Phylogeny of the Apodan Holothurians (Echinodermata) inferred from morphology

The Apodida is an order of littoral to deep-sea, largely infaunal sea cucumbers with about 270 extant species in 32 genera and three families, Synaptidae, Chiridotidae and Myriotrochidae. In this study, I perform the first phylogenetic test of the taxonomic and palaeontological hypotheses about evolutionary relationships within Apodida by using cladistic analyses of 34 morphological characters. I introduce several previously unconsidered synapomorphic characters, examine the relationships between all recognized suprageneric taxonomic groups and assess the assumptions of monophyly for each family. Maximum-parsimony analyses of type species from 14 genera and use of three rooting methods recovered identical topologies at the subordinal level and subfamily level within Synaptidae. Overall, the current higher-level classification of Apodida was well corroborated. Within Synaptidae, the relationships (Synaptinae, (Leptosynaptinae, Rynkatorpinae)) are well supported. The monophyly of Chiridotidae was not supported and appears paraphyletic at the subfamily level. Calibrating the phylogenetic hypothesis of Apodida against the fossil record indicated that most higher-level divergences occurred within the Palaeozoic, unlike that of extant non-holothuroid echinoderms, which radiated in the early Mesozoic. Synaptidae appears to have radiated during the Lower Cretaceous. Alternatively, and if one discounts the considerable ghost lineage duration that this hypothesis requires, they may have radiated during the Eocene.