‘SOURCING HARDGROUNDS’ USING ECHINODERM FRAGMENTS TO RECONSTRUCT RELICT HARD SUBSTRATE COMMUNITIES FROM THE BRITISH BATHONIAN

The hard substrates of the British Bathonian are some of the oldest scientifically studied in the world. The preserved hardground of the Bradford Clay at Bradford-on-Avon (near Bath, England) was first described by William Smith in 1816, as being rich in insitu Apiocrinites parkinsoni. The rare examples of these allochthonous assemblages are very useful, as examination of echinoderms from them can be used to reconstruct possible hard substrate communities that would not normally be preserved. Similar faunas to those from the Bradford Clay are known from the bioclastic limestones of the Forest Marble formation preserved at Watton Cliff, Dorset, which are rich in cementing organisms.

In order to sample these deposits, approximately 40kg of the soft unconsolidated sections are removed and sieved; the resulting residues are rich in echinoderm fragments. Analysis of these fragments reveals that some echinoderms possess hard substrate adaptations (such as a cementing holdfast). Such crinoid taxa include Apiocrinites parkinsoni, seen at the Bradford hardground, along with other cementing forms such as Millercrinus associated with starfish and regular echinoids. The presence of comatulid crinoids, (which in modern environments are observed in cryptic habitats), may indicate trophic layering developed in hard substrates.

Terrestrial and lagoonal elements within the deposit, along with the high diversity of marine forms, indicate transportation from heterogeneous source areas. Despite this, analysis of transported assemblages shows that it is possible to reconstruct hard substrate communities, even when the facies of origin has not been preserved.