BACKGROUND Polyplacophora, or chitons, have long fascinated malacologists for their distinct and rather conserved morphology and lifestyle compared to other mollusk classes. However, key aspects of their phylogeny and evolution remain unclear due to the few morphological, molecular, or combined phylogenetic analyses, particularly those addressing the relationships among the major chiton lineages. RESULTS Here, we present a mitogenomic phylogeny of chitons based on 13 newly sequenced mitochondrial genomes along with eight available ones and RNAseq-derived mitochondrial sequences from four additional species. Reconstructed phylogenies largely agreed with the latest advances in chiton systematics and integrative taxonomy but we identified some conflicts that call for taxonomic revisions. Despite an overall conserved gene order in chiton mitogenomes, we described three new rearrangements that might have taxonomic utility and reconstructed the most likely scenario of gene order change in this group. Our phylogeny was time-calibrated using various fossils and relaxed molecular clocks, and the robustness of these analyses was assessed with several sensitivity analyses. The inferred ages largely agreed with previous molecular clock estimates and the fossil record, but we also noted that the ambiguities inherent to the chiton fossil record might confound molecular clock analyses. CONCLUSIONS In light of the reconstructed time-calibrated framework, we discuss the evolution of key morphological features and call for a continued effort towards clarifying the phylogeny and evolution of chitons.

中文翻译：

---

石鳖的线粒体基因组系统发育（软体动物：多板纲）。


背景技术多板类或石鳖长期以来因其与其他软体动物类别相比独特且相当保守的形态和生活方式而令软体动物学家着迷。然而，由于形态学、分子或组合系统发育分析很少，特别是那些解决主要石鳖谱系之间关系的分析，其系统发育和进化的关键方面仍不清楚。结果在这里，我们基于 13 个新测序的线粒体基因组以及 8 个可用的线粒体基因组和来自另外四个物种的 RNAseq 衍生的线粒体序列，提出了石鳖的线粒体基因组系统发育。重建的系统发育在很大程度上与石鳖系统学和综合分类学的最新进展一致，但我们发现了一些需要分类学修订的冲突。尽管石鳖线粒体基因组中的基因顺序总体保守，但我们描述了可能具有分类学用途的三种新的重排，并重建了该组中基因顺序变化最可能的情况。我们的系统发育使用各种化石和宽松的分子钟进行了时间校准，并通过多项敏感性分析评估了这些分析的稳健性。推断的年龄在很大程度上与之前的分子钟估计和化石记录一致，但我们也注意到石鳖化石记录固有的模糊性可能会混淆分子钟分析。结论根据重建的时间校准框架，我们讨论了关键形态特征的演化，并呼吁继续努力阐明石鳖的系统发育和演化。