Celsiorthis bulancis, Paterson & Brock, 2003

Paterson, John R. & Brock, Glenn A., 2003, Early Ordovician Orthide Brachiopods from Mount Arrowsmith, Northwestern New South Wales, Australia, Records of the Australian Museum 55 (2), pp. 221-230 : 225-227

publication ID	https://doi.org/ 10.3853/j.0067-1975.55.2003.1384
DOI	https://doi.org/10.5281/zenodo.10529438
persistent identifier	https://treatment.plazi.org/id/03A1040B-FFB3-FFCF-9C17-FF3BFD42FF2A
treatment provided by	Carolina
scientific name	Celsiorthis bulancis
status	n.gen. and n.sp.

Treatment

Celsiorthis bulancis View in CoL n.gen. and n.sp.

Fig. 3A–N View Fig , Table 1

Type material. HOLOTYPE: AM F120718 , ventral valve ( Fig. 3L–N View Fig ). P ARATYPES: AM F120710 , dorsal valve ( Fig. 3A,B View Fig ); AM F120711 , dorsal valve ( Fig. 3C View Fig ); AM F120712 , complete shell ( Fig. 3D–F View Fig ); AM F120713 , ventral valve ( Fig. 3G View Fig ); AM F120714 , ventral valve ( Fig. 3H View Fig ); AM F120715 , ventral valve ( Fig. 3I View Fig ); AM F120716 , ventral valve ( Fig. 3J View Fig ); AM F120717 , ventral valve ( Fig. 3K View Fig ).

Type locality. Tabita Formation, TAB1 section, 30 m above base of section ( Figs. 1 View Fig , 2 View Fig ).

Etymology. Latin bu, prefix meaning large, and lancis, plate; referring to the large dental plates in the ventral valve.

Diagnosis. As for genus by monotypy.

Description. Exterior: ventribiconvex, subquadrate in outline. Ventral valve moderately and uniformly convex with slightly swollen umbo; hinge width approximately 90– 95% maximum valve width. Ventral interarea wide, high, flat, steeply apsacline to catacline. Delthyrium open. Dorsal valve moderately convex with shallow sulcus occupying median part of valve; maximum length about 60% of maximum width. Dorsal interarea low, anacline, with open notothyrium; hinge line mainly straight, but slightly deflected posteriorly in the posteromedian region. Commissure rectimarginate.

Shell finely costellate, becoming ramicostellate anteriorly; ribs low and rounded, becoming wider at anterior margin. Costae in umbonal area range between 25 and 35. Costellae arise principally by bifurcation and number 80– 90 along the anterior margin in adult shells. Fine, closely spaced concentric filae occur between costellae in larger specimens.

Ventral valve interior: dental plates large, divergent (approximately 20–30°), extending forward as raised lateral muscle bounding ridges. Muscle field large, well impressed, bilobed, about one-third valve length. Diductor scars large, elongate, subparallel; adductors slender, enclosed by diductors, and unbounded anteriorly. Costellae variably impressed around inner margin of shell.

Dorsal valve interior: cardinalia with broad, welldeveloped notothyrial platform which merges anteriorly with a short, low, broad median ridge. Cardinal process thick, ridge-like, with anterior portion merging with notothyrial platform. Brachiophores short, thick, tusk-like, anterolaterally divergent at 40–60°, grooved along inner faces, with narrow tops, rounded anterior ends, and thick bases joined to the notothyrial platform. Sockets very large with well rounded floors. Fulcral plates absent. Muscle field large, quadripartite, occupying about one-third valve length, bisected longitudinally by low median ridge. Posterior pair of adductors ovate, impressed below notothyrial platform, separated from smaller anterior pair by short transverse extension of the median ridge. Costellae impressed on anterior margin of shell.

Discussion. Celsiorthis bulancis can be distinguished from most other Ordovician members of the Orthidae (see Williams & Harper, 2000, p. 724–728), such as Orthis Dalman , Orthambonites Pander , Paralenorthis Havlíěek , Sulcatorthis Zeng , Sulevorthis Jaanusson & Bassett , and Trondorthis Neuman in Neuman & Bruton by its finely costellate to ramicostellate external ornament and steeply anacline to catacline interarea in the ventral valve.

Celsiorthis bulancis is similar to Sivorthis Jaanusson & Bassett , in particular the type species S. filistera Jaanusson & Bassett (1993, pl. 7, figs. 1–7), from the Ordovician (Caradoc) of Sweden. Both taxa possess a ventribiconvex shell, a high, wide ventral valve interarea, and have a similar costellate ornament. However, C. bulancis can be distinguished by its nearly flat, steeply apsacline to catacline ventral interarea ( Figs. 3D,F,M,N View Fig ), whereas S. filistera has a weakly apsacline ventral valve interarea. Celsiorthis bulancis is also more finely costellate (80–90 costellae along the anterior margin; Figs. 3L,K View Fig ) than S. filistera (40–50 costellae at anterior margin). The internal morphology of the ventral valve is very similar in both species; both have a large bilobed or subcordate muscle field bounded by relatively large dental plates ( Figs. 3G–J View Fig ). The interior of the dorsal valve of C. bulancis is also similar to S. filistera in having a well-developed notothyrial platform supported by a broad median ridge, and a large, quadripartite muscle field in which the posterior muscle scars are larger than the anterior pair ( Figs. 3A–C View Fig ). However, C. bulancis has considerably larger dental sockets, and tusk-like brachiophores with thick bases ( Figs. 3A–C View Fig ), unlike S. filistera which has simple, tabular brachiophores. Other species assigned to Sivorthis by Jaanusson & Bassett (1993, p. 46) differ greatly from C. bulancis , primarily in their coarser ornamentation and cardinalia, and thus do not warrant close comparison.

The genus Sinorthis , based on S. typica , described by Wang (1955) from the Lower Ordovician (Arenig) of south China is also finely costellate, but can be distinguished from C. bulancis by its widely divergent brachiophores and weakly apsacline, relatively small ventral valve interarea. Orthostrophia Hall , is a significantly younger genus and can be distinguished from C. bulancis by its coarser ramicostellate ornament, dorsibiconvex to resupinate shell and subtriangular ventral valve muscle scar.

Family Nanorthidae Havlíěek, 1977