Taxonomy of the bivalve Ptychomya in the Lower Cretaceous of the Neuquén basin (west-central Argentina)

In the Lower Cretaceous Agrio Formation, Neuquén Basin (west-central Argentina), the highly variable genus Ptychomya Agassiz has challenged traditional taxonomists for over a century. Here we apply a previously outlined quantitative protocol in order to settle the taxonomy of the genus based on specimens recorded from the base to the top of the Agrio Formation. The shell outline and ribbing pattern, two prominent external features of the valves, were quantified using geometric morphometric measurements of closed and open outlines, respectively. Morphological groupings were identified using model-based clustering, and the presence of significant morphological gaps between them was statistically tested. Our results show that the variation historically attributed to a single species, namely Ptychomya koeneni Behrendsen, corresponds to intra- and interspecific variability of at least four species, one of which was previously unknown (Ptychomya esbelta sp. nov.) whereas another was regarded ...

在阿根廷中西部内乌肯盆地（Neuquén Basin）的早白垩世阿格里奥组（Agrio Formation）中，高度变异的褶翅蛤属（Ptychomya Agassiz）一个多世纪以来一直对传统分类学家构成挑战。本文应用先前提出的定量方案，基于阿格里奥组底部至顶部记录的标本，解决该属的分类问题。壳轮廓与肋纹模式是壳瓣的两个显著外部特征，我们分别通过闭合轮廓与张开轮廓的几何形态测量对其进行量化。利用基于模型的聚类识别形态学分组，并对组间是否存在显著形态学间隙进行统计检验。结果表明，历史上归为单一物种——科恩褶翅蛤（Ptychomya koeneni Behrendsen）的变异，实际对应至少四个物种的种内与种间变异；其中一个物种此前未知（埃斯贝尔塔褶翅蛤，Ptychomya esbelta sp. nov.），而另一个物种曾被认为...