010). Ordovician faunas of Burgess Shale type Nature, 465 (7295), 215-218 DOI: 10.1038/nature09038