PALAEOENVIRONMENTAL SETTING AND SUBSTRATE AFFINITY IN CARBONIFEROUS ECHINOIDS

Echinoids (sea urchins) are a diverse and abundant clade occupying a range of habitats in both modern and ancient ecosystems. Echinoids in modern oceans tend to occupy different substrates dependent upon their life mode and many echinoids have evolved to live on or in certain substrates. Little is known about echinoid substrate affinities in the Paleozoic, and rigourous testing of hypotheses regarding substrate affinities has never been undertaken. Five families of echinoids are abundant in the Carboniferous, the archaeocidarids, lepidocidarids, lepidesthids, proterocidarids, and palaechinids. These differing families display a disparate array of morphologies, many of which may be adapted to specific substrates and life modes. In order to test for substrate affinities in these echinoids, a database was made of Carboniferous echinoids in museum collections comprising most of the known Carboniferous echinoid fossil record from America and Europe. Information on lithology (carbonate vs. siliciclastic) and grain size was recorded to test for substrate specificity in these taxa. The distribution of different clades of echinoids in the Carboniferous was then compared to the distribution of sediment types in the Carboniferous, so as to not bias the results by the abundance of certain types of sediment at a given time. The results show a clear affinity for carbonate sediments in all examined clades that is statistically significant. Results for grain size specificity were less clear. It appears that Carboniferous echinoids preferred carbonate sediments. Additionally specimens were scored for their taphonomic state of preservation to see how differing paleoenvironments and depositional settings affect Paleozoic echinoid taphonomy.