Saturday, July 5, 2014

[Paleontology • 2014] The Soft-tissue Attachment Scars in Late Jurassic ammonites from Central Russia

Fig. 14. Reconstruction of two Late Volgian ammonites from Craspeditidae family,
Kachpurites fulgens (A), Garniericeras catenulatum (B).

Ten arms are shown because Nautilus and representatives of Coleoidea exhibit five arm pairs in embryos, it can be a base number of arms in Cephalopoda (Kröger et. al. 2011). Two long tentacles are very speculative. However, such tentacles for external-shelled cephalopods could be very useful for catching pray which is at a distance; since rapid jumping forward could be difficult to these mollusks. The large hyponome were shown because of the presence of a funnel-locking apparatus and hyponomic retractors, and also due to the shape of aperture edges with lateral apertural sinuses and the presence of a large round opening between lappets in some ammonites (Westermann 1990: fig. 2). The eyes were drawn similar to coleoid eyes because Ammonoidea and Coleoidea were sister taxons (Jacobs and Landman 1993). The dark transverse bands on the shells correspond to the most common ammonites’ color pattern (Keupp 2000). The presence of such a color pattern in the Craspeditidae family is confirmed by findings of shells with transverse dark bands (AAM unpublished material). This picture, drawn by Andrey Atuchin, was based on the sketch of the author of this article.

Soft-tissue attachment scars of two genera and four species of Upper Jurassic (Upper Volgian) craspeditid ammonites from the Russian Platform are described. A previously suggested relationship between lateral attachment scars and ammonoid hyponome is confirmed, however, a new interpretation is proposed for dorsal attachment scars: they could have been areas not only for attachment of the dorsal (nuchal)  retractors, but also of the cephalic retractors. The new type of the soft-tissue attachment – anterior lateral sinuses, located between the lateral attachment scars and the aperture of the ammonite body chamber is described. Enclosed elliptical or subtriangular areas in apertural parts of the anterior lateral sinuses were found for the first time. Their presence and location suggest that this structure could have been used for attaching the funnel-locking apparatus, similar to those of coleoids. A transformation of shape and position of lateral attachment scars through the evolution of the Late Jurassic craspeditid lineage starting from platycones (Kachpurites fulgens) to keeled oxycones (Garniericeras catenulatum) is recognized. 

Keywords: Ammonoidea, Craspeditidae, Kachpurites, Garniericeras, soft-tissue attachment scars, paleobiology, Jurassic, Russia. 
Mironenko, A.A. 2014. The Soft-tissue Attachment Scars in Late Jurassic ammonites from Central Russia. Acta Palaeontologica Polonica. XX (X): xxx-xxx.