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 comprises Albian to early Cenomanian ammonoids whose phylogeny forms the basis of a biozonation for the upper Albian due to their highly evolving nature and worldwide distribution. However, both morphological plasticity of the group and the known common occurrences in many condensed successions call for accurate taxonomy and stratigraphy in order to ascertain the phylogeny and temporal/spatial relationships. Here, early late Albian Mortoniceratinae from mostly uncondensed successions, with a continuous record, are described from a hyperextended pericratonic rift basin in the Western Pyrenees. Although quite rare in the study area, we can here provide a detailed taxonomic study of 55 specimens, representing 19 species, with a single new genus and five new subspecies, all recovered over a period of 30 years from upper Albian strata in eight sedimentary areas covering nearly 1000 square kilometres and more than 4000 m of measured logs. Most of these mortoniceratines show morphologies that are in agreement with their known and accepted phyletic lineage in which tuberculation stages are key; however, we also record tri- and quadrituberculate forms that depart from the main and represent unsuccessful offshoots. A reassessment of Mortoniceras (M.) fallax is presented, inclusive of a more complete, emended diagnosis, and referral of the eponymous biozone to the lower upper Albian. The palaeobiogeographical distribution of species studied corresponds to the western Mediterranean area, part of the central Tethyan Realm, in the Basque-Cantabrian Basin (Western Pyrenees) which faces the 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 species studied suggests. The ammonoid fauna studied is proposed as support of the western Mediterranean biozonation; it is tentatively correlated interregionally as well.