Platycrinitid (Monobathrida) crinoid columnals from the Permian of Timor: Form, function, protection and intimate associations

Abstract

In any collection of Upper Palaeozoic crinoid columnals, dominated by the circular and the pentagonal, the elliptical articular facets of platycrinitid monobathrids stand out. One of the geologically youngest regions from which platycrinitids are known is the Permian (Artinskian to Roadian) of the Basleo area of West Timor, from which over a dozen species have already been described from thecae. Platycrinitid columnals and pluricolumnals from West Timor are dominated by nodals that are strongly tuberculated (= platycrinitid sp. D). These may have been to deter benthic vagile invertebrates, such as parasitic(?) platyceratid gastropods, from using the column as a route to the crown. One pluricolumnal (platycrinitid sp. A) is reminiscent of the proxistele of Platycrinites nikondaense Broadhead and Strimple, from Alaska. Two forms of nodals could not be more disparate, one (platycrinitid sp. B) having an unsculptured latus, in contrast to doughnut-like platycrinitid sp. C, where an epifacet is broad and circular. Episkeletozoans and epizoozoans are commonly bryozoans and tabulate corals; the boring Oichnus paraboloides Bromley is a domicile rather than evidence of predation or parasitism. In short, the platycrinitids were thriving and interacting with a diversity of invertebrate groups shortly before their demise at the end of the Permian.