Rhianastra isosceles, Mah, 2024
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publication ID |
https://doi.org/ 10.11646/zootaxa.5432.4.2 |
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publication LSID |
lsid:zoobank.org:pub:83AD2C59-8FC8-43AA-9576-68C34B88FE51 |
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DOI |
https://doi.org/10.5281/zenodo.10928255 |
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persistent identifier |
https://treatment.plazi.org/id/FD09D342-4827-FFC7-FF77-FF1DFEBC43AF |
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treatment provided by |
Plazi |
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scientific name |
Rhianastra isosceles |
| status |
n. gen. |
Rhianastra isosceles n. gen. n. sp.
FIGURE 13A–F View FIGURE 13 , 14A–B View FIGURE 14
Etymology
The genus is named for Dr. Rhian Waller, Biology Science Lead of EX2104 who collected the specimen. The epithet isosceles refers to the distinctive triangular arms present on this species.
Diagnosis
Strongly stellate body form with sharply triangular arms ending in strongly tapering tips ( Fig. 13A, E View FIGURE 13 ). Arms five, but with one specimen showing six. Abactinal plates tabulate ( Fig. 13B, C View FIGURE 13 ), papular region along six to seven series from disc to armtip along central radius along arm. Surface covered by dense covering of granules (3 count along a 1.0 mm line) ( Fig. 13C View FIGURE 13 ). Marginal plates, approximately 64–65 per interradius, quadrate in shape, covered by granules ( Fig. 13D View FIGURE 13 ). Furrow spines, 8 to 10, narrow, blunt with square tips in straight to weakly furrow formation ( Fig. 13E–F View FIGURE 13 ). Subambulacral spines adjacent to furrow spines, with 6 thick spines, approximately twice as thick as each furrow spine. Third and distalmost subambulacral “spines”, identical in stature to granules on actinal surface.
Comments
Rhianastra isosceles n. gen. n. sp. possesses a generalized Ceramaster -like goniasterid set of characters, including tabulate abactinal plates along the radial region ( Fig. 13A–C View FIGURE 13 ), the relatively well-developed radial fasciolar grooves and papular regions as well as the consistent, complete granule covering over the abactinal, marginal and actinal intermediate plate surface ( Fig. 13A–F View FIGURE 13 ). However, the body form and abactinal/marginal plate arrangement are reminiscent of taxa in the Paxillosida , especially in the Astropectinidae , which are known for having triangularshaped arms with elongate, tapering arm tips with lateral-facing marginal plate series.
Rhianastra n. gen. invites comparison with taxa such as Pseudarchaster ( Pseudarchasteridae ), which also have tabulate abactinal plates, and in some species elongate, triangular arms. However the actinal plate pattern in Rhianastra is distinctly chevron-like, unlike those in pseudarchasterines, and the marginal plates, while wide, do not encroach onto the abactinal surface as they do in most Pseudarchaster species.
Other than the comparison of tabulate plate and granule characters, which are generalized for goniasterids, it was difficult to find an obviously close taxon for comparison.
Occurrence
Only known specimen from Gosnold “Shallow” Seamount with second observation from Retriever Seamount, both in North Atlantic, 1752–1878 m.
Description
Body strongly stellate (R/r=4.2), arms sharply triangular, arm tips tapering, upturned ( Fig. 13A, E View FIGURE 13 , Fig. 14A, B View FIGURE 14 ). Interradial arcs angular. Disc, arms thick. Specimen shows five arms but another imaged, but not collected specimen shows six ( Fig. 14B View FIGURE 14 ). Marginal plate surfaces with strongly lateral facing, with strongly angular dorsolateral and actinolateral edges.
Abactinal surface with discrete radial papular regions ( Fig. 13A, C View FIGURE 13 ) with fasciolar regions present amidst low tabulate plates ( Fig. 13A View FIGURE 13 ) in 6 to 7 series beginning on disc, extending along arms, decreasing to a single series along distalmost arm tip ( Fig. 13A, C View FIGURE 13 ). Abactinal plates on disc and on adradial regions on arms becoming flush with surface distally along arm. Radial fasciolar grooves shallow but with distinct fasciolar grooves, proximally eventually “shallowing out” out distally. Single row of plates extending to arm tip. Surface covered by coarse, round granules, approximately 3 granules counted along a 1.0 mm line. Abactinal plates on radial regions, round to hexagonal in outline ( Fig. 13B View FIGURE 13 ). Each of these radial abactinal plates with 9 to 40 granules, widely spaced per plate. Peripheral granules, 7 to 30, more quadrate in shape, central granules round, 2 to 10. Granular cover on non-radial regions, round and homogenous in size and shape, more densely arranged than on radial regions, which are more widely spaced. Non-radial regions covered by granules that obscure plate boundaries. Madreporite irregularly round set within a deep impression below the granules with deep, well-defined sulci. Anus flanked by five round to elongate quadrate-shaped plates, each covered by 20–30 granules. No pedicellariae observed on abactinal surface.
Marginal plates approximately 65 per interradius, armtip to armtip (32 per arm with 1 interradially), Inferomarginals with an extra interradial plate in each interradius, triangular in shape, present in each interradius ( Fig. 13D View FIGURE 13 ). Both series of marginal plates quadrate in shape with distinct corners with W>L, inferomarginals wider than superomarginals. Inferomarginal lower plate extending onto actinal surface ( Fig. 13E View FIGURE 13 ). Marginal plate surfaces covered with uniformly distributed, round granules, approximately 150–400, identical to those on the abactinal surface ( Fig. 13D View FIGURE 13 ). No pedicellariae on marginal plates.
Actinal plates diamond to round in shape, in 4 full series in chevron arrangement ( Figs. 13E–F View FIGURE 13 ). Actinal plates covered by granules, 7 to 30 widely spaced, quadrate to polygonal. Simple two-piece pedicellariae present on proximal 3 to 6 actinal plates, 3 observed adjacent to the oral plates 1 to 2 pedicellariae on other actinal plates, tending to be present closer to the oral region.
Adambulacral plates with spines in three series. Furrow spines, 8 to 10, narrow, blunt with square tips in straight to weakly palmate formation ( Figs. 13E–F View FIGURE 13 ). Subambulacral spines adjacent to furrow spines, with 6 thick spines, approximately twice as thick as each furrow spine. Third and distalmost subambulacral “spines”, identical to actinal granules in stature. Each row of spines on adambulacral plates widely spaced from one another. Oral plate furrow spines 11, oral plates with 15–20 short, thickened spinelets, evenly spaced from one another ( Fig. 13F View FIGURE 13 ). Spinelets arranged on either half of the oral plates, separated by a wide central fossae.
Tube feet unpointed, in biserial series, decreasing to a single series distally. Each furrow wide ( Fig. 13E–F View FIGURE 13 ).
Color in life pale orange, darkening on radial regions and along lateral edges ( Fig 14A–B View FIGURE 14 ).
Material Examined
Holotype. USNM 1658841. Gosnold “shallow” Seamount. 38.2925 -62.5329, 1752 m. Coll. NOAA Ship Okeanos Explorer (R. Waller) 24 July 2021. 1 wet spec. R = 7.6 cm r= 1.8 cm.
EX2104_IMG_20210724T163944Z_ROVHD.jpg
Other images
North Atlantic Gosnold “shallow” seamount, 38.293253 -62.533149, 1778 m ( Fig. 14A View FIGURE 14 ). EX2104_IMG_20210724T155922Z_ROVHD.jpg
Retriever Seamount, 39.837256 -66.230123, 1878m.(6rayed form) ( Fig.14B View FIGURE 14 ).EX2104_IMG_20210727T173546Z_ ROVHD.jpg
| USNM |
Smithsonian Institution, National Museum of Natural History |
| R |
Departamento de Geologia, Universidad de Chile |
No known copyright restrictions apply. See Agosti, D., Egloff, W., 2009. Taxonomic information exchange and copyright: the Plazi approach. BMC Research Notes 2009, 2:53 for further explanation.