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Abstract <p>This paper discusses records of heteromorphic ammonites of the genus <i>Spiroceras</i> (family Spiroceratidae) in the Upper Bajocian of the interfluve of the Kuban and Urup rivers (Karachay-Cherkessia). The presence of a single species, <i>S. obliquecostatum</i> (Quenstedt), has been confirmed for the lower part of the <i>Parkinsoni</i> Zone (<i>sherstyukovi</i> faunal horizon). <i>S. obliquus</i> (Baugier et Sauzé) is identified for the first time from the upper part of the <i>Niortense</i> Zone (<i>bispinatum</i> faunal horizon). The most abundant and well-preserved material (also from the <i>bispinatum</i> horizon) consists of the species <i>S. annulatum</i> (Deshayes) and <i>S. bispinatum</i> (Baugier et Sauzé). Among the adult shells of these two species, two size groups are clearly distinguished, characteristic of both microconchs and macroconchs. Apparently, this is a classic case of intrapopulation heterochrony, expressed in the presence of normogerontic and tachygerontic shells.</p>

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The Genus Spiroceras (Ammonoidea) from the Upper Bajocian of the Northern Caucasus: Intraspecific Heterochronies

  • 期刊论文

  • 发表日期：

    2025年6月1日

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  • V. V. Mitta

摘要

Abstract

This paper discusses records of heteromorphic ammonites of the genus Spiroceras (family Spiroceratidae) in the Upper Bajocian of the interfluve of the Kuban and Urup rivers (Karachay-Cherkessia). The presence of a single species, S. obliquecostatum (Quenstedt), has been confirmed for the lower part of the Parkinsoni Zone (sherstyukovi faunal horizon). S. obliquus (Baugier et Sauzé) is identified for the first time from the upper part of the Niortense Zone (bispinatum faunal horizon). The most abundant and well-preserved material (also from the bispinatum horizon) consists of the species S. annulatum (Deshayes) and S. bispinatum (Baugier et Sauzé). Among the adult shells of these two species, two size groups are clearly distinguished, characteristic of both microconchs and macroconchs. Apparently, this is a classic case of intrapopulation heterochrony, expressed in the presence of normogerontic and tachygerontic shells.