ECHINODERM MORPHOLOGICAL EVOLUTION PRECEDES AND IS MORE VOLATILE THAN ECOLOGICAL INNOVATION DURING CAMBRIAN AND ORDOVICIAN RADIATIONS

Origins of higher taxa entail dramatic and nearly simultaneous changes in morphology and ecological function, limiting our ability to disentangle the causes of evolutionary diversification. However, because most studies utilize the same underlying data to interpret both evolutionary patterns, it is difficult to determine whether ecological or morphological novelty is the primary driver of a clade’s success. Here we compare the anatomy and life habits of all known early Paleozoic echinoderm lineages to demonstrate that each facet records a dynamically distinct evolutionary history.

Analyses are based on a stratigraphically calibrated informal supertree of 366 early Paleozoic echinoderm genera. Morphological and ecological data sets were coded independently of one another, and use 413 anatomical and 40 life-habit traits to comprehensively characterize the form and function of these early echinoderms (including inferred states for common ancestors).

Rates of morphological evolution are faster and involve more volatile trait changes, allowing morphological disparity to accrue faster and earlier in the Cambrian. However, persistent life-habit evolution throughout the early Paleozoic, combined with iterative functional convergence within adaptive strategies, results in ultimately greater expansion of ecospace and functional diversity. The interactions between tempo, divergence, and convergence demonstrate not only that anatomical novelty precedes ecological success, but that ecological innovation is constrained, even during a phylum’s origin.