The pseudoplanktonic crinoid Traumatocrinus from the Late Triassic of Southwest China — Morphology, ontogeny, and taphonomy

Abstract

The morphology of the pseudoplanktonic crinoid Traumatocrinus from the early Late Triassic (Carnian) Xiaowa Formation of Guanling County (South China, Guizhou Province) is described based on articulated and disarticulated material. Juvenile individuals of less than 10mm crown height with 20 isotomously branching arms prove the encrinid ancestry of the family Traumatocrinidae. However, even the smallest individuals have five large interradials. Endotomous branching commences at crowns of slightly more than 10mm at the outer arms of the rays. All brachial articulations are ligamentary (granosyzygies). The assignment of the Chinese material to one of the nominal species of Traumatocrinus is still left open. The diagnosis of the genus Traumatocrinus is emended. Traumatocrinus lived attached to driftwood and was widespread across the Palaeo-Tethys. In contrast to its benthic encrinid ancestors, Traumatocrinus fixed its distal column with countless anastomosing radicular cirri to its substrate, preferentially to the ends of the driftwood logs. Depending on the state of the hypothesized mutable collagene tissue, the columns are preserved in straight positions or curved like a rope. Crowns are preserved in star position with radially splayed arms or in lateral bell position with the cup disrupted in interbrachial position. As usual in black shales, the exposed upper sides of the crinoids are in a more or less progressive state of decomposition whereas the embedded lower sides are intact.