ARE MORPHOLOGICAL CHANGES IN DEVONIAN BRACHIOPODS AN EVOLUTIONARY RESPONSE TO PREDATION? A PHYLOGENETIC TEST OF THE SIGNOR-BRETT HYPOTHESIS

Signor and Brett (1984) suggested a global Mid-Paleozoic spike in predation, based on an increase in durophage diversity and a concurrent increase in the proportion of taxa with potentially anti-predatory morphologies (e.g. spines, ornament) in four different prey clades. Subsequent studies demonstrated that brachiopod ornament increased throughout the Devonian (Alexander, 1990) and that brachiopod ornament could inhibit predation (Leighton, 2001, 2003). However, it is unclear whether morphological change in brachiopods is a microevolutionary response to increasing predation or alternatively, is a case of species sorting favoring preadapted taxa. To explore this problem, the distribution of ornament on a phylogeny of stropheodontoid brachiopods was analyzed relative to the timing of the increase in durophage diversity. Stropheodontoids were among the most diverse and abundant groups during the Devonian, and were frequent victims of predation.

A heuristic maximum parsimony analysis of 23 stropheodontoids (plus one outgroup) using 26 unweighted, unordered, and reversible characters (17 binary, 9 multistate; 5 shape, 5 external, 16 internal), produced three equally most parsimonious cladograms (length=118, RI=0.56). The cladograms contain five clades: (1) stropheodontids, (2) douvillinids, and (3) pholidostrophids form one megaclade; and (4) amphistrophids and (5) leptostrophids form a second megaclade. A paraphyletic leptostrophid group is basal to both megaclades. Exclusion of the character of interest (ornament strength) did not significantly alter the results.

Four of the five clades are present during the Givetian-Frasnian, the interval when durophage diversity increases. All four of these clades convergently evolve stronger ornament at this time suggesting a common, and strong, selective pressure. The acquisition of stronger ornament concurrent with the increase in durophage diversity points to predation as the agent of selection and supports the Signor-Brett hypothesis.