Ventral Muscle

The scar of the unpaired ventral muscle was found in specimen SMNS 65533 (Figs 10.3-10.5). In this specimen, the surface of the internal mould is darkened anterior to the last formed septum ("Praseptalfeld" sensu Richter, 2002, compare pl. 7, Fig. 6 therein). This dark coating fades out anteriorly but posteriorly, it clearly delimits an oval spot which displays an area of lighter color. This spot is approximately 5 mm long and 3 mm wide and more or less symmetrical with the plane of symmetry being the same as that of the entire shell. According to its shape, position, and preservation, it is assumed that this spot represents the scar of the unpaired ventral muscle (Doguzhaeva and Mutvei, 1991, 1996; Richter, 2002). A similar structure consisting of a well-developed preseptal field (often dark-colored field in front of the last formed septum within the body chamber) with a lighter central oval spot is preserved in SMNS 65537. In this latter specimen, the preseptal field spans the external lobe and the adjacent ventrolateral lobe and fades out on the ventral flank of the subsequent saddle (Fig. 10.5).

Fig. 10.3 Ceratites flexuosus Philippi, 1901, SMNS 65533, col. A. Ehmann, atavus Zone, Upper Muschelkalk, late Illyrian, late Anisian, Middle Triassic, Neckarrems near Stuttgart, Germany. (A) Ventral view showing the dark preseptal Held (adoral of the last suture) surrounding the scar of the unpaired ventral muscle. x 2. (B) Detail of the ventrolateral part of the flank showing the pairs of dark lines in the lobes. x 5. (C) Detail of B showing a dark line with the ventral half of a lobe in a normal SEM image. x 20. (D) Same detail, BSEI image showing the different mineralogical composition of the dark lines. x 20. (E) Detail of D. x 200.

Fig. 10.3 Ceratites flexuosus Philippi, 1901, SMNS 65533, col. A. Ehmann, atavus Zone, Upper Muschelkalk, late Illyrian, late Anisian, Middle Triassic, Neckarrems near Stuttgart, Germany. (A) Ventral view showing the dark preseptal Held (adoral of the last suture) surrounding the scar of the unpaired ventral muscle. x 2. (B) Detail of the ventrolateral part of the flank showing the pairs of dark lines in the lobes. x 5. (C) Detail of B showing a dark line with the ventral half of a lobe in a normal SEM image. x 20. (D) Same detail, BSEI image showing the different mineralogical composition of the dark lines. x 20. (E) Detail of D. x 200.

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