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...产于阿根廷中西部内乌肯盆地（Neuquén Basin）、研究程度极高的阿格里奥组（Agrio Formation），时代涵盖凡兰今阶下部/上部至巴列姆阶最底部的褶蛤属（Ptychomya）双壳类，其复杂的形态演化历程为探究地质时间尺度下的演化模式与过程提供了理想契机。该地层中共产出4种褶蛤属物种，为全面厘清该类群的演化全貌，需对其形态变异开展时间尺度上的量化评估。本研究采用几何形态测量学（geometric morphometrics）方法，量化分析这些物种的壳形轮廓、肋纹样式与壳体大小的变异特征。本研究结合菊石生物地层学与绝对年代学数据限定样品的时代，并通过古生物时间序列分析与变化追踪手段，探究各性状的线系演化模式。研究结果显示，褶蛤属的演化以物种形成为主导，多数性状表现为演化停滞，仅P. coihuic……的壳体大小除外。