Early Triassic ophiuroids: Their paleoecology, taphonomy, and distribution

The Permian-Triassic transition was an important time for many marine groups, including echinoderms. However, the fossil record of ophiuroids through this interval is poorly understood. Recent discoveries in Lower Triassic rocks from northern Italy and western US suggest that ophiuroids were more common during this time than previously has been appreciated. Evidence from resting traces (Asteriacites lumbricalis) indicates that Early Triassic ophiuroids lived on fine-grained siliciclastic sediments in oxygenated, shallow-marine environments within storm wave base. No resting traces have been recorded from deeper and/or oxygen-restricted settings. However, following death and decay, ophiuroid skeletal elements sometimes were transported into offshore, low-oxygen environments. All known occurrences of Early Triassic ophiuroids fall within the paleotropics, but this is attributed to sampling bias. Articulated body fossils and their trace fossils are most common in latest Lower Triassic (Olenekian) rocks, but disarticulated ossicles occur throughout the Induan and Olenekian. At times and places, ophiuroids were the dominant marine benthos, carpeting the substrate in prodigious numbers. However, at other times, they also comprised minor components of diverse benthic assemblages, living alongside crinoids (Holocrinus), bryozoans, bivalves, gastropods, and brachiopods. Morphological comparison with extant ophiuroids suggests that all known Early Triassic taxa were small surface-dwellers.