We describe cameral membranes in prolecanitid and goniatitid ammonoids from the
  Lower Permian Arcturus Formation, Nevada, USA.  The membranes are preserved as
  phosphatic sheets and were originally composed of organic material such as conchiolin.
  Because the phragmocones are filled with micritic calcite, the cameral membranes can be
  exposed by etching with weak acetic acid.  The membranes are associated with the
  siphuncle and also coat the septal faces and chamber walls.  The siphuncular membranes
  are much more extensive in the prolecanitids than in the goniatites.  These membranes
  appear in the prolecanitids at the beginning of the third whorl, corresponding to a shell
  diameter of 3-4 mm, and become more complex through ontogeny.  Additional
  membranes, called transverse membranes, appear in some of the septal saddles on the
  ventrolateral side.  The siphuncular membranes in prolecanitids are very similar to those
  in the Ceratitina plus Mesozoic Ammonoidea, suggesting that such membranes are
  widely distributed in this group.  However, the origin and function of these membranes
  are unclear.  We argue that the siphuncular membranes were sequentially secreted by the
  rear mantle during forward movement of the body and were not produced by desiccation
  of cameral liquid after the Formation of the chambers.  The most compelling arguments
  for this interpretation are the abrupt appearance of these membranes at a shell diameter
  of approximately 3-4 mm in prolecanitids, ceratites, and ammonitids, coincident with
  the end of the neanic stage, and the uniform increase in complexity of the membranes
  through ontogeny.  The shape of the siphuncular membranes in prolecanitids suggests the
  presence of an invagination on the dorsal side of the siphuncle during part of the
  chamber formation cycle.  Cameral membranes may have served a variety of functions
  including stabilizing the cameral liquid to reduce rocking motion during swimming,
  anchoring the siphuncle to the chamber wall, and facilitating cameral liquid removal,
  permitting a faster rate of growth.

  [N. H. Landman [landman@amnh.org], Division of Paleontology (Invertebrates), American
  Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192; K.
  Polizzotto, Division of Paleontology (Invertebrates), American Museum of Natural History,
  Central Park West at 79th Street, New York, NY 10024-5192 and Department of Biological
  Sciences, Kingsborough Community College, 2001 Oriental Boulevard, Brooklyn, NY 1 1235; R.
  H. Mapes, Department of Geological Sciences, 316 Clippinger Laboratories, Ohio University,
  Athens, OH 45701-2979, USA; K. Tanabe, Department of Earth and Planetary Sciences,
  University of Tokyo, Tokyo 113-003, Japan; 12th December 2005, revised 27th june 2006.]