ENVIRONMENTAL CONTROLS ON THE DISTRIBUTION OF LATE PALEOZOIC BRYOZOAN COLONY MORPHOTYPES: AN EXAMPLE FROM THE PERMIAN OF TASMANIA, AUSTRALIA

Abstract

Paleozoic bryozoans differ significantly from modern forms in systematics and form, and in strength and occurrence of morphotypes. Nevertheless, the distribution of bryozoan morphotypes in modern oceans is often applied to the interpretation of fossil depositional environments. This study assesses the dominance of bryozoan morphotypes in the upper Paleozoic glaciomarine Tasmania Basin, and determines the environmental parameters that control their distribution by comparison with the environmental distribution and life habit of co-occurring brachiopod and bivalve biotas. Common Tasmanian late Paleozoic bryozoan morphotypes are erect-rigid fenestrate, erect-rigid branching, and erect-rigid foliose, with impersistent occurrences of encrusting forms. Water energy, or turbulence, is the primary control on bryozoan occurrence, with erect-rigid fenestrates occuring in low-energy settings, similar to reclining productid brachiopods, and erect-rigid foliose and branching forms in low- to moderate-energy settings, similar to nestling spiriferids. The secondary environmental parameter controlling bryozoan distribution is sediment accumulation rate. Both brachiopods and bryozoans need to keep their lophophores clear of sediment particles and are usually assumed to indicate low water turbidity. Differential epifaunal tiering, however, allows erect bryozoans to inhabit settings where sediment accumulation rates are sufficient to bury primary tier brachiopods at the sediment surface. With the caveat that water energy levels must remain at low-to-moderate levels, erect-rigid bryozoans are capable of dominating onshore settings where sedimentation rates may prohibit brachiopod dominance. The distribution of late Paleozoic bryozoan morphotypes examined here differs from modern distributions.