The origin of sponges and the Cambrian explosion
Dr. Jonathan Antcliffe
Department of Earth Sciences, University of Bristol
Thursday 13th October
Neil Chalmers Seminar Room (DC2, LG16)
Sponges are widely considered to be the animal group most likely to have evolved in the Precambrian. However reanalysis of all fossil candidates for Precambrian sponges shows that the oldest hitherto accepted specimens, Mongolian silica hexacts from c.545Ma, are abiogenic arsenopyrite crystals while all older candidates are abiogenic artefacts, microbialites, or variants of the Ediacaran biota. There are reliable sponge remains from the basal Cambrian represented by spicules from the Soltanieh Formation, Iran, reported in detail for the first time. Deep Precambrian divergences of Metazoa and particularly sponges have however been predicted based on molecular data. Yet the Ediacaran fossil record is abundant in soft bodied remains and does not yield any convincing evidence for sponges. Further geological data shows that chemically precipitated cherts and crystal fan fabrics are common and therefore the Ediacaran ocean is actively precipitating silica, and sponge spicules are not absent because of an unsuitable taphonomy as some have suggested. Sponges are complex organisms that require interactions with other animals in order to survive, a result of 540Ma of complex co-evolution with other animals. There is no reason why they should be thought more likely to be able to live outside of this context at a time before these ecosystems evolved that any other animal group. Sponges probably evolved at approximately the base of the Cambrian Period. Studying such problems can teach us general principles about how to analyse and thereby correctly interpret enigmatic fossils upon which so much macroevolutionary weight can be placed.
see http://www.nhm.ac.uk/research-curation/seminars-events/index.html for additional information