Arthraster Forbes, 1848

Gale, Andrew S., 2020, The fossil record of the asteroid (Echinodermata) family Chaetasteridae Sladen 1889 and subfamily Hyalothricinae Fisher, 1911, Zootaxa 4858 (1), pp. 144-150 : 145

Genus Arthraster Forbes, 1848

Type species. Arthraster dixoni Forbes, 1848 by original designation.

Diagnosis. Long-armed, robust chaetasterids, in which the extraxial arm is constructed of 7 rows of ossicles, including 1 row of large radials, paired rows of supero- and inferomarginals and broad actinals. Arm ossicles with tall transverse crest; abactinals of disc small, round, with raised central region.

Included species. In addition to the type species, A. cristatus Spencer, 1907 .

Remarks. Arthraster shares a number of similarities with Chaetaster , most obviously the long, subcylindrical arms and small disc, the fact that the abactinal, marginal and actinal ossicles are of similar size, and arranged in longitudinal rows. The extraxial ossicles of the arm (radials, marginal, actinals) of Arthraster ( Fig. 1A, B ) bear transverse ridges, similar to those present on the interradially positioned adradials and marginals of Chaetaster ( Fig. 1D ). The abactinal ossicles of the disc of Arthraster , revealed by preparation of the abactinal surface of the holotype of A. dixoni ( Fig. 1B ), are small and oval to rounded, as in Chaetaster . The differences between the genera include the smaller number of extraxial arm ossicle rows in Arthraster (7, rather than 13 in Chaetaster ), and the presence of crested ridges on the arm and abactinal ossicles; the raised central regions are flat-topped in Chaetaster .

Figures

FIGURE 1. A, B, Arthraster dixoni Forbes, 1848. Holotype specimen NHMUK 47000, “Lower Chalk”, probably Middle Turonian, Balcombe, Sussex, UK.A, actinal surface, B, enlargement of abactinal surface and base of arms. C–F, Chaetaster longipes (Bruzelius, 1805). C, enlargement of actinal arm. D, enlargement of abactinal disc. E, actinal view of specimen. F, abactinal view of specimen. Recent, Mediterranean.