Ptilocrinus (Ptilocrinus) pinnatus A.H. Clark 1907

Ptilocrinus (Ptilocrinus) pinnatus A.H. Clark, 1907

Synonymy: Ptilocrinus pinnatus A.H. Clark, 1907: 551 –553, 1 fig.; 1915a: 161; 1915b: 207, fig. 144; Bartsch, 1907: 555–556, pl. 53; Bather, 1908: 297; Gislèn, 1924: 24, 75, 93, 96, figs. 60, 118–122; 1939: 13; Roux, 1980: 35, pl. V (1–6); 1990: 1135; Marshall, Guille & Roux, 1981: 720; Roux et al., 2002: 822, figs. 2C, 10E–F; P. (Ptilocrinus) pinnatus Mironov & Sorokina, 1998b: 41 –42, fig. 13; P. (Ptilocrinus) ahearnae Mironov & Sorokina, 1998b: 48, fig. 17, pls. 7 (3–5), 10 (8); P. (Ptilocrinus) sp aff. s tukalinae Mironov & Sorokina, 1998b: 48.

Material examined. The holotype now disassociated (USNM 22603); another fragmented specimen (USNM E9478) and fragments from about 14– 16 specimens (including four intact thecas with proximalmost stalk and proximal arms) in a third jar (USNM E18363, previously 35978) all housed in the National Museum of Natural History, Smithsonian Institution, Washington, DC. One well preserved small specimen (BMNH 1935.1.15.1) from the Natural History Museum, London. One large dry specimen (ECPs 237) from MNHN (Paris).

Diagnosis. A relatively small species, usually with Dp <2.5 mm and maximum 30 pinnules per arm side. Aboral cup usually wider than high, forming a rounded pentagonal pyramid without ornamentation; basal ring entirely conical, upper edge without rib or inward bending of radials in inter-rays. Tegmen inflated usually up to Br6, Br 8 in largest specimens; Wr/Wb <1.75, with fewer than 50 thick polygonal plates in each inter-ray. Proximal arm pattern 1+2 3 4 5+6 7+8 (13%) or 7 8+9 (77%) with first pinnule always on Br4, successive brachial pairs exceptional; brachial pair 10+11 or 11+12 (55%); usually 6–10 successive muscular joints in mid arm (maximum 12). Adoral face of proximal arm not inflated; flattened projections absent on arms and pinnules. Proximalmost part of pinnules with one row of low lateral plates without inflation. Stalk symplexies with 7–8 crenular units of 1 crenula or 2–3 crenulae; number of crenulae per crenular unit independent of columnal diameter; small ligamentary pit frequently present between robust crenular units in proximal mesistele; distalmost syzygies with a regular radial crenularium of 26–28 crenulae; H/D of columnals up to 0.9 near Dm, usually>0.5 except in proximalmost and distalmost stalk.

Previous descriptions. The 24 specimens dredged by the steamer “Albatross” at station 3342 off British Columbia, Canada, remains the only record of P. pinnatus. A. H. Clark (1907) based his description on one specimen designated as holotype (Fig. 1a). Therefore, the other specimens from the same station are without status (neither cotypes nor paratypes). According to Clark’s short description and figure, the arms of the holotype are about 30 mm long with more than 25 pinnules on each side (maximum pinnule length about 12 mm); the proximal arm pattern is usually 1+2 3 4 5+6 7 8+9 with another non-muscular articulation “after about ten joints, and two or three at irregular intervals toward the tip of the arm”; “the longest pinnules are between the twentieth and twenty-fifth, these having about thirty joints”; “Radials are elongate, the median part of each one raised into a rounded ridge, giving the calyx a rounded pentagonal aspect when viewed dorsally”; two-thirds of the radial upper edge “is occupied by the first brachials” (A.H. Clark, 1907, p.552). Among the “Albatross” series, the longest stalk is 355 mm in length with 360 cylindrical columnals. Studying one specimen housed in the Uppsala Museum, Gislèn (1924) gave the first figures of brachial synostosis (p. 75, Fig. 60) and proximal articulations of pinnules (p. 93, Figs. 118 to 122); he notes that the proximal pattern is 1+2 3 4 5+6 7 8+ 9 10 11 … in the five arms (p. 24). SEM views of a stalk symplexy were published by Roux (1980) from the MNHN specimen and by Roux et al. (2002) from stalk fragments in USNM E18363. Roux (1990) provided additional preliminary information on arm pattern (first pinnule always on Br4 without exception, little variation in proximalmost arm pattern) from all the specimens housed in the USNM. Roux et al. (2002, Figs. 10e, f) figured the MNHN specimen. Mironov & Sorokina (1998b) described the USNM E9478 specimen and pointed out that the structure of pinnule genital expansion, and the form of cover and oral plates remained unknown.

Additional comments on description and variation in the “Albatross” series. Tegmen strongly inflated, about as high as aboral cup, lower in young specimens; top at level of Br 4–5 in small BMNH specimen, Br 6–7 in large MNHN specimen (Fig. 27), and Br 5–6 in others; tegminal plates relatively large, thick and regularly polygonal; 20–25 plates per interray in small BMNH specimen and more than 50 in large MNHN specimen (Figs. 27d, e). Hydropores more or less conspicuous, open in the upper half of the tegmen. Anal sac relatively inconspicuous, contiguous to C–D oral, slightly higher than oral cone (Fig. 27f); orals well developed with convex and smooth surface; first cover plates rectangular in regular rows. Aboral cup with rounded pentagonal aspect of radial ring variable, moderately developed in holotype, strongly marked in MNHN specimen (Figs. 27a, b), with coarse individualized ribs prolonging arm axes in BMNH specimen; radials and basals smooth without any ornamentation; basal ring flared and well-developed; basals fused or with up to 3 sutures, rarely conspicuous.

Using proximalmost diameter as a growth index in an ontogenetic sequence of five specimens, the main variations quantified in external morphological characters (Table 14, left) mainly related to growth except for the larger specimens, which have radial width (Wr), primibrachial width (Wb), aboral cup height (Hc) and radial height (Hr) variable independently of size. Ratios (Table 14, right) indicate variations that are independent of growth, especially Wr/Wb (interradial opening) and Hc/Dc (aboral cup shape), and those that reflect allometric changes through ontogeny, i.e., a decrease of Hr/Wr (elongation of radial shape), or an increase of Drb/Dp and decrease of Hr/Hc (relative development of basal ring). The MNHN specimen offers an example of intra-individual variation: Wr/Wb 1.66–1.79, Hr/Wr 0.80–0.85, and Hr/Hc 0.57–0.62.

FIGURE 27. Ptilocrinus pinnatus, theca and proximal crown of specimen MNHN ECPs 237. a and d: side view of A–B (anterior) inter-ray; b and e: C–D (anal) inter-ray; oblique view of tegmen from D to A rays (E ray removed); d–e: tegminal plates in inter-rays; f: top of tegmen with oral cone (oc) and inconspicuous anal cone (ac).

1+2 3 4 5 6+7 (1 arm within USNM-E18363 jar)

1+2 3 4 5+6 7 8 9+ 10 11 12 +13 14 15 16 17 18 19 20 21 22+ 23 24 1 +2 3 4 5+6 7 8+9 10 11 12 13 14 15 16 17 18 19 20 21+22 23 24 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 22 23 24 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 (1 arm in MNHN specimen)

FIGURE 28. Ptilocrinus pinnatus, pinnules in specimen MNHN ECPs 237 (SEM micrographs). a to c: proximal part of pinnules; a: view of two opposite pinnules attached on arm; b: pinnule side with small lateral plates up to Pn6 (location indicated in a); c: same side beyond Pn7; d: adoral view in middle part of pinnule showing the zigzag suture of cover plates; acv: arm cover plates; cv: pinnule cover plates; lp: lateral plates; Pn: pinnular.

FIGURE 29. Ptilocrinus pinnatus, brachials and pinnulars from isolated fragments in USMN 35978 (SEM micrographs). a–b: free brachials; c: proximal facet (transverse muscular synarthry) of proximalmost pinnular (Pn1); d: oblique adoral view of Pn1 with distal muscular synarthry in the upper part; e: proximal facet of Pn2 (muscular synarthry); F: distal view of Pn2 with synostosial facet; g: distal oblique view of Pn3 with flat synostosial facet; h: Pn from middle part of pinnule with muscular synarthry; i: Pn from distal part of pinnule with muscular synarthry.

On 38 arms preserved in their proximal part at least, the proximal pattern is usually 1+2 3 4 5+6 (90%) with the following brachial pair at 7+8 (13%) or 8+9 (77%); more than 55% of fourth synostoses occur at 10+11 or 11+12, and the fifth synostosis is always between Br17 and Br22. First pinnule is on Br4 without exception. Beyond Br4, 77% of Br7 and 80% of Br10 are free brachials. Between Br9 and Br21, a series of successive muscular articulations varies from 4 to 12 (Table 15). Brachial pairs become scarce distally. The BMNH specimen displays one abnormal proximal arm with a muscular articulation between Br1 and Br2, synostoses uniting Br2 to Br5, and no pinnule. The largest specimen in USNM-E18363 has Br1+2 fused. Four proximal brachials have nearly equal height and width; muscular synarthries are usually strongly oblique beyond Br9. Flattened projections are absent on arms and pinnules.

Genital pinnules not differentiated. Proximalmost part of pinnules slightly inflated with one row of small lateral plates on one side (Figs. 28a, b). Two rows of cover plates imbricate along a zigzag suture and form a regular roof over food groove; pairs of large and narrower, shorter plates alternate on each side, the suture between two pairs more conspicuous than within pairs (Figs. 28c, d). Distal architecture of pinnules not observed because of poor preservation. Ventral groove of arms covered by a roof of polygonal to rectangular plates. Brachial and pinnular articulations (Fig. 29) very similar to those of P. clarki n. sp.

FIGURE 30. Ptilocrinus pinnatus, columnal articulations along an isolated stalk preserved in USMN 35978 (SEM micrographs). a: proximal symplexy (arrow indicates claustrum); b–c: symplexy without ligamentary pits in mesistele; d: syzygy of irregular labyrinthic pattern at a distance of about 25 mm from attachment disk; e–f: syzygy with regular radial crenularium in distalmost columnal.

Proximal stalk prolonging distal part of the basal ring without discontinuity, very flexible, with thin discoid to cylindrical columnals articulated by symplexies. In USNM-E18363, an isolated stalk (length 265 mm) probably belonging to the largest specimen was examined in detail: proximal end diameter 1.92 mm with relative columnal thickness (H/D) 0.20–0.23; minimum diameter 1.79 mm with H/D 0.96 at a distance of 32 mm from proximal end; diameter 1.88 mm with H/D 0.83 at a distance of 50 mm from proximal end; diameter 2.36 mm with H/D 0.56– 0.58 in distal mesistele at 200 mm from proximal end; diameter increasing distally to 3.25 mm and H/D decreasing to 0.37; stalk broken distally near the attachment disk. Proximal symplexies with 8 crenular units of 2–3 short irregular crenulae and areolar lobes reaching outer facet border (Fig. 30a); mesistele symplexies near Dm with 8 crenular units of 1–2 relatively long crenulae and small crenulae underlying outer facet border, areolar lobes with small rudimentary pits (Figs. 30b, c); distal mesistele syzygies with irregular radial crenularium around a central juvenile symplexy with 7 crenular units (Fig. 30d); distalmost syzygies with a regular radial crenularium of 26–28 crenulae of labyrinthic design derived from 7 juvenile crenular units by up to four dichotomies through ontogeny (Figs. 30e, f). Additional observations on isolated columnals in USNM-E18363 indicate 7 crenular units more frequent than 8 in symplexies, young proximalmost columnals with crenular units of 1 crenula and pentalobate lumen (Fig. 31a); number of crenulae per crenular unit independent of columnal diameter; proximal mesistele symplexies with small deep ligamentary pits variously developed between robust crenular units (Figs. 31b to d). In the young BMHN specimen, columnals with maximum H/D 0.71 at a distance of 30 mm from aboral cup.

FIGURE 31. Ptilocrinus pinnatus, stalk symplexies from isolated fragments in USMN 35978 (SEM micrographs). a: young proximalmost columnal; b: mesistele symplexy with a few rudimentary pits in areola; c–d: well developed symplexial crenularium with conspicuous ligamentary pits in proximal mesistele.

Remarks. P. clarki n. sp. shares with P. pinnatus an inflated tegmen with flat oral ring, each oral at top of an angle formed by two rows of subrectangular, poorly differentiated, cover plates; no radial bridges elevated above the tegmen surface from mouth to arms, and a relatively inconspicuous anal sac coalescing with oral ring. P. a u s - tralis mainly differs in having a well-developed anal sac reaching above the oral ring, tip of each oral with curious globular projection, radial food groove slightly elevated above tegmen surface, well differentiated rounded cover plates, and first pinnule always on Br5 (Améziane & Roux 2011). The tegmen shape and pinnules with lateral plates restricted proximally or absent, suggest affinities with P. pinnatus rather than with P. clarki n. sp. Some characters suggest P. australis is intermediate between the subgenera Ptilocrinus and Chambersaecrinus, although its well differentiated crenularium of distal stalk syzygies is closer to the subgenus Ptilocrinus.

Table 16 compares P. pinnatus and P. clarki n. sp., showing their very close affinities despite differences related to the smaller size of P. pinnatus. However, a few characters clearly distinguish the two species, the most important of which is pinnule architecture. In P. clarki n. sp., proximal genital inflation develops with lateral plates on two sides, one side having plates sometimes up to mid pinnule. In P. pinnatus, genital inflation was never observed and only a few rudimentary lateral plates are restricted to the proximalmost part of pinnule as in juveniles of P. clarki n. sp. The development of genital inflation in pinnules of P. clarki n. sp. and its absence in P. pinnatus suggest two different strategies of reproduction.

Other distinguishing characters are proximal arm pattern beyond Br6, inter-ray space width, inward bending of radials in interrays of aboral cup, shape of basal ring, relative thickness of columnals and symplexies with or without areolar pits in mesistele. The shape of cover plates is not a discriminating character, probably because their low level of differentiation in the subgenus Ptilocrinus. Although variation is documented from a relatively small number of P. pinnatus specimens, many characters appear to have a wider range of variation in P. clarki n. sp. This suggests a response to heterogeneous environmental conditions where P. clarki n. sp. lives. The small ligamentary pits in stalk symplexies of P. pinnatus have previously been described in various hyocrinid taxa (Roux 1980; Mironov & Sorokina 1998b; Roux 2004). In stalk synarthries of bathycrinids, a ligamentary depression can occupy the whole areola, especially in species living in environments deeper than 2,000 m (Roux 1987). In hyocrinids, they are weakly or moderately developed and are present both in the deepest species such as P. stukalinae, as well as in the shallowest, i.e., Dumetocrinus antarcticus. In P. pinnatus, they are associated with high and robust crenularia (Figs. 31b to d), which indicate water flow of relatively high velocity. Robust crenularia of similar height and ligamentary pits are absent in P. clarki n. sp.

With its smaller size and characters, P. pinnatus tends to resemble juvenile P. clarki n. sp., suggesting that this species may represent a heterochronic development by paedomorphy (especially progenesis) in an environment about 1,000 m deeper than that of P. clarki n. sp., with lower food supply and currents of higher velocity but in the same geographical area. That could be interpreted as the result of sympatric speciation.

So this raises the question: are these two subspecies within a single species or two species isolated by depth and hydrodynamics? In the genus Feracrinus, Améziane & Roux (2011) distinguished two species with close affinities but living at different depths: F. k o s lo w i (1,310 to 1,815 m) and F. a c u l e a t u s (2,915 to 3,275 m). In pentacrinids of the subfamily Diplocrininae, after a detailed study of numerous specimens from different populations, David et al. (2006) and Roux et al. (2009) concluded that in each province (western Atlantic, north Indian and western Pacific) one species differentiated into morphotypes related to depth or hydrodynamics which were interpreted as subspecies. Given the present incomplete state of our knowledge of hyocrinids, and in agreement with the interpretation of Améziane & Roux (2011) for a similar case observed in Feracrinus, the simplest solution is to consider P. pinnatus and P. clarki n. sp. as two distinct species.

The single known specimen of P. ah e a r na e Mironov & Sorokina, 1998, falls within the range of variation of P. pinnatus. With its moderately inflated tegmen, proxistele with numerous thin columnals, mesistele symplexies with a juvenile pattern, and relatively small size, this specimen is interpreted as a young P. pinnatus. The incomplete specimen attributed to P. sp. aff. stukalinae by Mironov & Sorokina (1998b) was collected at the same station as the holotype of P. ahearnae (in 2,540 m off northeastern Japan), and exhibits no significant characters that justify its recognition as a different species. P. stukalinae lives significantly deeper than P. pinnatus. Therefore, we P. ahearnae as a junior synonym of P. pinnatus.

Occurrence. North Pacific, off British Columbia and northeastern Japan, at depths from 2,540 to 2,906 m.