Morphological variability patterns in the Balearites-Pseudothurmannia genera boundary (Ammonoidea, late Hauterivian): taxonomic and biostratigraphic implications

Abstract

As for many other ammonoid groups, the Hauterivian family Crioceratitidae has been traditionally approached from a strictly typological viewpoint, leading to considerable taxonomic oversplitting. Balearites angulicostatus (d’Orbigny, 1841) and Pseudothurmannia ohmi (Winkler, 1868) are two successive species within this lineage, which are used as index species in biostratigraphy. In order to clarify the taxonomic position of the two aforementioned species, the present study carries out morphometric analyses on 111 stratigraphically controlled specimens from the Arroyo Gilico section (SE Iberian Peninsula), enabling us to conclude that the diagnoses of both species should be broadened to encompass the continuous morphological variations occurring within their populations. A phenotypic variability model among eight extreme poles is described for B. angulicostatus, whereas P. ohmi ranges between two extreme poles. The detailed study of shell morphology through time recognizes the evolutionary changes between the two species: anagenetic processes such as the increase of whorl height growth rate and tachymorphosis of ontogenetic stages, coupled with punctuated changes such as the loss of heteromorphic coiling and an allometric hypermorphosis. Some of these may be inherent to the variation of the species themselves, but others such as the loss of heteromorphism seem to be related to palaeoenvironmental factors such as changes in nutrient supply to the sea.