Data from: 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 that the Miocene and Eocene reports
represent competing hypotheses. In the absence of additional independent
and decisive fossil data, I explored the relative plausibility of these
two scenarios with Bayesian relaxed molecular clock methods under a range
of fossil calibration schemes. Although I found no support for the Eocene
fossil dates, the two major freshwater colonization events probably
occurred much earlier than previously thought—as early as the Paleocene
for Cyclotella, followed by an Eocene origin for the cyclostephanoid
lineage. Much of the extant freshwater diversity in both lineages traces
back to the Miocene, however, giving the impression of a single Miocene
origin. Efforts to infer the timescale of Thalassiosirales evolution more
accurately would benefit from a systematic reevaluation of the marine
fossil record and formal integration of fossil species into existing
phylogenetic hypotheses.