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 : 779-780

THE PAXILLOSIDA , NOTOMYOTIDA, AND THE PSEUDARCHASTERINAE

The sister clade to the large, primary grouping of Valvatacea (node F in Fig. 1 ) includes a wellsupported clade (node G) with taxa that have been previously included in the Paxillosida ( Astropectinidae , Ctenodiscidae , Goniopectinidae , Luidiidae ), the monotypic Notomyotida (Benthopectinidae) , and genera that have been included in the Pseudarchasterinae , a group that has historically been placed with the Goniasteridae (Valvatida) . The overall taxonomic composition of this clade is very similar to the composition and subgroupings of the Archasteridae sensu Sladen (1889) , which included primarily members of the Benthopectinidae and the Astropectinidae . The Pseudarchasterinae was recently reviewed by Blake & Jagt (2005) and included several fossil taxa in addition to Pseudarchaster, Paragonaster, Perissogonaster , and Gephyreaster . Pseudarchasterines were separated from other goniasterids by Blake (1987) but have been retained in the Goniasteridae , without distinction, in several recent accounts (e.g. Clark & Downey, 1992; A.M. Clark, 1993). Pseudarchaster has been interpreted as being similar to certain goniasterid-like astropectinids, such as Plutonaster with which it shares several skeletal character similarities but differs in that most species of Pseudarchaster have nonpointed tube feet. Rowe & Gates (1995: 68) stopped short of moving Pseudarchaster jordani to the Paxillosida based on this single character. The addition of Pseudarchaster to this clade effectively isolates this body form to the Paxillosida . Pseudarchaster has been included in several other molecular phylogenies that have either (1) loosely supported affinities with other valvatidan taxa (e.g. Knott & Wray, 2000) or (2) been ambiguous (e.g. Janies, 2001).

The position of the Notomyotida (Benthopectinidae) as members of the same clade as the Paxillosida (node G in Fig. 1 ) is consistent with the phylogenies of Blake (1987) and Gale (1987b). Both supported the Notomyotida as the sister group to the larger, primary Valvatida clade. The remaining genera in the primary ‘Paxillosida’ clade (node I in Fig. 1 ) formed two subclades, one of which (node J) included several taxa with a marginal plate series that forms a distinctive periphery frequently with acute triangular arms bearing prominent spines ( Astropecten , Thrissacanthias , Bathybiaster , Persephonaster , and Psilaster ) versus those with more nondescript marginal plate series with smaller to absent spines ( Dipsacaster , Leptychaster, Mimasterella , Macroptychaster , and Luidia ). These morphological distinctions are reflected in several traditional identification keys and classifications (e.g. Fisher, 1911; Clark & Downey, 1992). The primary ‘Paxillosida’ clade includes Luidia supported as a derived lineage amongst the astropectinids (node K).

Historically, the Paxillosida has included a diverse assemblage of asteroids that occur on bottoms with unconsolidated sandy or muddy sediment. Because of this environmental constraint, this body form may be predisposed to being buried rapidly and is relatively well represented in the fossil record. Amongst the several ‘paxillosid-like’ forms observed in the fossil record are the Pseudarchasterinae , the Astropectinidae , the Luidiidae , the Ctenodiscidae , and the Radiasteridae . The Pseudarchasterinae has traditionally been classified as a subfamily of the Goniasteridae (in the Valvatida ), but is here supported as a member of the Paxillosida .

Pseudarchaster portlandicus was described from the Jurassic of England by Blake (1986) and showed close affinities with living Pseudarchaster . Breton (1992) summarized and figured several Pseudarchaster -like Jurassic and Cretaceous taxa in the genus Comptoniaster . Based on our results, this suggests that these taxa maybe properly placed within the Pseudarchasterinae in the Paxillosida rather than the Goniasteridae in the Valvatida . Tethyaster is living today (e.g. Clark & Clark, 1954) and is known from Jurassic fossils ( Blake, 1986).

Cretaceous paxillosidans are relatively well represented in the fossil record. Fossil taxa close to Mimastrella include the Cretaceous Betelgeusia in the Radiasteridae , which was described by Blake & Reid (1998). Blake (1988a) described a Cretaceous ctenodiscid, Paleoctenodiscus , and identified the presence of cribiform organs in the fossil record. Gale (2005) described the Cretaceous Chrispaulia as the first fossil record of the Goniopectinidae , represented in Figure 1 by Goniopecten . Villier, Breton & Atrops (2007) described Prothrissacanthias from the Cretaceous of Algeria, which they described as morphologically close to Thrissacanthias and Persephonaster , which are both included in Figure 1 . Breton, Bilotte & Sigro (1995) described Dipsacaster jadeti from the Cretaceous of France. Luidia has been documented with a Miocene to Recent fossil occurrence (e.g. Blake, 1982) and it is likely to have diversified relatively recently. Its phylogenetic position does not disagree with the relative timing of its fossil occurrence.

Fossil material tentatively identified as Cheiraster has been identified from the Cretaceous by Blake & Jagt (2005) in addition to other fossils that have been described from the Cretaceous ( Blake & Reid, 1998; Jagt, 2000). The Jurassic Paleobenthopectininae as described by Blake (1984) were reassigned by Villier et al. (2009) to the Velatida and are discussed elsewhere.

Figures

Figure 1. Maximum likelihood tree of valvatacean sea stars rooted against forcipulatacean + velatid sea stars. The analysis is based on 1084 bp of concatenated sequence data from two mitochondrial rDNA genes and one nuclear protein-coding gene, early-stage histone H3. As discussed in the text, solasterids are treated as part of the ingroup. The analysis was carried out in RAxML with bootstrap percentages indicated for nodes that had> 50% support. Boxed letters A–P are referred to in the text. Traditional family names are shown at the right; all families are in the Valvatida except as noted. The scale bar shows the expected number of substitutions.