Intraspecific variability, biostratigraphy and paleobiological significance of the Southern Gondwana ammonoid genus Lytohoplites Spath

Abstract

Lytohoplites Spath, 1925 is a late Tithonian-?early Berriasian ammonoid genus with a southern perigondwanean distribution. Two Lytohoplites species, L. burckhardti (Mayer-Eymar in Burckhardt, 1900) (type species) and L. vetustoides (Burckhardt, 1903), were originally described from carbonate successions of the Vaca Muerta Formation, Neuquén Basin, Argentina. Nevertheless, the holotype of L. burckhardti consisted of a single incomplete specimen that is currently missing. This situation compelled the search for new Lytohoplites specimens in Argentina and the selection of a neotype for L. burckardti. New Lytohoplites representatives were obtained through bed-by-bed collections performed at five localities in the Neuquén Basin. In addition to the taxonomic revision of the Lytohoplites species occurring in the basin, a paleobiological approach was preferred to conduct a paleontological analysis of L. burckhardti, including a description of its ontogeny, probable sexual dimorphism, and spectrum of intraspecific variability. Results of the qualitative and quantitative analyses supported the homogeneity of L. burckhardti as a taxonomic unit, thus implying that L. vetustoides should be considered its synonym. Lytohoplites in the Neuquén Basin is restricted to beds assigned to the Andean Substeueroceras koeneni Assemblage Zone (upper Tithonian-lower Berriasian), and not to the Andean Corongoceras alternans Assemblage Zone (upper Tithonian). Furthermore, the paleobiogeographic distribution of Lytohoplites around southern Gondwana and the herein reported occurrence of L. subcylindricus Collignon, 1962, otherwise only known from Madagascar, lend support to the existence of a functional trans-Gondawana seaway at least since the upper Tithonian.