The figure below shows the distribution of samples containing the nautiloid cephalopod Geisonoceras.
Geisonoceras is a common nautiloid in the MRV strata.
- Geisonoceras Haytt, 1884 [*Orthoceras rivulae ]
- Slowly enlarging orthocones or cyrtocones of circular to subcircular cross section and with straight transverse or slightly oblique sutures. Siphuncle subcentral; necks short or straight, connecting rings expanding slightly within camerae. Small adorally attenuated annulosiphonate deposits in adapical siphuncular segments; camarae with episeptal and hyposeptal deposits. Surface with broad transverse bands, between which are fine growth lines. Aperture transverse, with slight hyponomic sinus.
M. Ord. - M. Dev., North America, Europe, Asia
- Family Geisonoceratidae Zhuravleva, 1959
- Orthoconic or cyrtoconic conchs of subcircular cross section, with orthochoanitic to suborthochoanitic siphuncle occupied by endosiphuncular annuli restricted to septal foramen or growing anteriorly against connecting rings; in advanced species, endosiphuncular annuli fusing with those of adjacent segments to form continuous siphuncular lining.
M. Ord. - M. Dev.
- Order Orthocerida Kuhn, 1940
Treatise on Invertebrate Paleontology, K236.