BIOMINERALIZATION IN BRYOZOANS: BIOLOGICAL VS EXTRINSIC CONTROLS (Invited Presentation)

A perennial issue in biomineralization is the degree to which the microstructure, mineralogy, crystallography, and chemical compositions of skeletons are controlled by organisms as opposed to extrinsic factors such as environmental variables and the physicochemical properties of the biominerals themselves. Understanding the interplay of intrinsic and extrinsic factors is crucial if biomineralized skeletons are to be employed successfully as palaeoenvironmental indicators and also in phylogenetic interpretation. The complex calcareous skeletons of bryozoans are particularly challenging in this respect. Molecular phylogenetic findings imply that biomineralization has evolved from soft-bodied bryozoan ancestors twice in the phylum, first in the class Stenolaemata during the Early Ordovician (there are no indisputable Cambrian bryozoans), and subsequently in the order Cheilostomata during the Late Jurassic. All stenolaemates have calcitic skeletons, although the occurrence of thin phosphatic layers in some early species hints at the possibility of a calcium phosphate precursor. In contrast, an increasing number of cheilostome clades have switched from calcitic to aragonitic or biomineralic skeletons over time. The ubiquitous lamellar microstructures of stenolaemate skeletons appear more biologically controlled than the fibrous microstructures typical of cheilostomes that more closely resemble inorganic cements. Strong latitudinal gradients exist in both the mineralogy and Mg contents of cheilostome bryozoans, with aragonite and Mg both increasing towards the equator. Too few analyses of stenolaemates are available to know whether they too show a latitudinal gradient in Mg. The primitively calcitic skeletons in both groups appeared during times of calcite seas, matching expectations. However, there is no indication of hypercalcification at times when skeletal mineralogy corresponded to seawater chemistry, and the transition to aragonite among cheilostomes apparently began too early in geological time to have been triggered by the advent of aragonite seas.