Leoniorthis robusta, 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 : 36-37

Leoniorthis robusta sp. nov.

Fig. 5 .

Holotype: PMU In 110 ( Fig. 5A ), dorsal valve from the upper clay lens ofthemudmoundinPutilovoQuarry,lowermost P.originalis conodont Zone.

Paratypes: 44 specimens from Putilovo Quarry (22 ventral valves, 14 dorsal valves and 8 conjoined specimens, PMU In 110, 111, 120, 130–132, 134–171), 4 dorsal valves and 3 ventral valves from the Popovka River ( PMU In 121–127), and 1 dorsal and 1 ventral valve from Tallinn, Estonia ( PMU In 128, 129) .

Diagnosis.—Moderately ventribiconvex, subrectangular orthide with weakly sulcate anterior commissure. Ventral valve 72% as long as wide and 26% as deep as wide. Dorsal valve 70% as long as wide and 20% as deep as wide. Ventral muscle field suboval, confined to delthyrial cavity by low ridge.Cardinalprocessformshigh,sometimespointedridge.

Description.—Up to 9 mm wide and 8 mm long, moderately ventribiconvex and slightly sulcate shell with maximum width at, or slightly anterior to, hinge line; cardinal extremities moderately acute to obtuse. Lateral margins straight to convex, anterior margin rounded. Radial ornament coarsely costellate with triangular ribs, frequently branching into smaller costellae, which flank the coarser first one. Number of ribs per mm shell width varies between 2 and 5, depending on the branching frequency. Concentric ornament consists of irregularly spaced growth lamellae and very faint fila, not visible on all specimens.

Ventral valve on average 72% as long as wide and 26% as deep as wide. Interarea short, about 90% as wide as maximum width of shell, moderately concave, apsacline to orthocline, horizontally striated. Hinge teeth small, pointed, with crural fossettes, dental plates recessive. Ventral muscle field suboval, confined to delthyrial cavity by low ridge, individual muscle scars not readily distinguished, broad adductor trackslightlylongerthandiductorscars;lowroundedmedian ridge anterior to the delthyrial cavity present in some specimens. Interior surface of valve smooth; mantle canal system obscure on most specimens, when visible—saccate with slightly divergent vascula media.

Dorsal valve on average 70% as long as wide and 20% as deep as wide. Interarea short, about 85% as wide as maximum width of shell, planar, anacline, striated horizontally. Cardinal process forms a high, sometimes pointed ridge; brachiophores moderately divergent on thick notothyrial platform, rather short and stout with sharp posterior edges, upperandanteriorsurfacesblunt.Dorsalmusclefieldquadripartite, impressed on booth sides of a low, rounded median ridge which extends as long as muscle field; each half of muscle fieldabouttwiceaslongaswide,anterior muscle scar about twice as long as the posterior one ( Fig. 5E ). Mantle canal system digitate.

Remarks.—This species occurs in the lower part of the Vassilkovo Member (Billingen Stage) at Popovka River and in approximately corresponding layers at the Suhkrumägi road cut in Tallinn, Estonia (both Oepikodus evae Zone ). In Putilovo Quarry a few specimens are from the O. evae and B. navis zones in the standard section (Billingen Stage), whereas the majority are from the B. triangularis and lower P. originalis zones in the mud mound (Volkhov Stage). The specimens from the first two localities differ somewhat from those from Putilovo, mainly in respect to the number of ribs and depth and thickness of the shell ( Fig. 6 ). The nine specimens from Popovka River and Suhkrumägi have on average 4 ribs per mm width, compared with 3 ribs per mm for the Putilovo specimens. They also have on average 29 percent deeper shells than the mud mound specimens. However, these quite small differences are not sufficient to justify separation into two different species; the number of ribs seems to vary a lot also among the mud mound specimens and the thickeningoftheshellmightbeduetoenvironmentalfactors.

Figures

Fig. 5. Leoniorthis robusta gen. et. sp. nov. A–F. Specimens from Putilovo mud mound. G–J. Specimens from Popovka River. A. Holotype, PMU In 110, dorsal exterior (A1) and interior (A2). B. PMU In 111, ventral exterior. C. PMU In 145, ventral interior. D. PMU In 144, ventral interior. E. PMU In 120, dorsal interior (E1); enlargement of cardinal region (E2). F. PMU In 153, oblique view of hingeline and dorsal valve of conjoined specimen. G. PMU In 121, dorsal exterior. H. PMU In 122, dorsal interior. I. PMU In 125, ventral exterior. J. PMU In 123, ventral interior. Scale bars 1 mm.

Fig. 6. Bivariate plots of 51 measured specimens of Leoniorthis robusta. Solid rhombi represent specimens from Putilovo mud mound, open squares represent specimens from Popovka River, triangles represent specimens from Suhkrumägi road cut.