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（bivalve）Ptychomya在研究程度较高的阿根廷中西部内乌肯盆地（Neuquén Basin）Agrio组（Agrio Formation，时限为凡兰吟阶下部/上部至巴列姆阶最底部）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。该地层中产出的Ptychomya共包含4个物种，为全面阐明该类群的演化历程，需对其形态变异开展时间维度的评估。本研究采用几何形态测量学（geometric morphometrics）方法，量化分析这4个物种的壳轮廓、肋纹样式与壳尺寸变异。本研究结合菊石生物地层学（ammonoid biostratigraphy）与绝对年龄数据限定样品的时代，并通过古生物学时间序列分析与变化追踪方法，探究各性状的线系演化模式。研究发现，Ptychomya的演化以物种形成式为主，多数性状表现为停滞状态；仅P. coihuicoensis的壳尺寸与P. windhauseni的壳轮廓呈现定向演化趋势，分别向更大壳型与更高壳形方向发展。Ptychomya的平均形态与形态差异度均发生变化，这是分类更替与性状镶嵌演化共同作用的结果。物种形成脉冲由生态机遇所触发，这类脉冲发生于影响盆地的缺氧事件之后，浅栖双壳类动物群的复苏阶段。与之相反，P. coihuicoensis与P. windhauseni的定向演化模式，似乎是盆地在豪特里维阶上部至巴列姆阶最底部时期普遍存在的向上变浅趋势所导致的；而凡兰吟阶早/晚期至豪特里维阶早期时期被推断为周期性古环境稳定阶段，这一环境背景使得P. koeneni与P. esbelta的性状呈现停滞状态。