Why is the tropical Cenozoic fossil record so poor for bryozoans?

Diverse bryozoan assemblages have been recorded from widely across the Cenozoic palaeotropics, including the West Indies and Central America (Miocene-Pleistocene), Arabia (Oligocene), East Africa (Miocene), India (Eocene-Miocene) and the East Indies (Eocene-Miocene). However, records of Cenozoic tropical bryofaunas are relatively few compared with higher latitudes, and bryozoan limestones seem to be lacking. Insights into the reasons for this poor fossil record can be gleaned from comparisons with modern tropical bryozoan faunas, and consideration of the effects of diagenesis on fossil preservation and other factors that bias against the tropical Cenozoic fossil record. At least for the North Atlantic, bryozoan assemblage diversity in the modern tropics is not significantly less than in higher latitudes. However, a survey of colony-forms shows that encrusting species of small biomass are more dominant in tropical assemblages (mean 78% of species) than they are outside the tropics (mean 60% of species). These encrusting colonies may be difficult to observe and study when, as is often the case in tropical carbonate settings, diagenetic cement binds sediment firmly to the colony surfaces. Most erect bryozoan species living today in the tropics have weakly mineralized skeletons with a poor potential for fossilization; robust species capable of generating large quantities of carbonate sediment are uncommon. In addition, a higher proportion of cheilostomes in the tropics have metastable skeletons of aragonite: a Raman spectroscopic survey of 23 bryozoan species encrusting the undersides of platy corals from Puerto Rico and Malaysia showed 30% to be aragonitic and 27% bimineralic. Along with the typically higher Mg levels in the calcite of tropical cheilostomes, this further biases against preservation of bryozoans in the Cenozoic fossil record.