Early Triassic recovery of echinoderms

The Permian–Triassic mass extinction interval was an important time in the evolutionary history of the echinoderms. Details of the extinction and, in particular the immediate post-extinction recovery in the Early Triassic, are seldom addressed because of a perception that the Permian–Triassic echinoderm fossil record is too poor. However, only the Holothuroidea and Asteroidea lack any Early Triassic fossil representatives. Even in these groups, details of the extinction and recovery can be inferred from recent cladistic analyses. The Holothuroidea are unique amongst the echinoderms in showing no family level extinction through the Permian–Triassic interval, possibly due to their deposit-feeding lifestyle. In contrast, the Echinoidea, Crinoidea and probably the Asteroidea underwent severe evolutionary bottlenecks during that time. In the echinoids, significant post-Permian radiation occurred from the Late Triassic (Carnian), although it may have begun in the Early Triassic. In the Crinoidea, fossil diversity increases dramatically from the Late Ladinian, although cladistic analyses suggest that initial diversification took place in the Earliest Triassic (Induan). Many undescribed crinoid remains from Lower Triassic strata worldwide also imply that the post-Permian radiation in this group may have been more rapid than currently thought. Locally in the Spathian, crinoid ossicles may approach rock-building densities. The presence of at least seven Early Triassic fossil ophiuroid species may indicate rapid post-Permian radiation in the Ophiuroidea, although the higher level affinities of these taxa are presently unresolved and the Late Permian record is poorly known. Ophiuroid remains are the most diverse echinoderm fossils during the Early Triassic, comprising both complete body fossils and disarticulated ossicles. Holothuroids possibly radiated in the Early Triassic, but current evidence from cladistic analysis favours a largely Anisian age for the post-Permian radiation in this group. All known Early Triassic echinoderms were small-sized animals that inhabited very shallow, oxygenated, low palaeolatitude environments within wave base.