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.