Plinthaster Verrill 1899

Plinthaster Verrill, 1899

Verrill 1899: 161; Fisher 1906: 1052 [as Tosia section B (Plinthaster)]

Verrill 1899: 166; Fisher 1911: 165 (as Pyrenaster)

Fisher 1910: 172; 1911: 165; Halpern 1970a: 131; Halpern 1970b: 244; McKnight 1973: 185; Downey in Clark & Downey 1992: 259; H.E.S. Clark in Clark & McKnight 2006: 110 (as Plinthaster)

Diagnosis. Body weakly stellate to pentagonal. Abactinal plates, flat, bare central surface, polygonal, no surficial granulation but with crystalline nodules/tubercles or radiating patterns present on subsurface. Marginal plates relatively few per interradius, with widely spaced coarse granules but otherwise smooth or bare. Actinal surface covered by granules.

Comments. Plinthaster along with Astroceramus and Sibogaster are among several Goniasteridae which display crystalline nodules embedded in the abactinal plate surface and which also possess bald, smooth abactinal plates with few to no granules on the marginal surface. These differ from others, such as Stellaster and various members of the Ferdininae, such as Kanakaster which display crystalline nodules in conjunction with a granulated tegument covering the surface. Based on historical definitions from the literature (e.g Doderlein, 1924), Plinthaster, Astroceramus and Sibogaster have shown relatively distinct morphological boundaries, but examination of further diversity within each suggests overlap between them. Smaller sized (R=5.0) Sibogaster, for example superficially resemble very stellate Plinthaster [e.g. “ Eugoniaster ephemeralis ” in Macan (1938)]. Similarly, pentagonal shaped or small-sized Astroceramus can also superficially resemble Plinthaster.

Plinthaster dentatus, which occurs widely in the Atlantic and possibly in the south Pacific is morphologically varied leading to a wide synonymy. A comprehensive survey of Indo-Pacific Plinthaster has never been undertaken but it seems likely, based on unpublished observations, that there is considerable, if not greater variation among Pacific species.

What are Plinthaster (ex. Pentagonaster) döderleini, P. investigatoris and P. cuenoti?. Koehler (1910) described three Indian Ocean species of Pentagonaster, which have found current assignment to Plinthaster including Plinthaster ephemeralis, Plinthaster investigatoris, Plinthaster döderleini and Plinthaster cuenoti. Plinthaster ephemeralis was found to be a synonym of Sibogaster digitatus Döderlein 1924 by Mah (2015). Evaluation of these other two species further suggests that assignment to Plinthaster is unwarranted. In both species, the number of marginal plates (~25–34) per interradius is high and exceeds the number present in any of the known Plinthaster species. The abactinal plate patterns and plate morphology, as described for P. investigatoris and P. döderleini as figured by Alcock (1893) and Koehler (1910) respectively, suggest that these species have been incorrectly assigned to Plinthaster.

The description of Pentagonaster investigatori s Alcock indicates that the abactinal plates are tabulate, and are distinctly isolated from one another but with a fringe of squamous membrane clad granules flush with the general surface and with marginal plates bearing a small central, elevated patch of deciduous granules. A type specimen for this species is unavailable and is most likely lost, but based on the description, this species seems best assigned to Pillsburiaster. Other species of Pillsburiaster display the same general body shape, small, irregular abactinal plate pattern, raised granulated marginal patches and granules on the abactinal surface. The marginal plate number and furrow spine number are also more similar to other known Pillsburiaster species.

Pentagonaster döderleini ’s description in Koehler (1910) is also very compatible with placement in Pillsburiaster. Similarities are similar to those observed in P. investigatoris in terms of marginal plate number and irregular abactinal plate pattern. Pentagonaster cuenoti Koehler that Fisher (1919: 260) thought was similar to Ceramaster also displays characters, such as the abactinal plate pattern and marginal plate morphology, granulated raised marginal patch and marginal plate number that are most compatible with Pillsburiaster. Absence of granules on the surface of the abactinal plates can be attributed to abrasion during collection or possibly variation as has been observed in collections of the Atlantic Pillsburiaster geographicus.

Neither description of the above species mentions the presence of crystalline nodules in the abactinal plate surface which is a synapomorphy of Plinthaster. Also noteworthy is the extreme occurrence depth of both species, with Plinthaster döderleini collected from 2160 m and P. investigatoris from 1380 m which is consistent with collection depths (800 to 2200 m) of accepted Pillsburiaster spp. These two species are also consistent with Ceramaster and Peltaster but descriptions of abactinal and marginal plate morphology are not as identical.

A search for the types of these species in the MNHN collections was unsuccessful and it is likely that this material is lost.