Pachyceratidae is a family of

Perisphinctoidea Perisphinctoidea, formerly Perisphinctaceae, is a superfamily of Middle Jurassic (Bajocian) to Lower Cretaceous (Barremian) ammonites, commonly with evolute shells with strong ribbing that typically divides about mid flank before crossing the ven ...

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 cuttlefish) ...

s from the upper Middle - and lower Upper Jurassic. Genera within the Pachyceratidae have shells that are in general moderately involte but with most of the inner whorls exposed; whorl sections subquadrate to subtrapezoidal, with rounded venter. Ribbing is strong, in some sharp. Primary ribs typically branch above mid flanks into twos, threes, and even fours. The Pachyceratidae, according to Donovan et al. (1981), were derived from the Perisphinctidae in the mid

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 ...

at about the same time as the longer lived

Aspidoceratidae The Aspidoceratidae comprise a family of middle and upper Jurassic ammonites that make up part of the superfamily Perisphinctoidea, characterized by evolute shells, commonly stocky, that tend to develop tubercles. The Aspidoceratidae are thought ...

, lasting only into the subsequent Oxfordian. This differs from the perspective of Arkell, et al. (1957) in the

Treatise A treatise is a formal and systematic written discourse on some subject, generally longer and treating it in greater depth than an essay, and more concerned with investigating or exposing the principles of the subject and its conclusions."Treat ...

wherein the Pachyceridae is included in the

Stephanoceratoidea Stephanoceratoidea, formerly Stephanocerataceae, is a superfamily of middle- upper Jurassic ammonoid cephalopods within the order Ammonitida containing diverse forms, generally with sharp ribbing and complex suture lines. Aptychi are believed t ...

, derived from the Tulitidae and giving rise to the Mayaitidae. Genera, as described in the Treatise on Invertebrate Paleontology, Part L. (1957): *'' Erymnoceras'': moderately involute; primary ribs branch into multiple secondaries. (M Callov). *'' Erymnocerites'', subgen of ''Erymoceras'' with more numerous secondary ribs. (M Callov). *'' Pachyerymnoceras'': involute, last whorl becoming compressed and ''Pachyceras''-like; subgen of ''Erymnoceras''. (U Callov) *'' Rollierites'': evolute, more or less planulate; ribs coarse, branching from tubercles at umbilical edge. (M Callov). *'' Pachyceras'': basically involute; umbilical edge rounded, non tuberculate; ribs fade on inner half of whorl sides; last whorl commonly smooth or with ventral folds only. (U Callov - L Oxf). *'' Tornquistes'': differs from ''Pachyceras'' ''s.s.'' by coarsening ribs on last whorl and developing eccentric coils; subgenus(?) of ''Pachyceras''. (L- U Oxf).

References

* Arkell, et al., 1957. Mesozoic Ammonoidea, in the Treatise on Invertebrate Paleontology, (Part L); Geological Soc. of America and University of Kansas press * Donovan, Callomon and Howarth 1981 Classification of the Jurassic Ammonitina; Systematics Association

Ammonitida families Perisphinctoidea Jurassic ammonites Callovian first appearances Late Jurassic extinctions {{ammonitina-stub