Article: Silurian crinoids from the Pentland Hills, Scotland
The crinoid fauna of the Pentland Hills consists of six species, of which five are new: Macrostylocrinus silurocirrifer, Ptychocrinus longibrachialis, Dimerocrinites pentlandicus, Herpetocrinusparvispinifer, and Dendrocrinus extensidiscus. Pisocrinus campana Miller is also known from the Silurian (upper Llandovery to Ludlow) of the North American Mid-continent. Pentland Hills crinoids are generally rare and show low taxonomic diversity compared to many crinoid faunas of similar age, suggesting a marginal habitat for crinoids. The most likely unfavourable conditions are quiet water, high suspended sediment content, rapid sedimentation, and perhaps influxes of brackish water. Study of the phylogeny of Dimerocrinites pentlandicus shows that the zygodiplobathrid base can be derived from the eudiplobathrid type by reducing the growth rates of the basals. Macrostylocrinus silurocirrifer, Ptychocrinus longibrachialis, and Dendrocrinus extensidiscus belong to archaic lineages which began in the Ordovician. Elsewhere, their relatives became extinct before the upper Llandovery.