Data from: Assessing the evolutionary history of the class Synurophyceae (Heterokonta) using molecular, morphometric, and paleobiological approaches

Premise of the study: Heterokont algae of the class Synurophyceae, characterized by distinctive siliceous scales that cover the surface of the cell, are ecologically important in inland waters, yet their evolutionary history remains enigmatic. We explore phylogenetic relationships within this group of algae relative to geologic time, with a focus on evolution of siliceous components. Methods: We combined an expansive five-gene and time-calibrated molecular phylogeny of synurophyte algae with an extensive array of fossil specimens from the middle Eocene to infer evolutionary trends within the group. Key results: The group originated in the Jurassic approximately 157 million years ago (Ma), with the keystone genera Mallomonas and Synura diverging during the Early Cretaceous at 130 Ma. Mallomonas further splits into two major subclades, signaling the evolution of the V-rib believed to aid in the spacing and organization of scales on the cell covering. Synura also diverges into two primary subclades, separating taxa with forward-projecting spines on the scale from those with a keel positioned on the scale proper. Approximately one third of the fossil species are extinct, whereas the remaining taxa are linked to modern congeners. Conclusions: The taxonomy of synurophytes, which relies extensively on the morphology of the siliceous components, is largely congruent with molecular analyses. Scales of extinct synurophytes were significantly larger than those of modern taxa and may have played a role in their demise. In contrast, many fossil species linked to modern lineages were smaller in the middle Eocene, possibly reflecting growth in the greenhouse climatic state that characterized this geologic interval.

研究背景
本研究针对异鞭藻（Heterokont）门下的硅鳞藻纲（Synurophyceae）：该类群以覆盖细胞表面的独特硅质鳞片为典型特征，在内陆水域中具有重要生态意义，但其演化历史仍不明晰。本研究旨在探究该藻类类群基于地质时间尺度的系统发育关系，重点聚焦硅质结构的演化。

研究方法
本研究结合了针对硅鳞藻的大规模五基因位点时间校准分子系统发育分析，以及来自中始新世的海量化石标本，以此推断该类群的演化趋势。

主要结果
该类群起源于侏罗纪，距今约1.57亿年（Ma，百万年）；其中关键属类群钟罩藻属（Mallomonas）与合尾藻属（Synura）于早白垩世的1.3亿年前发生分化。钟罩藻属进一步分化为两个主要亚支，这一分化事件标志着被认为可助力细胞被膜上鳞片排布与组装的V形肋（V-rib）的演化。合尾藻属同样分化为两个主要亚支，将携带前伸棘刺的类群与鳞片本体带有龙骨突的类群相区分。约三分之一的化石物种已灭绝，剩余类群则均与现生同属物种存在关联。

研究结论
硅鳞藻的分类学体系主要依赖硅质结构的形态特征，其分类框架与分子系统发育分析结果整体吻合。已灭绝硅鳞藻的鳞片显著大于现生类群，这或为其灭绝的潜在诱因。与之相反，诸多与现生支系相关联的化石物种在中始新世时期体型更小，这或许反映了该地质时期典型温室气候状态下的体型演化趋势。