Eocene crinoids from Seymour Island, Antarctic Peninsula: paleobiogeographic and paleoecologic implications

Abstract

On the basis of recent collections from the Upper Eocene La Meseta Formation of Seymour Island, Antarctic Peninsula, the morphology, systematic position, taphonomy, and paleoecology of the isocrinid Metacrinus fossilis are investigated. A new species, Notocrinus rasmusseni, is described as the first comatulid crinoid known from the Antarctic fossil record. The systematic assignment of M. fossilis is maintained. Basal abrasion of calyxes and absence of long attached columns suggest that M. fossilis might have lost most of the column in adult stages and lived directly on the substratum, supported by some arms and a few cirri, similar to comatulids. About 10 percent of M. fossilis individuals show brachial regeneration, in contrast to regeneration frequencies of 70–90 percent among modern Japanese isocrinids. The anomalous occurrence of isocrinids in shallow-water facies of the La Meseta is attributed to a combination of reduced predation pressure, the presumed stalkless mode of life, and a favorable temperature regime in Antarctic surface waters prior to the onset of cooling at the close of the Eocene.