Opis

Diagnosis. — Conch aperture with auricles and relatively high whorl expansion rate, septum of simple geometry developing only small lateral vaultings (expressed as one set of flank lobes in the suture) with a tendency to simplification in secondarily evolute forms.

Remarks. — The main difficulty with the taxonomy of generalized tornoceratids is the simplicity of their external conch morphology and geometry of the septum. To interpret the latter it seems enough to invoke the “balloon concept” of the cephalopod septum as an interplay of the whorl cross-section and uniformly distributed hydraulic pressure within the animal soft body. Only in more advanced tornoceratids vaulted areas in the septum developed, suggestive of a more complex pattern of muscular attachments of the body to the internal conch wall. The septal and conch morphology of early tornoceratids seems to be sensitive of evolutionary reversals and homeomorphy an thus of low taxonomic value. In fact, the conch shape and the suture line controlled by it seem variable in both the early Frasnian and latest Frasnian species represented by a large number of specimens.
In such a situation the morphology of the first larval septum, deeply concave in at least some tornoceratids  may be of much taxonomic value, because of its uniqueness. The first septum is very different from the succeeding septa and apparently originated under different conditions of secretion. One may speculate that this was a result of secretion of the phragmocone chamber liquid being delayed in respect to secretion of the septal tissue. The volume of the first regular phragmocone chamber was much smaller than that of the protoconch, so subsequent septa originated in a more regular rhythmic pattern. Anyway, this peculiar morphology of the larval (ammonitella) conch may be a useful diagnostic character (synapomorphy) for the least derived trornoceratids.
The problem thus emerges when this concave first septum originated and how long it persisted in the evolution of the tornoceratids. This character was first identified in a typical Tornoceras species from the early Frasnian Domanik Formation of Timan. House (1965), while interpreting pyritized protoconchs of the type species of the genus from the Givetian Alden Marcasite horizon within the Ledyard Shale of New York, suggested the presence of an unusually large caecum. It seems possible that this was actually the first septum which developed such a swollen structure. If true, this character might define the origin of the tornoceratid clade more precisely than the conch morphology.
This bizarre first septum morphology is shown by silicified tornoceratid specimens from the topmost Frasnian strata at Miedzianka in the Holy Cross Mountains associated with a normally developed (not swollen) siphuncle. Among postlarval tornoceratids of generalized conch morphology only Linguatornoceras has been identified in these strata. A similar, but not so strongly concave first septum characterizes a tornoceratid from the significantly younger Upper Łagów Beds at Dule (sample Ł-38). Associated protornoceratids show normal development of the first septum. This suggests a gradual disappearance of this character and explains why the clymeniids do not show any signs of its presence, despite their probable tornoceratid ancestry.

Synonimy

Zasięg czasowy

Literatura

Dzik, J. 2006. The Famennian "Golden Age" of conodonts and ammonoids in the Polish part of the Variscan sea. Palaeontologia Polonica 63, 1-359.