Bathycrinus rozhnovi Mironov 2019, n. sp.

Bathycrinus rozhnovi n. sp.

Figs 7 A–J, 8A–I, 9A–H

Synonymy: Bathycrinus sp. D — Belyaev 1966: 119.

Etymology: The species is named for Sergey V. Rozhnov, Borissiak Paleontological Institute, Russian Academy of Sciences, Moscow, in recognition of his contributions to our knowledge on echinoderm evolution.

Holotype. IO RAS, No. XV-63-8, RV Vityaz, St. 7499, aboral cup with arms and proximal stalk, RRD = 3.84 mm.

Type locality. RV Vityaz, cruise 59, St. 7499, 21.06.1976, the Japan Trench, 37°85′5 N, 144°12′1E, 6480–6640 m.

Material examined. RV Vityaz, cruise 59, St. 7499, TS, 21.06.1976, Japan Trench, 37°85′5 N, 144°12′1 E, 6480–6640 m, holotype and 17 paratypes, IO RAS Cat.no. Ech 00762– Ech 00764; RV Vityaz, cruise 19, St. 3114, TS, 27.08.1954, east of Kuril-Kamchatka Trench, 48°43.2′ N, 160°55.9′ E, 5670–5680 m, 1 sp. + fr-ts, IO RAS Cat. no. Ech 00759; RV Vityaz, cruise 19, St. 3214, TS, 24.10.1954, Japan Trench, TS, 38°10.9′ N, 143°56.2′ E, 6156–6207 m, 8 spms., IO RAS Cat. no. Ech 00760; RV Vityaz, cruise 45, St. 6138, TS, 11.06.1969, south-east of Aleutian Trench, 53°11,0′ N, 163°12,0′ W, 5595–5660 m, 1 sp., paratype, IO RAS Cat. no. Ech 00761. RV Sonne, KuramBio II, St. 86, AGT, 15.09.2016, east of Kuril-Kamchatka Trench, 45°00.43′ N, 151°06.01′ E – 45°01.37′ N, 151°06.00′ E, 5493 m, paratype 18, SMF. Total: 15 aboral cups with proximal arms and stalks; 6 aboral cups with proximal arms; 7 RR-rings with proximal arms; arm and stalk fragments; RRD from 1.41 to 4.47 mm.

Description of the holotype (Figs 7A, C, D). CH 2.37 mm, RRD 3.84 mm, BBd 1.44 mm; CH /RRD 0.62. RRring is slightly funnel-shaped, forming with BB evenly sloping line (Fig. 7A); RRH/RRD 0.42; BBH/RRH 0.49. Distinct sutures between BB; BBH/BBD 0.44. Surface of BB and RR smooth. Tegmen rising to base of Br3. Sides of tegmen without plates. Height of primibrachitaxis (IBr1+2ax) 3.48 mm. Ratio IBr1H/RRH 1.06, Br1H/Br2H 0.94, Br1H/W 0.72, Br2H/W 0.70. First and second primibrachs very short, with smooth surface and low longitudinal median convexity; their sides flattened into wide lateral flanges. All free arms incomplete. Best preserved arm about 44 mm in length and almost complete; it is composed of 65 Brs. Proximal IIBrs low, IIBr1H/W about 0.77. Their longitudinal median convexity low, rounded, with smooth surface. Outer sides of Brs 1–7 and inner sides of Brs 1–5 flattened into lateral flanges. Profile of arm (viewed from side) smooth. Proximal free arm pattern 1+2 4+5 7+8 10+11 13 (7 cases), 1+2 4+5 8+9 11+12 14 (2 cases) and 1+2 4+5 7+8 9+10 12 (1 case). P1 on Br8 (8 cases) and Br7 (2 cases). Pattern in distal arm similar to pattern in proximal arm with free brachials throughout the length. Every third brachial without pinnule. For example, pattern of arm on Fig. 7D: 24 26+27 29+30 32 with pinnule on Brs 24, 25, 27, 28, 30, 31. Number of pinnulars and length of Ps increase slowly from 7 pinnulars, 3.3 mm in P1 to 12 pinnulars and 7.04 mm in P7. More distal Ps are incomplete. Relative length of pinnulars slightly variable. In P2, length rations of pinnulars 2–7 to pinnular 1 are as successively: 1.02, 1.29, 1.31, 1.25, 1.11, 0.63, 0.29. In P4, the same rations (including pinnulars 2–9) are 1.14, 1.38, 1.30, 1.36, 1.25, 1.15, 0.90, 0.57, 0.31; in P7, the same rations (including pinnulars 2–12) are 1.02, 1.18, 1.29, 1.18, 1.18, 1.12, 1.00, 0.91, 0.80, 0.51, 0.39. Pinnulars V-shaped in cross section, with slightly sinuous edges in proximal Ps and with strongly sinuous edges in distal Ps. Cover plates from proximal Brs do not differing conspicuously from those on distal Brs and pinnules. Side plates absent. Ambulacral tube feet plates x-shaped. Preserved proxistele and mesistele attached to aboral cup composed of 44 columnals including 18 short proxistele columnals, 77 mm in length including 7.4 mm for proxistele (Fig. 7A). D decreases between 1 st and 22 th columnals from 1.44 mm to 0.96 mm, then increases slowly up to 1.41 mm in 44 th columnal. H increases between 1 st and 32 rd from 0.1 to 3.41 mm, then decreases. Maximum H/D 2.75 in 32–33 rd columnals. Columnals 16–22 barrel-shaped, with slight median thickening (visible from side); median constriction noticeable since 29 th columnal. Facets are round in proxistele and proximal mesistele, slightly elliptical with transverse position of fulcral ridge in mesistele (along the small facet diameter).

Other specimens. RR-ring slightly funnel-shaped or conical, forming with BB evenly sloping line. Sutures between BB distinct or indistinct. Each radial plate with two long distal processes on its inner side (Fig. 8D); 10 distal processes of radials form a ring around axial depression. CH /RRD varies from 0.62 to 0.82; RRH/RRD 0.42–0.60; BBH/RRH 0.39–0.58; BBH/BBD 0.41–0.72. Tegmen rising to base of Br2 or Br3. Oral opening rounded by elevation of soft tissue. Ambulacral grooves approach circumoral elevation without reaching oral opening. Large (not cover) plates and tube feet (Fig. 9A) located along aboral margin of circumoral elevation. Anal sac low and well differentiated. IBr1H/RRH 0.87–1.12; IBr1H/IBr2H 0.96–1.20; IBrH1/IBr1W 0.77–0.97; IBrH2/IBr2W 0.61–0.76. Number of knobby processes varies from 8 to 10; always absent on IBrs 1. Three of five IBrs 2 with well developed knobby processes; two other IBrs 2 with single knobby process (Fig. 8B), or second process rudimentary. Best preserved arms in specimen from St. 86, RRD 4.10 mm. Almost complete free arm 57 mm in length, composed of 70 Brs. P10 is the largest: 9.16 mm long with 15 pinnulars. Proximal pattern examined in 88 arms, and position of P 1 in 109 arms (excluding 10 arms of the holotype). Most frequent proximal arm pattern 1+2 4+5 7+8 10+11 13+ (49 cases or 56%) and 1+2 4+5 8+9 11+12 14+ (5 cases or 6%). P1 on Brs 6–11; most frequent positions on Br8 (68 cases or 62%) and Br7 (16 cases or 15%; see also Table 1). Successive alternation of free brachial and brachial pairs in middle and distal arm. Muscular synarthrial facets in free arms with large semicircular aboral ligament fossa (Fig. 8F). Nonmuscular (ligamentary) trifascial synarthry with bilateral symmetry through ambulacral — abambulacral axis; thick fulcral ridge interrupted by axial canal (Fig. 8E). Best preserved stalk in the specimen from St. 86. Preserved proxistele and mesistele attached to BB-ring composed of 66 columnals including 20 short proximal columnals, 153 mm long including proxistele length 6.8 mm, BBD 1.81 mm (Figs 7 E–G). Three other fragments (mesistele, mesistele — proximal-dististele and distal dististele, likely belonging to the same stalk (Fig. 7H, I), corresponding to the length of complete stalk likely more than 300 mm. Number of short proxistele columnals varies from 12 to 28, usually <20. Thin stalk is characterized by maximum H/D 4.62 (D of most elongate column 0.47 mm), thick stalk displays maximum H/D 2.50 (D of most elongate columnal 1.31 mm). Height of columnal usually increases from proxistele to proximal mesistele, more or less constant in middle mesistele, decreases from distal mesistele to proximal dististele, then increases in middle dististele and again decreases in distalmost columnals. Facets round in proxistele and proximal mesistele (Figs 9 C–F), slightly elliptical with transverse position of fulcral ridge in mesistele; large fulcral ridge in distal mesistele with slightly concave (almost parallel) borders including axial canal (Fig. 3G). Facets of distalmost columnals strongly elliptical (D/d up to 1.8) with regular relief on fulcral ridge (Fig. 9H). In distal proxistele ligament fosse appear on even surface as shallow elongate depressions (Fig. 9E).

Remarks. The absence of pinnule on every third brachial in B. rozhnovi is unique feature within the family Bathycrinidae. Other species of the family are characterized by the absence of pinnule on every second brachial. In addition, in B. rozhnovi free brachials are present throughout the whole arm length, whereas in other Bathycrinus species they are absent or rare in distal arms. These features seem to be of great taxonomic value. Most other characters of B. rozhnovi are more similar to those of the bathyal and abyssal species (B. pacificus and B. complanatus) than to hadal Bathycrinus (Table 2).

Distribution. Japan and Aleutian trenches, east of Kuril-Kamchatka Trench, south-east of Aleutian Trench; 5595–6865 m.