Data from: 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. coihuicoensis and shell outline in P. windhauseni, which seem to evolve directionally towards larger and higher shells, respectively. Ptychomya displays changes in its average morphology and disparity, which are the result of a mixture of taxonomic turnover and mosaic evolution of traits. Pulses of speciation would have been triggered by ecological opportunity, as they occur during the recovery of shallow-burrowing bivalve faunas after dysoxic events affecting the basin. On the other hand, the presence of directional patterns of evolution in P. coihuicoensis and P. windhauseni seems to be the result of a general shallowing-upward trend observed in the basin during the upper Hauterivian – lowest Barremian, as opposed to the cyclical paleoenvironmental stability inferred for the early/late Valanginian – early Hauterivian, which would have prompted stasis in P. koeneni and P. esbelta.双壳类褶喙蛤属（Ptychomya）在研究程度较高的阿根廷中西部内乌肯盆地阿格里奥组（下/上凡兰吟阶—最低巴列姆阶）中的复杂形态演化，为研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种褶喙蛤属物种，为全面阐明该类群的演化全貌，需对其形态变异开展时间维度的量化评估。本研究采用几何形态测量学方法，对这4个物种的壳形轮廓、肋纹样式以及壳体大小的变异进行定量分析。研究人员结合菊石生物地层学与绝对年代学方法限定了采样样品的地质年代，并通过古生物时间序列分析与变化追踪手段，探究各性状的前进演化模式。研究结果显示，褶喙蛤属的演化整体以物种形成为主导，多数性状表现为进化停滞；仅科伊惠科褶喙蛤（P. coihuicoensis）的壳体大小与温德豪森褶喙蛤（P. windhauseni）的壳形轮廓分别呈现出向更大壳体与更高壳体定向演化的趋势。褶喙蛤属的平均形态与形态分异度均发生了变化，这是分类群更替与性状镶嵌进化共同作用的结果。物种形成脉冲事件由生态机遇所触发，此类事件发生于受缺氧事件影响的盆地内浅栖双壳类动物群的复苏阶段。另一方面，科伊惠科褶喙蛤与温德豪森褶喙蛤的定向演化模式，似乎是盆地在豪特里维阶上部—最低巴列姆阶期间普遍存在的向上变浅沉积趋势所导致的；而在凡兰吟阶早/晚期—豪特里维阶早期，盆地被推断为处于周期性古环境稳定状态，这使得科恩褶喙蛤（P. koeneni）与纤细褶喙蛤（P. esbelta）呈现进化停滞特征。