Feracrinus heinzelleri Bohn

Feracrinus heinzelleri Bohn in Eléaume et al., 2012

Figures 4–8, Tables 5–8.

Synonymy: Ptilocrinus antarcticus John, 1937: 3 –8, fig. 3; Gislén, 1939: 3–5, figs. 1–7; Feracrinus heinzelleri Bohn in Eléaume et al., 2012: 8–12, figs. 4–6; Eléaume et al., 2014: 210–211.

Material examined. USNM E48124, Weddell Sea, Queen Maud Land, W of Cape Norwegia, 71°40’S – 15°35’W, depth 1,445 m (1 specimen with nearly complete stalk and broken calyx and arms); USNM 1005435, Ross Sea, Marie Byrd Land, E of Pennell Bank, 74°06’ to 74°06’S –175°05’ to 174°58’W, depth 2,350 m (1 well-preserved crown with proximal stalk); USNM 1126892, Shetland Islands, 57°50’ to 57°54’S –56°51’ to 56°51’W, depth 3,788–3,944 m (1 crown with proximal stalk, and isolated arms only from one larger specimen); USNM 1126896, center of Ross Sea, 75°46’ to 75°52’S –168°51’ to 168°53’W, depth 2,049–2,089 m (2 specimens; one calyx with proximal arms, another with proximal arms and proximal half of stalk); USNM 1126873, Weddell Sea, 72°49’36”S – 30°29’42”W, depth 3,697 m (1 theca with proximal arms and proximal half of stalk).

Emended diagnosis. A small to moderate size species with robust arms of proximal pattern 1+2 3 4 5+6 (92% of arms), with first pinnule on Br4 (6% on Br5); first free brachial beyond Br4 varying from Br7 to Br15 and usually fewer than 4 successive muscular articulations. Pinnules relatively robust and rigid; ratio of pinnule to arm length about 0.4; insertion deep, carved on arm side and indenting the following brachial; synarthrial articulation on brachial with strong symmorphy; concave cover plates varying from lanceolate to rounded; genital pinnule inflated with one row of additional H-shaped plates. Ratio of distal radial to primibrachial width 1.2–1.8 (usually 1.3–1.5). Tegmen with orals large and erect around mouth; anal cone more or less conspicuous in C–D inter-ray and bearing large bulging plates; ratio of anal cone to oral cone height 0.7–1.3 (usually 0.8–1.1). Conical aboral cup with ratio of cup height to distal diameter 1.0–2.1 (usually 1.2–1.3); basals fused; radials trapezoidal. Columnals barrel-shaped distally; ratio of height to diameter up to 0.9 in distal mesistele; stalk symplexies with 6 to 8 crenular units of 1–3 crenulae (usually 1–2); distal syzygies with irregular radial crenularium.

Description. Small to moderate size species; largest specimens with proximal stalk diameter 2.6 mm. Proximal arm pattern usually 1+2 3 4 5+6 with first pinnule on Br4 (24/27 cases); one arm 1+2 3+4 5+6 with first pinnule on Br4 (USNM 1126892-1), and two arms 1+2 3 4+5 6+7 with first pinnule on Br5 (USNM 1126892-2); first free brachial beyond Br6 frequently Br11 or Br13 (5 cases each) but variable with Br7 (1 case), Br9 (2 cases) and Br12 (1 case); free brachials usually alternating with brachial pairs in mid and distal arm, always fewer than 4 successive muscular articulations except in one abnormal arm (broken after Br15) with 7 successive muscular articulations and free Br9 without pinnule (USNM 1126896-1). Only two arms nearly complete were observed: USNM 1005435: arm length 37 mm with 11 pinnules on each side; maximum pinnule length 15.6 mm; ratio of pinnule to arm length 0.4; distal arm pattern 21+22 25+26 28+29 31+32 35+36; USNM 1126892-1: arm length 34.9 mm, of 33 brachials; distal arm pattern with brachial pairs alternating with free brachials. Pinnule socket deeply incising side of brachial and indenting following brachial especially in large specimens (Fig. 5 A–C) but not in unique USNM 1126873 (see below). Cover plates of arms and pinnules (Fig. 5 D–G) rounded, of same size, with more-or-less developed lateral plate at their base. Genital pinnules usually with one regular row of H-shaped plates (Fig. 5 E). USNM 1126896-2 with proximalmost genital pinnules short, with smaller and more irregular cover plates (Fig. 5 G). Table 5 lists main arm pattern variations; tables 6 and 7 list morphological parameters. USNM 1126892-1 illustrates the most frequent feature of the theca (Fig. 4 A–C) with anal cone slightly higher than welldeveloped oral cone and anus at tegmen center just above mouth.

1+2 3 4 5+6 7 8+9 10+11 12+13 14 15+16 17 18+19 ** 1+2 3 4 5+6 7+ 8 9 10 11 12 13 14 15

1+2 3 4 5+6 7+ 8 9 10 + 11 12 13 +14 15 16+17 18 19+20 1+2 3 4 5+6 7+8 9+ 10 11 12 +13 14 15+16 17 18+19 20 21 22+ § 1+2 3 4 5+6 7+8 9+10 11+ 12 13 14 15+16 17

1+2 3 4 5+6 7+8 9+10 11+ 12 13 14 +15 16 17 18+19 20 21+22 §§ 1+2 3 4 5+6 7+8 9+10 11+12 13+14 15 16+17 18 19 20+21 * 1+2 3 4+ 5 6+7 8+9 10+ 11 12 13 +14 15 16+ 17

1+2 3+ 4 5+6 7 8+9 10+11 12

Both large USNM 1126896 specimens (Fig. 4 D–H) differ in having anal sac inflated, anus open laterally to oral cone, numerous hydropores visible on D–E inter-radius (Fig. 4 G–H), anal cone lower than oral cone in one case (Fig. 4 H), arm bases relatively narrow (Wr/Wb 1.61 to 1.76), and tegmen clearly visible between two adjacent arms (Fig. 4 D and F). Number of tegminal plates per inter-radius increasing with growth, reaching 15–17 in larger specimens.

Largest specimen (USNM 1126873) unique in having very high aboral cup with Hc/Dc 2.08 (maximum 1.30 in other specimens), Hr/Wr 2.83 (maximum 1.43 in other specimens), and large arm base (Wr/Wb 1.17) as in smaller paratype; proximal pinnules very thin and without genital inflation.

USNM E48124 retains the most complete stalk (length 186 mm). The stalk is broken probably at a short distance from the distal attachment disk. Table 8 lists detailed measurements. Distal columnals are barrel shaped (Fig. 6 F and I). The maximum value of H/D (0.93) is reached at a distance of about 115 mm from the aboral cup. Distally, columnal height continues to increase, to 1.4 mm 130 mm from the aboral cup. Symplexies in proxistele and proximal mesistele have 8 crenular units of 1 short crenula (Fig. 6 A–D). Distally, the portion of the articular surface occupied by symplexy decreases, and the syzygial area develops with irregular radial crenularium (Fig. 6 E–I). In distal mesistele, the facet center preserves the symplexial pattern with 6 crenular units of the juvenile columnal (Fig. 6 H). In USNM 1126892-1, the proxistele and proximal mesistele exhibits the symplexial pattern with 6 crenular units of 1 (rarely 2) well developed crenulae (Fig. 7). In the other specimens, symplexies display 7 crenular units. As the columnal diameter grows, proximal symplexies vary (Fig.8), crenular units can become irregular (Fig. 8 B and E), and the number of crenulae tends to increase up to 3 (Fig. 8 D). Only USNM 1126896-2 exhibits well-developed ligamentary pits in columnals (Fig. 8 D and F).

Remarks. Feracrinus heinzelleri was previously known from two specimens collected off Queen Maud Land (69°40.3’ to 69°40.5’S –00°51.1’ to 00°54.8’E) at a depth of 1393–1398 m (Eléaume et al. 2012). The mature holotype and juvenile paratype were both small (Table 6), and stalk articulations were documented only on the juvenile paratype. The collection described here includes a wider range of sizes and locations, which allows documentation of stalk and crown character variation and changes through ontogeny.

Stalk symplexies vary from 6 to 8 crenular units of 1 to 3 crenulae. The only stalk with distal part preserved (USNM E48124) displays the increase in crenular units from the juvenile stage (6 to 8). However, the paratype, which is the smallest specimen, has 8 crenular units in the proximal stalk, whereas the larger specimen (USNM 1126892-1) has 6 and the largest (USNM 1126896-2 and 1126873) 7. Therefore, this character can vary independently of growth. The number of crenulae per crenular unit seems to increase as columnal diameter grows, as in some other hyocrinids. Arm pattern varies both within and between specimens. Including the holotype and paratype (Eléaume et al., 2012), 94.4% of Br4’s bear the first pinnule; 91.7% of proximal arms have 1+2 3 4 5+6; the first free brachial beyond the brachial bearing the first pinnule ranges from Br7 to Br15, most frequently Br7, Br11 or Br13 (22.7% each). A wide range of variation in stalk, theca and arm characters is also known in Feracrinus koslowi Améziane & Roux, 2011.

John (1937: 8, fig. 3) interpreted a small specimen collected during the Belgica Expedition in the Bellingshausen Sea at a relatively shallow depth of about 500 m as a young Dumetocrinus antarcticus (Bather, 1908). Gislén (1939, fig. 1–7) pointed out that it clearly differed from D. antarcticus and gave a detailed description, which now suggests an attribution to F. heinzelleri. The discovery of F. heinzelleri from the Amundsen Sea at a similar depth of 577 m (Eléaume et al. 2014) confirms this interpretation.

Occurrence. Southern Ocean (off Shetland Islands; Amundsen, Bellingshausen, Ross and Weddell Seas), depth 577–3,798 m, possibly 500–3,944 m.