Ptilocrinus australis, 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: 160-166

publication ID

http://doi.org/ 10.1080/00222933.2010.520825

persistent identifier

http://treatment.plazi.org/id/03E687BE-FF8F-5173-FE22-FA3252DBFD71

treatment provided by

Felipe

scientific name

Ptilocrinus australis
status

sp. nov.

Ptilocrinus australis   sp. nov.

Etymology

Named for the Austral Ocean.

Material

A single specimen ( Figure 13A, B View Figure 13 ), the holotype, with one isolated arm, proximal crown and complete stalk collected in 1988 during a cruise of FV Adelaide Pearl, station 28, 33 ◦ 45 ′ S, 129 ◦ 19 ′ E, off South Australia, depth 999–1110 m GoogleMaps   , housed in the collections of South Australian Museum , Adelaide, Australia. One stalk in four fragments without its proximal part from Tasmanian seamounts, station 40, hill J1, 44.24 ◦ S, 147.36 ◦ W to 44.27 ◦ S, 147.32 ◦ W, depth 1200–1450 m GoogleMaps   .

Diagnosis

A species of Ptilocrinus   with inflated tegmen attached to arms up to Br5–6, anal sac not conical and much higher than oral cone. Orals with spherical expansions at top. Proximal arm pattern 1+2 3 4+5 6 7+8 with first pinnule on Br5, followed usually by brachial pairs united by synostosis, brachial pairs separated by one to three muscular joints, rarely four, pattern of distal arm a+b c d e+f g h i+j. Arms robust bearing more than 25 pinnules on each side. Ten proximal brachials nearly equal in height and width. Proximal part of genital pinnules with one row of very irregular lateral plates on one side, largest ones separating two successive cover plates, cover plates rounded. Proximal stalk symplexies with 8–10 crenular units of one or two crenulae transitioning distally to regular multiradiate pattern.

Holotype

South Australian Museum registration number: SAMK2480. Aboral cup with wide ribs prolonging arm axis, height of aboral cup 8.7 mm, diameter at top of radial ring 11.8 mm, ratio of aboral cup height to upper radial ring diameter 0.74, height of radials 4.9 mm, diameter at base of radial ring 7.6 mm, width of radials 6.4 mm, ratio of radial upper width to first primibrachial width 1.4, height of basal ring 3.7 mm, diameter at base of basal ring 2.2 mm. Basal fused, aboral border of basal ring slightly flanged without morphological continuity with proximal stalk ( Figure 13C View Figure 13 ). Tegmen moderately inflated and attached on arms up to Br5–6 with first pinnules remaining free ( Figure 13D View Figure 13 ), height at top of anal cone 12.7 mm, height at top of oral cone 7.8 mm, 15–20 tegminal plates per inter-radius, regularly polygonal and convex, hydropores only at top of tegmen, orals with two or three spherical expansions ( Figure 13E View Figure 13 ).

Arm bearing more than 25 pinnules on each side, arm length about 9–10 cm (size extrapolated from arm fragments). Five proximal brachials nearly equal in height, width of primibrachials 4.5 mm, width of Br10 2.8 mm. Proximal arm pattern 1+2 3 4+5 6 7+8 with first pinnule on Br5 (five cases), 9+10 (one case), 10+11 (three cases), only one arm preserved beyond Br12 with 1+2 3 4+5 6 7+8 9 10 11 12+13 14 15+16 17 18+19 20 21+22, fragment of distal arm with a+b c d e+f g h i+j. Pinnules welldifferentiated and relatively rigid ( Figure 13B View Figure 13 ), maximum length> 45 mm. Proximal part of genital pinnules with very irregular lateral plates on one side never forming a row ( Figure 14A View Figure 14 ), more distally large diamond-shaped plates separating two successive cover plates ( Figure 14B View Figure 14 ), rounded cover plates alternating with those of the other side.

Stalk complete with its distal attachment disc, total length 50.8 cm, proximalmost diameter 1.95 mm, minimum stalk diameter 1.78 mm at a distance of 1.8 cm from aboral cup; maximum stalk diameter 3.4 mm near attachment disc decreasing to 2.5 mm 6.5 cm above disc. Proxistele with distinctive nudinodals and internudinodals of slightly different diameters, external surface of primicolumnals with a row of tubercles or a ridge around middle ( Figure 15B View Figure 15 ). Nudinodals with external surfaces conspicuous convex up to 9 cm below aboral cup. ProximalProximal symplexies with 10 crenular units of one or two irregular crenulae ( Figure 15A View Figure 15 ) and irregular lobated lumen; additional crenulae appearing between crenular units 0.8 cm away from aboral cup, more distally multiradiate crenularium developed as columnal diameter increases ( Figure 15C–E View Figure 15 ); in the juvenile pattern observed at the facet centre of distalmost columnals ( Figure 15F View Figure 15 ) number of crenular units eight or nine.

Stalk from Tasmanian seamount, station 40

Museum Victoria registration number: MV F171346. Total preserved stalk length 28.1 cm; distal end fixed on coral fragment. Diameter of proximalmost preserved columnal 2.19 mm, articulation intermediate between symplexy and syzygy with inner part corresponding to juvenile symplexy having eight short crenular units of one or two raised crenulae and with outer part of lower radial crenulae of syzygial stereome prolonging inner crenular units or developed between them ( Figure 16A, B View Figure 16 ), diameter of inner part 1.4 mm. At a distance of 38 mm from aboral cup, columnal diameter 2.25 mm, same articulation with diameter of symplexial inner part 1.3 mm. At a distance of 113 mm, columnal diameter 2.50 mm; ratio of columnal diameter to height 2.4, articulation same as preceding with diameter of symplexial inner part 1.1 mm ( Figure 16C View Figure 16 ). At a distance of 188 mm, columnal diameter 2.80 mm, same pattern of articulation but with two rings in syzygial crenularium, the inner one more irregular, and diameter of inner symplexial part 0.88 mm ( Figure 16D, E View Figure 16 ). At a distance of 248 mm, columnal diameter 3.53 mm, multiradial syzygy partly anchylosed, three rings in syzygial crenularium, crenulae width weakly increasing from inner to outer ring, diameter of inner symplexial part 0.47 mm ( Figure 16F, G View Figure 16 ). Just before attachment on coral, columnal diameter 3.6 mm, ratio of columnal diameter to height 2.25. Distal attachment curled around coral ( Figure 17A View Figure 17 ).

Remarks

The distal stalk from station 40 and the holotype clearly belong to the same species both have a similar ontogenetic trajectory of columnals. The holotype is a younger specimen with a narrower distal stalk which limits the development of syzygial crenularium. Its greater number of symplexial crenular units in proximal articulations documents an intraspecific variation of this character.

Along the stalk, the pattern of articular facets progressively changes from symplexy (only functional in the proximal stalk) to anchylosed distal syzygy. Distal stalk remains with a relatively regular multiradial syzygial crenularium. It differs from the genus Feracrinus   by the absence of syzygies with labyrinthic pattern. The trend towards dense and regular multiradial crenularium in middle stalk articulations was described in Dumetocrinus antarcticus   , Calamocrinus diomedae   , Hyocrinus biscoitoi   and Analchalypsicrinus nefertitii ( Agassiz 1892; Holland et al. 1991; Mironov and Sorokina 1998b; Roux 1980, 2004) but never with the same feature.

Ptilocrinus australis   sp. nov. shares with Ptilocrinus pinnatus   the main crown and tegmen characters: proximal brachials nearly equal in height and width without ornamentation, genital pinnules with irregular lateral plates of various size, inflated tegmen with numerous polygonal tegminal plates of about the same size. The two species seem to be closely related. Ptilocrinus australis   sp. nov. mainly differs in having the first pinnule on Br5, the aboral border of the basal ring slightly flanged, marking a morphological discontinuity with the proximal stalk, and 8–10 shorter crenulae in symplexial stalk articulations. The proximal and distal stalk articulations of P. pinnatus   remain unknown. Mironov and Sorokina (1998b) documented the frequency of first pinnule on Br 5 in the genus Ptilocrinus   sensu stricto. So, for this character, the holotype of P. australis   sp. nov. falls within the range of intrageneric variation.

Occurrence

South Australian margin and Tasmanian Seamounts, depth 999–1450 m.