Leoniorthis, Evaegerquist, 2003

Evaegerquist, 2003, New brachiopods from the Lower-Middle Ordovician (Billingen-Volkhov stages) of the East Baltic, Acta Palaeontologica Polonica 48 (1), pp. 31-38 : 35-36

Genus Leoniorthis gen. nov.

Derivation of name: In honor of brachiopod specialist Dr. Leonid Popov.

Type and only species: Leoniorthis robusta sp. nov.

Diagnosis.—Smallorthoid.Radialornamentcoarselycostellate with angular ribs; concentric ornament weakly developed. Shell nonpunctate with smooth internal surface. Ventral mantle canal system saccate with moderately divergent vascula media. Dorsal muscle field quadripartite, anterior muscle scar longer than posterior; a thick notothyrial platform continues anteriorly into low median ridge.

Discussion.—This brachiopod is assigned to the family Orthidae on account of its short, curved apsacline ventral interarea and anacline dorsal interarea, the poorly differentiated, broad ventral adductor track slightly longer than diductor scars, and its saccate ventral and pinnate dorsal mantle canal systems. It also has a ridge−like cardinal process, short, stout and moderately divergent brachiophores, and quadripartite dorsal muscle field with anterior scars longer than posterior. The weakly developed concentric ornament and simple, ridge−like cardinal process exclude reference to other possible orthidine families such as the Glyptorthidae Schuchert and Cooper, 1931 and Orthidiellidae Ulrich and Cooper, 1936 .

With a recorded maximum width of 9 mm the new brachiopod is much smaller than the other coarsely ribbed genera belonging to this family. Orthis Dalman, 1828 , Orthambonites Pander, 1830 , and Paralenorthis Havlíček and Branisa, 1980 are distinguished by having costate radial ornament with capillae and rounded costae. Orthambonites as well as Sivorthis Jaanusson and Bassett, 1993 and Sulevorthis Jaanusson and Bassett, 1993 are strongly filate, which is not the case with the new genus. Furthermore, Sivorthis differs in having a subcordate ventral muscle field, whilst Sulevorthis has a different shaped cardinal process and possesses exopunctae on the sides of the costae.

The Silurian genus Orthokopis Baarli, 1995 from Norway shows quite close external resemblance to Leoniorthis . However, internal features such as the lack of a notothyrial platform, different development of the dorsal muscle field, and the ribs impressed all over the inner surface of the ventral valve, clearly distinguish Orthokopis from the new genus.

The slightly younger (Arenig–Llanvirn) genus Trondorthis Neuman, 1974 is also similar to Leoniorthis externally, but also in this case the internal structures such as ventral adductor scars terminating anteriorly in an elevated ridge and strongly divergent vascula media distinguish it from the new genus.