Crimites elkoensis Miller et al

The membranes in this species consist of a series of small, adorally concave sheets that extend from the outer layer of the siphuncle to the inner surface of the ventral part of the chamber (Figs. 9.6, 9.7).

They are usually evenly spaced along the siphuncle in each chamber, although in some specimens they are more common in the adapical part of the chamber. The

Fig. 9.6 Crimites elkoensis Miller et al., 1957, AMNH 51223, Arcturus Formation, Buck Mountain, Nevada. Dorsoventral section showing the siphuncle (s) and siphuncular membranes (sm). The membranes form sheets between the siphuncle and the ventral floor of the chamber (v).

Fig. 9.7 Crimites elkoensis Miller et al,, 1957, AMNH 51240, Arcturus Formation, Buck Mountain, Nevada, Median views of the siphuncle and siphuncular membranes, A, The siphuncle (s) is visible in two chambers, Adoral direction is to the right, B, Close-up of the siphuncular membranes (sm) in A, A fragment of a siphuncular membrane (sm-l) that connects multiple siphuncular membranes is visible.

Fig. 9.7 Crimites elkoensis Miller et al,, 1957, AMNH 51240, Arcturus Formation, Buck Mountain, Nevada, Median views of the siphuncle and siphuncular membranes, A, The siphuncle (s) is visible in two chambers, Adoral direction is to the right, B, Close-up of the siphuncular membranes (sm) in A, A fragment of a siphuncular membrane (sm-l) that connects multiple siphuncular membranes is visible.

membranes sometimes appear to have two layers (Fig. 9.6), although in most instances, they appear as one solid sheet. The bilayered structure may indicate that the original membrane was coated with phosphate on both sides during early diagenesis. When the original membrane decomposed, the two phosphatic layers remained with a small space between them.

Figure 9.7 indicates that the membranes may merge dorsally into a thin sheet parallel to the floor of the chamber. However, the fragmentation of the specimen makes it difficult to determine this for certain. Such sheets, formed by the merging of s everal elements, are common in ceratites and prolecanitids (Weitschat and Bandel, 1991; Landman et al., 2006).

Perhaps because goniatites are relatively rare in the Buck Mountain concretions, we were unable to find a specimen with preservation adequate to determine the morphology of the siphuncular membranes throughout ontogeny. The goniatite Crimites elkoensis is present in the same deposits as the prolecanitid Akmilleria electraensis Plummer and Scott, 1937, whose membranes do not appear until the end of the neanic stage (at or near the beginning of the third whorl, corresponding to a shell diameter of approximately 3 mm). The membranes in A, electraensis first appear as simple sheets, similar to those of the goniatites, and then become more complex during ontogeny (Landman et al., 2006). Until better-preserved specimens are examined, it is impossible to state whether there is any consistent ontogenetic pattern in C, elkoensis.

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