MUSCLE SCARS OF AMMONOIDS

Abstract

Hitherto unknown muscle scars, which can be interpreted as places of attachment of retractor muscles, have been found in Cretaceous ammonites Aconeceras trautscholdi Sinz and Deshayestites deschayesi Leym. This discovery suggests that in certain ammonites strong retractor muscles were developed and took part in jet propulsion. The nautilus, the only Recent cephaloped with an external shell, has paired retractor muscles, which are attached to the shell on the lateral sides of the posterior part of the living chamber, and at their attachment points produce large scars. Muscle scars were observed in the living chambers of 35 specimens of A. trautsscholdi and 16 specimens of D. deshayesi, from the collection of Lower Cretaceous ammonits from the Ul'yanovsk area of the Volga region. The two species in which muscle scars were found (and no other ammonites were found with them) belong to the order Ammonitida, to the families Aconeceratidae and Deshayesitidae respectively. The muscle scars were observed in shells ranging in size from 1.5 to 9.0 cm, in various stages of ontogenetic development. Additional aspects of the subject are discussed.