PHYLOGENETIC ANALYSIS OF A LIKELY PRODUCTIDE FROM THE TRIASSIC, AND THE IMPLICATIONS FOR SURVIVAL THROUGH THE PERMO-TRIASSIC EXTINCTION

Strophalosiidine (Order Productida) brachiopods were among the most diverse, morphologically disparate and ecologically predominant benthic invertebrates during the Permian. Some strophalosiidines capable of cementation formed reefs. With rare, possible exceptions, all productides are thought to have gone extinct during the Permo-Triassic mass extinction. Discovery in the Middle Triassic of Croatia of a silicified, diminutive (1-2mm), concavo-convex brachiopod possessing a complex brachidium and hollow spines capable of cementing prompts the re-evaluation of the stratigraphic range of the order Productida, and the phylogenetic relationships of this new-found taxon.

We analyzed the characters of 44 Late Permian species and genera representing more than 26 productide subfamilies, as well as a Triassic thecideide species, from specimens and the literature. Features of the new species such as a conical shape, ventral spines, the retention of teeth and brachial ridges extending to the anterior margin suggest an affinity with the suborder Strophalosiidina. Indeed, preliminary phylogenetic results indicate that the new species belongs within the Strophalosiidina, though it is not yet clear to which strophalosiidine the new species is most closely related. There are two primary possibilities: parsimony analysis nests the new species within the Aulostegidae, whereas neighbor joining nests it within the Cooperinidae. Each has different morphological implications, and thus different implications for the eventual survival of the lineage. If a derived aulostegid, then small body size and the prominent brachidium of the new species are derived, suggesting a shift in the ecological requirements of the lineage. If the new species is a derived cooperinid, then small size and a short growth period may have pre-disposed it to living in cryptic or marginal environments.

We will increase the taxon sampling density of included strophalosiidines, add Triassic spire-bearing species and thecideide species to better document and increase confidence in character evolution within these taxa and improve phylogenetic resolution. We will also add new characters to help distinguish between the two hypotheses.