Arthraster dixoni 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 : 145-148

Arthraster dixoni Forbes, 1848

Figures 1A, B , 2A, B

1848 Arthraster dixoni Forbes : p. 467.

1850 Arthraster dixoni Forbes : p. 336, pl. 23 fig. 1.

p 1907 Arthraster dixoni Forbes. —Spencer: p. 91, pl. 18: fig. 1 only.

1913 Arthraster dixoni Forbes. —Spencer: p. 140.

1918 Arthraster dixoni Forbes. —Spencer: p. 168.

1966 Arthraster dixoni Forbes. —Spencer & Wright: U74, fig. 64.8.

Diagnosis. Arthraster in which the crest on the extraxial arm ossicles is low and weakly sculptured.

Type. NHMUK 47000, Middle Turonian of Balcombe , Sussex, U.K.

Description. Arms cylindrical, elongated, disc small ( Fig. 1A ). Arms made up of 7 rows of extraxial ossicles, including radials (1), marginals (2 pairs) and actinals (1 pair). Actinals, inferomarginals, superomarginals in longitudinally and transversely aligned rows, bearing central, transverse crest. Radials with tall crest, concave distally. Abactinal surface of disc ( Fig. 1B ) rimmed by broad supero- and inferomarginals, including single interradial pair. Abactinal ossicles of disc small, round to oval, bearing central process. Adradials small, 4 pairs in each arm, only extending into base of arm.

Remarks. The well-preserved actinal surface of the holotype has been illustrated a number of times (see synonomy, above), and the basic construction of the arm has been described. However, Spencer (1918) and Spencer & Wright (1966: fig. 64.8) mistook the two rows of actinal ossicles for adambulacrals, which appear on the edges of the ambulacral groove. In 2005, I was permitted to develop the part of the abactinal surface of the disc and basal arms of the holotype, which showed considerable similarities with the same parts of the living genus Chaetaster . These include the ridged interradial marginal ossicles, and the small abactinal ossicles of the disc.

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.

FIGURE 2. A, B, Arthraster dixoni Forbes, 1848, small individual, in abactinal (A) and actinal (B) views; Middle Turonian, Dieppe, France; Collection of Nicolas Cottard, Offranville, Normany, France. C–I, Chaetasterina gracilis Hess, 1970. Holotype (NMB Hess A 94), Upper Hauterivian, Neuchâtel, Switzerland, original of Hess (1970: figs 9–14, pl. 3: fig. 5); C, abactinal view; D, actinal view; E–G enlargement of abactinal ossicles; H, I, enlargement of adambulacrals (ad) and inferomarginal (im). J–M, Hyalinothrix sp., Recent, Philippines, AS Gale coll. J, actinal view; K, abactinal view; L, enlargement of ossicles of denuded ambulacral groove; M, enlargement of denuded abactinal surface.