Soft-Part Anatomy of the Siphuncle in Ammonoids

Abstract

Exceptionally well-preserved remains of phosphatized siphuncles were discovered in four specimens of a Permian prolecanitid ammonoid Akmilleria electraensis (Plummer and Scott) from Buck Mountain, Nevada. These structures occur as truncated segments within the siphuncular tube. The outer surface of the siphuncle is sculptured by numerous equally spaced longitudinal ridges and furrows; the ridges represent an infolded basement membrane of epithelial cells which corresponds to the distal ends of individual canaliculi between epithelial cells. In cross-section, the siphuncle of A. electraensis consists of a large central vein, possibly two pairs of arteries, porous connective tissue with reticulate hemocoelic spaces, and a thin epithelium. In the presence of two pairs of arteries and porous connective tissue, the siphuncle of A. electraensis is more like that of Nautilus pompilius than that of Spirula spirula, which has nine arteries and dense connective tissue. However, Nautilus possesses relatively smaller and more numerous epithelial cells around the siphuncle than does Akmilleria. These observations strongly suggest that the siphuncular epithelium of Akmilleria served as the salt-concentrating organ for buoyancy regulation of the living animal, just as in Nautilus and Spirula. square Akmilleria, anatomy, Nautilus, Nevada, Permian, Prolecantid ammonoids, siphuncle.