Herbivorous dinosaur jaw disparity and its relationship to extrinsic evolutionary drivers

Morphological responses of nonmammalian herbivores to external ecological drivers have not been quantified over extended timescales. Herbivorous nonavian dinosaurs are an ideal group to test for such responses, because they dominated terrestrial ecosystems for more than 155 Myr and included the largest herbivores that ever existed. The radiation of dinosaurs was punctuated by several ecologically important events, including extinctions at the Triassic/Jurassic (Tr/J) and Jurassic/Cretaceous (J/K) boundaries, the decline of cycadophytes, and the origin of angiosperms, all of which may have had profound consequences for herbivore communities. Here we present the first analysis of morphological and biomechanical disparity for sauropodomorph and ornithischian dinosaurs in order to investigate patterns of jaw shape and function through time. We find that morphological and biomechanical mandibular disparity are decoupled: mandibular shape disparity follows taxonomic diversity, with a steady increase through the Mesozoic. By contrast, biomechanical disparity builds to a peak in the Late Jurassic that corresponds to increased functional variation among sauropods. The reduction in biomechanical disparity following this peak coincides with the J/K extinction, the associated loss of sauropod and stegosaur diversity, and the decline of cycadophytes. We find no specific correspondence between biomechanical disparity and the proliferation of angiosperms. Continual ecological and functional replacement of pre-existing taxa accounts for disparity patterns through much of the Cretaceous, with the exception of several unique groups, such as psittacosaurids that are never replaced in their biomechanical or morphological profiles.

非哺乳类植食动物对外部生态驱动因子的形态响应，尚未在长期时间尺度上得到量化研究。植食性非鸟类恐龙是开展此类响应研究的理想类群：它们主导陆地生态系统长达1.55亿年，且包含了地球历史上体型最庞大的植食动物。恐龙的演化辐射进程穿插了多起生态关键事件，包括三叠纪/侏罗纪（Tr/J）与侏罗纪/白垩纪（J/K）界线处的集群灭绝、苏铁类植物的衰落，以及被子植物的起源，上述所有事件均可能对植食动物群落造成深远影响。本研究首次针对蜥脚形亚目与鸟臀目恐龙开展形态差异度（disparity）与生物力学差异度分析，以探究颌骨形态与功能随时间的演化规律。研究发现，下颌的形态差异度与生物力学差异度呈现解耦关系：下颌形状差异度随分类多样性同步变化，在中生代呈现持续上升趋势。与之相反，生物力学差异度在晚侏罗世达到峰值，该峰值对应蜥脚类类群的功能多样性提升。该峰值之后的生物力学差异度下降，与J/K灭绝事件、伴随发生的蜥脚类与剑龙类多样性丧失，以及苏铁类植物的衰落时间相吻合。本研究未发现生物力学差异度与被子植物的扩张存在明确关联。在白垩纪的大部分时期，已有类群的持续生态与功能替代主导了差异度的演化模式，仅鹦鹉嘴龙类等若干特殊类群例外——这类类群从未在生物力学特征或形态特征轮廓上被其他类群所替代。