CONICET | Buscador de Institutos y Recursos Humanos

The complex morphological evolution of the bivalve Ptychomya throughout the well-studiedAgrio Formation in the Neuquén Basin (west-central Argentina, lower/upper Valanginian?lowestBarremian) constitutes an ideal opportunity to study evolutionary patterns and processes occurring atgeological timescales. Ptychomya is represented in this unit by four species, the morphological variationof which needs to be temporally assessed to obtain a thorough picture of the evolution of the group. Herewe use geometric morphometrics to measure variation in shell outline, ribbing pattern, and shell size inthese species. We bracket the ages of our samples using a combination of ammonoid biostratigraphy andabsolute ages and study the anagenetic pattern of evolution of each trait by means of paleontologicaltime-series analysis and change tracking. We find that evolution in Ptychomya is mostly speciational, asthe majority of traits show stasis, with the exceptions of shell size in P. coihuicoensis and shell outline inP. windhauseni, which seem to evolve directionally toward larger and higher shells, respectively.Ptychomya displays changes in its average morphology and disparity, which are the result of a mixture oftaxonomic turnover and mosaic evolution of traits. Pulses of speciation would have been triggered byecological opportunity, as they occur during the recovery of shallow-burrowing bivalve faunas afterdysoxic events affecting the basin. On the other hand, the presence of directional patterns of evolution inP. coihuicoensis and P. windhauseni seems to be the result of a general shallowing-upward trend observedin the basin during the upper Hauterivian?lowest Barremian, as opposed to the cyclical paleoenvironmentalstability inferred for the early/late Valanginian?early Hauterivian, which would have promptedstasis in P. koeneni and P. esbelta.