Biomineralization, structure and diagenesis of the coelenterate skeleton

Review of biomineralization and microstructure in major coelenterate groups leads to generalizations regarding the locus and method of skeletogenesis. The Hydrozoa, which include the most primitive skeleton-bearing coelenterates, generally have an aragonitic skeleton formed externally of varying combinations of spherulitic crystallites modified by organic matrix material. Living Anthozoa show two markedly differing plans of skeletogenesis. In Octocorallia, internal crystallization of calcite closely controlled by organic matrix forms spicules, while the Scleractinia have external crystallization of aragonite with microstructure likewise closely controlled by envelopes of organic matrix. Fossil corals (Rugosa) followed the same architectural plan as the Scleractinia, although building of calcite. As a result of differing biogenic mineralogy, diagenetic structures differ greatly between the two especially where vadose or fresh water diagenesis was involved. Both groups are characterized by biogenic structures of a trabecular or fibro-normal nature

Darwin's dilemma: the realities of the Cambrian ‘explosion’

The Cambrian ‘explosion’ is widely regarded as one of the fulcrum points in the history of life, yet its origins and causes remain deeply controversial. New data from the fossil record, especially of Burgess Shale-type Lagerstätten, indicate, however, that the assembly of bodyplans is not only largely a Cambrian phenomenon, but can already be documented in fair detail. This speaks against a much more ancient origin of the metazoans, and current work is doing much to reconcile the apparent discrepancies between the fossil record, including the Ediacaran assemblages of latest Neoproterozoic age and molecular ‘clocks’. Hypotheses to explain the Cambrian ‘explosion’ continue to be generated, but the recurrent confusion of cause and effect suggests that the wrong sort of question is being asked. Here I propose that despite its step-like function this evolutionary event is the inevitable consequence of Earth and biospheric change