Morphogenesis Rate Variability and Modularity of Development in Jurassic Ammonites of the Genus Arcticoceras Spath, 1924

The high variability of ammonites is largely due to the diversity of combinations of individual heterochronies (bradimorphy, tachymorphy, bradygeronty, and tachygeronty) and the modularity of development. Using the genus Arcticoceras as an example, it was shown that heterochronies of various directions can be combined in any number of characters, and individual heterochronies differ from phylogenetic ones by their smaller range, and a larger number of combinations. The study of such combinations allowed intraspecific morphs to be recognized, and available Arcticoceras species to be revised. During the Early and Middle Bathonian, the family Cardioceratidae evolved through a series of paedomorphosis, and after the appearance of Cadoceratinae, through accelerations.