Pteropelta Eisenack, 1939

Hints, Olle & Eriksson, Mats E., 2010, Ordovician polychaeturid polychaetes: Taxonomy, distribution and palaeoecology, Acta Palaeontologica Polonica 55 (2), pp. 309-320 : 310-313

Genus Pteropelta Eisenack, 1939

Type species: Pteropelta gladiata Eisenack, 1939 ; Ordovician , Baltic region .

Discussion.—The genus Pteropelta was erected by Eisenack (1939) for isolated carriers of bifurcate morphology (the carriers comprise the posteriormost elements of the maxillary apparatuses; for a review of the general architecture of polychaete jaw apparatuses see, e.g., Kielan−Jaworowska 1966; Szaniawski 1996). Eisenack (1939) diagnosed Pteropelta as carriers with short median peaks (Medianhöcker in German), situated in the proximal part, and lateral spines (Lateralstacheln in German) with two posteriorly directed wing−like excrescences on the sides. In that same paper he described three new species: Pteropelta gladiata , P. thomsoni , and P. glossa , of which the first was designated as type species.

In 1956, Kozłowski described an articulated jaw apparatus, possessing carriers morphologically identical to those of Pteropelta , and named it Polychaetura gracilis . Kozłowski noted that with regard to the species of Pteropelta distinguished by Eisenack (1939), Pteropelta thompsoni seems to correspond to the carriers of Polychaetura gracilis . Thus Kozłowski (1956) apparently did not fail to notice the great similarity between Pteropelta and his new genus Polychaetura . He did, however, share the belief of Lange (1947, 1949) that the isolated elements (scolecodonts) are of little taxonomical value and that a different set of names had to be applied for articulated apparatuses.

Subsequently Kielan−Jaworowska (1966) obtained additional jaw apparatuses and numerous isolated elements belonging to Polychaetura , and erected a monogeneric family Polychaeturidae , which included two species; Polychaetura gracilis and Polychaetura sp. a (the latter has subsequently been described as Pteropelta kielanae , see below). She also noted the similarity between the carriers of Polychaetura and the scolecodonts described as Pteropelta by Eisenack (1939). Nonetheless, for the same reasons as Kozłowski (1956) she did not consider them as congeneric (discussed in closer detail by Kielan−Jaworowska 1968).

Tasch and Stude (1965), referring to the rules of the International Code of Zoological Nomenclature ( ICZN), regarded eight apparatus−based generic names, Polychaetura included, as junior synonyms to Eunicites Ehlers, 1868 . The latter amendment was erroneous and strongly criticised by Kielan−Jaworowska (1968). Moreover, Jansonius and Craig (1971) suggested abstaining from assigning fossil polychaete jaws under the generic name Eunicites because of the very poor preservation of the type specimen of its type species.

Kozur (1970) was first to regard Polychaetura as a junior synonym to Pteropelta . For the emended diagnosis of Pteropelta , he proposed to use those provided by Kozłowski (1956) and Kielan−Jaworowska (1966) for Polychaetura . The question of the type material was, however, left open and Kozłowski’s (1956) species P. gracilis was kept separate from those described by Eisenack (1939). Perhaps due to the strong criticism on Kozur’s (1970) work (e.g., Jansonius and Craig 1971; Szaniawski and Wrona 1973), several authors were reluctant to use Pteropelta as opposed to Polychaetura . Edgar (1984) provided a cursory discussion on the element−based versus apparatus−based classification with regards to polychaeturids. He did not consider those two generic names as synonymous, but preferred to maintain both classification systems. Hints (1998) also used Polychaetura instead of Pteropelta but noted that some of the names employed may turn out to be junior synonyms. More recently, Hints (2000), Hints and Eriksson (2007a), and Eriksson and Hints (2009) have illustrated a few polychaeturid jaws (MI, basal plate and carriers) under the generic name Pteropelta .

Eisenack’s (1939) collection, including the type material of Pteropelta , was obtained in the 1930s but became lost during World War II. Neotypes have subsequently been designated for several species of chitinozoans, foraminiferans, and acritarchs (e.g., Eisenack 1959), but not for scolecodonts. With respect to the diagnostics and applicability of a genus, the validity of the type species and its corresponding type specimen are of particular significance. Thus designation of a neotype for Pteropelta gladiata is a prerequisite for solving the above described synonymy problem.

The International Code for Zoological Nomenclature recommends that neotypes should derive from a horizon and locality as close as possible to that of the holotype ( ICZN; Article 75). In the case of Pteropelta this is complicated because Eisenack’s (1939) material came from erratic boulders found in Germany, but with a Baltic provenance. The precise age of these boulders are not known, but most likely they are of Ordovician age. Eisenack’s (1939) type specimen of Pteropelta gladiata is morphologically identical to the carriers of the type specimen of Polychaetura gracilis (a well preserved apparatus illustrated by Kozłowski 1956: fig. 17), except being larger. Like Eisenack’s (1939) material, Kozłowski’s (1956) type specimen was derived from an erratic boulder, the specific age of which is ambiguous. However, the entire apparatus, complemented with isolated jaws recovered from the same boulder, provides much better basis for taxonomy than the single carriers. Considerable variation in size of the jaws of P. gracilis was noted by Kielan−Jaworowska (1966: 105) and confirmed by the present authors to be regarded as intraspecific variability. We therefore believe that Pteropelta gladiata and Polychaetura gracilis represent one and the same species and designate the holotype of Polychaetura gracilis as neotype for Pteropelta gladiata , thus also making the former name a junior synonym of the latter.

Concerning the two other species of Pteropelta described by Eisenack (1939), the single specimen of Pteropelta glossa is regarded to be conspecific with Pteropelta gladiata . The main difference between the two is the shape of the shaft, which is narrower in P. gladiata . The shaft is, however, very thin along the longitudinal axis and its shape thus depends largely on the state of preservation.

We agree with the opinion of Kozłowski (1956: 197) and Kielan−Jaworowska (1966: 102), that Pteropelta thompsoni is conspecific with P. gladiata . However, one of the specimens assigned to P. thompsoni by Eisenack (1939: pl. B: 2, specimen No. 3), has an anterior margin that is nearly straight or slightly convex as opposed to being distinctly concave in those of P. gladiata . Similar carriers are found in P. huberti described below. The carriers of P. kielanae ( Hints, 1998) and Pteropelta sp. A herein are hitherto unknown, but their left MI share similar, straight posterior margins. Accordingly the anterior margins of the carriers (which fit with the posterior terminations of the MI) of all three species are also expected to be similar. Two of the P. thomsoni specimens (including No. 3) were not recovered from an erratic boulder but from an outcrop in Rakvere town (formerly Wesenberg) of northern Estonia. Eisenack (1939: 165) noted that they were derived from “Stufe DII”, which corresponds to the Keila Regional Stage. This age determination nevertheless remains ambiguous since most of Rakvere town is actually confined to the outcrop area for strata belonging to the Rakvere Stage. This uncertainty interval, from the Keila to Rakvere stages, overlaps with the known ranges of the three above mentioned species and therefore specimen No. 3 of Eisenack (1939) could belong to any of these.

In conclusion, studies of appropriate type material as well as new scolecodont collections have supported the taxonomic amendment originally proposed by Kozur (1970). The doi:10.4202/app.2009.0086

name Polychaetura Kozłowski, 1956 is regarded as a junior synonym of Pteropelta Eisenack, 1939 , and Polychaetura gracilis Kozłowski, 1956 as a junior synonym of Pteropelta gladiata Eisenack, 1939 .