Internal features of Ammonitellas of Tornoceratids from the Middle Devonian Cherry Valley Limestone, New York State, USA

A previous study on the ammonitellas of Agoniatites vanuxemi ( Hall, 1879) from the Middle Devonian Cherry Valley Limestone of New York State, USA, showed remarkable preservation of the internal morphology. In this paper, the ammonitellas of another ammonoid, a member of the Tornoceratidae, from the Cherry Valley Limestone are studied. The tornoceratids come from a weathered friable shale layer. The ammonitellas examined are from juvenile specimens, many of which appear to be loose in the matrix. Specimens were studied in median cross section using a Scanning Electron Microscope. Shell walls and septa appear as more heavily etched grooves surrounded by large blocks of calcite. Membrane-like structures are associated with several embryonic septa in one specimen. In one specimen, a primary varix marks the end of the ammonitella. The first two septa join the shell wall of the initial chamber at its dorsalmost point. The first septum (= proseptum) is asymmetrically concave adapically and attaches to the dorsal wall of the initial chamber, which extends into the first chamber. The second septum attaches at the end of this extension. The third septum attaches to the dorsal wall of the ammonitella coil. This arrangement differs from that in Agoniatites, where all three septa join the dorsal wall of the ammonitella at a single point. This difference cannot be easily explained by the tighter coiling of the ammonitella of the tornorceratids because they are not closely related to the agoniatites. In addition, although the tornoceratid ammonitella is similar to that of the agoniatids in many ways, it is half the size.