Exceptionally preserved fossils reveal the pattern of early

Exceptionally preserved fossils reveal the pattern of early sponge evolution Alex Page, Tom Harvey & Nick Butterfield Dept. of Earth Sciences, University of Cambridge, UK The sponges are the most primitive of all animals, with the divergence between the calcisponges (sponges with carbonate spicules) and silicisponges (sponges with opaline spicules) occuring during the Ediacaran period. Although the ancestor of these two groups can be regarded as the ancestor of all animals, little is known about early sponge evolution. This is due in part to the susceptibility of sponge spicules to diagenetic dissolution or replacement, but also to the enigmatic nature of Cambrian and Ediacaran fossil sponges. In a taphonomic study of exceptionally preserved Early-Middle Cambrian sponges, we have identified a variety of extinct character combinations that link calcareous and siliceous sponges to a common ancestor. This work presents a new model for early sponge evolution and provides a ”search image” for recognizing the earliest animals in the pre-Cambrian fossil record. Taphonomy of exceptionally-preserved sponges from the Burgess Shale Fm (Mid Cambrian: British Columbia, Canada): (left) photomicrograph of the stem-group silicisponge Eiffelia globosa preserved as a flattened organic compression; (right) backscattered scanning electon micrograph of a section through a spicule of the stemgroup calcisponge Protospongia hicksi; preserved in high-Mg carbonate (mid grey) with secondary replacement by chlorite (light grey) and mica (dark grey). a Character acquistion in early animal evolution (right): [1] water canal, soft organic spicules with Protospongia-like organisation; [2] opal-organic spicules, axial canals; [3] loss of organic layer & Protospongia-like organisation; [4] Mg-carbonate spicules; [5] loss of Protospongia-like spicules; [6] loss of spicules; acquistion of gut. ge on sp i lic Si a ea o az et ng Eiffelia 3 Ca m Eu i lc 5 o sp Protospongia 6 4 SPONGE 2 1 SPONGE Positions of fossil sponges plotted on cladogram based on molecular phylogenies.