Interpretation of sponge fossil faunas: A neontological approach to a paleontological problem

Abstract

The tempo and mode of early animal evolution remains one of the biggest conundrums in biology. Stratigraphy shows that there is a gap, not attributable to poor preservation, of at least ~100 Myr between the oldest animal fossils and the divergence times implied by molecular phylogenies. Sponges, due to their position in the metazoan tree, are a good candidate for the earliest fossil evidence for the diversification of animals. Nevertheless, the search for the earliest animals represents a major challenge due to the lack of criteria by which to recognize such organisms in the fossil record. Here I describe a way to quantify the sponge filtration or ‘sponge pump character’ by analyzing the ratio between the two major components of the aquiferous system: external surface area–SA and osculum cross-sectional area–OSA, which allows comparison between extant sponges and many fossil forms. Second, I addressed the question of whether sponges make use of induced flow, an often-made assumption when interpreting fossils as sponges. The analysis includes the three major classes of Porifera: Demospongiae, Hexactinellida and Calcarea, both are represented by extant and fossil taxa from two extremes of the Phanerozoic, the Cambrian and Eocene. The data show that this ratio is narrow, 0.01 – 0.001 in modern sponges, and the slope of the ratio can be used to distinguish classes of sponges. I then examined the ratio of OSA/SA for the putative Ediacaran sponge Thectardis avalonensis, and found it to be similar to some Cambrian sponge genera. Moreover, the slope of the ratio across different sizes of T. avalonensis is similar to that of demosponges. However, the original argument for the Poriferan affinity of T. avalonensis was based on the idea that the main mechanisms that sponges use to filter water is through current induced flow, meaning that shape alone drives the flow in and out of a sponge at no extra metabolic cost. I tested this hypothesis in the second chapter of this thesis by analyzing data from tank experiments in the demosponge Geodia barretti. The analysis of the filtration to respiration ratio showed increased filtration at ambient currents below 10 cm s-1 but a reduction at higher ambient current speeds, which contrast the traditional view of shape induced flow. Instead, these results support the evidence that sponges have a high degree of control over their filtration, with the consequence that induce flow should not be taken as a criterion for ascribing to the sponges any structure based on this assumption. Overall, my work shows that a more solid understanding of the biology of modern sponges provides a wealth of information with which to examine the Precambrian record of sponges in particular, and the greater picture of early animal evolution in general. This is because having a correct interpretation of the fossil record is essential to properly calibrating molecular phylogenies.