PORIFERAN PARAPHYLY AND ITS IMPLICATIONS FOR PRECAMBRIAN PALEOBIOLOGY

Well-supported molecular phylogenies combined with knowledge of modern biology can lead to new inferences about the sequence of character acquisition in early animal evolution, the taxonomic affinity of enigmatic Precambrian and Cambrian fossils, and the Proterozoic earth system in general. In this study we demonstrate, in accord with previous molecular studies, that sponges are paraphyletic with calcisponges more closely related to eumetazoans than they are to demosponges. In addition, our Bayesian analysis finds the Homoscleromorpha, previously grouped with the demosponges, to be even more closely related to eumetazoans than are the calcisponges. Hence there may be at least three separate extant “poriferan” lineages each with their own unique skeleton. Because spiculation is convergent within “Porifera,” differences between skeletonization processes in enigmatic Cambrian taxa such as Chancelloria and modern sponges does not mean that these Problematica are not organized around a poriferan body plan, namely a benthic, sessile organism feeding with a water canal system. The shift from an anoxic and sulfidic deep ocean that characterized the mid-Proterozoic to the well-ventilated Phanerozoic ocean occurs before the evolution of planktonic bilaterian predators, and cannot have been caused by the advent of fecal pellets. The evolution and ecological dominance of sponges provide an alternative mechanism for the drawdown and sequestration of dissolved organic carbon within the sediment.