Timing marine-freshwater transitions in the diatom order Thalassiosirales

With species found throughout both marine and fresh waters, the diatom order Thalassiosirales is one of the most phylogenetically and ecologically diverse lineages of planktonic diatoms. A clear understanding of the timescale of Thalassiosirales evolution would provide novel insights into the rates and patterns of species diversification associated with major habitat shifts, as well as provide valuable context for understanding the age and evolutionary history of the model species, Cyclotella nana (= Thalassiosira pseudonana). The freshwater fossil record for Thalassiosirales is extensive, well characterized, and generally supportive of a Miocene origin for the major freshwater lineages. The marine record is, by comparison, more sparse and in many cases, unverified. The discovery of freshwater thalassiosiroids in Eocene sediments pushed the freshwater fossil record considerably further back in time, highlighting an apparent gap of some 30 million years. An alternative interpretation is ...海链藻目（Thalassiosirales）是一类广泛分布于海洋与淡水生境的硅藻类群，亦是浮游硅藻中系统发育与生态多样性最为丰富的支系之一。明确海链藻目演化的时间尺度，既能为解析与主要生境转变相关的物种分化速率与模式提供全新见解，也可为理解模式物种小环藻（Cyclotella nana，=假微型海链藻Thalassiosira pseudonana）的起源年代与演化历史提供重要参考。海链藻目的淡水化石记录较为丰富且特征清晰，整体支持其主要淡水支系起源于中新世的结论。相较之下，其海洋化石记录则更为零散，且多数未得到验证。在始新世沉积物中发现的淡水海链藻类群，将淡水化石记录的年代大幅提前，凸显出其间存在约3000万年的明显化石记录空白。另一种解读则为……