During the 46-million-year Ordovician Period (489–443 mya), a phenomenal array of adaptive radiations of "Paleozoic- and Modern-type" biotas appeared in marine habitats, the first animals walked on land, and the plants appeared.
This Great Ordovician Biodiversification Event represents a tripling of diversity at the family level, and a quadrupling at the genus level, from those in the Cambrian.
The number of families reached by the end of the Ordovician remained fairly constant (except for a number of mass extinction events) for almost 200 million years. Articulate brachiopods, conodonts, graptolites molluscs, crinoids, and trilobite groups all became established of diversified greatly during this time.
A major question is what happened to the Cambrian faunas - as typified by the Burgess Shale, the 'soft-bodied', or poorly mineralised, organisms that are found in numerous Cambrian deposits. They are by-and-large absent from the Ordovician.
We do have a large number of Cambrian sites with exceptional preservation (the Bugress Shale being the ‘type’ example), but from the Ordovician, not so much. In fact very few. In fact, bugger all!
What Ordovician exceptional preservation we have comes from the middle and late Ordovician, the Beecher’s Trilobite Bed, New York, the Soom Shale, South Africa, Winneshiek, Iowa, and a couple of localities in Manitoba, Canada.
All localities represent restricted faunas from extreme environments (low oxygen) and not examples of more diverse, open marine environments. (Yes, the Burgess Shale may well represent a low oxygen environment, but the biota it contains represents a diverse open marine environment.)
No examples of exceptional preservation have been found from the early Ordovician.
Given this extreme lack of exceptional preservation, our understanding of this great biodiversification event is almost entirely based on the shelly fossil record. What we get, is a spectacular rise in the shelly fossil record.
This leads to the obvious question. What happened to the Cambrian critters?
There are two main theories to explain that wholescale change in faunas. One is that the Cambrian forms were replaced by Ordovician forms. The other is that the exceptional preservation we see in Cambrian deposits simply does not occur in the Ordovician, and so the missing forms are due to a taphanomic, or preservational, bias. So which is it? replacement of preservational?
Introducing the Fezouata biota from the lower Ordovician of Morocco.
This represents the first site of exceptional preservation from the lower Ordovician, and (if that wasn’t important enough) the first from a ‘normal’ open marine environment.
Over 50 different taxa have been collected so far indicating an open marine, deep water assemblage that has been trapped beneath, or brought in by, storm deposits. Limited bioturbation indicates low oxygen which would account for the exceptional preservation. The fauna is a mix of typical Ordovician forms and typical Cambrian Burgess Shale-types, although there are similarities with the Chengjiang fauna from China, both in terms of fossils and depositional environment.
One cool find is a xiphosurid (a horseshoe crab - everyone's favourite chelicerate). The horseshoe crab fossils are the oldest yet found, suggesting that their roots may dip into the Cambrian. What is really cool is that one of the two species of horseshoe crab has a fully segmented opisthosoma (it is either partially or fully fused in other forms), suggesting evolution from a segmented ancester. The other has a fully fused preabdomen (quite a derived feature) and is similar to existing forms (though not the same.)
This has major implications for the transition from Cambrian fauna to the Palaeozoic fauna represented by Ordovician forms. Namely that the Cambrian critters hung on for a considerable time after the end of the Cambrian and intermingled with Ordovician forms. So there was no wholesale replacement at the end of the Cambrian. Both sets mixed.
However it is interesting to note that this deep water deposit was close to the South Pole during lower Ordovician times. The deposits represent a deep, cold water environment. Cambrian deposits with exceptional preservation, by contrast, were primarily from low latitudes, near the paleoequator, and from shallow water.
It may be that the Fezouata biota represents a refuge for the Cambrian forms - much like modern brachiopods are excluded from tropical areas and are mainly found in temperate and cold deep water today.
So this biota shows that Cambrian forms persisted well into the Ordovician, but a shallow water site of exceptional preservation is needed to see if the Cambrian critters were mixing it with the Ordovician upstarts, or were just hanging on along the margins.
Thanks to Paul H.
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