Middle Ordovician – Late Ordovician
Orthonybyoceras is an extinct genus of nektonic carnivores
Identification in Hand Sample:
- Medium-sized, moderately to rapidly expanding orthoconic longicones
- Circular to slightly depressed in cross section with straight, transverse sutures
- Septa shallow
- Septal necks short to recumbent
- Siphuncle somewhat distant from venter, tending to be central or subcentral in young
- Teichert and others (1964) included in the genus Orthonybyoceras Shimizu & Obata those specimens others (for example, Aronoff, 1979) had referred to Treptoceras. Hence, the name Orthonybyocerascommonly has been used for specimens here included in Treptoceras. To differentiate specimens of the two genera, one must carefully examine internal features of the two shells.
- Medium-sized, moderately to rapidly expanding orthoconic longicones; circular to slightly depressed in cross section with straight, transverse sutures. Camerae short, with cameral lengths typically from one-fourth to one-sixth the diameter of the shell, decreasing in length with ontogeny. Body chamber incompletely known. Shell exterior smooth.
- Siphuncle subcentral to midventral in position, becoming more ventral with ontogeny. Siphuncle segments expanded, compressed in outline, segment diameters from one-third to one-fourth the diameter of the shell. Septal necks cyrtochoanitic, short, with recumbent brims. Connecting rings thin, adnate dorsally to both adoral and adapical septa; not adnate ventrally to adapical surface of adoral septum. Endosiphuncular deposits well developed, consisting of dorsal annuli developed more or less symmetrically around septal necks and more massive ventral annulosiphonate deposits that thicken adapical of the septal necks. Broad central canal having dorsal radial canals approaching connecting rings at the midpoint of the segment, while ventrally, canals enter in the adoral half of segment. Adoralmost cameral deposits consisting of discrete episeptal and hyposeptal deposits, which are more strongly developed ventrolaterally.
Treatise on Invertebrate Paleontology, Part K (1964):
- Small to medium sized straight shells with circular to slightly depressed cross sections; sutures straight, either transverse or sloping; septa shallow; septal necks short to recumbent. Siphuncle somewhat distant from venter, tending to be central or subcentral in young; segments broadly inflated , almost Armenoceras-like in early stages, then passing through globular Ormoceras– like stage to elongated Deiroceras-like segments in anterior part of the phragmocone; endosiphuncular structures, when present, resembling those of Ormoceras and Deiroceras.