THE AFON GAM BIOTA – AN EARLY ORDOVICIAN BURGESS SHALE-TYPE FAUNA FROM WALES

In contrast to the Cambrian Lagerstätten, there are few known Ordovician exceptionally preserved biotas. They preserve a diverse range of environments and faunas, preventing meaningful palaeoecological comparisons between regions. The only currently described Ordovician Burgess Shale-type fauna (diverse, open-marine soft-bodied assemblage preserved in fine sediments) is the Tremadocian-Floian Fezouata Biota of Morocco, which yields a combination of Cambrian-like taxa, typical Ordovician groups, and unexpectedly advanced representatives of other groups. This shows that for non-biomineralised animals, the Great Ordovician Biodiversification Event (GOBE) had begun by at least the latest Tremadocian, and raises the possibility that the GOBE was only a continuation of the Cambrian Explosion, rather than a discrete phenomenon.

The Afon Gam Biota (early Late Tremadocian) is a new Burgess Shale-type fauna from North Wales, which slightly pre-dates the Fezouata Biota. The fossils include abundant algae and sponges, echinoderms, arthropods, worms, and shelly taxa such as lingulate brachiopods, tergomyans and hyolithids. Labile tissue remains include three-dimensionally pyritised sponge tissues, hyolithid muscle bases and gut, and brachiopod pedicles. Many of the families and genera recovered so far are typical of Cambrian biotas, including several genera of sponges (e.g. Choia, hazeliids, Valospongia), mollisoniid arthropods, and a variety of bivalved carapaces. Groups characteristic of the GOBE are almost entirely absent, and although sponges and echinoderms show evidence of diversification beyond Cambrian faunas, these mostly do not correspond to known later lineages. This confirms the separation of the Cambrian Explosion and the GOBE, and implies either a sudden beginning to diversification during the late Tremadocian, or rapid spread of derived faunas from lower latitudes.