THE ORIGIN OF TEREBRATULIDA (BRACHIOPODA): ARE THEY NEOTENIC SPIRE-BEARERS?

Investigating the evolution of major animal body plans, how transitions from one to another occur, and why they occur when they do is a topic of long-standing interest in paleontology (Valentine, 2006). Determining the evolutionary relationships among major groups of extinct organisms is often fraught with uncertainty, as the evolutionary process itself eliminates certain shared traits while originating novel traits over time. With no direct access to genomic or developmental data, paleontologists must make inferences based on the information we do have: morphology, stratigraphy, and paleobiogeography. In this study, we focus on one particular evolutionary transition – the origin of the most diverse clade of extant brachiopods, the Terebratulida, from its extinct spire-bearing ancestors. How can we make robust inferences about terebratulide ancestry?

We have completed a series of parsimony-based phylogenetic analyses at the genus level on the major groups of extinct neoarticulates (in Rhynchonellata). With some degree of confidence, we can conclude that punctate Spiriferinida evolve from paraphyletic impunctate Spiriferida; punctate retziidine athyridides evolve from within impunctate athyrididines, which themselves evolve from impunctate atrypides. Testing these patterns with Bayesian inference and fossilized birth-death (FBD) models allows us to evaluate the effect of utilizing different methods that involve different assumptions about evolution, and to examine the effect of incorporating stratigraphic information into morphological phylogenetic analysis. Does the terebratulide sister-group lie within the atrypides or athyridides, or elsewhere? We focus on testing previously untested hypotheses about the neotenic origin of Terebratulida in the Devonian.

Testing a paedomorphic origin requires investigating data on ontogenetic size increase and morphologic trait change in putative ancestors and descendants. By constructing a phylogenetic supertree with data from three separate FBD analyses -- 73 atrypide genera, 69 athyridides, and 71 Devonian terebratulides – we will deepen our understanding of ontogenetic and evolutionary changes in body size and shape, develop a more comprehensive hypothesis of relationship among all these taxa, and help clarify the origin of Terebratulida.