TRIASSIC ASTEROIDEA (ECHINODERMATA) AND THE DIVERGENCE OF CROWN-GROUP ASTEROIDS

All known Jurassic and younger asteroids belong to the crown group, and all known Paleozoic asteroids belong to the stem group. For many years, the only Triassic asteroid known from more than scattered skeletal elements was Trichasteropsis ECK, from the Muschelkalk of Germany. Three species of Trichasteropsis and a fourth trichasteropsid species belonging to a new genus are recognized. Another asteroid genus, Noriaster (Poraniidae), recently has been described from the Triassic of Italy. Parsimony analysis treated the full (Ordovician to Recent) history of asteroids using a somasteroid outgroup. Ambulacral evolution is critical in echinoderm history; the ambulacral arrangement known from crown-group asteroids first appeared in Paleozoic sister lineages, and a new subclass will be proposed for Paleozoic and younger taxa with these apomorphies. Trichasteropsis is assigned to the Forcipulatacea, which includes Asterias, whereas Noriaster represents the second major phylogenetic branch (Valvatacea and Spinulosacea) of the crown-group infraclass Neoasteroidea. Trichasteropsis and Noriaster share similarities of the ventral disk inferred to represent apomorphies of crown-group diversification. Presence of both branches in the Triassic indicates that crown-group diversification already was well underway although the fossil record does not allow precise determination of timing. Morphological similarities suggest crown-group asteroids returned to stem-group life modes. Trichasteropsis is robust, suggesting protection from wave impact or predators. It is found in sediments associated with shell banks but not from within the banks. Trichasteropsis senfti commonly occurs with brachiopods whereas T. weissmanni does not, although brachiopods are found in associated strata. Aspects of morphology of both species are similar to those of recent predatory Asteriidae suggesting similar behavior, although feeding habits are not verified.