Ptilocrinus tasmaniaensis, Ameziane & Roux, 2010

Ameziane, Nadia & Roux, Michel, 2010, Stalked crinoids from Tasmanian seamounts. Part 1: Hyocrinidae, Journal of Natural History 45 (3 - 4), pp. 137-170 : 166-169

Ptilocrinus tasmaniaensis sp. nov.

Etymology

Species discovered off Tasmania.

Material

Single specimen poorly preserved collected during 1997 cruise on Tasmanian seamounts on Hill W, station 45, 44.42 ◦ S, 147.24 ◦ W to 44.46 ◦ S, 147.20 ◦ W, depth 1715–1815 m. Museum Victoria registration number: MV F171347 .

Diagnosis

A species of Ptilocrinus with relatively narrow arm base; ratio of radial width to primibrachial width about 2.5; aboral cup with coarse ribs prolonged along arm axis and convex surface between ribs; tegmen not inflated with anal cone slightly taller than oral cone and upper part of oral resembling a parrot’s beak. Proximal part of genital pinnules with numerous long additional plates not in rows and more numerous on one side, small cover plates with needle-like stereome projections on inner face. Proximal stalk prolonging basal ring without discontinuity and with symplexies having seven crenular units of two (rarely three) crenulae widening out towards columnal facet edge.

Holotype

Robust specimen with proximal stalk attached to theca ( Figure 17B ), broken proximal fragments of one arm and a few genital pinnules. Aboral cup conical with one very wide and rounded rib prolonged along each arm axis; radial ring surface convex between two ribs; upper radial border strongly folded back to tegmen in inter-radius; basal ring more widely open than in radial ring, diameter of upper radial ring 15.8 mm, diameter at base of radial ring 10.1 mm, ratio of upper radial ring diameter to lower radial ring diameter 1.56, diameter of basal ring base 3.4 mm, no morphological discontinuity between basal ring and proximalmost columnals, basal fused, cup height at suture between radials 17.0 mm, cup height at arm axes 15.1 mm, basal height 5.3 mm, ratio of cup height to upper radial ring diameter 0.96 to 1.08, ratio of cup height to radial height 1.46 to 1.52, radial width 9.87 mm, primibrachial width 3.9 mm, ratio of radial width to primibrachial width 2.5. Tegmen not inflated, partly collapsed ( Figure 17B, C ), height of tegmen about 6.4 mm, anal cone probably slightly taller than oral cone, tegmen granulated except on anal cone and on upper part of orals making it difficult to estimate tegminal plate number (probably <15 per inter-radius), upper part of oral resembling a parrot’s beak.

One proximal arm attached to aboral cup showing pattern 1+2 3 4 with first pinnule on Br4, arms and pinnules without ornamentation. Only proximal part of genital pinnules preserved, lateral plates elongated and numerous but not in rows ( Figure 18A–C ), more numerous on one side, irregular small polygonal cover plates with needle-like stereome on their inner part ( Figure 18B ). Brachial facet regularly semi-circular, pinnule socket on upper part of brachial and having fulcral ridge parallel to those of brachial in proximal arm ( Figure 18D ) and oblique more distally ( Figure 18E ), synostosis uniting brachial pair with almost bilaterally symmetrical flat facet ( Figure 18F ), proximal articulation (synarthry) of first pinnular on brachial strongly flexible with sharp fulcral ridges and deep ligamentary pits ( Figure 18G ), distal muscular articulation of first pinnular well-developed ( Figure 18H ), other pinnulars united by synostoses with regular bilateral symmetry ( Figure 18I ).

Proximal stalk fragment attached to aboral cup with diameter decreasing from 3.4 mm to 2.3 mm at its distal end, length 46 mm, regularly cylindrical columnals articulated by symplexies with seven crenular units of two (rarely three) crenulae, crenulae widening out from inner to outer part, short subsidiary crenulae appearing between crenular units near outer border of facet ( Figure 17E, F ).

Remarks

Mironov and Sorokina (1998b) distinguished two subgenera within the genus Ptilocrinus , i.e. Chambersaecrinus and Ptilocrinus . As its genital expansion of pinnules is well-developed, P. tasmaniaensis sp. nov. could belong to the subgenus Chambersaecrinus . But, in Chamberseacrinus, the tegmen is high, inter-radial plates tend to be arranged in a median row and the number of columnal crenular units is nine to 10. So, characters of P. tasmaniaensis sp. nov. appear to be intermediate between the subgenera; recognition of the subgenera Chambersaecrinus and Ptilocrinus based upon such characters is questioned.

Proximal part of genital pinnules with numerous lateral plates without regular rows is a character shared by Ptilocrinus brucei Vaney, 1908 , and Calamocrinus diomedae and also by Gephyrocrinus grimaldii Koehler and Bather, 1902 ( Mironov and Sorokina 1998b; Roux 2004; Bohn and Roux, in preparation). That confirms the close affinities among these three genera which were previously pointed out by Roux (2004). However, the peculiar feature of cover plates with needlelike inner face is presently known only in P. tasmaniaensis sp. nov. among hyocrinids.

Ptilocrinus tasmaniaensis sp. nov. and F. koslowi sp. nov. were collected together at station 45 (Hill W). We suggest that the two species may have developed different feeding strategies in same environment.

Conclusions

The hyocrinid specimens described above are the first found off southeastern Australia and southern Tasmania. Previous southwestern Pacific records ranged from Macquarie Island to New Caledonia, mainly at depths greater than 2000 m. The nearest record is Ptilocrinus stukalinae Mironov and Sorokina, 1998b , dredged between New Zealand and Macquarie Island at a depth of 5400 m. McKnight (1973) attributed to? Ptilocrinus sp. numerous isolated ossicles (mainly columnals and a few brachials) found off Macquarie Island at a depth of 1280 m. Three taxa were collected during submersible investigations, i.e. Thalassocrinus mironovi Roux, 2002 at a depth of 1870 m and Hyocrinus cyanae Bourseau et al., 1991 at a depth of 2536 m both off New Caledonia, and Laubiericrinus pentagonalis Roux, 2004 on the North Fiji Rise at a depth of 2765 m. As each sampling revealed a new taxon and each southwestern Pacific species is known by a single specimen, we suspect the real hyocrinid diversity to be greater than presently listed (at least six species attributed to five different genera). Feracrinus koslowi sp. nov. is the first species described from eight specimens, including three specimens from the same dredge which illustrate an ontogenetic sequence. Except for Ptilocrinus stukalinae Mironov and Sorokina, 1998b , all these species live between about 1000 and 3000 m. Thalassocrinus mironovi comes from a passive margin and Ptilocrinus australis sp. nov. from a passive margin and seamount. The other species live on rises and seamounts. Southern Tasmanian species as well as T. mironovi live at depths less than 2000 m.

The genus Feracrinus is now known both from northeastern Japan ( F. aculeatus ) and southern Tasmania ( F. koslowi sp. nov.), which are two distant areas of the western Pacific. The two species have close affinities but live at different depth ranges: 1310– 1815 m for F. koslowi sp. nov. and 2915–3275 m for F. aculeatus . The wide intraspecific variations in middle-distal arm pattern and stalk articulations observed in F. koslowi sp. nov. are related to the heterogeneity of the rocky substrate and the current regimen in the seamount environment. Such an opportunistic adaptive strategy of deep-sea stalked crinoids seems to explain their frequent widespread dispersion despite the absence of knowledge on larvae ( Roux 1987; Améziane and Roux 1997).

The genus Ptilocrinus (sensu stricto) was clearly identified from the northern Pacific, off Macquarie Island, and off the eastern side of the Antarctic Peninsula at depths ranging from 2450 to 6145 m ( Mironov and Sorokina 1998b). Only a few species like P. brucei or P. stukalinae live at depths greater than 4000 m so P. tasmaniaensis sp. nov. and P. australis sp. nov. are the species that live at the shallowest depths (less than 1820 m). Ptilocrinus stukalinae is the deepest species (5397–6145 m) with the widest distribution and occurs throughout the range of the entire genus except in the northwestern Pacific. It colonizes both seamounts (Imperator Chain) and trenches (South Orkney Trough, Macquarie Trench).

Améziane and Roux (1997) pointed out the favourable environment of rises and seamounts for widespread dispersal in the deep sea of suspension feeders like hyocrinid crinoids. In our current state of knowledge and despite that they are new to science, the hyocrinid species collected off southern Tasmania seamounts provide no evidence of endemism.

Figures

Figure 17. Ptilocrinus australis sp. nov. from station 40 (A) and holotype of Ptilocrinus tasmaniaensis sp. nov. (B–F). (A) Distal stalk attached on coral fragment, (B) theca and proximal stalk of the holotype, (C) tegmen showing oral cone, (D) tegmen showing anal cone, (E, F) columnal symplexy presumed at minimum stalk diameter. Scale bars: A, B, 2 cm; C, D, 5 mm; E, 1 mm; F, 0.1 mm.

Figure 18. Ptilocrinus tasmaniaensis sp. nov. brachials and pinnules. (A) Genital expansion at base of a proximal pinnule, (B) cover plate showing its inner face, (C) detail of A more distally, (D) distal facet of proximal brachial, (E) distal facet of the upper brachial of a brachial pair (more distally than D), (F) proximal synostosial facet of E, (G) pinnule socket of E, (H, I) proximal facet of second pinnular, (J) synostosial facet of pinnular supporting genital expansion. Scale bars: A, D–J, 1 mm; B, C, 0.1 mm.