Biostratigraphy and evolution of crinoid columnals from the Ordovician of Britain

Only about 30 crinoid species based on calyces have been described from the Ordovician of Britain, yet crinoid columnals are present in many Ordovician beds. Columnals are relatively rare fossils before the Upper Ordovician, where they greatly increase in both numbers and diversity. Columnals can broadly be divided into two groups, meric (composed of more than one plate) and holomeric (columnals formed from a single ossicle). Only two British crinoids have tetrameric stems, Ramseyocrinus cambriensis (Lower Arenig) and Colpodecrinus forbesi (Caradoc). Bimeric and trimeric columnals are only known from the Ashgill. Pentameric columnals, and circular and pentagonal holomeric coluffinals, are common throughout the Ordovician. In the Upper Ordovician several new morphotypes appear, including columnals with a pentastellate lumen (Bystrowicrinus) and columnals with synarthrial articulating ridges (Ristnacrinus). The first cirriferous roots are Caradoc in age but Ordovician attachment structures are rare.