Soft‐tissue imprints in fossil and Recent cephalopod septa and septum formation

Abstract

D. 2008: Soft-tissue imprints in fossil and Recent cephalopod septa and septum formation. Lethaia , Vol. 41, Several soft-tissue imprints and attachment sites have been discovered on the inside of the shell wall and on the apertural side of the septum of various fossil and Recent ectocochleate cephalopods. In addition to the scars of the cephalic retractors, steinkerns of the body chambers of bactritoids and some ammonoids from the Moroccan and the German Emsian (Early Devonian) display various kinds of striations; some of these striations are restricted to the mural part of the septum, some start at the suture and terminate at the anterior limit of the annular elevation. Several of these features were also discovered in specimens of Mesozoic and Recent nautilids. These structures are here interpreted as imprints of muscle fibre bundles of the posterior and especially the septal mantle, blood vessels as well as the septal furrow. Most of these structures were not found in ammonoids younger than Middle Devonian. We suggest that newly formed, not yet mineralized (or only slightly), septa were more tightly stayed between the more numerous lobes and saddles in more strongly folded septa of more derived ammonoids and that the higher tension in these septa did not permit soft-parts to leave imprints on the organic preseptum. It is conceivable that this permitted more derived ammonoids to replace the chamber liquid faster by gas and consequently, new chambers could be used earlier than in other ectocochleate cephalopods, perhaps this process began even prior to mineralization. This would have allowed faster growth rates in derived ammonoids.

Cameral Membranes, Pseudosutures, and Other Soft Tissue Imprints in Ammonoid Shells

“…Drag lines are often reported together with pseudosutures ( Fig. 4.1b, c, 4.2, Zaborski 1986;Lominadze et al 1993;Richter 2002;Klug et al 2007Klug et al , 2008Polizzotto and Landman 2010).…”

“…Pseudosutures have been described and discussed in numerous ammonoid groups (John 1909;Hölder 1954;Vogel 1959;Schindewolf 1968;Bayer 1977;Hagadorn and Mundlos 1983;Zaborski 1986;Seilacher 1988;Hewitt et al 1991;Bandel 1991, 1992;Westermann 1992;Lominadze et al 1993;Bucher et al 1996;Checa 1996;Checa and Garcis-Ruiz 1996;Doguzhaeva and Mutvei 1996;Tanabe et al 1998;Keupp 2000;Richter 2002;Richter and Fischer 2002;Klug et al 2007;Polizzotto et al 2007;Klug et al 2008;Polizzotto 2010;Polizzotto and Landman 2010). The groups in which pseudosutures have been most widely reported include ceratitids, lytoceratids, phylloceratids, perisphinctids, vascoceratids, scaphitids, and goniatitids, but they are found fairly often and probably Weitschat and Bandel (1991) occur to one degree or another in most ammonoid groups.…”

“…In contrast to the portion of the rear mantle that secreted pseudosutures, however, the parts of the mantle that formed drag lines must have remained in continuous contact with the shell wall. Alternatively, Klug et al (2008) hypothesized that some spirally arranged drag lines might represent impressions of muscle fiber bundles in the posterior mantle (rather than secretions).…”

“…This model, however, would not explain the evidence of the attachment-like ultrastructure in at least some pseudosutures. Klug et al (2008) introduced a "tension model" of septal morphogenesis that incorporates elements of both the tie-point model and the viscous fingering model, in which muscle fibers at the edge of the rear mantle attached to the inner shell, and the more complex the shape of the septum, the more tension could develop in the rear mantle and in the organic pre-septum prior to mineralization. While Klug et al (2008) did not elaborate specifically on the consequences of this model for the formation of pseudosutures, the model implies that the shape of temporary attachment points would have depended on components of translocation that were not so much genetically influenced, but mainly affected by changes in chamber pressurization.…”

A Silurian oncocerid with preserved colour pattern and muscle scars (Nautiloidea)

“…It is interesting that in Euryrizocerina and its ancestor "Oonoceras" fraternum the ventral furrow is commonly preserved, while in Oonoceras and allied genera, it has not been observed yet. According to Chirat & Boletzky (2003, p. 167), the ventral furrow represents a taxonomically unimportant developmental by-product originating "from the inner part of the initial, calcified shell apex, in line with the ventral termination of the central linear depression of the cicatrix" (see also Klug et al 2008). This means that the ventral furrow is shared by all nautiloids.…”