Morphological evolution of the bivalve Ptychomya through the Lower Cretaceous of Argentina

The complex morphological evolution of the bivalve Ptychomya throughout the well studied Agrio Formation in the Neuquén Basin (west-central Argentina, lower/upper Valanginian – lowest Barremian) constitutes an ideal opportunity to study evolutionary patterns and processes occurring at geological timescales. Ptychomya is represented in this unit by four species, the morphological variation of which need to be temporally assessed in order to obtain a thorough picture of the evolution of the group. Here we use geometric morphometrics to measure variation in shell outline, ribbing pattern and shell size in these species. We bracket the age of our samples using a combination of ammonoid biostratigraphy and absolute ages, and study the anagenetic pattern of evolution of each trait by means of paleontological time-series analysis and change tracking. We find that evolution in Ptychomya is mostly speciational as the majority of traits show stasis, with the exceptions of shell size in P. coihuic...