''Duashnoceras'' is an extinct ammonoid cephalopod genus included in the Stephanoceratidae that lived during the

Bathonian In the geologic timescale the Bathonian is an age and stage of the Middle Jurassic. It lasted from approximately 168.3 Ma to around 166.1 Ma (million years ago). The Bathonian Age succeeds the Bajocian Age and precedes the Callovian Age. Strat ...

stage of the

Middle Jurassic The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations co ...

. ''Duashnoceras'' was established by Westermann in 1983, based on ''Stephanoceras floresi'' Burchhardt (1927) and originally placed in the Bathonian Zigzagiceratinae as a subgenus of ''Zigzagiceras'' but returned to the Stephanoceratidae by Westermann in 1984 Duashnoceras is a stephanoceratid of variable size; evolute with a very slight uncoiling of the last whorls. Juvenile whorls are depressed subtrapezoidal to subrectangular in section, with slightly convex flanks, arched venter, and well defined ventrolateral shoulders. With further growth the whorl section becomes more elliptical to subcircular with flanks more convex and without ventrolateral shoulders. Primary ribs, which form on the umbilical and dorso-lateral part of the whorls, appear almost immediately. At first the primary ribs are dense but become wide spread on the intermediary whorls, then dense again on the last whorls. Tubercles are conical, usually raised as spines on the intermediary whorls, and are independent of ribs which curve around them. Secondary ribs become thicker on the body chambers of microconchs but thinner of body chambers of macroconchs. Microconch and macroconch variations, presumed male and female respectively, also differ in their apertures. The aperture of macroconch ''Duashnoceras'' is simple, a constriction followed by an elevated lip. That of microconch ''Duashnoceras'' has a moderately long lappet, an extension on either side, sometimes preceded by a constriction. The suture of Duashnoceras is intermediate between that of ''

Stephanoceras ''Stephanoceras'' (meaning ''crown horn'') is an extinct genus of Stephanoceratoid ammonite which lived during the Bajocian (Middle Jurassic). It is the type genus of the family Stephanoceratidae. Type species ''Ammonites humphriesianum'' J ...

'' and ''

Cadomites ''Cadomites'' is an extinct ammonite genus from the superfamily Stephanoceratoidea that lived during the Middle Jurassic (upper Bajocian – lower Callovian). Description ''Cadomites'' is directly descended from '' Stephanoceras'', with a sim ...

''. in coiling and whorl section ''Duashnoceras'' is similar to ''Stephanoceras'', but differs in ornamentation.

References

*Sandoval, J. & Westermann, G. E. G. 1986. The Bajocian (Jurassic) ammonite fauna of Oaxaca, Mexico; Jour Paleo V.67, N.6, p. 1220, Nov. 1986 Jurassic ammonites Fossils of Mexico Bathonian life {{ammonite-stub