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First published online January 5, 2010; doi:10.3732/ajb.0900322 American Journal of Botany 97: 268-275 (2010) © 2010 Botanical Society of America, Inc.

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Structural, physiological, and stable carbon isotopic evidence that the enigmatic Paleozoic fossil Prototaxites formed from rolled liverwort mats¹

² Department of Botany, University of Wisconsin, Madison, Wisconsin 53706-1381 USA ³ School of Biological Sciences, Illinois State University, Normal, Illinois 61790-4120 USA ⁴ Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131-0001 USA ⁵ School of Life Sciences, Arizona State University, Tempe, Arizona 85287-4501 USA

ABSTRACT

New structural, nutritional, and stable carbon isotope data may resolve a long-standing mystery—the biological affinities of the fossil Prototaxites, the largest organism on land during the Late Silurian to Late Devonian (420–370 Ma). The tree trunk-shaped specimens, of varying dimensions but consistent tubular anatomy, first formed prior to vascular plant dominance. Hence, Prototaxites has been proposed to represent giant algae, fungi, or lichens, despite incompatible biochemical and anatomical observations. Our comparative analyses instead indicate that Prototaxites formed from partially degraded, wind-, gravity-, or water-rolled mats of mixotrophic liverworts having fungal and cyanobacterial associates, much like the modern liverwort genus Marchantia. We propose that the fossil body is largely derived from abundant, highly degradation-resistant, tubular rhizoids of marchantioid liverworts, intermixed with tubular microbial elements. Our concept explains previously puzzling fossil features and is consistent with evidence for liverworts and microbial associates in Ordovician-Devonian deposits, extensive ancient and modern marchantioid mats, and modern associations of liverworts with cyanobacteria and diverse types of fungi. Our interpretation indicates that liverworts were important components of Devonian ecosystems, that some macrofossils and microfossils previously attributed to "nematophytes" actually represent remains of ancient liverworts, and that mixotrophy and microbial associations were features of early land plants.

Key Words: C isotope • liverwort • mixotrophy • Marchantia • marchantioid • Prototaxites • rhizoid

Received for publication 15 October 2009. Accepted for publication 3 December 2009.

FOOTNOTES

¹ The authors thank A. Pope-Heinrich, B. Smith, N. Neff, R. Powell, and R. Afandi for laboratory assistance and K. Elliot for art expertise. P. Gensel provided helpful discussion, and D. M. McConnell suggested a useful reference. The authors thank Lee Wilcox for making montages of the fossil transect images, for photographing the fossil slide, and for making a map of the fossil transects, as well as for the idea of including this supplemental data.

⁶ Author for correspondence (e-mail for correspondence: lkgraham{at}wisc.edu)

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