Department of Earth Sciences, Palaeobiology, Uppsala University, Villavägen 16, Uppsala 752 36, Sweden.
School of Biological Sciences, University of Adelaide, North Terrace, Adelaide, South Australia 5005, Australia.
Proc Biol Sci. 2021 Dec 22;288(1965):20212131. doi: 10.1098/rspb.2021.2131. Epub 2021 Dec 15.
The exceptional fossil record of trilobites provides our best window on developmental processes in early euarthropods, but data on growth dynamics are limited. Here, we analyse post-embryonic axial growth in the Cambrian trilobite from the Emu Bay Shale, South Australia. Using threshold models, we show that abrupt changes in growth trajectories of different body sections occurred in two phases, closely associated with the anamorphic/epimorphic and meraspid/holaspid transitions. These changes are similar to the progression to sexual maturity seen in certain extant euarthropods and suggest that the onset of maturity coincided with the commencement of the holaspid period. We also conduct hypothesis testing to reveal the likely controls of observed axial growth gradients and suggest that size may better explain growth patterns than moult stage. The two phases of allometric change in , as well as probable differing growth regulation in the earliest post-embryonic stages, suggest that observed body segmentation patterns in this trilobite were the result of a complex series of changing growth controls that characterized different ontogenetic intervals. This indicates that trilobite development is more complex than previously thought, even in early members of the clade.
三叶虫异常完整的化石记录为我们提供了研究早期真节肢动物发育过程的最佳窗口，但有关生长动态的数据有限。在这里，我们分析了来自南澳大利亚埃姆湾页岩的寒武纪三叶虫的后生轴生长。使用阈值模型，我们表明不同身体部位的生长轨迹的突然变化发生在两个阶段，与变态/后变态和幼期/后期变态的转变密切相关。这些变化与某些现存真节肢动物的性成熟进展相似，表明成熟的开始与后期变态期的开始同时发生。我们还进行了假设检验，以揭示观察到的轴向生长梯度的可能控制因素，并表明大小可能比蜕皮阶段更好地解释生长模式。在后胚胎阶段的两个阶段的异速生长变化，以及早期阶段可能存在不同的生长调节，表明该三叶虫观察到的身体分段模式是一系列复杂的变化生长控制的结果，这些控制特征在不同的个体发育间隔中表现出来。这表明三叶虫的发育比以前想象的更复杂，即使在该类群的早期成员中也是如此。
