Panglaocrinus isseliformis, Roux, 2024

Roux, Nadia Améziane Marc Eléaume Michel, 2024, Classification of Isocrinida (Echinodermata: Crinoidea) with the description of a new extant genus and species from the western Pacific, Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) 200 (4), pp. 994-1012 : 998-1000

publication ID	https://doi.org/ 10.1093/zoolinnean/zlad101
publication LSID	lsid:zoobank.org:pub:UMR7205CNRS-MNHN-EPHE-SUUA
DOI	https://doi.org/10.5281/zenodo.11354767
persistent identifier	https://treatment.plazi.org/id/5D0E87C1-BD65-FF86-D787-57F8C531FA37
treatment provided by	Plazi
scientific name	Panglaocrinus isseliformis
status	gen. nov., sp. nov.

Treatment

Panglaocrinus isseliformis View in CoL gen. nov., sp. nov.

( Figs 2–8 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 )

Etymology: Displaying morphological characters similar to those found in the extinct genus Isselicrinus .

Material examined: A single specimen (catalogue number MNHN-IE-2007-13 ), Deep Sea Cruise PANGLAO 2005 (P. Bouchet, L. Labe, and P.K.L. Ng chief scientists), 22 May 2005 to 1 June 2005 on board N/O DA-BFAR, Station CP 2386, depth range 2120–2149 m, 08°49ʹN–123°02ʹE ( Table 2 View Table 2 ) .

Diagnosis: Isocrinid genus with IBr2ax, IIBr15-17ax, synostosis at IBr1 + 2, IIBr1 + 2, and occasionally at IIBr3 + 4, trace of previous axial synarthry on IBr1 + 2 synostosis, large and deep muscular areas and rectangular adoral ligament area along fulcral ridge in proximal muscular synarthries; short pinnules of 10–12 pinnulars, large and round cover plates alternating with small isolated lateral plates; tegmen with well-developed anal cone reaching height of IIBr3; stalk with columnals pentalobate proximally and rounded pentagonal distally, maximum number of internodals per noditaxis 21, internodals and nodals tending to equal height distally (H/D up to 0.8), nodals with two cirrus sockets on their distal edge, small cirri always oriented downwards (distally) and alternating along the stalk; infranodal cryptosymplexy; symplexies with open areolas, without inner crenularium and with five pairs of dense calcite patches at base of interpetaloid crenularium.

Description of the holotype: Small specimen with maximum arm length 38 mm (all distal arms broken), proximal stalk diameter 2.3 mm and preserved stalk length 69 mm ( Fig. 2 View Figure 2 ). Aboral cup with radial ring shaped as an inverted cone, cup diameter at top of radial ring 5.4 mm, cup height 3.6 mm, radials substantially taller than basals (ratio of radial to basal height 3.1), basals pentagonal and in contact laterally, each radial regularly convex. Tegmen poorly preserved with well-developed anal cone attaining IIBr3 and having sub-lozenge to oval imbricated plates ( Fig. 3 View Figure 3 ).

All arms with primibrachitaxes of two brachials (IBr1 + 2ax), second axillary at IIBr15 (4 cases) ( Fig. 2A View Figure 2 ) or IIBr17 (one case), arm broken at IIBr16 (one case), other arms broken before IIBr7. Proximal pattern of secondibrachitaxes IIBr1 + 2 (synostosis) with first pinnule on IIBr2, occasionally second synostosis at IIBr3 + 4 (two cases for seven arms broken beyond IIBr4); tertibrachitaxes without proximal synostosis ( Fig. 4A View Figure 4 ). One arm not completely regenerated (from autotomy at IBr1 + 2) and smaller than the others ( Fig. 2B View Figure 2 ). Maximum width of IBr2ax 3.6 mm, height of primibrachitaxis 4.0 mm, ratio maximum width to height of primibrachitaxis 0.89. Muscular synarthries of primibrachials well developed with deep and large muscular areas ( Fig. 5A, C, D View Figure 5 ), articulation of IBr on radial also displaying wide sub-rectangular areas corresponding to the inner ligamentary area ( Fig. 5A View Figure 5 ). Markedly concave synostosis at IBr1 + 2 with traces of axial synarthry and small aboral pit of earlier ontogenetic stage ( Fig. 5B View Figure 5 ). Synostosis at IIBr1 + 2 differing in having a thicker synostosial stereom developed along its external border and around the axial canal ( Fig. 6D View Figure 6 ). Flat synostosis occasionally present at IIBr3 + 4. Proximal facet of IIBr4 corresponding to a moderately asymmetrical sub-circular muscular synarthry with adjacent muscular and inner ligamentary areas of nearly the same size ( Fig. 6C, E, F View Figure 6 ). Pinnule socket relatively large and located on the upper (distal) half, deforming distal brachial facet ( Fig. 6A, B View Figure 6 ). Pinnule articulated on brachial by an asymmetrical muscular synarthry with a deep muscular fossa on brachial corresponding to the single muscular area developed on P1 proximal facet ( Fig. 7C View Figure 7 ). P1-2 muscular synarthry symmetrical ( Fig. 7D View Figure 7 ). Other pinnular synarthries becoming asymmetrical distally with marked fulcral ridge and small muscular areas ( Fig. 7E, F View Figure 7 ). Large and round, more or less oval, cover plates with radiating stereom arrangement, alternating with small isolated triangular lateral plates ( Fig. 7B View Figure 7 ). Pinnules short ( Fig. 7A View Figure 7 ) having maximum length of 8.1 mm for 10–12 pinnulars.

Stalk pentalobate in cross-section proximally and rounded pentagonal distally. Stalk diameter (D) decreasing from 2.3 mm proximally to 1.8 mm at 13.8 mm away from the proximalmost columnal articulation; distalmost diameter 1.9 mm. Seven nodals (N) are identified before the broken end, with number of internodals per noditaxis varying from aboral cup as follows (N: nodal): 17, N1, 13, N2, 21, N3, 10, N4, 8, N5, 15, N6, 15, N7, 2, broken distal end. Two proximal noditaxes (flexible proxistele) displaying numerous conspicuous inter-articular pores and corresponding to main growth zone of the stalk in which new columnals appear and rapidly increase in height (H). Maximum number of 21 internodals displayed in the third noditaxis (length 7.0 mm, ratio H/D 0.42 in N3 and> 0.38 in internodals). Distally, number of internodals varying from 8 to 15, columnal height increasing, internodals and nodals tending to be equal in height, and H/D ratio to be nearly 0.8 ( Fig. 4C View Figure 4 ). Inter-articular pores remaining widely open up to broken stalk end. Each nodal with two cirri, socket position alternating regarding pentamerous symmetry along the stalk. On N1 and N2 cirri small, restricted to a rudimentary bud; their growth directed towards the distal part of stalk, as seen distinctly on N3 ( Fig. 4B View Figure 4 ); length progressively increasing up to 5.1 mm on N6 with nine cirrals ( Fig. 4C View Figure 4 ). Cirrus sockets located near the distal margin of nodals. Middle and distal stalk appearing relatively rigid and all columnal articulations nearly flat ( Fig. 8 View Figure 8 ). Articular facets always with conspicuous interpetaloid grooves corresponding to widely open inter-articular pores on external stalk surface. One pair of dense calcite patches present at the inner end of each groove. Nodal cryptosymplexy with facets entirely covered by synostosial stereom (secondary synostosis) without trace of syzygial stereom ( Fig. 8D View Figure 8 ). Symplexies between internodals ( Fig. 8E, F View Figure 8 ) displaying a crenularium of low relief with up to four crenulae per petaloid zone, each petaloid zone without marginal crenulae (open areola); galleried stereom having meshes significantly larger in inner than in outer areola.

Remarks: The small size, large inter-articular pores distributed along the entire length of the stalk, the high ratio H/D in distal columnals, proxistele with rudimentary cirri, and short pinnules, all suggest that the holotype is a juvenile specimen. The small rudimentary distal cirri cannot provide anchorage on a substrate and this specimen is likely to have been attached with the help of an incrusting disk on a rocky substrate or pebble such as observed for juveniles of Neocrinus naresianus and Endoxocrinus alternicirrus (see: Carpenter 1884: pl. 30, fig. 4; Tunnicliffe et al. 2016: fig. 11). In addition, the variable number of internodals per noditaxis beyond N3 is also interpreted as a juvenile character, and the high number of internodals (21) in noditaxis N3 may be considered as the most peramorphic character. This number is probably higher in larger, more mature, specimens because in isocrinids it usually tends to increase during ontogeny. However, in Panglaocrinus isseliformis gen. nov., sp. nov., the mosaic of morphological characters significantly differs from all other extant genera of Isocrinida known to date. The highest number of secundibrachials (14, mode 5) is found in Cenocrinus asterius among extant species with IBr2ax ( David 1998), and in Metacrinus levii (21, mode 11) for extant species with more than two primibrachials ( Améziane-Cominardi 1991). The number of secundibrachials varies between 8 and 14 in the only two crowns of the genus Isselicrinus known to date from the Lower Eocene ( Rasmussen 1972). Columnal symplexies with dense calcite patches and without inner crenularium as seen in Panglaocrinus isseliformis gen. nov., sp. nov. are known only from Proisocrinus ruberrimus (see: Bourseau et al. 1991). Cirrus sockets on the distal half of nodals is a character shared with the three species of Neocrinus , a genus that has cirri that are well developed, relatively delicate, and more or less oriented downwards (distally). In Proisocrinus , cirri are restricted to the proxistele. In juvenile specimens, cirri are better developed than in adults, and are oriented directly upwards (proximally) ( Bourseau et al. 1991). A reduced number of cirri per whorl (three or two) is frequent in Endoxocrinus alternicirrus , particularly in the subspecies E. alternicirrus alternicirrus inhabiting the deepest depth range ( David et al. 2006), and Rasmussen (1978) pointed out a similar reduction in the number of cirri in the Late Cretaceous–Paleocene genus Doreckicrinus . The fossil genus Isselicrinus typically displays one to three cirri (usually two or three) per nodal that are inserted on the distalmost border of nodals, indicating a downward-directed growth, and placed in alternating positions along the stalk, a character named ‘alternicirration’ by Bather (1917). Solely, the Late Cretaceous genus Austinocrinus commonly displays one or two cirrus sockets per nodal, carrying robust cirri oriented upwards ( Hess 2011b). Panglaocrinus isseliformis gen. nov., sp. nov. displays the closest morphological affinities to the extinct genus Isselicrinus , which ranged from the Upper Cretaceous ( Rasmussen 1961) to the Oligo-Miocene (Klikushin 1992, Kirilova and Georgiev 2018) and occupied a wide depth range during the Eocene ( Roux et al. 2006). Panglaocrinus isselliformis gen. nov., sp. nov. was dredged from a depth of 2149 m, which corresponds to the greatest depth attained by extant isocrinids, i.e. 2070 m for Endoxocrinus wywillethomsoni in the Bay of Biscay, north-east Atlantic (Roux 1985) and 2468 m for Neocrinus naresianus on the Tonga –Kermadec slopes ( Carpenter 1884).