Jacksonaster, Lambert in Lambert & Thiery, 1914

Arachchige, Gayashan M., Jayakody, Sevvandi, Mooi, Rich & Kroh, Andreas, 2019, Taxonomy and distribution of irregular echinoids (Echinoidea: Irregularia) from Sri Lanka, Zootaxa 4541 (1), pp. 1-100 : 47-49

Jacksonaster sp. 2

Figures 39 , 40B , 41

Material studied. Two denuded tests: WUSL/EI/25, EI/29 from Silavathurai, northern coast of Sri Lanka.

Description. Shape and size —Test elliptical to rounded; medium-sized, 32.1 mm and 43.0 mm TL, longer than broad, width 87% and 89% TL; greatest width coinciding with anterior paired petals; TH 18% TL; oral side flat; edge thick, anterior edge thickness approximately 12% TL, posterior edge slightly thinner than anterior edge of test.

Apical system —Subcentral, 49% and 50% TL from anterior margin of test; with five circular gonopores; hydropores in sinuous groove.

Ambulacra —Petaloid area large, 73% and 77% TL, slightly elevated; petals narrow, closed distally; outer pore of each pair more elongate than inner pore; pores conjugate, but conjugating furrows inconspicuous; anterior unpaired petal slightly longer than others, 35% and 38% TL; anterior paired petals shortest, 31% and 32% TL; posterior paired petals slightly shorter than petal III 34 % and 35% TL; all petal widths similar, approximately 12% TL; width of poriferous zone IIIa 128% and 103% of interporiferous zone at 2/3 of the petal length; food grooves simple, unbranched, reaching up to 64% and 71% of corresponding test radius of ambulacrum III.

Interambulacra —On oral side, interambulacra narrow, forming straight bands separating broad ambulacra.

Tuberculation —Aboral primary tubercles small and densely distributed among numerous miliary tubercles; on oral surface, primary tubercles larger and sparse relative to those on aboral surface.

Peristome —Small, length approximately 7% TL, width approximately 9% TL; rounded to pentagonal; situated slightly anterior of centre, 46% and 48% TL from anterior margin of test.

Periproct —Small, length 5% and 7% TL; transversely elongate; width 6% and 8% TL; lying on oral surface 13% and 14% TL from posterior margin of test.

Observed occurrence in Sri Lanka. Denuded tests were collected from Silavathurai, northern coast of Sri Lanka at 28 m ( Fig. 41 ).

Remarks. The two specimens of Jacksonaster sp. 2 were collected from a different locality to Jacksonaster sp. 1. They can be distinguished from Jacksonaster sp. 1 in having a more elliptical, rounded test outline, thicker test margin, relatively larger petaloid area, relatively closed petals, wider interporiferous zones ( Fig. 40B ) and the periproct situated further from the posterior margin.

Using the key of Clark and Rowe (1971), these specimens resemble Jacksonaster depressum . However, as stated in Mortensen (1948b), the posterior end of Jacksonaster depressum is usually “reenteringly curved” (indented). This feature was not observed in Jacksonaster sp. 2.

These specimens are similar in outline to Laganum joubini , but L. joubini has a more anterior periproct and more anterior apical system. Jacksonaster in Sri Lanka requires further examination and revision, perhaps through future molecular analyses.

Figures

FIGURE 39. Jacksonaster sp. 2 (A–C, WUSL/EI/25; D, E, WUSL/EI/29): A, D, aboral view; B, E, oral view; C, lateral view.

FIGURE 40. Comparison of apical system and petaloid area of Jacksonaster sp. 1 (A, WUSL/EI/26) and Jacksonaster sp. 2 (B, WUSL/EI/29).

FIGURE 41. Distribution map of Jacksonaster sp. 2 recorded in this study.