Pteropelta sp. A

Hints, Olle & Eriksson, Mats E., 2010, Ordovician polychaeturid polychaetes: Taxonomy, distribution and palaeoecology, Acta Palaeontologica Polonica 55 (2), pp. 309-320 : 316

Pteropelta sp. A

Fig. 2D–I.

2009 Pteropelta sp. ; Eriksson and Hints 2009: fig. 5m, o (n tentatively).

Material.—Three right MI, three left MI, two Bp from the Saku quarry and the Vasalemma 758 borehole of Estonia and the Stormyr−2 and Båticke−3 boreholes of Sweden.

Description.—Right MI: L = 1.0– 1.2 mm, L/W ratio = 2.3– 2.5. Outer margin runs straight postero−dextrally and just above mid−length bends, enclosing a sub−rounded ramus extending to approximately 0.5 of jaw length from anterior; bight, vaguely concave anteriorly. Posterior of ramus outer face comparably wide, tapering gently and evenly. Posteriormost termination of jaw bluntly subtriangular and dorso−ventrally compressed compared to the elevated denticles. Dentary houses 18–19 tightly packed denticles that decrease evenly in size posteriorly. Fourth and sixth denticles may be slightly smaller than surrounding ones. Anterior part of dentary arranged sub−transversally to length−axis. Elevation of the dentary more pronounced in anterior half of jaw. Inner face is narrow and steep, very narrow inner wing representing c. 0.85 of jaw length may be visible. In ventral view cover comprises 0.1 of jaw length.

Left MI: L = 0.75–1.6 mm, L/W ratio = 2.2–2.9. Sub−rectangular sturdy jaw with wide, nearly straight transverse posterior margin. Outer margin runs straight postero−sinistrally and bends at approximately 1/3 to half of jaw length, sometimes enclosing a relatively narrow ramus−like feature, and continuing postero−dextrally and sub−parallel to dentary in posterior half of jaw. Dentary houses 16–18 sub−triangular, relatively tightly packed denticles. First three denticles arranged sub−transversally to length axis and decrease evenly in size, following one or two are considerably smaller whereas the next one is as big as the first denticle. Posterior of this, denticles decrease evenly and gently in size. Denticulated ridge particularly elevated and well−developed in posterior half of jaw. Long, sub−rectangular inner wing extending for 0.65–0.77 of jaw length. Its inner margin is straight and parallel to dentary. Anterior of inner wing, inner face is not visible in dorsal view. In ventral view cover comprises approximately 0.1 of jaw length.

Basal plate: Sub−trapezoidal element. L= 0.36–0.58 mm, L/W = c. 2.2. Inner margin is longer than outer margin. Anterior margin wider than posterior one. Posteriormost termination usually has posteriorly−directed, narrow and blunt extension. Dentary houses 12–14 sub−triangular denticles which are slightly bigger in posterior half of jaw.

Remarks.—Although this species is distinctive and differs from all other polychaeturids, it is here kept under open nomenclature because of the limited amount of material available. The jaws of Pteropelta sp. A are most similar to those of P. kielanae . The main difference in the right MI is the shape of the posteriormost termination; the undenticulated ridge is longer and somewhat pointed in Pteropelta sp. A , as a posterior continuation of the outer face. The few jaws hitherto recovered of Pteropelta sp. A are, moreover, bigger and have an over−all sturdier appearance than to those of P. kielanae . The MI dentary of Pteropelta sp. A is more differentiated (especially in the left MI where small, intermediate denticles occur) and has more denticles than that of P. kielanae . By contrast to the other polychaeturids that have a left MI most reminiscent of those of the polychaetaspid genus Oenonites , the left MI of Pteropelta sp. A has a general morphology superficially resembling those of certain ramphoprionids ( Protarabellites ) or even kalloprionids (see, e.g., Eriksson 2001, 2006). See also discussion by Kielan−Jaworowska (1966: 102–103).

Stratigraphic and geographic range.— Pteropelta sp. A is recorded from the Keila and Oandu stages (lowermost Katian) of northern Estonia and from the Pirgu Stage (upper Katian) of Gotland, Sweden.