EVOLUTION OF ARCHAEOCIDARIS ECHINOIDS DURING THE PALEOZOIC PRECURSOR TO THE MMR

Archaeocidaris echinoids diversified during the Late Paleozoic Precursor to the Marine Mesozoic Revolution, an interval in which new and more powerful durophagous predators radiated. Although archaeocidarids bore large and robust spines, little research has been done to explore the possibility that the evolution of these urchins was influenced by the concurrent evolution of predators. To test this hypothesis, we performed a phylogenetic analysis of North American Archaeocidarisspecies and mapped potentially anti-predatory characters onto the phylogeny. Although 37 species have been described from North America, many species are known only from isolated plates and spines, resulting in a fairly depauperate character matrix. Lack of character information resulted in the removal of six taxa; an additional “wildcard” species was also culled. The resulting matrix included 30 species plus two Devonian outgroup taxa and 26 multi-state, unordered, reversible characters. Characters were measured from previously published photographs and camera-lucida drawings and separated into discrete character states at natural breaks in their distributions. A branch-and-bound analysis using maximum parsimony (in PHYLIP 3.695) produced a single most-parsimonious tree.

Because spine-ornament complexity and test-size were both coded as unordered and multistate (6 and 8 states respectively), complex spine-ornament and large size were unlikely to be synapomorphic, or to be congruent with the increase in durophages diversity, by random chance. Despite this conservative coding, the results indicate that both spine-ornament complexity and test-size increased through time and along evolutionary branches. Derived taxa, primarily from the Pennsylvanian and Permian, had more complex and sharp spine-ornament than did older, basal species; in addition, different types of spine-ornament evolved convergently, and possibly in different basins. Similarly, more derived taxa were generally larger than more basal taxa, excepting outgroups. Thus, two potentially anti-predatory characters, spine-ornament complexity and test size, both evolved concurrently with increases in predator size, strength, and diversity, suggesting that the Archaeocidarislineage responded morphologically to the Paleozoic Precursor.