Macroevolutionary patterns of body plan canalization in euarthropods

Reconstructing patterns of macroevolution has become a central endeavor in palaeobiology, because it offers insight into evolutionary models shaping the history of life. As the most diverse and abundant animals since the Cambrian period, arthropods provide copious data to elucidate the emergence of body plans in metazoan lineages. However, information provided by fossils on the tempo and mode of this phenomenon has lacked a recent synthesis. Here, I investigate macroevolutionary patterns of morphological evolution in Euarthropoda using a combined extinct and extant dataset optimized for multivariate analyses. Overall ordination patterns between the main morphogroups are consistent with another, independently-coded, extant-only dataset providing molecular and morphological rates of evolution. Based on a “deep split” phylogenetic framework, total-group Mandibulata and Arachnomorpha emerge as directional morpho-anatomical lineages, with basal fossil morphogroups showing heterogeneously spread-out occupations of the morphospace. In addition to a more homogeneous morphological variation, new morphogroups arose by successive reductions of translation distances; this pattern was interrupted only by terrestrialization events and the origin of pancrustaceans. A displaced optimum type of model is proposed to explain the fast assembly of canalized body plans during the Cambrian, with basal fossil morphogroups fitting intermediate fitness peaks in a moving adaptive landscape. Given time constraints imposed by the palaeontological evidence, and owing to the interplay between canalization and modularity, as well as a decoupling between molecular and morphological rates, the rise of euarthropods would support the view that the swiftness of the Cambrian explosion was mostly associated with the build-up of genetic regulatory networks.

重建宏观演化（macroevolution）模式现已成为古生物学（palaeobiology）的核心研究议题，因其可为解析塑造生命演化历程的各类演化模型提供关键视角。作为寒武纪以来物种多样性与丰度最高的动物类群，节肢动物（Arthropoda）为阐释后生动物（Metazoa）谱系中躯体构型（body plans）的起源提供了海量研究数据。然而，现有化石记录中关于该现象的演化速率与模式（tempo and mode）的相关数据，迄今仍缺乏系统性的综合研究。本研究基于经多变量分析优化的灭绝与现生类群联合数据集，探讨了真节肢动物（Euarthropoda）的形态演化宏观模式。主要形态类群（morphogroups）间的整体排序分析（ordination）结果，与另一项仅基于现生类群、经独立编码且包含演化分子速率与形态速率的数据集所得结论一致。基于“深度分化”的系统发育框架，大颚类总群（Mandibulata）与蛛形形类（Arachnomorpha）呈现出定向的形态解剖学演化谱系；其基部化石形态类群在形态空间（morphospace）中的分布呈现出异质性的弥散特征。除了形态变异趋于均一化之外，新形态类群的出现伴随形态演化位移距离的持续缩减；该演化模式仅在陆生化事件（terrestrialization）以及泛甲壳类（Pancrustacea）起源时发生中断。本研究提出一种“最优位移型”演化模型，用以解释寒武纪时期受渠限化（canalization）约束的躯体构型快速成型过程；其中基部化石形态类群在动态适应性景观（adaptive landscape）中，契合于中间适应度峰（fitness peaks）位。考虑到古生物学证据所限定的时间尺度，加之渠限化与模块化（modularity）效应的相互作用，以及分子演化速率与形态演化速率之间的解耦（decoupling）现象，真节肢动物的兴起佐证了如下观点：寒武纪大爆发（Cambrian explosion）的快速性，主要与遗传调控网络（genetic regulatory networks）的逐步构建密切相关。