Iterative ontogenetic development of ammonoid conch shapes from the Devonian through to the Jurassic

We measured longitudinal growth in conch cross-sections of 177 Devonian to Jurassic ammonoid species to test whether conch ontogenetic development parallels the iterative evolution of pachyconic or globular conch shapes. Ontogenetic trajectories of two cardinal conch parameters, conch width index and umbilical width index, show a few common recurring ontogenetic pathways in terms of the number of ontogenetic phases. The most common, with three phases in the conch width index (decrease–increase–decrease) and umbilical width index (increase–decrease–increase), is termed here C-mode ontogeny (after the Carboniferous genus Cravenoceras). Many of the studied globular Palaeozoic and Triassic species (of the latter, particularly the arcestid ammonoids) share principal patterns in the triphasic C-mode conch ontogeny in closely related groups but also between unrelated groups as well. The repetition of conch growth patterns is an example of convergent evolution of the entire life history of glob...

我们对177个泥盆纪（Devonian）至侏罗纪（Jurassic）的菊石（ammonoid）物种的壳体横截面（conch cross-sections）开展了纵向生长（longitudinal growth）测量，以验证壳体个体发育（ontogenetic development）是否与厚壳形（pachyconic）或球形壳体形态（globular conch shapes）的迭代演化相一致。两项核心壳体参数——壳体宽指数（conch width index）与脐宽指数（umbilical width index）的个体发育轨迹（ontogenetic trajectories），在个体发育阶段（ontogenetic phases）的数量上呈现出若干共通的重复发育路径。其中最常见的模式为：壳体宽指数呈现“减小—增大—减小”的三阶段变化，脐宽指数则呈现“增大—减小—增大”的三阶段变化，本文将其命名为C型个体发育（以石炭纪（Carboniferous）克拉文菊石属（Cravenoceras）命名）。诸多被研究的球形古生代（Palaeozoic）及三叠纪（Triassic）菊石物种（其中三叠纪类群尤以弧菊石类（arcestid ammonoids）为代表），不仅在近缘类群中，甚至在远缘类群之间，均共享三阶段C型壳体个体发育的核心模式。壳体生长模式的重复出现，是球形（globular）壳体完整生活史趋同演化（convergent evolution）的一个典型案例……