Graham, Linda E. , Cook, Martha E. , Hanson, David T. .
Evidence that the enigmatic Paleozoic macrofossil Prototaxites formed from wind-rolled liverwort mats.
The enigmatic Paleozoic fossil known as Prototaxites has excited considerable interest because of its unusual anatomy and size–the largest organism present on land during the Late Silurian to Mid-Devonian, when early vascular plants were diversifying. Found in various world regions, the unbranched, tree trunk-shaped specimens range substantially in size, with some being 8.8 m long and 1.37 m in diameter, though others were much narrower at 20 cm or less. Anatomical analyses reveal that the Prototaxites body consisted mainly of unbranched, thick-walled, non-septate tubes that were 18-50 mm diameter and more than 2.5 mm long, occurring in “growth rings”. Though early investigators linked Prototaxites with various marine algae, more recent anatomical studies suggested that Prototaxites was a member of an enigmatic group of extinct plant-like Nematophytales, a lichen, or the fruiting body of a perennial basidiomycete fungus. The latter concept is supported by some stable carbon isotopic data, but enormous size and absence of spores, basidia, and clamp connections expected of a basidiomycete sporocarp, together with presence of chemical constituents similar to those of plants argue against the fungal hypothesis. We provide structural and isotopic evidence that Prototaxites formed when extensive asexual mats of thalloid, mixotrophic liverworts similar to modern Marchantia polymorpha were rolled-up like a rug by wind. This inference is consistent with phylogenetic and microfossil evidence for the existence of liverworts from the mid-Ordovician and our previous demonstration that abundant tubular parts of modern marchantioid liverworts resist decay. Our hypothesis explains variations in 1) fossil “trunk” diameter and length, 2) diameter and density of constituent tubes, 3) radial width of “growth rings,” 4) epidermal structure, and 5) stable C isotopic ratio, as well as growth ring borders, sparse occurrence of branching septate tubes, medullary spaces, presence of vascular plant fragments within the body, and absence of spores.
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1 - University of Wisconsin Madison, Department of Botany, Birge Hall, 430 Lincoln Drive, Madison, Wisconsin, 53706-1381, USA
2 - Illinois State University, Department of Biological Sciences, Campus Box 4120, Normal, Illinois, 61790-4120, USA
3 - University of New Mexico, Department of Biology, 167 Castetter Hall, Albuquerque, New Mexico, 87131, USA
Presentation Type: Oral Paper:Papers for BSA Sections
Location: Superior A/Cliff Lodge - Level C
Date: Monday, July 27th, 2009
Time: 1:45 PM