INTRACAMERAL MEMBRANES IN PERMIAN AMMONOIDS FROM THE BUCK MOUNTAIN, NEVADA LAGERSTÄTTE

The Buck Mountain (Permian-Artinskian) Arcturus Formation contains a cephalopod Lagerstätte that preserves intracameral membranes as phosphatic coatings.  Prolecanitid ammonoids dominate with some goniatites, and orthoconic and coiled nautiloids.  The fossils occur in carbonate concretions. Some concretions contain thousands of carbonized cephalopod mandibles, with fewer ammonoid phragmocones and shell fragments.  The extraordinary number of mandibles suggest that there should be many more ammonoids than have so far been recovered, and together with the unusual preservation of the membranes in the ammonoids, indicates that these organisms experienced unusual taphonomic pathways.

  We document the membranes in the prolecanitid ammonoid Akmilleria electraensis (Plummer and Scott).  The fossils were etched in weak (1-5%) acetic acid. Three intracameral membranes are present: siphuncular sheets, horizontal sheets, and transverse sheets at the tips of the sutural saddles.  The intracameral membranes exhibit regular placement and increasing complexity from juvenile to mature growth stages. The membranes are generally bilaterally symmetrical at the same stages of ontogeny among different specimens.

We presently interpret these membranes as having been produced by the animal during chamber formation.  We tentatively reject the interpretation that the membranes were formed by desiccation and/or chemical alteration of the cameral liquid or gel. These membranes are similar to those described in some Mesozoic ammonoids and may imply a similar function.  They may also represent useful characters for phylogenetic analysis.