AN OVERVIEW OF THE CENOZOIC FOSSIL RECORD OF BRYOZOANS IN THE TROPICS

Surprisingly few Cenozoic bryozoans have been described from the palaeotropics as defined geographically by the 23.5° lines of latitude. Insights into the reason/s can be obtained through comparison with modern tropical bryozoan faunas. Whereas it was once considered that bryozoans are rare in the tropics, especially in coral reef environments, it is becoming increasingly clear that they are just as diverse, if not more so, than in higher latitudes. However, tropical bryozoans are generally low in relative abundance and biomass. Bryozoans from tropical reefs consist overwhelmingly of inconspicuous encrusting colonies inhabiting cryptic settings (e.g. reef rubble and the undersides of corals). A published checklist of coral-associated bryozoans shows 80% of species to be encrusting, 9% erect and articulated, 5% rigidly erect phidoloporid cheilostomes (most with reticulate colonies), and 6% lightly calcified erect species. Free-living lunulitiform bryozoans can also be occur on fine-grained substrates in the tropics, and a variety of erect forms with greater biomass inhabit regions of upwelling or below the photic zone. Scant available data shows that a significant proportion of tropical cheilostomes secrete aragonitic skeletons, including all extant lunulitiform species, many others being bimineralic. The typically small biomasses and metastable skeletons of tropical bryozoans act as major biases against their discovery in the fossil record. Furthermore, pervasive cementation can make it difficult to observe the surfaces of substrates encrusted by bryozoans, while neomorphism of aragonitic substrates adversely effects the preservation of encrusting biota such as bryozoans. Mud-rich environments offer the best prospects for finding tropical Cenozoic bryozoans. These include deeper water, offshore settings (e.g. Pliocene Bowden Fm. of Jamaica), as well as muddy coral reefs (e.g. Miocene of East Kalimantan). However, the profound effects of diagenesis on the dominant small encrusting bryozoans from carbonate-rich settings will likely remain an impediment to our understanding of Cenozoic bryozoan evolution in the tropics.