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Intraspecific variability, sexual dimorphism, and non-sexual polymorphism in the ammonite genus Larcheria Tintant (Perisphinctidae) from the Middle Oxfordian of western Europe Meléndez, G.; Fontana, B. (1993). Intraspecific variability, sexual dimorphism, and non-sexual polymorphism in the ammonite genus Larcheria Tintant (Perisphinctidae) from the Middle Oxfordian of western Europe, in: House, M.R. (Ed.) The Ammonoidea: environment, ecology, and evolutionary change. pp. 165-186 In: House, M.R. (Ed.) (1993). The Ammonoidea: Environment, ecology, and evolutionary change. The Systematics Association Special Volume, 47. Clarendon Press: Oxford. ISBN 0-19-857765-6. 353 pp. In: The Systematics Association Special Volume

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Meléndez, G. Fontana, B.

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Abstract

Middle Oxfordian representatives of the ammonite subfamily Perisphinctinae are traditionally known by their high variability and diversified morphology. This has led to a clasical, 'vertical' systematics pattern for this group formed by a large number of morphospecies evolving in parallel. A systematic approach based on the concept of horizontal species regards successive homogeneous, coeval assemblages as monobiospecific, and the many morphological varieties within each assemblage as the expression of intraspecific variability, be it continuous or discontinuous (polymorphism). According to this approach, the 'genus' Larcheria, from the Middle Oxfordian, Transversarium Zone, is regarded as a possible case of intraspecific polymorphism in relation to Perisphinctes. Main arguments to support this view are: the discontinuous character of the morphological differences between both forms, such as the style of ribbing, regularly biplicate in Perisphinctes and 'lithacoceratoid', multi divided in Larcheria; the parallel evolution shown by both groups during the middle and upper part of the transversarium Zone, and the similarity in the early stages of the ontogeny in the representatives of these forms. Besides this, and super-imposed on this kind of intraspecific polymorphism, sexual dimorphism, known for a long time in Perisphinctes, is also evidenced within the group Larcheria.

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