Origin of Membranes

The origin of siphuncular membranes has been debated (Kulicki, 1979; Weitschat and Bandel, 1991; Westermann, 1992; Checa, 1996; Landman et al., 2006).

Fig. 9.11 Cravenoceras fayettevillae Gordon, 1965, AMNH 51243, median cross section, lower Fayetteville Formation, Durham, Arkansas. Because the specimen is fragmented, it is unclear whether this is the adapical part of the body chamber or a more adoral portion. The shell wall exhibits two layers ofphosphatized, presumably originally organic, layers. One of these layers (p) is assumed to be the periostracum, while the other (cl) is likely to be the chamber lining, similar to the chamber lining found in the phragmocone.

Fig. 9.11 Cravenoceras fayettevillae Gordon, 1965, AMNH 51243, median cross section, lower Fayetteville Formation, Durham, Arkansas. Because the specimen is fragmented, it is unclear whether this is the adapical part of the body chamber or a more adoral portion. The shell wall exhibits two layers ofphosphatized, presumably originally organic, layers. One of these layers (p) is assumed to be the periostracum, while the other (cl) is likely to be the chamber lining, similar to the chamber lining found in the phragmocone.

Fig. 9.10 (continued) view of siphuncular membranes (sm) in A. C. Fragmented siphuncular membranes along the siphuncle (s). Adoral direction is up. D. Close-up view of siphuncular membranes in C (sm). E. Etched phragmocone with the siphuncle exposed. Adoral direction is up. F. Close-up view of siphuncular membranes in E (sm). G. Same specimen as in E, F rotated 90° clockwise to show the siphuncular membranes in another chamber. Adoral direction is to the right. H. Close-up view of siphuncular membranes in G (sm).

Fig. 9.12 Pseudosutures in Cravenoceras fayettevillae Gordon, 1965, AMNH 51244, lower Fayetteville Formation, Durham, Arkansas. The adoral direction is down and to the left. A. The inner surface of the shell is exposed, showing the ridges or pseudosutures (ps). The specimen was not etched, st = suture. B. Close-up view of pseudosutures (ps), which appear on the adoral side of the suture (st). C. Close-up view of the most prominent pseudosuture, showing mineralization and an asymmetrical slope similar to the mural ridge in Nautilus. D. Close-up view of weaker pseudosutures, demonstrating the wide variability in the amount of material comprising each pseudosuture.

Fig. 9.12 Pseudosutures in Cravenoceras fayettevillae Gordon, 1965, AMNH 51244, lower Fayetteville Formation, Durham, Arkansas. The adoral direction is down and to the left. A. The inner surface of the shell is exposed, showing the ridges or pseudosutures (ps). The specimen was not etched, st = suture. B. Close-up view of pseudosutures (ps), which appear on the adoral side of the suture (st). C. Close-up view of the most prominent pseudosuture, showing mineralization and an asymmetrical slope similar to the mural ridge in Nautilus. D. Close-up view of weaker pseudosutures, demonstrating the wide variability in the amount of material comprising each pseudosuture.

Landman et al. (2006) argued that these membranes were secreted by the rear part of the mantle, and are not solely the result of desiccation of a cameral liquid or gel after formation of the septum. As evidence for this view, they noted: (1) the abrupt appearance of complex membranes at the end of the neanic stage; (2) the consistency of the ontogenetic pattern among individuals; (3) the surface morphology of the membranes, which lack features characteristic of desiccated gels; and (4) the presence of membranes of similar composition in the adapical portions of the body chambers of Cretaceous ammonoids (Tanabe et al. 2005: Fig. 9.1), which could not have formed by desiccation of cameral liquid after chamber formation.

The similarity in structure and composition of goniatite membranes to prolecan-itid membranes strongly suggests that they were formed by similar processes. The four points of Landman et al. (2006) in support of the secretion hypothesis were also evaluated for the goniatites we studied.

Fig. 9.13 Pseudosutures on the external molds (A and B) and corresponding internal molds (C and D) of an unetched specimen of Cravenoceras fayettevillae Gordon, 1965, AMNH51245, lower Fayetteville Formation, Durham, Arkansas. A, B. Overview and close-up of the pseudosutures (ps) on the inside surface of the shell (external mold). These pseudosutures are located on the adapical side of the saddle, and appear as ridges. The adoral direction is to the upper left. C, D. Pseudosutural grooves on the internal mold of the specimen. When specimens are broken, the pseudosutures appear as ridges on the external mold and as grooves on the internal mold.

Fig. 9.13 Pseudosutures on the external molds (A and B) and corresponding internal molds (C and D) of an unetched specimen of Cravenoceras fayettevillae Gordon, 1965, AMNH51245, lower Fayetteville Formation, Durham, Arkansas. A, B. Overview and close-up of the pseudosutures (ps) on the inside surface of the shell (external mold). These pseudosutures are located on the adapical side of the saddle, and appear as ridges. The adoral direction is to the upper left. C, D. Pseudosutural grooves on the internal mold of the specimen. When specimens are broken, the pseudosutures appear as ridges on the external mold and as grooves on the internal mold.

Fig. 9.14 Cravenoceras fayettevillae Gordon, 1965, AMNH 51246, lower Fayetteville Formation, Durham, Arkansas. Internal mold with pseudosutures (ps) on the adapical side of the suture (st). The adoral direction is to the bottom right. A. Overview. B. Close-up of the pseudosutures (ps). The position and spacing of the pseudosutures do not correspond to the position and spacing of the siphuncular membranes.

Fig. 9.14 Cravenoceras fayettevillae Gordon, 1965, AMNH 51246, lower Fayetteville Formation, Durham, Arkansas. Internal mold with pseudosutures (ps) on the adapical side of the suture (st). The adoral direction is to the bottom right. A. Overview. B. Close-up of the pseudosutures (ps). The position and spacing of the pseudosutures do not correspond to the position and spacing of the siphuncular membranes.

(1) If membranes were produced by desiccation after chamber formation, membranes would have formed in all chambers. However, the abrupt appearance of siphuncular membranes in Cravenoceras fayettevillae in the fourth whorl suggests that these features did not form by desiccation. Chamber linings are present starting from the initial chamber in both prolecanitids and in the goniatites we examined, which leaves open the question of whether the linings (as opposed to the siphuncular membranes) were formed by secretion or desiccation (but see point 4).

(2) In the goniatites we examined, the data are insufficient to determine whether a consistent ontogenetic pattern of membranes exists, although the uniform structure of the membranes in each chamber suggests secretion rather than the vagaries of desiccation.

(3) The surface of the membranes in goniatites is generally smooth, similar to that of membranes observed in other ammonoids. Features characteristic of a desiccated gel, such as wrinkles or tessellation, are absent.

(4) No body chambers were preserved in specimens of Crimites elkoensis, and so we cannot comment on the presence or absence of an organic lining in the body chambers of individuals of that species. Such linings, however, have been observed in body chambers of some individuals of Cravenoceras fayettevillae (Fig. 9.11). Thus, at least some chamber linings probably were secreted by the animal.

In summary, although additional data are needed, there is some evidence that the membranes formed by secretion rather than by desiccation.

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