Gephyrocrinus grimaldii Koehler & Bather, 1902

Roux, Michel & Bohn, Jens Michael, 2010, Revision of the genus Gephyrocrinus Koehler & Bather, 1902 (Echinodermata, Crinoidea, Hyocrinidae), Zoosystema 32 (3), pp. 425-437 : 429-433

Gephyrocrinus grimaldii Koehler & Bather, 1902

Gephyrocrinus grimaldii Koehler & Bather, 1902: 68-79 , figs 1-4. — Koehler 1909: 265, 266, pl. I, fig. 12 and pl. XXXII, figs 1-9. — A. H. Clark 1915: 160. — A. M. Clark 1973: 274; 1980: 208. — Roux 1977: 31; 1980: 33, 34, 40, fig. 1, pl. I figs 1-6. — Mironov & Sorokina 1998b: 30. — Roux et al. 2002: 816, 817, fig. 10c, d.

Hyocrinus (Gephyrocrinus) grimaldii – Roux 1980: 42; 1985: 481.

AMENDED DIAGNOSIS. — A relatively small species with crown length less than 30 mm, robust arms and pinnules without ornamentation, width of proximal brachials subequal. Pinnules mostly rigid except distalmost part, fewer than 10 on each arm side of each arm, and weakly differentiated from arms. Genital pinnules with numerous irregular polygonal lateral plates of nearly equal size in proximally inflated portion and with rarely conspicuous lanceolated cover plates distally. Middle and distal arm pattern usually with successive brachial pairs (e.g., a+b c+d e+f), rarely free brachials or triplets. Tegmen with about 12 or slightly more polygonal plates in each interradius; orals convex without projection; tubercles bearing hydropores on upper tegminal plates and oral base sometimes present, and food groove elevated above adjacent interradial surfaces forming a bridge between orals and Br4. Ratio of radial upper width to primibrachial width 1.2-1.8. Conical to weakly bowl-shaped aboral cup with trapezoidal radials; ratio of cup height to maximum cup diameter 0.7-1.0; basals fused. Proximalmost stalk diameter up to 2.0 mm; minimum stalk diameter up to 0.9 mm; proximal symplexies with chiefly galleried stereom and 6-8 small crenular units of 1 (rarely 2) crenulae; symplexies near the minimum stalk diameter with 6 or 7 well-developed crenular units of 1-2 crenulae and galleried stereom restricted to inner part of articular facet; crenularium of distal syzygies with radial or moderately labyrinthic pattern.

DISTRIBUTION. — Northeastern Atlantic from the north slope of the Bay of Biscay to Madeira ( Roux 1985) and

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…

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+16 17 18+19…

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…27+28 29+30+31 …

1+2 3 4 5+6 7+8 9+10 11+12 13+14…19+20 21+22+23 24+25…

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 18+19…

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…

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…

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

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

from the Canary Islands to the Mid-Atlantic Rise at depths ranging from (1360?) 1420 to 1968 m. Th e stalk fragment first identified as belonging to Hyocrinus by Carpenter (1884) and attributed to Gephyrocrinus by A. H. Clark (1915) came from a location in mid-Atlantic (01°47’N, 24°26’W, at a depth of 3330 m) significantly deeper than the depth range of G. grimaldii but corresponding to the depth range of Hyocrinus . A specimen attributed to G. grimaldii from off Newfoundland ( Haedrich & Maunder 1985) was later described as Ptilocrinus atlanticus Roux, 1990 , and transferred to Anachalypsocrinus by Mironov & Sorokina (1998b). So, specimens undoubtedly attributed to G. grimaldii have only been dredged in a relatively restricted area and depth range in the northeastern Atlantic ( Fig. 1).

DESCRIPTION

All specimens characterized by small to medium size, robust arms and pinnules with thick brachials and pinnulars, widely separated inter-rays, conical aboral cup with basals fused, and usually large conspicuous ribs prolonging arm axis ( Fig. 2). Proximal brachials subequal in width and height; following arm decreasing slowly and progressively in width. Muscular articulations becoming moderately oblique in mid and distal arm. Trapezoidal radials never forming an angle with the basal ring. Basals fused. Base of basal ring usually flanged; proximal stalk flexible and multilobated ( Fig. 2 B-D), both characters becoming inconspicuous in a few large specimens ( Fig. 2A). Variation of the main external morphological characters that can be quantified ( Table 2) are either related to growth (arm and pinnule lengths, number of pinnules on each arm side, and proximalmost stalk diameter) or independent. Variation not related to growth clearly observed at least for largest specimens with proximalmost stalk diameter more than 1.6 mm; ratios of primibrachial width to radial upper width and of cup height to maximum cup diameter are the most variable. In the smallest specimen, primibrachials jointed when crown closed, except in the anal inter-ray. Increase in primibrachial width slowing rapidly with growth relative to radial width (ratio ranging from 0.54 to 0.68), so that primibrachials are much narrower than radials and the inter-ray space much wider in larger specimens when crown closed.

Proximal arm pattern usually 1+2 3 4 5+6 (92.8% of 72 arms observed) with Br4 bearing first pinnule (95.8%). Th ree cases with first pinnule on Br5 and 1+2 3+4 5 6+7 (1 case) or 1+2+3 4+5 6+7 (2 cases). In two cases arm pattern 1+2 3+4 5+6 with first pinnule on Br4. Brachial triplets also observed in other arms at various places (one case each) from 1+2+3 to 29+30+31 ( Table 3). Distal to brachial bearing first pinnule, usual pattern consisting of successive brachial pairs (i.e. a+b c+d) producing regular alternation of ligamentary (synostosis) and muscular (synarthry) articulations. Free brachials scarce and observed (one case each) at various places from Br5 to Br18 ( Table 3). Patterns other than successive brachial pairs representing less than 1% of brachial articulations. Specimen Th A with arm division at IBr4ax (one case) with two branches of equal size and well-developed pinnules ( Fig. 3); distal to axillary, series of brachial pairs beginning immediately (IIBr1+2) in the left branch and later (IIBr2+3) in the right one.

Proximal part of genital pinnules inflated ( Fig. 4A) with numerous imbricating lateral plates not in rows ( Fig. 4B, C). Irregular lateral plates present to the pinnule tip ( Fig. 4D) and on the ambulacral face of arms. Cover plates usually variable and difficult to distinguish from lateral plates, lanceolate shape ( Fig. 4E) only observed in distal part of pinnule.

Muscular brachial synarthries with prominent fulcral ridge allowing wide amplitude of movements ( Fig. 5A, B). As in other hyocrinids ( Holland et al. 1991), muscular and ligamentary areas of the adoral part of brachial facet not clearly delimited by galleried stereom associated with ligament. In Gephyrocrinus grimaldii ( Fig. 5D, E), galleried stereom only present near ambulacral side of fulcral ridge. Ligament mainly attached to labyrinthic stereom covered by small globular extensions as in synostosial stereom ( Macurda et al. 1978). Muscle attached either to thin layer of labyrinthic stereom of small meshes or to stereom with radially thickened frame near facet outer edge ( Fig. 5E). All brachial synostoses with flat undifferentiated facets ( Fig. 5F). Pinnule on episynostosial ossicle of brachial pair ( Fig. 4A); pinnule socket ( Fig. 5C, G) located ventrally with fulcral ridge parallel to the brachial fulcral ridge in proximal arm. Pinnule articulation with arm a transverse muscular synarthry with a symmorphy that produces a convex pinnular facet ( Fig. 5H). Two proximalmost pinnulars united by classical muscular synarthry and following pinnulars by flat synostoses ( Fig. 5I). Muscular synarthry articulating primibrachial to radial wider than high and symmetrical ( Fig. 6B). Inner surface of radial ( Fig. 6B, C) with two radial nerves running in parallel grooves covered by labyrinthic stereom in the distal part of radial only ( Fig. 6A, C), as previously observed by Gislèn (1939) in Ptilocrinus .

Tegmen reaching Br 4-5 in medium-sized specimens and inflated to Br6-7 and not attached to first pinnule in the largest one. Subconical anal sac located near external border of tegmen and taller than oral cone ( Fig. 7A).Tegminal plates polygonal and convex ( Fig. 7B, C), usually about 12 or slightly more per interradius (except anal interradius), up to 18 in specimen BM2 . Hydropores frequently inconspicuous except at base of oral plates as in the holotype ; a few specimens showing series of conspicuous hydropores open at top of small tubercles in upper half of tegmen ( Fig. 7A). Orals always well developed, convex and smooth, usually bearing one tubercle with hydropore at base.

Stalk relatively gracile; length 78 mm in the single complete specimen (ThD). Diameter decreasing rapidly from 1.7 to 1.1 mm in the proximal 5 mm below aboral cup, and reaching minimum (0.92 mm) at 17 mm, increasing slowly to 1.42 mm at 2 mm before the distal end, and rapidly to 1.48 mm in the few last columnals indicating the proximity of the attachment disk. Proximal columnals thin, multilobated and with variable thickness and diameter (as in Figure 2A, C, D) providing maximum flexibility. In largest specimens, proxistele more regular and cylindrical ( Fig. 2B). Columnals increasing in thickness and becoming cylindrical ( Fig. 8A) or weakly barrel-shaped in middle and distal stalk of large specimens ( Fig. 8B, C); ratio of columnal height to diameter up to 0.7 in proximal mesistele, decreasing to <0.6 in distal stalk and <0.5 in distalmost columnals. Middle stalk articulations (symplexies) of young specimens with ligament area (areola) predominating, and 6 or 7 crenular units of 1 short crenula restricted to outer border of facets ( Fig. 8A, D). In larger specimens, well-developed crenularium restricting areola to facet centre ( Fig. 8B, E); up to 8 crenular units of 2 crenulae in proxistele of largest specimens; radial crenulae variable, more or less regular in size and form. Distal articulations with thin syzygial crenularium predominating ( Fig. 8C), varying from a radial to labyrinthic pattern, and juvenile symplexial pattern with 6 or 7 crenular units preserved in facet centre ( Fig. 8F).