Softtissue Attachment Structures Large Scar of the Cephalic Retractor

The cephalic retractor muscles represent the most massive muscles in Nautilus and probably also in most fossil ectocochleate cephalopods, i.e., nautiloids, bactritoids, and ammonoids. In Nautilus, they serve to retract the soft body and also play an important role in locomotion. These muscles insert in the capito-pedal cartilage and form a roof above the mantle cavity (see figures in Doguzhaeva and Mutvei, 1991). By contraction of its longitudinal muscle fibers, the head is pulled back into the shell, exerting pressure on the mantle cavity and thus expelling water out of it.

In contrast to the Germanonautilus figured by Klug and Lehmkuhl (2004), there is only one specimen displaying traces of this muscle scar and fewer details are preserved (MHI, 1856: Fig. 10.1). The specimen received attention because of a shining and reflective surface at the posterior end of the body chamber. This surface is subdivided in several irregular spots on both sides of the steinkern. This subdivision was caused by a burrowing organism which left its traces in a polygonal pattern in the posterior two-thirds of the body chamber. It is important to note here that exactly the same ichnotaxon co-occurs with phosphatized soft-tissues in schizodont bivalves of the same age (Klug et al., 2005). The presence of this trace fossil in combination with the shining surface (which is distributed over both sides of the body chamber including also dorsum and venter) led to the presumption that this structure is actually phosphatized. Some EDAX-analyses confirmed this presumption, showing the presence of a significant amount of phosphorus. The shining surface was analyzed at two spots (samples 1 and 2 in Fig. 10.1). These analyses yielded the following results for the elements oxygen, phosphorus, and calcium (in weight percent):

Consequently, it appears reasonable that this structure actually represents a formerly, at least partially, organic part of this organism. A map of all the phosphatic spots and surfaces (Fig. 10.2) roughly revealed an outline that very closely resembles the

Fig. 10.1 Ceratites philippii philippii Riedel, 1916, MHI1856, col. H. Hagdorn, 4-8 m below the Spiriferina-bed, pulcher to robustus Zone, Upper Muschelkalk, late Illyrian, late Anisian, Middle Triassic, Schön and Hippelein quarry at Neidenfels near Crailsheim, Germany. (A) Lateral view of the posterior part of the body chamber showing the phosphatized remains of the annular elevation and the traces in the internal mould. The rectangle approximately demarcates the detail shown in B. x 2. (B) Detail of A to show the smooth surface of the phosphatized remains of the annular elevation. x 5.

Fig. 10.1 Ceratites philippii philippii Riedel, 1916, MHI1856, col. H. Hagdorn, 4-8 m below the Spiriferina-bed, pulcher to robustus Zone, Upper Muschelkalk, late Illyrian, late Anisian, Middle Triassic, Schön and Hippelein quarry at Neidenfels near Crailsheim, Germany. (A) Lateral view of the posterior part of the body chamber showing the phosphatized remains of the annular elevation and the traces in the internal mould. The rectangle approximately demarcates the detail shown in B. x 2. (B) Detail of A to show the smooth surface of the phosphatized remains of the annular elevation. x 5.

arrangement of muscle scars in other ammonoids such as Aconeceras (Doguzhaeva and Mutvei, 1991). According to this map, the shape and position of the "large scar" of the cephalic retractor could be identified in detail. It has a roughly oval outline with a semicircular indentation at its posterior end. The specimen investigated shows the scar of the right cephalic retractor of approximately 17 mm length from the posterior indentation to the most anterior spot and also from its dorsal and ventral limits demarcated by the scars of the myoadhesive band where it is running subparallel to the dorsum and venter, respectively.

In this context, a rare but repeatedly occurring structure found in ammonoids needs to be mentioned. As described and figured by Keupp (2000), internal moulds of ammonoids sometimes display remains of the - sometimes questionably aragonitic- nacreous layer in spots of varying size and shape. In some cases, they are associated with shell injuries. Keupp (2000) explains this fragmentary shell preservation by an increased amount of organic matter in the shell, somehow conserving the aragonite around the injury. Nevertheless, it is striking, that these spots with shell remains often are located in the posterior portion of the body chamber.

In the specimen figured by Keupp (2000: 142), the aragonitic spot is actually very clearly delimited at its posterior end. We suggest that these structures in some cases actually represent phosphatized parts of healed shell injuries and in some other cases they represent aragonitic or phosphatized shell remains of the annular elevation.

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