Mortoniceratinae (Ammonoidea) from the lower upper Albian of the Basque-Cantabrian basin (Western Pyrenees): New records, new taxa and their taxonomic and biostratigraphical value

Abstract

The subfamily Mortoniceratinae is an Albian to lower Cenomanian ammonoid group whose phylogeny is the base for the biozonation of the upper Albian due to its highly evolving nature and worldwide distribution. However, the morphological plasticity of the group and the known common occurrences in many condensed successions imply the use of an accurate taxonomy and stratigraphy in order to ascertain the phylogeny and the temporal/spatial relations. We report here lower upper Albian Mortoniceratinae from mostly uncondensed successions, with continuous record, of a hyperextended pericratonic rift basin in the Western Pyrenees. Despite their scarcity in the studied area, we provide a detailed taxonomic study of 55 specimens divided into 19 species, with a single new genus and 5 new species, all obtained after three decades of research in upper Albian outcrops from eight sedimentary areas covering near 1000 square kilometres and more than 4000 meters of measured sedimentary series. Most of these Mortoniceratinae show morphologies in agreement with their known and accepted phyletic lineage in which the tuberculation stages are key; however, we report trituberculate and quadrituberculate forms departed from the common lineage, which represent unsuccessful offshoots. Taxonomic discussions led to the proposal of new taxa and precisions in the species understanding and their corresponding phyletic zonation. Accordingly, we present the reassessment of the Mortoniceras (M.) fallax species, give an amended more complete diagnosis, and propose the inclusion of its eponymous biozone into the lower upper Albian. The palaeobiogeographical distribution of the studied species corresponds to the Western Mediterranean area, part of the central Tethyan Realm, in the Basque-Cantabrian Basin (Western Pyrenees), facing Northern European basins of the Boreal Realm. This strategic location is interpreted to be a nexus between worldwide basins as the striking cosmopolitan character of most of the studied species suggests; regarding this we propose the studied ammonoids as a support of the Western Mediterranean ammonoid biozonation and present a tentative interregional correlation.