Thalassometra A. H. Clark, 1907

Eléaume, Marc, Améziane, Nadia & Park, Young-Hyang, 2004, Re-evaluation of the systematics of two deep-sea species of Thalassometra (Echinodermata: Crinoidea) and its biogeographical implications, Journal of Natural History 38 (15), pp. 1949-1968 : 1953-1963

Genus Thalassometra A. H. Clark, 1907

Diagnosis. A genus of Thalassometridae having cirri long and slender with more than 25 segments; distal cirrals much shorter than proximal cirrals, broader than long with dorsal spines or processes; transition segment present. Radials 5; arms 10–15, aborally rounded; IBr, IIBr (when present) and proximal brachials aborally beset with spines, at least along the edges; IIBr (when present) 4(3z4); P 1 larger and stouter than P 2; P 2 resembles P 3; proximal pinnulars on genital pinnules not expanded.

Thalassometra bispinosa (P. H. Carpenter, 1888)

(figures 2–4; tables 2, 3)

Antedon bispinosa: Carpenter, 1888: 115 , pl. 20, figures 3, 4.

Thalassometra bispinosa: Clark, 1950: 151–156 .

Diagnosis. IBr and first brachials bordered with coarse blunt spines or tubercles; br 6–8 and following each with two or three long, curved, overlapping spines near the distal edge; distal pinnules spiny; cirri arranged in 10 definite columns on the centrodorsal; 10 arms; XX cirri with 45 segments.

Description. Centrodorsal conical (EcCh 237), almost columnar (1888.11.9.16) or columnar (EcCh 236) with projecting interradial tubercles; oral diameter (D) 2.75–3.80 mm (table 2); radial height (H1) 2.18–2.40 mm; interradial height (H2) 2.50–2.70 mm. Aboral pole conical (1888.11.9.16, EcCh 237) or flat (EcCh 236), papillose (1888.11.9.16, EcCh 237) or smooth (EcCh 236). Cirrus sockets arranged in 10 columns, two per radial area, with two to three functional sockets per column. Sockets in close apposition all around the centrodorsal.

Cirri XIX–XXV, 30–45, 30– 35 mm long; c 1 –c 3–4 shorter than wide (L/Wv 1) with distal edge strongly everted (1888.11.9.16, EcCh 237) or not (EcCh 236), slightly spinous (1888.11.9.16, EcCh 237) or smooth (EcCh 236); next segment longer than wide with reduced eversion of distal edge; c 6–8 longest (L/W~2.00); following cirrals gradually shorter; distal cirrals short (L/Wv 1). Transitional cirral usually c 6–8. Conspicuous aboral spine first appearing after c 8 –c 10, increasing in height over the next several cirrals. Opposing spine prominent. Terminal claw strongly curved (figure 2h).

Radials (figure 3a) distally concave, well exposed; distal margin with coarse spines (NHM 1888.11.9.16), tubercles (EcCh 237) or smooth (EcCh 236); width (Wd) 2.00–3.00 mm (table 2).

Brachitaxes (IBr2) and first brachials rounded dorsally; dorsal surface spinose (1888.11.9.16) or smooth (EcCh 236, EcCh 237); proximal and distal borders smooth (EcCh 236), or fringed with blunt spines (1888.11.9.16) or tubercles (EcCh 237); lateral margins flattened and in close apposition. Ibr 1 short, W/L~2.50 (1888.11.9.16) to 3.00 (EcCh 236) (table 2), with the proximal border convex, everted and fringed by coarse blunt tubercles (1888.11.9.16, EcCh 237) or not everted and smooth (EcCh 236); distal border concave, everted and fringed with coarse blunt spines (1888.11.9.16, EcCh 237) or not everted and with slight synarthrial tubercle (EcCh 236). Ibr 2 (axillary) pentagonal, W/L~1.46–1.58, with a curved proximal margin fringed with coarse spines (1888.11.9.16), tubercles (EcCh 237) or smooth, straight proximal edge (EcCh 236); distal margins with coarse spines (1888.11.9.16), tubercles (EcCh 237) or smooth (EcCh 236) (figure 3a).

Arms 10; ray length 110 (EcCh 236) to 140 mm long (EcCh 237). LBr~ 56–64 mm (table 2). First brachial (br 1) short, W/L~1.75–3.00, rounded dorsally; proximal and distal edges everted and fringed with blunt tubercles (1888.11.9.16, EcCh 237) or not everted and smooth (EcCh 236) (figure 3a). Second brachial (br 2) squarish, W/L~0.94–1.19, rounded dorsally; proximal and distal edges everted and fringed with blunt tubercles (1888.11.9.16, EcCh 237) (figure 3a). Third brachial (br 3z) short W/L~2.93–3.38. Syzygial pair (br 3z4) strongly constricted at articulation (figures 3a, b, 4a, f); br 5 (figures 3a, c, 4b, g), br 6 (figures 3a, d, 4c, h), br 7 (figures 3a, e, 4d, i) and br 8 squarish; br 9 oblong. Following brachials cuneate (figure 2a–g). Distal edge of br 5–8 (figures 3a–e, 4a–j) bearing two long spines; following brachials with about three long curved spines (figure 2a–c, e–g). First syzygy at br 3z4; second usually at br 12z13 or br 13z14; distal intervals 4.

P 1 up to 14 segments, 7.00 mm long, more robust than following pinnules; four basal segments squarish (L/W~1); following increasing in length; tapered to the tip (figure 2i). P 2 resembling Pa, much smaller than P 1, up to eight segments, 3.00 mm long (figure 2j). P 3 up to 10 segments, 4.40 mm long (figure 2k). P 4 up to 11 segments, 4.50 mm long (figure 2l). Following pinnules gradually increasing in length. Distal pinnule up to 18 segments, 12.50 mm long; first segment short with two large lateral spines; following segments longer than wide with long aboral overlapping spine on distal edge (figure 2m). All pinnules except P 1–2 heavily plated with large triangular alternating covering plates. Mean spacing of tube feet 11.4 tf mm 21 (table 3).

Disk densely covered by small rounded nodules; anal tube prominent.

Geographic and bathymetric distribution. Southern Indian Ocean : sub-Antarctic Marion and Crozet Islands (2925 m); West Pacific: Peru (3949 and 4265 m) .

Affinities. Among the 18 species of the genus Thalassometra previously described (table 1), only five display coarse spines or tubercles at least on their proximal brachials: T. bispinosa , T. electrae , T. agassizi , T. villosa and T. tara (A. H. Clark, 1950; McKnight, 1977). Only two of these, T. bispinosa and T. electrae , have the cirrus sockets arranged in 10 columns. A re-examination of the holotype of T. electrae (also the only known specimen) indicates that it is badly damaged with all arms broken at the first hypozygal segment (br 3); the first pinnules (P 1 and Pa) are the only ones remaining, but only one of these is nearly complete.

Thalassometra bispinosa and T. electrae differ only in that P 1 is slightly longer with more pinnulars in the latter (up to 14 segments and 7.00 mm long in T. bispinosa versus 17 segments and 8.00 mm in T. electrae ). In T. electrae , the centrodorsal is conical with produced interradial corners; the dorsal pole is papillose or with spines, and the cirrus sockets are arranged in 10 apposed columns with two functional sockets per column. A number of characters fall within the range of variation of T. bispinosa : the number of cirri (XIX–XXV in T. bispinosa versus XXII in T. electrae ), number of cirrals (30–45 in T. bispinosa versus 29–43 in T. electrae ), the number of basal cirrals for which L/Wv 1 (c 1 –c 3–4 in T. bispinosa , c 1 –c 3 in T. electrae ), the position of the transitional cirral (c 6–8 in T. bispinosa , c 5–7 in T. electrae ). The terminal claw is strongly curved in both. Ornamentation, when present, is similar in size, shape and distribution in both species. Spines are long rod-like processes, sometimes tubercles, densely concentrated on proximal and distal edges of brachials, more scattered on ossicle surfaces. Thalassometra electrae is probably conspecific with T. bispinosa , but we await additional, better preserved material for verification.

Thalassometra villosa (A. H. Clark, 1907)

(figures 4, 5; tables 3, 4)

Antedon villosa: A. H. Clark, 1907a: 138 .

Thalassometra villosa: A. H. Clark, 1907b: 360 .

Thalassometra bispinosa: Speel and Dearborn, 1983: 24 .

Diagnosis. IBr and first brachials covered with stout, well-spaced, sharply conical spines; cirri arranged in 15–20 columns on centrodorsal; 10 arms; XX– XL cirri with 40–50 segments.

Description. Centrodorsal conical (1 003 963 No. 4 and No. 5), hemispherical (1 003 963 No. 3) or cylindrical (1 003 963 No. 1 and No. 2; 22 630— holotype); oral diameter (D) 4.40–4.95 mm (table 4); radial height (H1) 2.40–4.10 mm; interradial height (H2) 2.60–4.60 mm. Aboral pole truncated, irregularly flattened. Cirrus sockets arranged in 15–20 columns with two columns of two to four cirri in each radial area and one or two columns of one to two cirri in each interradial area. Sockets in close apposition all around the centrodorsal (1 003 963 No. 3, No. 4 and No. 5) or separated by an interradial ridge extending from the centrodorsal margin to the base (1 003 963 No. 2) or centre of the apex (1 003 963 No. 1, 22 630— holotype).

Cirri XX–XL, 40–50, 33 mm long; c 1 –c 3 shorter than wide (L/Wv 1) with distal edge everted and slightly spinous; c 4 L/W~1, with distal edge everted and slightly spinous; c 5–7 longest, L/W~2; following cirrals gradually shorter; distal cirrals short (L/Wv 1). Transitional cirral usually c 7. Aboral spine not conspicuous, first appearing after c 10 (figure 5i).

Radials concealed, distally concave, occasionally with spines on lateral edges; width (Wd) 3.10–3.50 mm (table 4).

IBr2 and first brachials of free arm rounded aborally; aboral surface beset with stout, well-spaced, sharply conical spines; proximal and distal borders fringed with blunt spines; lateral margins flattened and in close apposition. Ibr 1 short, W/L ~3.89–7.00 (table 4); proximal edge convex, slightly everted with small spines or tubercles; distal edge concave, everted, with spines becoming larger on oral–aboral axis; lateral edges everted with spines in close apposition with adjacent ossicles; dorsal surface smooth. Ibr 2 (axillary) diamond-shaped, broader than long: W/L~1.44–1.75; proximal edge convex, fringed with spines becoming larger on synarthrial tubercle; distal edge concave and fringed with small scattered spines; aboral surface, especially on oral–aboral axis, with high, well-separated spines.

Arms 10; ray length 95 mm. LBr~ 60–75 mm (table 4). First brachial (br 1) trapezoidal, W/L~2.31–3.10; proximal edge convex, fringed with small spines that become larger on oral–aboral axis; distal edge concave, with scattered spines, incised by synarthrial tubercle of br 2; lateral edges parallel to oral–aboral axis,

T. bispinosa T. villosa

Specimen registration number EcCh 236 EcCh 237 1888.11.9.16 1003 963 Mean tube feet spacing (tf mm 21) 10.5 11.4 12.5 12.2

Tube feet (tf) were inferred from the number of lappets on one side of the ambulacrum of the pinnules, each lappet corresponding to a triad of tube feet.

slightly everted with no spine; aboral surface smooth, occasionally with one spine or tubercle. Second brachial (br 2) pentagonal, W/L~1.56–1.93; proximal edge convex with synarthrial tubercle occasionally bearing spines; distal edge slightly concave, fringed with small spines; interior lateral edge not everted, lacking spines; exterior lateral edge slightly everted, bearing spines. Third brachial (br 3z) short, W/L~2.21–3.00. Following brachials (br 3–9) oblong, broader than long, rounded dorsally; proximal and distal edges everted, fringed with spines (figure 5b, c, e, f); lateral margins flattened and in close apposition. Following brachials wedge shaped, as long as broad with distal edges produced (figure 5a, d).

P 1 very stout, up to 20 segments, 10 mm long, more robust than following pinnules; p 2–5 roughly square; p 6 roughly square, W/L~1, with aboral spines on distal edge; following pinnulars tapering to the tip, with aboral spines on distal edges (figure 5g). P 2 smaller and slender than P 1, up to 15 segments, 7 mm long. Following pinnules shorter, but gradually increasing in length to 14 mm distally (figure 5h). Mean spacing of tube feet 12.2 tf mm 21 (table 3).

Disk densely covered by small elongated nodules; anal tube prominent, plated with elongated nodules.

Geographic and bathymetric distribution. Northern Pacific: Aleutian Islands (1860 m) and Southern Pacific: sub-Antarctic Campbell Island (1940 m).

Remarks. The six specimens dredged by the Eltanin assigned to T. bispinosa ( Speel and Dearborn, 1983) are all badly damaged; all lack most of the arms, which prevents the use of the long overlapping spines on the distal edge of the brachials (from br 5 on) as a discriminating character. Nevertheless, characters of the centrodorsal, proximal brachials and pinnules provide enough evidence for reassigning these specimens.

The cirrus sockets are arranged in 15–20 columns (one very small specimen displays 10 cirrus sockets) with two to six cirri per column (usually three). Sockets are usually separated interradially by a ridge that runs from oral part of centrodorsal to the base of the apex. Cirri are XVI–XL (mode~XXXVI), more numerous than in T. bispinosa . Brachitaxes (IBr2) and the first brachials of the free arm have aboral surfaces beset with stout, well-spaced tubercles. Ibr 2 (axillary) is diamond-shaped, W/L~1.45–1.75 with proximal edge convex and fringed with small spines; spines larger on synarthrial tubercle; distal edge concave and fringed with small scattered spines. P 1 very stout is up to 20 segments and 10 mm long, more robust than following pinnules.

According to A. H. Clark (1950) and McKnight (1977), the species of Thalassometra not displaying an arrangement of cirri in 10 columns are: T. agassizi (Hartlaub, 1895) , T. multispina (Carpenter, 1888) , T. omissa ( Koehler, 1909) , T. peripolos (A. H. Clark, 1929) , T. setosa (A. H. Clark, 1913) , T. tara McKnight, 1977 and T. villosa (A. H. Clark, 1907) . Among these, only T. agassizi , T. tara and T. villosa have coarse spines on the proximal brachials. Compared with T. villosa , T. agassizi and T. tara display a greater number of cirrals per cirrus, smaller and more slender spines on proximal brachials and enlarged proximal pinnulars. Characters displayed by the Eltanin specimens are in good agreement with the characters of T. villosa (table 5).

However, Speel and Dearborn (1983) offered strong evidence in assigning the Eltanin specimens to T. bispinosa . They argued that ossicles of a detached arm from one of the Eltanin specimens displayed spines on the distal border of the brachials, overlapping the base of the next brachial. Contrary to Speel and Dearborn’s interpretation, however, distal brachials of the detached arm have the distal edge everted, slightly overlapping the following brachial (figure 5a–f) but lacking the well-developed spine characteristic of T. bispinosa (figure 2a–g).

Comparison of articular facets of specimens of T. bispinosa (figures 2b–d, f, g, 3b–e, 4a–j) with those of T. villosa (figures 4k–s, 5b, c, e, f), including the type specimens (figures 4f–j, p–s, 5e–f), supports placement of the Eltanin specimens in T. villosa . Preliminary studies of articular facets of antedonids (Carpenter, 1888; Clark, 1921), comasterids, mariametrids, thalassometrids and atelecrinids ( Macurda and Meyer, 1975; Messing et al., 2000), suggest that endoskeletal microstructure displays diagnostic characters at the family level and, among thalassometrids, at the generic level. However, the variability of these traits, as well as the limits of their relevance, still await a rigorous analysis.

The groove between the muscular fossae on the brachial articular facets is generally wider and deeper in T. bispinosa than in T. villosa and the Eltanin specimens. In T. bispinosa , br 3z (figures 2b, 4a, f) displays an almost circular facet, with a marked concavity running from the lumen to the ambulacral edge. The corresponding facet (br 4z) of the Eltanin specimen (figure 4k) is semicircular and lacks the ambulacral groove. On the following ossicles, the ventral incision separating the two ventral muscular fossae articular facets is wider and deeper in T. bispinosa (figure 4b–e, g–j) than in T. villosa (figure 4l–o) and the Eltanin specimen (figure 4l–o). The muscular fossae in T. bispinosa display two distinct areas. Adjacent to the intermuscular septum is an area composed of numerous rods pointing into the muscular fossa, giving an impression of tight unordered stereom. Between this area and the interarticular ligament fossa lies an area of galleried stereom with a looser appearance. No growth bands are visible. In T. villosa and in the Eltanin specimen, the microstructure of the muscular fossae consists of homogenous galleried stereom with visible growth bands. Therefore, the brachial articular facets of the Eltanin specimen are similar to those of T. villosa , whereas all the specimens of T. bispinosa are similar to each other and different from both T. villosa and the Eltanin specimen.

Affinities. In the genus Thalassometra , T. bispinosa (Carpenter, 1888) , T. electrae John, 1937 , T. agassizi (Hartlaub, 1895) and T. villosa (A. H. Clark, 1907) are the only representative species with numerous coarse spines or tubercles (Clark, 1950). Thalassometra agassizi displays 15 columns of cirrus sockets on the centrodorsal whereas T. bispinosa and T. electrae have only 10 sockets.