Pachechinus bajulus is an endemic Australian sea urchin with an unusual mode of brooded larval development. We used mitochondrial and nuclear gene sequences to estimate the phylogenetic relationships among Pachechinus and other Echinometridae, including wellstudied species of Heliocidaris with planktonic development. We found strong evidence for the planktotrophic species Heliocidaris tuberculata as the sister group to a clade of three closely related species in which development is known (Heliocidaris erythrogramma, P. bajulus) or suspected (Pachechinus australiae) to be lecithotrophic. Clade support values and likelihood ratio tests rejected monophyly of Heliocidaris species. The sister group to H. erythrogramma is most likely the two Pachechinus species. We resolve the paraphyly problem by reassigning the Pachechinus species to the genus Heliocidaris (the senior synonym), which has six extant species including Heliocidaris australiae and Heliocidaris bajulus. The phylogeny has potentially important implications for comparative studies of developmental morphology and genetics that have assumed a close sister-group relationship between H. erythrogramma and H. tuberculata, and highlights the need for such data from H. bajulus and other Heliocidaris species.Additional key words: Pachycentrotus, bindin, COI, 16S Invertebrate Biology 130(2): 175-185. r

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.