Acta Palaeontologica Polonica

Remarks on the embryogeny and postembryonal development of ammonites

Cyprian Kulicki

Acta Palaeontologica Polonica 19 (2), 1974: 201-224

Previous views on the type of embryogeny in ammonites are discussed by the present writer who also points out some morphological characters of the initial chamber, along with the first whorl, which are indicative of a non-larval type of embryogeny. The swelling of shell wall on the nepionic constriction is analyzed in detail and the manner of forming this swelling explained, together with the hypothesis on an adaptative character of the swelling. The diagrams of a relative density of septa in the genus Quenstedtoceras are analyzed and used as a basis for showing certain similarities in the postembryonal development of ammonites and the development of the Recent Decapoda. The occurrence of the morphological hiatus - lack of one whorl - characteristic for the sexual dimorphism of type "A" is explained, on the basis of the Quenstedtoceras, by the difference of one whorl in the ending of the only sexual cycle in males and the first sexual cycle in females.

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