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(American Journal of Botany. 2009;96:487-497.) doi: 10.3732/ajb.0800307 © 2009 Botanical Society of America, Inc.

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Three extant genera of freshwater thalassiosiroid diatoms from Middle Eocene sediments in northern Canada¹

² Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada ³ Botany Department, Connecticut College, New London, Connecticut 06320 USA

ABSTRACT

The evolutionary history of diatoms is only constrained partially by the fossil record. The timing of several key events, such as initial colonization of freshwater habitats by marine taxa, remains poorly resolved. Numerous specimens of the genera Cyclotella, Discostella, and Puncticulata (Ochrophyta: Thalassiosirales) have been recovered in Middle Eocene lacustrine sediments from the Giraffe Pipe locality in the Northwest Territories, Canada. These diatoms extend the fossil record of the family Stephanodiscaceae to at least 40 million years before present (Ma) and thus provide a new evolutionary milepost for the thalassiosiroid diatoms, an important clade of centric diatoms with global representation in both marine and freshwater environments. The quality of the fossil material enables detailed investigations of areolae, fultoportulae, and rimoportulae, revealing direct morphological affinities with a number of extant taxa. These observations extend the antiquity of several characters of phylogenetic importance within the thalassiosiroid diatoms, including the fultoportula, and imply that the entire lineage is considerably older than prior constraints from the fossil record, as suggested independently by several recent molecular phylogenies.

Key Words: Cyclotella • Discostella • Eocene • evolution • fossil diatoms • fultoportula • morphology • Puncticulata • Thalassiosirales

Received for publication 11 September 2007. Accepted for publication 21 October 2008.

FOOTNOTES

¹ The authors thank BHP Billiton Diamonds Inc. (Kelowna), the Geological Survey of Canada (Calgary), J. Westgate (University of Toronto), A. Lizarralde (Connecticut College), G. Braybrook (University of Alberta), and B. Perren (University of Toronto) for Greenland Puncticulata specimens. Comments by A. Alverson (Indiana University), M. Edlund (Science Museum of Minnesota), D. Harwood (University of Nebraska), two anonymous reviewers, and the editorial staff have greatly improved the manuscript. This work was supported by the Natural Sciences and Engineering Research Council (Canada) and the National Science Foundation (USA, NSF-DEB-0716606).

⁴ Author for correspondence (e-mail: awolfe{at}ualberta.ca)

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