Rhabdomeson Young & Young, 1874

Ernst, Andrej, Wyse Jackson, Patrick N. & Aretz, Markus, 2015, Bryozoan fauna from the Mississippian (Visean) of Roque Redonde (Montagne Noire, southern France), Geodiversitas 37 (2), pp. 151-213 : 176-178

Genus Rhabdomeson Young & Young, 1874

Coeloconus Ulrich, 1889: 298 .

TYPE SPECIES. — Rhabdomeson progracile Wyse Jackson & Bancroft, 1995 by subsequent designation of Wyse Jackson & Bancroft (1995) ( ICZN Opinion 1854; ICZN 1996). Lower Carboniferous, Mississippian (Visean-Serpukhovian); Britain and Ireland.

DIAGNOSIS. — Rhabdomesid with delicate dendroid colony with irregularly dichotomizing branches. Autozooecia regularly budding around central axial cylinder in an annual or spiral manner. Hemisepta common. Autozooecial apertures elliptical, pyriform or rhombic, closely spaced, arranged in quincunx on colony surface; of constant or variable dimensions around branch. Stylets abundant and structurally diverse (after Wyse Jackson and Bancroft, 1995).

OCCURRENCE. — Middle Devonian to Upper Permian; worldwide.

COMPARISON

Rhabdomeson Young & Young, 1874 differs from other rhabdomesines in the presence of a central axial cylinder, from Silenella Gorjunova, 1992 in absence of aktinotostyles, from Pseudorhabdomeson Gorjunova, 2002 in having a rounded axial tube instead of a polygonal one and in the presence of differentiated styles instead of aktinotostyles.

Rhabdomeson progracile Wyse Jackson & Bancroft, 1995 ( Fig. 19 A-F; Appendix)

For synonymy see Wyse Jackson & Bancroft (1995: 30, 31).

MATERIAL. — SMF 21.872- SMF 21.883.

OCCURRENCE. — Carboniferous, Mississippian (Visean-Serpukhovian); Britain, Ireland. Carboniferous, Mississippian (upper Visean); Roque Redonde (Montagne Noire, southern France).

DESCRIPTION

Branched colonies with narrow axial cylinder. Branches 0.60- 1.63 mm in diameter. Axial cylinder circular to polygonal, 0.1- 0.2 mm in diameter. Autozooecia budding in a spiral pattern diverging at angles of 27-40° from the axial cylinder, arranged 10-14 in the first row around the axial cylinder, hexagonal to rhomboid in transverse in the endozone. Autozooecial apertures oval, arranged in regular diagonal rows. Macroacanthostyles large, long, 4 arranged around each aperture building a regular hexagonal pattern. Microacanthostyles small, 1-2 arranged between macroacanthostyles. Paurostyles scattered on the colony surface. Superior hemisepta small, blunt, positioned at distal end of the autozooecia, visible only in occasional sections; inferior hemisepta absent. Diaphragms absent.

COMPARISON

Rhabdomeson progracile Wyse Jackson & Bancroft, 1995 is similar to R. regularis Nekhoroshev, 1932 , and R. spinosum Morozova, 1955 , in terms of size, and in the positioning of a solitary prominent acanthostyle situated proximally of autozooecial apertures. R. progracile differs from R. regularis in having larger hemisepta and wider autozooecial apertures, and from R. spinosum in acanthostyles that are smaller and a wider exozone ( Wyse Jackson & Bancroft 1995: 34).

Figures

FIG. 19. — A-F, Rhabdomeson progracile Wyse Jackson & Bancroft, 1995; A, longitudinal section showing axial cylinder, SMF 21.882; B, longitudinal section showing autozooecial chambers with hemisepta (arrow), SMF 21.882; C, tangential section showing autozooecial apertures, acanthostyles and paurostyles (arrow), SMF 21.872; D, tangential section showing acanthostyles and paurostyles (arrow), SMF 21.872; E, F, branch transverse section, SMF 21.875; G-I, Saffordotaxis incrassata (Ulrich, 1888), SMF 21.889; G, tangential section showing autozooecial apertures and aktinotostyles; H, I, branch longitudinal section. Scale bars: A, 2 mm; B, C, E, H, 1 mm; D, 0.2 mm; F, G, I, 0.5 mm.