Coordinated shifts to non-planktotrophic development in spatangoid echinoids during the Late Cretaceous.

Despite widespread interest in the interplay between evolutionary and developmental processes, we still know relatively little about the evolutionary history of larval development. Many clades exhibit multiple shifts from planktotrophic (feeding) to non-planktotrophic (non-feeding) larval development. An important question is whether these switches are scattered randomly through geological history or are concentrated in particular intervals of time. This issue is addressed using the Cretaceous spatangoid sea urchins, which are unusual in that larval strategy can be determined unambiguously from abundantly fossilized adult tests. Using a genus-level phylogeny, we identify five clades of non-planktotrophic taxa, each of which first appears in the fossil record in the Campanian or Maastrichtian (the final two Cretaceous stages). No examples of non-planktotrophy have been identified in any of the earlier stages of the Cretaceous. This strongly suggests that shifts to non-planktotrophic development are clustered in certain episodes of geological history, and this, in turn, implies that extrinsic factors operating at these times are responsible for driving shifts in developmental strategy.