Data from: Evidence for heterochrony in the cranial evolution of fossil crocodyliforms

The southern supercontinent of Gondwana was home to an extraordinary diversity of stem-crocodylians (Crocodyliformes) during the Late Cretaceous. The remarkable morphological disparity of notosuchian crocodyliforms indicates that this group filled a wide range of ecological roles more frequently occupied by other vertebrates. Among notosuchians, the distinctive cranial morphology and large body sizes of Baurusuchidae suggest a role as apex predators in ecosystems in which the otherwise dominant predatory theropod dinosaurs were scarce. Large-bodied crocodyliforms, modern and extinct, are known to have reached large sizes by extending their growth period. In a similar way, peramorphic heterochronic processes may have driven the evolution of the similarly large baurusuchids. To assess the presence of peramorphic processes in the cranial evolution of baurusuchids, we applied a geometric morphometric approach to investigate ontogenetic cranial shape variation in a comprehensive sample of notosuchians. Our results provide quantitative morphological evidence that peramorphic processes influenced the cranial evolution of baurusuchids. After applying size and ancestral ontogenetic allometry corrections to our data, we found no support for the action of either hypermorphosis or acceleration, indicating that these two processes alone cannot explain the shape variation observed in Notosuchia. Nevertheless, the strong link between cranial shape variation and size increase in baurusuchids suggests that peramorphic processes were involved in the emergence of hypercarnivory in these animals. Our findings illustrate the role of heterochrony as a macroevolutionary driver, and stress, once more, the usefulness of geometric morphometric techniques for identifying heterochronic processes behind evolutionary trends.

晚白垩世（Late Cretaceous）时期，南部超大陆冈瓦纳大陆（Gondwana）孕育了物种多样性极为丰富的基干鳄形类（stem-crocodylians），该类群隶属于鳄形超目（Crocodyliformes）。诺托鳄类（Notosuchia）鳄形超目动物展现出显著的形态分异度，显示该类群占据了诸多生态位，而这类生态位通常由其他脊椎动物更为频繁地占据。在诺托鳄类中，包鲁鳄科（Baurusuchidae）物种凭借其独特的颅骨形态与庞大体型，推测在原本占据主导地位的兽脚类恐龙（theropod dinosaurs）较为稀缺的生态系统中充当了顶级捕食者。

现生及已灭绝的大型鳄形超目动物均已被证实，可通过延长生长周期达到巨型体型。与之类似，增态异时发育过程（peramorphic heterochronic processes）或许推动了体型同样庞大的包鲁鳄科物种的演化。为评估包鲁鳄科颅骨演化中是否存在增态异时发育过程，我们采用几何形态测量法（geometric morphometric approach），针对涵盖全面诺托鳄类类群的样本，开展个体发育颅骨形状变异的研究。

我们的研究结果提供了定量形态学证据，证实增态异时发育过程对包鲁鳄科的颅骨演化产生了影响。在对数据开展体型与祖先个体发育异速生长校正后，我们未发现支持超生长（hypermorphosis）或加速发育（acceleration）任一过程的相关证据，这表明仅依靠这两种过程无法解释诺托鳄类中观测到的颅骨形状变异。尽管如此，包鲁鳄科物种的颅骨形状变异与体型增长之间存在的强关联提示，增态异时发育过程参与了这类动物超肉食性（hypercarnivory）特征的演化形成。

本研究阐明了异时发育作为宏观进化驱动力的作用，并再次强调了几何形态测量技术在识别演化趋势背后异时发育过程方面的应用价值。