''Macrocephalites'' is a genus of the stephanoceratoid

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefis ...

family

Macrocephalitidae ''Macrocephalitidae'' is an extinct family of marine invertebrate animals belonging to the superfamily Stephanoceratoidea, order Ammonitida. Etymology The Macrocephalitidae is one of the 11 families listed in the Treatise, 1957 Part L in the sup ...

, diagnostic of the

Callovian In the geologic timescale, the Callovian is an age and stage in the Middle Jurassic, lasting between 166.1 ± 4.0 Ma (million years ago) and 163.5 ± 4.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the ...

stage of the Middle

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

. Three subgenera, ''Dolikephalites'', ''Kamptokephalites'', and ''Pleurocephalites'' are recognized in addition to ''Macrocephalites'' itself, with ''Indocephalites'' tentatively included as the fourth.

Diagnosis ''Macrocephalites'' s.s.

''Macrocephalites'' itself has a strongly ribbed, involute, subglobose to discoidal shell with a somewhat small open umbilicus, compressed whorl section, higher than wide, and rounded venter. Early whorls are more strongly ribbed than later, which become smoother, ending with a smooth body chamber. Primary ribs, which emanate radially from the umbilicus, bifurcate and sometimes trifurcate mid flank and cross the venter uninterrupted. Shells grew to be large, even gigantic. Distribution is widespread.

Subgenera

''Dolikephalites'' is similar in general form to ''Macrocephalites'' but has fine ribbing that continues to the end and is never large. ''Kamptokephalites'' is more globose than ''Macrocephalites proper, with a depressed whorl section, wider than high, broadly arched venter, coarse plicate ribbing and wider umbilicus. ''Pleurocephalites'' also has a broad shell with depressed whorl section and sharp ribbing that continues to the end. Primary ribs on the inner part of the whorls slope dorso-ventrally to the rear. ''Indocephalites'' is broad and coarsely ribbed in early growth stages but becomes smoother and more compressed in the adult.

Gallery

File: Macrocephalitidae - Macrocephalites (Dolikephalites) perseverans.JPG, ''Macrocephalites (Dolikephalites) perseverans'' File: Kamptokephalites.JPG, Subgenus ''Kamptokephalites'' File:Macrocephalites diadematus - Naturhistorisches Museum Nürnberg - Nuremberg, Germany - DSC04165.jpg, ''Macrocephalites diadematus''

References

* Arkell et al. 1957. Mesozoic Ammonoidea. Treatise on Invertebrate Paleontology, Part L. Geological Society of America and Univ of Kansas Press, R.C. Moore (ed)
Macrocephalites verus
Buckman 1922, illustrated.

aff. cossmani Petitclerc 1915, illustrated. Jurassic ammonites Ammonites of Europe Stephanoceratoidea Ammonitida genera {{Ammonitina-stub