Soft-tissue attachment scars of two genera and four species of Late Jurassic 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.
The article is devoted to the description of the muscle scars in the body chambers of ammonites of the family Craspeditidae: Kachpurites and Garniericeras. For the first time, muscle scars have been studied in ammonites whose shells differ by shape and body chamber length. One of these ammonite genera (Kachpurites) is a direct ancestor of another (Garniericeras). This allows the author to study the changes of muscle scar location and shape and their dependence on the shape of the ammonite shell during phylogenesis. The interpretation of lateral sinuses is given for the first time in this article: it is possible that these areas were used for attachment of funnel-locking apparatus.
1. I tried to refer to all the existing articles which are devoted to ammonoid muscle scars, but unfortunately left out the publication Palframan, D.F.B. 1969. Taxonomy of sexual dimorphism in ammonites: morphogenetic evidence in Hecticoceras brightii (Pratt). International Union of Geological Sciences, Series A, Number 1, pp. 126–154 in which the lateral muscle scars and lateral sinuses of ammonites Hecticoceras brightii (the genus Brightia in the modern classification) are described and depicted (see pages 142-143). The shape of these attachment structures is very similar to that of Kachpurites and Garniericeras.
2. The muscle scars of Kachpurites were previously mentioned in the literature, but they have never been depicted or described. The ammonite depicted in the publication Doguzhaeva L. A., Mapes R. H. 2015. The body chamber length variations and muscle and mantle attachments in ammonoids //Ammonoid Paleobiology: From anatomy to ecology. – Springer, Dordrecht, pp. 545-584 on Fig. 14.10 as "Kashpurites" actually belongs to the genus Craspedites, and the large spot on its surface, which looks similar to a muscle scar, is actually a result of nonuniform dissolution of an aragonitic shell (note that it is located not on the body chamber, but on the phragmocone).
Ammonoidea, Craspeditidae, Kachpurites, Garniericeras, attachment scars, paleobiology, Jurassic, Russia.