Friday, April 3, 2020

[Paleontology • 2020] New Rhenopyrgid Edrioasteroids (Echinodermata) and Their Implications for Taxonomy, Functional Morphology, and Paleoecology

Rhenopyrgus viviani Ewin, Martin, Isotalo & Zamora, 2020 
Silurian (lower Telychian), Jupiter Formation, Anticosti Island, Canada.

 (1, 2) Rhenopyrgus grayae (Bather, 1915), Upper Ordovician, Ayrshire, Scotland
(holotype E23470);  
(6–7) Rhenopyrgus indet. 3; (9) Rhenopyrgus coronaeformis Rievers, 1961, Lower Devonian, Bavaria, Germany
(holotype SNSB-BSPG 1958 XV 50)

Illustration: Virgil Tanasa

Rhenopyrgids are rare, turreted edrioasterid edrioasteroids from the lower Paleozoic with a distinctive and apparently conservative morphology. However, new, well-preserved rhenopyrgid edrioasteroid material from Canada, along with a review of described taxa, has revealed broader structural diversity in the oral surface and enabled a re-evaluation of rhenopyrgid functional morphology and paleoecology.

The floor plates in Rhenopyrgus viviani n. sp., R. coronaeformis Rievers, 1961 and, R. flos Klug et al., 2008 are well fused to each other and the interradial oral plate and lack obvious sutures, thereby forming a single compound interradial plate. This differs from other rhenopyrgids where sutures are more apparent. Such fused oral surface construction is only otherwise seen in some derived edrioblastoids and in the cyathocystids, suggesting homoplasy.

Our analysis further suggests that the suboral constriction could contract but the flexible pyrgate zone could not. Thus, specimens apparently lacking a sub-oral constriction should not necessarily be placed in separate genera within the Rhenopyrgidae. It also supports rhenopyrgids as epifaunal mud-stickers with only the bulbous, textured, entire holdfasts (coriaceous sacs) anchored within the substrate rather than as burrow dwellers or encrusters.

Rhenopyrgus viviani n. sp. is described from the Telychian (lower Silurian) Jupiter Formation of Anticosti Island, Québec, Canada and is differentiated by a high degree of morphological variability of pedunculate plates, broader oral plates, and narrower distal ambulacral zones. Specimens lacking or with obscured diagnostic plates from the Ordovician of Montagne Noire, France, and the Ordovician and Silurian of Girvan, Scotland are also described.

Systematic paleontology 

Phylum Echinodermata de Bruguière, 1791 (ex. Klein, 1734) 
Class Edrioasteroidea Billings, 1858 
Order Edrioasterida Bell, 1976 
Suborder Edrioblastoidina Fay, 1962 

Family Rhenopyrgidae Holloway and Jell, 1983 

Genera included.— Rhenopyrgus Dehm, 1961; Heropyrgus Briggs et al., 2017.

Figure 3. Rhenopyrgidae; all whitened with ammonium chloride.
 (1, 2) Rhenopyrgus grayae, Upper Ordovician, Lady Burn Starfish Bed, Girvan, Ayrshire, Scotland (holotype E23470): (1) details of oral surface with prominent collar plates and no suboral constriction apparent; black arrows highlight exposed floor plates in distal ambulacra; white arrow points to a small, exposed part of a plate of the suboral constriction, suggesting that suboral constriction could be contracted behind collar plates; (2) lateral view.
 (3, 4) Rhenopyrgus indet. 1, Silurian, Newland Formation, Newlands, Ayrshire, Scotland (E 62753): (3) lateral view of pyrgate zone, arrow points to enlarged plates interpreted here as collar plates; (4) oral view, black arrow highlights disarticulated large D-shaped oral ossicle.
(5) Rhenopyrgus indet. 2, Ordovician, Drummuck Series, Ardmillan, Girvan District, Ayrshire, Scotland (EE 16254); lateral view of pyrgate zone; note difference in size and morphology of the pyrgate ossicles suggesting it is different than R. grayae, which is found in similar age rocks that are geographically close; also note disarticulated ridged deltoidshaped plate closely associated with articulated pyrgate plates.
(6–8) Rhenopyrgus indet. 3: (6, 7) Foulon Formation (middle Floian), La Croix de Roquebrun, Saint-Nazaire-de-Ladarez, Hérault, France (UCBL-FSL 713312); (6) lateral view of whole specimen; (7) enlargement of the oral surface, showing confused plate articulation of this region; black arrow highlights possible oral ossicle; white arrows highlight collar plate series; (8) late Tremadocian, beneath Saint-Chinian Formation, Saint-Chinian, SW of Donnadieu, Babeau-Bouldoux, Hérault, France (UCBL-FSL 713316), lateral view.
(9–11) Rhenopyrgus coronaeformis Rievers, 1961, Lower Devonian, Emsian, Hunsrück Slate, Bavaria, Germany (holotype SNSB-BSPG 1958 XV 50): (9) detail of oral surface and proximal structures; (10) detail of oral surface; note complicated cover plate articulation surfaces; black arrow highlights grooved adambulacral margin of oral plate; (11) detail of coriaceous sac.
Abbreviations, O = oral plate. All scale bars represent 1 mm.

Genus Rhenopyrgus Dehm, 1961 

Type species.— Pyrgocystis (Rhenopyrgus) coronaeformis Rievers, 1961; Hunsrück Slate, Lower Devonian, (Emsian) of Germany.

Other species.— Rhenopyrgus coronaeformis Rievers, 1961; R. flos Klug et al., 2008; R. viviani n. sp.; R. sp. indet.1, formerly Pyrgocystis procera (Aurivillius) Bather, 1915; R. sp. indet. 2, R. sp. indet. 3, R. sp. indet. 4.; R. grayae (Bather, 1915); R. whitei Holloway and Jell, 1983; and R. piojoensis Sumrall et al., 2013.

Figure 5. Idealized reconstruction of Rhenopyrgus viviani n. sp. Silurian (lower Telychian), Jupiter Formation, Jupiter River, Anticosti Island, Canada. Note individuals with extended and contracted suboral constrictions and with only the coriaceous sac and very distal part of the pyrgate zone buried in the substrate.
Illustration: Virgil Tanasa

Rhenopyrgus viviani new species

Occurrence.— Cybèle Member to Pavillon Member, Jupiter Formation, Telychian, upper Llandovery, lower Silurian, Anticosti Island, Québec, Canada.

Etymology.— Named for the initial discoverer Mr. Travis Vivian.

Timothy A. M. Ewin, Markus Martin, Phillip Isotalo and Samuel Zamora. 2020. New Rhenopyrgid Edrioasteroids (Echinodermata) and Their Implications for Taxonomy, Functional Morphology, and Paleoecology. Journal of Paleontology. 94(1); 115-130.  DOI: 10.1017/jpa.2019.65

Newest member of echinoderm family revealed