Belyaevicrinus latipinnulus Mironov & Sorokina 1998

Belyaevicrinus latipinnulus Mironov & Sorokina, 1998

Figures 1–3, Tables 2–4.

Synonymy: Belyaevicrinus latipinnulus Mironov & Sorokina, 1998: 58 –61, fig. 23, pl. 9 (figs. 2–6) and 11 (fig. 3); Roux et al., 2002: 822; Roux & Lambert, 2011: 48, fig. 33–34; Hess, 2011: T174, fig. 85-2; Eléaume et al., 2014a: 210–211.

Material examined. USNM 1005436, off Chile, Drake Passage, S of Tierra del Fuego, 56°57’S to 56°56’S – 74°54’W to 74°43’W, depth 4209 m (1 specimen consisting of fragmented proximal crown and stalk). USNM 1005453 (2 dissociated specimens [B & C] consisting of basal rings, radials, columnals, brachials), South Orkney Islands, Scotia Ridge, 62°09’ to 62°16’S –38°11’ to 38°24’W, depth 3056–3459 m.

Emended diagnosis. Number of pinnules on each arm side> 10 in largest specimens; genital expansion wide with two rows of lateral plates on one side; cover plates with long and narrow projection. Pinnule socket large and close to distal margin of brachial. Brachial synostoses flat. Tegmen with interradial fields wide but not equal in their height; oral cone as tall as or taller than anal cone. Stalk symplexies with 7–8 crenular units of 1–3 crenulae, sometimes irregular and with hieroglyphic shape, especially in proxistele. Distal stalk syzygies with thin and irregular multiradiate crenularium.

Description. Measurements are given in Tables 2–4.

USNM 1005436 (Figs 1–2): Aboral cup height 11.9 mm; upper diameter at distal edge of radials 9.25 mm; basal ring height 5.8 mm; aboral edge of basal ring slightly flanged. About 5–6 large and convex tegminal plates per interradius; hydropores at top of small tubercles; orals 5, about half the height of the tegmen, sharp and concave (Fig. 1 A); oral cone taller than anal cone; anal cone rounded, well-differentiated, and located peripherally on tegmen (Fig.1 B).

Proximal arm pattern 1+2 3+4 5+6 (Fig. 1 A), with first pinnule on Br6 (5 cases) (Fig. 1 C); longest preserved arm pattern distally with 7+8+9 10+11+12 13+14+15+16; two other arms with 7+8+9+10 or 7+8 9+10. Largest arm with primibrachial width 2.24 mm; length of triplet (7+8+9) 3.42 mm; length of quadruplet (13+14+15+16) 5.19 mm; Br12 width 1.36 mm; Br16 width 1.29 mm. Smallest arm with Br1 width 1.76 mm. Ratio of radial to primibrachial width 2.7. Adoral architecture of arm multiplated with festooned cover plates in a gathered arrangement proximally (Fig. 1 E–F). Articular facets of brachial rounded; symplexies without conspicuous boundary between inner ligament and muscular areas (Fig. 1 G); brachial series without functional articulations united by flat synostosis (Fig. 1 H); pinnule socket large, close to distal margin of brachial (Fig. 1 I). Pinnules poorly preserved.

Length of preserved stalk 108 mm with ~60 columnals; length of proxistele 6.6 mm, proximalmost diameter 2.5 mm; other measurements given in Table 4. Proxistele partly damaged, composed of alternating thick (~ 0.5 mm) and thin columnals, and showing trend to chaotic pattern in proximalmost part (Fig. 1 D). Mid and distal stalk with columnals moderately barrel-shaped (Fig. 2 D, G–H); ratio of columnal height to diameter up to 0.8 in mid and distal mesistele. Symplexies slightly concave with 8 crenular units of 2 irregular crenulae in proximal part of proxistele (Fig. 2 A–B) and 1 short regular crenula in distal proxistele and mesistele (Fig. 2 C–E). Distal syzygies flat with thin and irregular multiradiate crenularium (Fig. 2 G–H), and with juvenile symplexial stage of 8 crenular units visible at center (Fig. 2 F).

USNM 1005453B–C (Fig. 3): Fragments of two entirely dissociated specimens; specimen C is a small juvenile, and larger specimen B is significantly smaller than USNM 1005436 (Table 2). Radials nearly rectangular; distal articular facet rounded, typical (Fig. 3 A); two parallel nerve grooves on inner surface of radials, distally masked by stereom development in specimen B (Fig. 3 B), and absent in juvenile (Fig. 3 C); inter-radial space between arms wider in specimen B than in juvenile. Basal ring conical, regularly everted, with possibly a single inconspicuous suture. Proximalmost heptagonal columnals preserved with 7 crenular units of 1 crenula of irregular hieroglyphic shape in specimen B (Fig. 3 D–G).

Remarks. This species was previously described from two poorly preserved specimens collected off the eastern side of the Scotia Arc at depths ranging from 5,530 to 5,651 m (Mironov & Sorokina, 1998). The Eltanin specimen USNM 1005436 differs in having a shorter and more rounded anal sac, larger and more concave orals, fewer tegminal plates, and 8 short crenular units of 1–2 crenulae in stalk symplexies. As in the holotype, columnals of the USNM 1005453 specimens display 7 crenular units of 1 irregular crenula each. The juvenile symplexial pattern of 8 crenular units is visible within the center of the distal syzygies in specimen 1005436. In USNM 1005453-B, the large proximalmost symplexies have 7 crenular units. The variation in number and length of crenular units is therefore due to intraspecific variation rather than to change during ontogeny. The main change during ontogeny was observed in radial morphology (Fig.3). The absence of nerve grooves on the inner surface of juvenile radials (Fig. 3 C) suggests that most of the aboral nerve complex remains attached to the visceral cavity, and begins to be partly embedded into the skeleton (grooves) at a later stage (Fig. 3 B). Bohn & Heinzeller (1999) and Roux et al. (2013) recently confirmed the importance of aboral nerve complex patterns in crinoid taxonomy. Ubaghs (1978) noted that nerves were progressively embedded into the aboral cup skeleton and arms during crinoid evolution. Therefore, the incomplete embedding of aboral nerves in the hyocrinid cup is a remarkable plesiomorphic character (Roux & Bohn, 2010). Unfortunately, the exact pattern of hyocrinid aboral nervous system remains unknown. The Eltanin juvenile USNM 1005453-C is the only known hyocrinid specimen without nerve grooves on the inner surface of the radials.

Occurrence. Off southern Chile, south of Tierra del Fuego to east of Scotia Arc, depth range: 3,459 to 5,530 m, possibly 3,056 to 5,651 m.