title: Assessing the evolutionary history of the class Synurophyceae (Heterokonta) using molecular, morphometric, and paleobiological approaches.
abstract: OBJECTIVE
  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.
  
  
  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.
authors.affiliation: Department of Botany, Connecticut College, New London, Connecticut 06320-4196 USA.
authors.forename: Peter A
authors.initials: PA
authors.lastname: Siver
authors.affiliation: Department of Biology, Chungnam National University, Daejeon, Korea.
authors.forename: Bok Yeon
authors.initials: BY
authors.lastname: Jo
authors.affiliation: Department of Biology, Chungnam National University, Daejeon, Korea.
authors.forename: Jong Im
authors.initials: JI
authors.lastname: Kim
authors.affiliation: Department of Biology, Chungnam National University, Daejeon, Korea.
authors.forename: Woongghi
authors.initials: W
authors.lastname: Shin
authors.affiliation: Department of Botany, Connecticut College, New London, Connecticut 06320-4196 USA.
authors.forename: Anne Marie
authors.initials: AM
authors.lastname: Lott
authors.affiliation: Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9, Canada.
authors.forename: Alexander P
authors.initials: AP
authors.lastname: Wolfe
mesh_terms: D001483:Base Sequence; D005075:Biological Evolution; D002138:Calibration; D048429:Cell Size; D005580:Fossils; D019684:Magnoliopsida / Q000033:anatomy & histology* / Q000166:cytology / Q000648:ultrastructure; D010163:Paleontology; D010802:Phylogeny; D013997:Time Factors
keywords: Mallomonas; Synura; Synurales; Synurophyceae; bristles; evolutionary history; fossil synurophytes; morphology; phylogeny; scales