Early Maastrichtian ammonites and nautiloids from Hrebenne, southeast Poland, and phenotypic plasticity of Acanthoscaphites tridens (Kner, 1848)

Marly siliceous limestones (opokas) of late early Maastrichtian (Belemnella sumensis Zone) age exposed temporarily at Hrebenne (southeast Poland) have recently yielded seven species of ammonite and four nautiloid taxa, namely Hauericeras sulcatum (Kner, 1848), Diplomoceras cylindraceum (Defrance, 1816), Baculites vertebralis de Lamarck, 1801, Baculites knorrianus Desmarest, 1817, Baculites sp., Acanthoscaphites tridens (Kner, 1848) s. str., Hoploscaphites constrictus (Sowerby, 1817), Cymatoceras loricatum (Schlüter, 1876), Cymatoceras patens (Kner, 1848), Eutrephoceras aff. darupense (Schlüter, 1876), and Eutrephoceras sp. nov. A. Of these, four heteromorphs (B. knorrianus, B. sp., B. vertebralis, and A. tridens s. str.) predominate in these rich assemblages, while Eutrephoceras aff. darupense is the most common nautiloid. The wide range of variation in both size and ornament seen in Acanthoscaphites tridens s. str. ( = formae trispinosus, innodosus, trinodosus; ranging from the uppermost Belemnella obtusa Zone into the Bn. sumensis Zone), is held to be intimately linked to phenotypic plasticity of that species, rather than to the previously widely accepted concept of marked sexual dimorphism or an expression of hypothetical polymorphism. The presence of small representatives (< 200 mm in diameter) of Cymatoceras loricatum at Hrebenne shows that this species cannot be considered endemic to northern Germany. The Hrebenne locality is the first in Poland to provide a rich and diverse macrofauna, inclusive of cephalopods, of Bn. sumensis Zone age. The ammonites from Hrebenne comprise typically early Maastrichtian forms; their specific composition is similar to that recorded from both western (e.g., Kronsmoor and Rügen, northern Germany, Limburg and Liège provinces, Belgium; southern Limburg, the Netherlands; Møns Klint, Denmark), and eastern Europe (e.g., Nagoryanў, the Ukraine; Russian Platform).