A COMPARISON OF THREE EARLY ORDOVICIAN LAGERSTäTTEN

Many early and middle Cambrian Lagerstätten are known, and are striking in the similarity of their faunas on widely spaced palaeocontinents. It is not known if the same pattern applies in the Ordovician, as Ordovician Lagerstätten are rare and usually represent atypical faunas. The recently discovered Early Ordovician (Tremadocian-Floian) Fezouata Biota contains a typical Ordovician community, along with Burgess Shale-type elements and taxa more characteristic of modern faunas (Van Roy et al. 2010). However the Fezouata formations were deposited at high latitude, and the faunas may not be typical for the time. Two recently discovered Lagerstätten of the same age as the Fezouata Biota, both from lower latitudes, offer for the first time the opportunity of understanding the distribution of Ordovician soft-bodied taxa.

One of the new Lagerstätte is from the Dol-cyn-Afon Formation (late Tremadocian) of North Wales, UK. Initial collecting has found a diversity of over thirty species. The fauna is dominated by worms, sponges, inarticulate brachiopods and algae, with rarer trilobites, tergomyans, unmineralised arthropods, articulate brachiopods, one species of echinoderm, and hyolithids. This fauna is strikingly different from the Fezouata Biota, and lacks many typical Ordovician groups.

The second Lagerstätte occurs in the early Floian Tonggao Formation from near Sandu, Guizhou Province, China. This deposit contains a diverse (at least 50 species) assemblage of dendroid and graptoloid graptolites, algae, articulate and inarticulate brachiopods, echinoderms, trilobites, gastropods, hyolithids, agglutinated tubes, ceratiocarids and other unmineralised arthropods. This assemblage is partly similar to those in the Fezouata formations, but lacks the Burgess Shale-type elements.

These new faunas indicate that by the Early Ordovician there were already substantial differences in the broad composition of open shelf communities, in contrast to the early and middle Cambrian.

Reference

Van Roy, P., Orr, P.J., Botting, J.P., Muir, L.A., Vinther, J., Lefebvre, B., el Hariri, K. & Briggs, D.E.G., 2010. Ordovician faunas of Burgess Shale type. Nature, 465, 215-218.