Size matters – Analysis of shell repair scars in endocerid cephalopods

Endocerids are the dominant cephalopod group in the lower–middle Darriwilian limestones in Baltoscandia which historically were named “Vaginatum Limestone” (“Vaginatenkalk”, Schmidt 1858). These limestones range from south-eastern Sweden to the St. Petersburg region, Russia, and are today subsumed under the Kunda Regional Stage limestones in Estonia, the Lynna, Obukhovo, and Simonkovo formations in the St. Petersburg region (Ivantsov 2003), and the Formation D in land, Sweden (Stouge 2004). Boulders of these formations are common in Northern Germany and northern Poland in the Pleistocene erratics, here the older stratigraphic terms “Lower Grey Orthoceratite Limestone” and “Lower Red Orthoceratite Limestone” are still in use by private fossil collectors. The predominant endocerids in these limestones are the name giving straight, irregularly annulated longicones, that were subsumed under Anthoceras vaginatum (Schlotheim, 1820) by earlier authors. The conch of these endocerids is characteristically ornamented with prominent transverse striae. Five or more striae occur between two ribs. The largest specimens known measure nearly 50 mm in diameter. Adult size was usually reached at diameters exceeding 40 mm (indicated by septal crowding). A considerable variation in conch ornamentation occurs between different specimens. In some specimens the transverse striae appear more irregularly spaced, while in others, they are very regularly spaced. The amplitude of the annulation varies between different specimens and in some specimens irregularly spaced constrictions occur on the steinkerns (Fig. 1). These cephalopods exceptionally often preserve traces of shell breakage and repair. Nearly a third of all conchs (29%) in the collection of the Naturhistoriska Riksmuseet, Stockholm, Sweden, have traces of healed breakages. Traces of failed predation are reported from other Ordovician mollusks (gastropods, monoplacophorans and tergomyans: Ebbestad & Peel 1997; Ebbestad 1998; Isakar & Ebbestad 2000; Alexander & Dietl 2003; Lindstr m & Peel 2005; Frisk & Ebbestad 2007; Ebbestad & Stott 2008). The shell repair frequencies documented in these reports range from 5–36%. Ebbestad & Stott (2008) demonstrated that differences in shell repair frequency can vary strongly between gastropod species in coeval rock samples. The authors inter-