Porambonites havliceki, Mergl, 2013

Mergl, Michal, 2013, Porambonites Havliceki Sp. Nov., A New Brachiopod From The Šárka Formation (Darriwilian) From Bohemia And Its Contribution To Early History Of The Porambonitidae, Acta Musei Nationalis Pragae Series B 69 (1 - 2), pp. 87-92 : 88-89

Porambonites havliceki sp. nov.

Pl. 1, figs 1-10

H o l o t y p e. Complete, partly broken shell preserved as internal and external moulds in siliceous nodule, illustrated on Pl. 1, figs 1-10 (PCZCU 1842).

T y p e h o r i z o n. Darriwilian, Šárka Formation, Corymbograptus retroflexus Zone.

T y p e l o c a l i t y. Mýto, a field NW of the Svatoštěpánský pond.

E t y m o l o g y. In honour of Vladimír Havlíček, a late outstanding Czech brachiopodist.

M a t e r i a l. Only the holotype.

D e s c r i p t i o n. Shell astrophic, equally biconvex, thin-shelled, with weakly uniplicate commissure, 22 mm wide.

Dorsal valve subsubcircular, with evenly curved anterior and lateral margins. Posterior margin almost evenly rounded with a poorly defined beak. Transverse profile evenly convex. Axial profile unevenly convex, with maximum height at posterior third. A very poorly developed median fold apparent from midvalve, becoming well defined only near the anterior margin.

Ventral valve subcircular, with well defined beak and palintrope. Anterior margin and sides evenly curved. Palintrope low, occupying some 40 % of the valve width, apsacline. Delthyrium unknown.

Dorsal valve interior with thin parallel brachiophore plates, which extend to 20–25 % of the valve length. Brachiophore plates gently converging toward the valve floor. Mantle canal system digitate, distinctly impressed. Vascula media are thin canals, evenly wide along whole length, gently divergent and almost straight. Their distal ends are not clearly divided and almost touch the anterior margin of the valve. Vascula myaria has the same width of canals as vascula media, and also almost touch the shell margins, but they are more strongly diverging anteriorly and are slightly curved laterally along whole length. Posteriorly, both canal systems extend near the anterior termination of the brachiophore plates. Vascula genitalia are formed by three pairs of short and weakly impressed canals in posterior flanks. Two pairs of elongate oval, small adductor scars weakly impressed posterior to mid valve. Posterior pair located between proximal parts of vascula myaria and vascula media, anterior scars impressed between proximal parts of vascula media. A broad periphery along anterior and lateral margins by radially arranged, evenly sized and spaced striae which correspond to external multicostellate ornamentation.

Ventral valve interior with thin, long parallel dental plates confining an elongate, undivided sessile ventral muscle field. A thin myophragm developed anteriorly. Adductor imprints extend anteriorly by two acute lobes leaving a triangular valve floor in between. Teeth large. Mantle canal system digitate, distinctly impressed. Vascula media are thin canals, evenly wide along whole length, moderately divergent and slightly curved anterolaterally along the length. Their distal ends not clearly divided, nearly touching the shell margin (Pl. 1, Fig.7). Internal radial striation of the same size and density as on the dorsal valve.

External ornamentation is multicostellate, uniform over whole shell surface. Capillae almost evenly sized, flattopped, numbering 5 per 1 mm anteriorly. Interspaces narrow and deep, with deep subrhomboidal pits aligned in distinct radial rows. Pits are evenly spaced numbering 6 per 1 mm. New costellae originate exclusively by bifurcation. New costellae narrower in their proximal parts than the mother costella, but rapidly attain nearly the same width. Flat crest of costellae surface bears radially arranged and wrinkled striation. Concentric ornament restricted to very low, short, but distinct concentric lamellae. The single examined shell bears only one lamella situated anteriorly, with a few obscure concentric bands in posterolateral sector.

S i z e. The dimensions are re-calculated from incomplete partly broken shell to entire unbroken shell: DvW = 22mm, DvL = 19mm, DvH = 4mm, VvL = 20mm, VvH = 5mm.

R e m a r k s. The new species is nearest to Porambonites intermedius PANDER, 1830 and P. trigonus PANDER, 1830 from the the East Baltic ( Popov et al. 2005). The new species has similar shell shape, weakly uniplicate commissure, similar shapes and arrangements of brachiophore plates and dental plates, respectively. The main difference is a distinct multicostellate ornament with radial rows of pits (fenestrate structure: Popov et al. 2005) of the new species, whereas P. intermedius and P. trigonus shells are smooth ( Popov et al. 2005). More regularly rounded anterior margin and lower convexity of the new species are further differences to East Baltic species. Porambonites trigonus is also smaller species having less than 20 mm long shell. Porambonites intermedius differs from the new species by longer dental plates and anteriorly divergent brachiophore plates.

Porambonites dubius WILLIAMS et CURRY, 1985 from the Tourmakeady Limestone ( Ireland) is very similar to the new species by outline, size and ornamentation. A feature shared with the new species is presence of subparalell bosses between the inner flanks of the dental plates. The main difference is presence of broad, rectangular sulcus in the ventral valve of the Irish species. Williams and Curry (1985) noted a unusually fine pitted radial ornamentation, but they provide no detailed information about its shape.

O c c u r r e n c e. Only the type locality.