''Stephanoceras'' (meaning ''crown horn'') is an extinct genus of Stephanoceratoid ammonite which lived during the

Bajocian In the geologic timescale, the Bajocian is an age and stage in the Middle Jurassic. It lasted from approximately 170.9 ±0.8 Ma to around 168.2 ±1.2 Ma (million years ago). The Bajocian Age succeeds the Aalenian Age and precedes the Bathonia ...

(Middle Jurassic). It is the

type genus In biological taxonomy, the type genus (''genus typica'') is the genus which defines a biological family and the root of the family name. Zoological nomenclature According to the International Code of Zoological Nomenclature, "The name-bearin ...

of the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Stephanoceratidae Stephanoceratidae is a family of planulate and coronate ammonites within the Stephanoceratoidea Stephanoceratoidea, formerly Stephanocerataceae, is a superfamily of middle- upper Jurassic ammonoid cephalopods within the order Ammonitida cont ...

Type species

''Ammonites humphriesianum'' J. de C. Sowerby, 1825, subsequent designation by

Buchman Buchman (or Buchmann, ) is the surname of: Buchman * Alex Buchman (1911–2003), American activist * Dana Buchman, American fashion designer * Frank N. D. Buchman (1878–1961), Christian evangelist * Heather Buchman, American conductor and trombo ...

, 1898

Selected species

*''S. humphriesianum'' (J. de C. Sowerby 1825) - Type species *''S. blagdeni'' (Sowerby) *''S. brodeaei'' (J. Sowerby) *''S. mutabile'' (Quenstedt 1886) *''S. umbilicum'' (Quenstedt 1886) *''S. nodosum'' (Quenstedt 1886) *''S. macrum'' (Quenstedt 1886) *''S. pyritosum'' (Quenstedt 1886) *''S. triplex'' Weisert *''S. freycineti'' (Bayle) *''S. kreter'' (Buckman) *''S. leptogyrale'' (Buckman) *''S. boulderense'' Imlay 1982 *''S. coronatus'' (Bruguiere 1789) *''S. quenstedti'' Кoche 1939 *''S. skidegatense'' Whiteaves 1876

Description

''Stephanoceras'' has an evolute shell, as characteristic of the family, with well-developed ribbing and tubercles. The shell is coiled so whorls barely touch in most, but some have notable overlap. Primary ribs emerge from the umbilical seam, the line marking the inner edge of the particular exposed whorl, and divide on the flanks, usually in two, occasionally in three, forming secondary ribs that cross the outer rim of the shell, known as the venter, uninterrupted. Turbercles, elevated projections, form at the ends of the primary ribs where they bifurcate or sometimes trifurcate. ''Stephanoceras'' grew to be fairly large with a shell diameter as much as 27 cm and width as much as 6 cm across the outer whorl. Like many ammonites, ''Stephanoceras'' is dimorphic with a large, macroconch, form thought to be female and a small, microconch, form thought to be male. In most aspects the microconch is simply a smaller version of the macroconch. The primary difference lies in the aperture, which in the microconch has planar extensions on either side, lappets. That of the macroconch is generally simple, preceded by a constriction.

Distribution

Fossils of ''Stephanoceras'' are found in the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

marine strata of Canada, Mexico, Saudi Arabia and Spain. Some species, including ''S.coronatus'' and ''S.humphriesianum'' can also be found in rocks of the same age from England and Germany.

Gallery

File:S. humphriesianum - Clatcombe Dorset.jpg, ''Stephanoceras humphriesianum'', Jurassic, Inferior Oolite, ''Humphriesianum'' Zone, Clatcombe, Dorset, UK. Size of ammonite 275mm. File:Stephanoceratidae - Stephanoceras blagdeni.JPG, ''Stephanoceras blagdeni'' from Mid

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''References''

*Arkell, ''et al.'' 1957. Mesozoic Ammonoidea, Treatise on invertebrate Paleontology, Part L, Mollusca 4. Geol Soc of America and Univ of Kansas press, Lawrence. *Galacz, A. 1991. Bajocian Sephanoceratid Ammonites from the Bakony Mountains, Hungary. Palaeontology, Vol 34, Pt 4, 1991, pp 859–885. *Sandoval, J. & Westermann, G. E. G. 1986. The Bajocian (Jurassic) ammonite fauna of Oaxaca, Mexico; Jour Paleo V.67, N.6, p. 1220, November 1986 {{Taxonbar, from=Q3726721 Jurassic ammonites Jurassic ammonites of Europe Jurassic ammonites of North America Ammonitida genera Stephanoceratoidea