Enigmatic branching structures within Upper Devonian crinoids, north Devon, UK

摘要

The ichnology of Palaeozoic crinoids has largely concentrated on the crown. Features that have been recognized so far include predation on the anal series and arms (Lane 1984; Baumiller & Gahn 2003), borings (Baumiller 1990; Donovan 1991; Gahn et al. 2003; Donovan et al. 2010, pl. 31, fig. 1) and growth deformities(Donovan & Webster 2013) produced by epizoozoans (sensu Taylor & Wilson 2002). The stem has received less attention, but is particularly known for preserving small round holes produced by episkeletozoans; these may be associated with growth deformities of the column, indicating that the crinoid was alive when infested (Donovan & Lewis 2010). That crinoid stems were used as hard substrates as early in the group's history is well known(see, e.g., Donovan 1995, pl. 19, figs 1, 3). The geometry of borings in the crinoid stem, if beyond a simple hole or pit, can commonly only be studied by thin section or modern 3D scanning techniques. But many horizons in the Palaeozoic of the British Isles preserve excellent moulds of crinoid columnals and pluricolumnals, some containing casts of borings. The examples described below have a distinct morphology that would not have been recognized without such mouldic preservation.