Asterodiscides sp.

Mah, Christopher L., 2021, The East Pacific / South Pacific Boundary: New taxa and occurrences from Rapa Nui (Easter Island), New Caledonia and adjacent regions, Zootaxa 4980 (3), pp. 401-450 : 410-412

Asterodiscides sp.

Figure 4A–B, E–F

Comparison of Small Sized Asterodiscides with Pauliastra

Newly discovered specimens of Asterodiscides at small sizes (R=0.8 and 0.2) suggest further insight into the phylogenetic history of the Asterodiscides and related taxa. These specimens were immediately comparable to individuals of Pauliella (now Pauliastra ) aenigma ( Ludwig, 1905) , sharing several characters including the bullet-shaped tubercles on the abactinal and actinal surface and the strongly arched superomarginal plates, conical intermarginal spines and pentagonal body shape. However, the New Caledonia specimens differed in lacking granules on the abactinal plates around the bullet-shaped tubercles as Pauliella does. The arched superomarginals, also differ in only having the central area projecting rather than the entire surface as in Pauliella . Furrow spines in small-sized Asterodiscides were about 25% the length of those in Pauliella . Subambulacral spines which were also more slender being only 50% of the thickness of those in Pauliella . Rowe (1977) has described similar examples of comparably sized Asterodiscides .

Fig. 2 shows a preliminary phylogeny of the Asterodiscididae showing Pauliella (= Pauliastra ) as the sister taxon to a clade containing Asterodiscides and Uokeaster n. gen.

This could suggest a heterochronic explanation for Pauliastra , perhaps displaying retained traits from the shared ancestor of Asterodiscides and Uokeaster .

Significance to other Valvatida . The smallest sized individual of Asterodiscides sp. (R= 0.2 cm) displays an oval- shaped, strongly-arched body with angular actinal plates in a chevron-like pattern with a relatively indistinct marginal plate series ( Fig. 4E–F ). This body form invites immediate comparison with the enigmatic sphere shaped valvatidan asteroid Podosphaeraster (Podosphaerasteridae) which also has indistinct marginals and angular actinal plates in a chevron formation. Affinities of Podosphaeraster have been controversial, but it is hoped that observations such as this will be useful in understanding the phylogeny and evolution of the group.

Description. Body pentagonal (R/r=1.0–1.6), arms triangular, interradial arcs curved, arm tips upturned. Abactinal plates round to polygonal on radial and proximal regions, quadrate with angular sides interradially. Primary circlet with largest, disk plates circular, plates along radial regions round to polygonal. Smaller interradial plates becoming smaller more irregular in shape distally adjacent to superomarginals. Plates tabulae-like elevated above body surface forming distinct fasciolar grooves, which are deeper proximally and along radial regions, becoming shallower interradially. Papulae single present at each plate angle, six per plate along radial regions, absent interradially. Each plate with one to three, mostly one, large, smooth dome-shaped tubercle occupying center surface. Plates with 2 to 30, mostly 10 to 20 widely-spaced, round to quadrate shaped granules. Plate surface between tubercle and peripheral granules, smooth and bare. No pedicellariae.

Marginal plates, smooth, bare, with convex surface with no central surface accessories. Superomarginals seven, inferomarginals twelve with both plate series offset forming zigzag contact between them. Interradial superomarginal number variable, six on four sides, seven on the fifth, with enlarged, strongly convex pre-terminal superomarginal adjacent to the terminal plate. Enlarged pre-terminal superomarginal plates in direct contact. Remaining superomarginal plate strongly convex forming a table-like raised surface on each. Three or four round spherical tubercles present intermarginally between superomarginals and inferomarginals interradially. Inferomarginals each with large, prominent conical spine extending from the surface of the five or six distalmost plates adjacent to the terminus. Marginal plates each with 20 to 30 large, widely spaced, coarse, quadrate to round granules. Peripheral granules on interradial marginals slightly enlarged adjacent to the enlarged tubercles/spines. Fasciolar groove welldeveloped between superomarginal plates. No pedicellariae.

Actinal surface in three complete chevrons in weakly, rounded series. Each plate quadrate to polygonal in outline, each with one to ten coarse, widely spaced granules. Most actinal plates with one to five large granules present centrally on each plate and one to eight smaller irregularly sized granules forming widely-spaced, inconsistent periphery around them. Proximal plates with greatest number of central, large granules with distalmost plates reduced to a single enlarged tubercle or granule, adjacent to inferomarginal contact. Fasciolar groove around actinal plates well-developed.

Furrow spines three or four, blunt-tipped, widely spaced, cylindrical in cross-section in weakly palmate series. Two subambulacral spines immediately adjacent to furrow spines, with one or both enlarged. When enlarged, subambulacrals are twice as thick as the furrow spines and slightly taller. When subambulacrals are not enlarged, they are similar in thickness to furrow spines but about 50% of furrow spine height. A third subambulacral series of two or three, primarily two, granules is also present on each adambulacral plate.

Oral plates each with five furrow spines, identical to those on other adambulacral plates, with one enlarged spine per plate directed into mouth, forming a pair in each interradius. Oral plate surface with two differently sized subambulacrals adjacent to furrow spines, in addition to five paired round to irregularly shaped granules present along the fossae created by the paired oral plates.

Specimen deep-red or orange along radial regions, which extend to the oral surface forming irregular blotches on actinal interradius surface.

Material examined. MNHN IE-2007-6499 . New Caledonia, 22º53′S, 169º26′E, 100– 280 m. Coll. EXBODI DW3865, 16 Sept. 2011. 1 wet spec., R =0.8, r=0.5 . MNHN IE-2013-7014 New Caledonia, 24° 11′S, 159°42′E to 24°12′S, 159°41′E, 300 m. Coll. KANADEEP DW4954, 3 Sept. 2017. 1 wet spec. R =0.2, r=0.2.

Figures

FIGURE 2. Phylogenetic tree for Asterodiscididae. Tree shows all known genera (n = 7) including Goniaster and Uokeaster n. gen. and two outgroup taxa. Analysis outlined in Methods section. Character matrix and morphological characters listed in Appendix I. OG = Outgroup.

FIGURE 4. Asterodiscides small specimens. MNHN IE-2007-6499 (New Caledonia): A. Abactinal. Scale bar equals 0.5 cm. B. Actinal surface. Scale bar equals 0.5 cm. MNHN IE-2013-7014 (New Caledonia). E. Abactinal. Scale bar equals 0.1 cm. F. Actinal surface. Scale bar equals 0.1 cm. Inset shows lateral view of specimen. Total length = 4.0 mm. Pauliastra sp. USNM 1020006 (Costa Rica): C. Abactinal. D. Actinal surface. Scale bar equals 0.5 cm.