Anomalorthis rossi Hansen & Holmer 2011, sp. nov.

Anomalorthis rossi sp. nov.

Pl. 24, Figs. 8–12

1970 Anomalorthis n. sp. A―Ross, p. 62, Pl. 5, Figs. 26–31; Pl. 6, Figs. 2, 3, 5.

Derivation of name. Named after Reuben James Ross, Jr., who first described the species.

Holotype. Pl. 24, Fig. 11; USNM 160805, dorsal valve; Antelope Valley Limestone, Orthidiella zone, 1,272 feet below top, sample D1583 CO; southern Groom Range, Nevada.

Material. Fragments of one deformed dorsal valve from sample JH-161 and three ventral valves from samples F4804 and JH-87. The figured speciments are F4804, TSGF 16769, TSGF 16882 and USNM 160806b.

Diagnosis. Small Anomalorthis with diamond-shaped notothyrial platform; steeply procline ventral interarea; rectangular to obtuse cardinal extremities in larger specimens; low but distinct median ridge dividing pseudospondylium.

Description. Shell semicircular to transversely subangular. Cardinal extremities acute in early growth stages but becoming rectangular to obtuse with increasing size. Valves moderately thick-walled. Holotype 7.5 mm long and 9.5 mm wide. Ornamentation finely multicostellate with widely and unevenly spaced growth lines and additicules; 6–8 slightly rounded costellae per mm at 1-mm growth stage. Ornamentation not impressed on valve floor.

Dorsal valve slightly convex, with weak sulcus. Interarea low, planar, anacline. Notothyrium wide and open. Brachiophores rather low, diverging at about 120˚. Notothyrial platform low and broad, diamond-shaped. Cardinal process simple and thin. Median ridge broad and poorly developed.

Ventral valve subpyramidal. Maximum depth located at umbo. Anterior flank moderately convex. Ventral interarea high, steeply procline and planar or concave. Delthyrium open, moderately wide and rounded, U-shaped, with an angle of about 35–40°. Teeth strong and subtriangular. Dental plates broad, recessive and forming a pseudospondylium. Muscle scars slightly diverging, cordate and bilobed. Diductor scars separated by a low adductor ridge reaching its apex at front of pseudospondylium. Posterior part of delthyrial chamber with strong pedicle callist.

Remarks. The available fragments from Spitsbergen show the same diagnostic features as Anomalorthis n. sp. A described by Ross (1970) from the uppermost Orthidiella zone of Nevada. Ross (1970) did not formally name the new species because he only had three valves available. However, with the availability of new specimens showing the same features, we propose to formally name the species here. Together with the species A. lambda Ross, 1968 from the Dapingian of Utah, A. rossi sp. nov. is the oldest known species within the genus. The species A. oklahomensis Ulrich & Cooper, 1936 from the Darriwilian of Oklahoma is similar but has longer brachiophores and a concave anterior margin of the notothyrium. Among the other species within the genus, A. lonensis (Walcott, 1884) from the Darriwilian of Nevada and A. fascicostellatus Ross, 1970 from the Darriwilian of Nevada differ by having coarser ornamentation and a non-procline ventral interarea. A. juabensis Jensen, 1967 from the Middle Ordovician of Utah is alate and has finer costellae. A. lambda Ross has an extremely procline ventral interarea, acute cardinal extremities, and concave anterior margin of the notothyrial platform. A. nevadensis Ulrich & Cooper, 1938 from the Darriwilian of Nevada has a convex ventral interarea and concave anterior slope of the apex and lacks the median ridge in the pseudospondylium. A. resoi Ross, 1970 from the Darriwilian of Nevada has an undercut anterior part of the pseudospondylium, which is not divided by a distinct median ridge. The type species, A. utahensis Ulrich & Cooper, 1936, is distinguished by its apsacline ventral interarea and wide U-shaped delthyrium. A. vermontensis Ulrich & Cooper, 1938 from the Darriwilian of Vermont differs by its apsacline ventral interarea and small shell.

Occurrence. 59, 65 and 83 m above base of Profilbekken Member, Valhallfonna Formation, Basissletta in northeastern Ny Friesland, Spitsbergen.