The crinoid Jimbacrinus bostocki from the Lower Permian Cundlego Formation near Gascoyne Junction, Western Australia

Abstract

Invertebrate macrofossils are superbly preserved in slabs of calcareous silty sandstone exposed in the bed of the Gascoyne River just west of Gascoyne Junction, Western Australia. Based on regional mapping, these rocks have been assigned to the shallow marine Cundlego Formation, which has been dated to the Early Permian. Many of the slabs have preserved the cladid, inadunate crinoid Jimbacrinus bostocki in large numbers, with densities of 150 individuals or more per square metre. Articulated crowns with complete or partial stems attached are often haphazardly aggregated in pods. Details on the morphology of J. bostocki are provided, particularly on juveniles which have cirriferous stems. Adult stems lack any obvious holdfast and are acutely geniculate below the calyx before being preferentially directed linearly in the homocrinid (E–BC) symmetry plane in the E ray direction. Crowns are commonly preserved with their arms arranged in a protective manner, also referred to as the ‘trauma’ posture. It is inferred that such individuals were likely transported alive by storm induced turbidity currents from a feeding position on a marine shelf to deeper, possibly more anoxic, waters, often accumulating en masse in syndepositional lows such as swales. They were subsequently smothered by sediment and rapidly buried. These fossiliferous deposits are interpreted as an “obrution” Lagerstätte. Using taphonomic and functional morphology analysis, possible options for the feeding posture of J. bostocki are proposed. Because of its advanced arm morphology comprising mainly oblique muscular articulations between brachials, the question whether adults were motile, rather than sessile, is addressed. Robbert J Willink [robblink@ozemail.com.au], 11 Coral Sea Court, Sunshine Beach, Queensland 4567, Australia. Tom Kapitany [tomk@crystal-world.com] Crystal World, 13 Olive Road, Devon Meadows, Victoria 3977, Australia.