'Screwstones' from the Lower Carboniferous (Mississippian; Visean, Brigantian) at Bradford Dale, Youlgrave, Derbyshire, and a new species of Gilbertsocrinus Phillips (Echinodermata, Crinoidea)

摘要

Screwstones are chert clasts from the Lower Carboniferous (Mississippian) of Derbyshire in which the internal moulds of the axial canal of crinoid pluricolumnals have a screw-like appearance. They are commonly regarded as curiosities, but chert can preserve the morphological details of crinoid ossicles exquisitely. The thecae of Gilbertsocrinus occur only in the Tournaisian (Ivorian to Chadian) of the British Isles, yet their distinctive columnals range from Upper Devonian (Famennian) to Visean (Brigantian) in the same region. The youngest such columnals, rounded in outline with a narrow to moderately broad marginal crenularium, a broad pentalobate lumen, and heteromorphic with two orders of internodal, are described as Gilbertsocrinus fionae sp. nov. from cherts of the Monsal Date Limestone Formation of Youlgrave, Derbyshire.