Current paleobiological models hold that predators eliminated populations of epifaunal suspension feeders from shallow, soft-substrate marine environments beginning in the Mesozoic. Among the suspension feeders affected were dense populations of ophiuroids, which are rare in shallow water today, and isocrinid crinoids, which today occur only in the deep sea. The La Meseta Formation on Seymour Island, Antarctic Peninsula, represents an ecological anomaly: this deposit contains localized, autochthonous, dense assemblages of ophiuroids and isocrinids in a late Eocene, shallow-water setting. The rare occurrence of sublethal arm injuries in both the ophiuroid and crinoid populations suggests low predation levels, as seen in similar populations before the Mesozoic. Sporadic return to a Paleozoic community structure was apparently provoked by changes in temperature and productivity in Antarctica during the late Eocene.

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