A combined morphological and molecular phylogeny for sea urchins (Echinoidea: Echinodermata)

Phylogenetic relationships of higher taxa of echinoids have been investigated using a 163 character morphological data base and molecular sequences from large and small subunit (LSU and SSU) ribosomal RNA (rRNA) genes. The complete SSU rRNA gene has been sequenced for 21 taxa, with representatives from nine of the 14 extant orders of Echinoidea. Partial LSU sequences, representing the first 400 base pairs (b.p.) from the 5' end were also sequenced for three taxa to complement an existing data base of ten taxa. The two molecular sequences provided a total of 371 variable sites, of which 143 were phylogenetically informative (compared to 145 phylogenetically informative sites from morphological data). Morphological, LSU and SSU data have been analysed separately and together. Morphological and SSU sequence data generate topologies that are not significantly in conflict (under Templeton's test), but the strong signal pairing arbaciids with clypeasteroids in the LSU derived tree marks the LSU sequence data as anomalous for this taxon. A 'total evidence' approach derived a tree very similar in topology to that derived from morphological data. Rooted on the stem group echinoid Archaeocidaris, our total evidence tree suggested relationships of higher taxa as follows: Cidaroida (Phormosomatidae (Echinothuriidae (Diadematidae ((Spatangoida (Clypeasteroida, Cassiduloida)) ((Calycina, Arbacioida) (Stomopneustidae (Glyphocidaridae (Temnopleuridae (Echinometridae (Echinidae, Stronglyocentridae)))))))))). Phylogenetic analyses run both with and without key fossil taxa yielded slightly different topologies. It is important to include fossil taxa in a phylogenetic analysis where there are long stem-group branches or where the crown group is highly derived.