Meeting Abstract
The Clypeasteroida (sand dollars and sea biscuits) is among the most easily recognized of irregular (burrowing) echinoid groups. These animals are of increasing interest due to their excellent fossil record, and for their ecological responses to climate change over geologic time. Morphological phylogenies of clypeasteroids and their closest relatives, an informal group known as “cassiduloids” (lamp urchins), have questioned clypeasteroid monophyly — a clade supported by many morphological features. Recent molecular trees suggest that some features have evolved more than once. For example, most urchins have an Aristotle’s lantern, a five-part jaw apparatus used in morphological analyses. Outgroups of Clypeasteroida, including “cassiduloids” and heart urchins (Spatangoida and Holasteroida), lack a lantern as adults. Distantly related “regular” urchins and clypeasteroids have a lantern, suggesting that it reappeared in a monophyletic Clypeasteroida. The molecular work suggests reacquisition of the lantern separately in sea biscuits and sand dollars. A weakness in those analyses has been the lack of a focus on clypeasteroids, usually because of inadequate taxon sampling. We address this using Sanger sequencing methods on a set of key clypeasteroids and “cassiduloids”. Several other problems, such as the phylogenetic position of miniaturized, bean-shaped clypeasteroids such as fibulariellids with the bizarre, but otherwise sand dollar-like rotulids, are also explored. Problems with, and possible solutions for resolving the phylogeny are discussed in view of differing evolutionary rates and geologic depth of nodes implied by our phylogenetic results.
