PAXILLOSIDA, sensu Blake, 1987

Mah, Christopher & Foltz, David, 2011, Molecular phylogeny of the Valvatacea (Asteroidea: Echinodermata), Zoological Journal of the Linnean Society 161 (4), pp. 769-788 : 780-781

THE PAXILLOSIDA : PHYLOGENETIC PERSPECTIVES AND INSIGHT ON THE ‘GREAT DEBATE’

The Paxillosida have occupied a prominent position in the discussions surrounding asteroid evolution and the early studies seeking out the ‘primitive’ asteroid ancestor. This discussion began with discussions between Mortensen (1922, 1923) and MacBride (1921, 1923) regarding the ‘primitive’ status of the Astropectinidae (Paxillosida) because of a number of morphological specializations, including, but not limited to pointed tube feet and the absence of a brachiolaria larvae. Mortensen argued for the primitiveness of the Astropectinidae whereas MacBride argued the opposite. Morphology-based phylogenies continued this discussion. Gale (1987b), following Mortensen, supported the Paxillosida as the basal sister taxon to the other living Asteroidea , whereas Blake (1987, 1988b) echoed MacBride, and supported the Paxillosida as having derived adaptations to living on a soft-bottom or unconsolidated, sediment type setting. Because of the focus on this argument, all subsequent phylogenetic studies have included paxillosidans, but most have given ambiguous results. Many of these studies have not supported a monophyletic Paxillosida (e.g. Wada et al., 1996; Knott & Wray, 2000; Matsubara et al., 2004) and those that have, include relatively limited taxon sampling (e.g. Lafay et al., 1995; Matsubara et al., 2005; Yasuda et al., 2006).

Although we do not have a definitive result supporting a basal lineage within the Asteroidea , our results do not place the Paxillosida as basal amongst the Valvatacea (following Mortensen, 1922, 123; Gale, 1987b) relative to our outgroup choice. The Paxillosida occur as derived relative to the Poraniidae and the clade containing the Ophidiasteridae + Mithrodiidae .

All of the included taxa on node G possess a ‘Paxillosida’ type morphology, which is associated with unconsolidated, soft-bottom substrate settings. All included taxa possess characters, such as the presence of paxillar abactinal and marginal plates, pointed tube feet and well-developed fasciolar grooves between the abactinal, marginal, and actinal plates, which imply a derived morphology (sensu MacBride, 1921, 1923 and later Blake, 1987, 1988b). It should be noted that our tree includes not only the Paxillosida sensu Blake (1987) but also the Notomyotida and the Pseudarchasterinae .

Archaster closely resembles Astropecten and possesses several similar morphological characteristics. Archaster was not included in our analysis because of a lack of histone H3 sequence data, but all prior studies from morphology (e.g. Blake, 1987) and molecules ( Knott & Wray, 2000; Matsubara et al., 2004) do not suggest that it shows any close relationship with the Paxillosida , suggesting that its Astropecten - like appearance is a result of convergence.