Asterodiscididae Rowe 1977

ASTERODISCIDIDAE Rowe, 1977

Rowe 1977: 190; 1985: 532; Rowe & Gates 1995: 42.

Diagnosis. Abactinal surface with conical spines with broad bases or tubercles. Superomarginal spines strongly convex to strongly arched, demonstrating a spine-like process in some genera. Intermarginal plates in all save Kionaster. Intermarginal papulae present in all but Goniaster and Kionaster.

The Asterodiscididae. The Asterodiscididae was established by Rowe (1977) to accommodate genera showing morphological affinities intermediate between the Goniasteridae and the Oreasteridae. The group included genera, such as Asterodiscides, which was originally classified within the Oreasteridae and others such as Amphiaster, which was originally placed with the Goniasteridae. The current Asterodiscididae includes four genera, Asterodiscides, which includes 18 species distributed throughout the Indo-Pacific, as well as two monotypic taxa, Amphiaster insignis from Baja California and Mexico, Paulia horrida, known primarily from the Galapagos, and Pauliella aenigma from the Cocos Islands (Costa Rica) (Ludwig 1905). Blake & Portell (2011) later added the Miocene fossil Kionaster from Florida.

Rowe (1977) summarized and discussed consideration of Pauliella aenigma Ludwig, 1905 as a juvenile of Paulia. This account follows Hendler (1996) who documented two adult specimens of Pauliella aenigma from Rocas Alijos and listed them as a distinct species rather than a synonym of Paulia. Rowe (1977) further outlined that Pauliella Ludwig, 1905 was a homonym of Pauliella Munier-Chalmas, 1895 (Mollusca) and would need to be reconciled should the genus be found to be valid. This is dealt with herein.

Mah (2005) in an unpublished analysis of the Goniasteridae found Goniaster tesselatus was recovered on the same clade as other asterodiscidid genera. Goniaster was not considered among asterodiscidid taxa as outlined by Rowe (1977, 1985). Fisher (1911: 168) observed that Amphiaster was more closely related to Goniaster than to other Oreasteridae. Blake & Portell (2011) ran a subsequent analysis including the newly described Miocene Kionaster petersonae from Florida and also supported Goniaster as a closely related sister taxon to other asterodiscidid genera.

Character discussion for Goniaster in the Asterodiscididae. The monotypic Goniaster tesselatus has not been historically considered a member of the Asterodiscididae, save for the aforementioned work by Mah (2005) and Blake & Portell (2011). Several distinctive characters support the placement of Goniaster with the Asterodiscididae, including the strongly convex marginal plates (characters 2.11, 2.13) which vary from strongly arched (e.g., Pauliastra) to having a spine-like process (e.g., Goniaster, Paulia) to round tubercles (e.g., Asterodiscides) especially those in the superomarginal series. Thus, “spines” (character 2.11-state 1) in asterodiscidids are essentially solid, conical to spine-like processes formed of the marginal plates. Thus, the tubercles present on the body of Asterodiscides appears be an expression of the marginal plate rather than an accessory on the plate, as it is in other goniasterids. Proper spines in other goniasterids, such as on Hippasteria or Calliaster are closely-articulated with the marginal, or corresponding basal plate. These latter, “goniasterid” spines, given dissolution of the connective tissue present, disarticulate from the underlying basal plate. This is reflected in character coding summarized in Appendix I.

Other characters placing Goniaster among the Asterodiscididae include the presence of intermarginal plates (character 2.9), which are figured by Blake & Portell (2011: Fig. 4b) in Goniaster and present in all asterodiscidids but the Miocene Kionaster petersonae. The large conical spines present in Goniaster are also present in Kionaster and all of the Eastern Tropical Pacific genera, including Amphiaster, Paulia, Pauliastra and Uokeaster.

Goniaster and the Asterodiscididae and/or Goniasteridae. A full phylogenetic analysis and overview of the historical Goniasteridae is beyond the scope of this treatment. However, as argued herein, distinctive characters on Goniaster are argued as synapomorphies for the Asterodiscididae. Placement of Goniaster within the Asterodiscididae is accompanied by two possible phylogenetic scenarios, which have been suggested by available data. Discussions of Goniaster being unaffiliated with the Asterodiscididae or being sister clade to the Asterodiscididae are not addressed here. The scenarios herein are contingencies based on what I argue are conclusions from the data presented. This is presented to facilitate future research in this area as new data becomes available.

The first possible phylogenetic scenario is the historical and current taxonomic classification presented by Rowe (1977, 1985) and summarized in Clark (1993) and Mah (2018b), which present the Asterodiscididae and Goniasteridae as separate, nominally monophyletic groupings with no nested relationship, and assumes that the Asterodiscididae is present within the Valvatacea, but distinct from the Goniasteridae. If the nominotypical Goniaster is placed within the Asterodiscididae within this context, then the Asterodiscididae would be synonymized within the Goniasteridae, leaving the historical definition of the Goniasteridae in need of a new designation. Possible names for this grouping suggest the oldest of possible groupings within the historical Goniasteridae, such as the Pentagonasterinae Perrier, 1884, but a comprehensive account would require identification of groupings within the Goniasteridae. Other subfamilies within the Goniasteridae could present other appropriate designations.

The second possible phylogenetic scenario suggests that the Asterodiscididae is a subfamily within the Goniasteridae. This was suggested by an unpublished phylogenetic analysis of both families, based on morphological data by Mah (2005) and was implied by data presented in Blake & Portell (2011). Also consistent with this notion was preliminary molecular phylogenetic data presented by Arai and Fujita (2018) showing Asterodiscides as lodged within the Goniasteridae. If Goniaster is placed within the Asterodiscididae as a subfamily of the Goniasteridae, this would result in the “Asterodiscidinae” becoming the “ Goniasterinae ” joining several other subfamilies within the Goniasteridae, as currently summarized by Mah (2018a).

Phylogenetic analysis: Taxonomy and Biogeography. A survey of asterodiscidid genera shows the group as monophyletic as supported by the presence of intermarginal papulae and intermarginal plates with smooth, pointed to rounded superomarginal plates. The tree generated herein is shown in Fig. 2 and is based partially on data from Mah (2005) but differs in that an additional taxon, Uokeaster, n. gen. is included. The tree presented in Fig. 2 and by Blake & Portell (2011) both support Goniaster as a member of the Asterodiscididae. Blake & Portell (2011) show Kionaster as the sister clade to a second branch containing the other included genera (Asterodiscides, etc.) with Goniaster as the sister group. Figure 2 shows Goniaster as the sister clade to a branch with Kionaster and the other “asterodiscidid” genera. In this respect both datasets agree that Kionaster and Goniaster occupy morphologically distant locations relative to the other “asterodiscidid” genera which occur on a single clade in both Fig. 2 and Blake & Portell’s (2011) trees.

Blake & Portell (2011) propose an east to west (Atlantic to Indo-Pacific) trend of diversification within the Asterodiscididae with more taxa, such as Kionaster and Goniaster with western Atlantic affinities as the basal taxa relative to the more derived Asterodiscides in the Indo-Pacific. This is in contrast to Rowe (1985) and Lane and Rowe (2009) who posited origination of asterodiscidids in the central Pacific with subsequent spread to the Atlantic. Phylogenetic patterns inferred from data herein (Fig. 2) agree broadly with those of Blake & Portell (2011) in that both basal taxa, Goniaster and Kionaster show occurrence in the western Atlantic followed by westward diversification with subsequent diversification of Asterodiscides throughout the Indo-Pacific region. Uokeaster n. gen. from Easter Island approximates the biogeographic boundary between more stemward asterodiscidids and the diversification of Asterodiscides.

(0) Actinal plates with prominent spherical tubercles or conical spines. Superomarginal plate number <9 per interradius (arm tip to arm tip)............................................................................................ (2)

(0) Actinal plates lacking tubercles or any primary ornamentation. Superomarginal number 10-18 per interradius........... (1)

(1) Conical spines present on abactinal plates. Abactinal plate surfaces which are lacking spines are flat and covered by close-set granules. Secondary plates granular, quadrate, close-set interspersed between larger abactinal plates. Disk plates flat. Spine-like projections on all superomarginals, especially in larger specimens (R>1.0 cm). Superomarginals, quadrate in outline, numbering 8 at R=1.2 to 18 at R=9.5. Actinals in two to three full chevron-like series. Tropical Atlantic............................................................................................. Goniaster tessellatus (Lamarck, 1816)

(1’) Conical spines absent from abactinal surface. Primary plates smooth, enlarged with broad, flattened tops. Disk plates weakly convex. Paired spine-like projections on interradial and basal superomarginals. Actinals few, 10–12 in a single series. Known only from the Miocene of Florida....................................... Kionaster petersonae Blake & Portell, 2011

(2) Penultimate superomarginals not enlarged, superomarginals forming gradual series to terminus....................... (4)

(2’) Enlarged penultimate superomarginals................................................................... (3)

(3) Abactinal, marginal, actinal surface intermediate plate surfaces covered by large, round, spherical tubercles with body surface covered by coarse polygonal granules. Indo-Pacific (Hawaii, southern Japan to New Caledonia and east to the Indian Ocean)...................................................................................... Asterodiscides spp.

(3’) Abactinal, marginal, actinal surface covered by prominent, conical spines, with body surface covered by a distinct dermal tissue. Rapa Nui (Easter Island)....................................................... Uokeaster ahi n. gen. n. sp.

(4) Abactinal, marginal and actinal plates with prominent conical spines. Body stellate (R/r= 2.2–2.5).................... (5)

(4’) Abactinal plates with short, pointed tubercles. Marginal plates strongly tumid to arched.Actinal plates with short, bullet-shaped tubercles and coarse granulation. Body pentagonal (R/r=1.4). Cocos Island, Costa Rica.......................................................................................... Pauliastra enigma (Ludwig, 1905) n. gen, n. comb.

(5) Superomarginal plate surface (non-spine) covered by coarse granules. Superomarginal plates lateral facing, boundaries indistinct, covered by granules. Subambulacral spines in a two series. Galapagos and Peru............ Paulia horrida Gray, 1840

(5’) Superomarginal plate surface (non-spine) smooth. Superomarginal plates form dorsal-facing frame. Subambulacral spines in a single series, but two in larger individuals. Coastal Baja California to Panama............. Amphiaster insignis Verrill, 1868