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Abstract Detail


Paleobotanical Section

Hoffman, Laurel [1], Tomescu, Alexandru [2].

An early origin of secondary growth in the Early Devonian of Gaspé.

Secondary growth may have evolved independently in as many as five plant lineages. The earliest occurrences are seen in Middle Devonian euphyllophytes - progymnosperms and cladoxyllaleans. By the Late Devonian, sphenopsids, some ferns, and lycopsids had also evolved secondary growth. Secondary tissues arising from a vascular cambium are recognizable based on a suite of characters, including presence of axial and radial components, radially aligned cells as seen in cross sections, and addition of radial cell files by multiplicative divisions. Investigation of anatomically preserved plants in the Early Devonian (Emsian, ca. 405 Ma) Battery Point Formation (Gaspé, Quebec) has yielded a specimen exhibiting features of the secondary growth syndrome. The specimen consists of a cylinder of xylem 1.7 mm in diameter, devoid of extraxylary tissues, and is associated with Psilophyton dawsonii material. Although its anatomy is reminiscent of P. dawsonii, the specimen cannot be indisputably assigned to that species due to its lack of extraxylary tissues and because it falls outside the range of defining parameters of P. dawsonii. The specimen is protostelic with a small amount of crushed metaxylem around the central protoxylem. Primary tissues make up only a small fraction of the xylem, and are surrounded by a significant amount of tissue with secondary growth characteristics: conspicuous radial arrangement of tracheids and numerous instances of multiplicative division. The specimen is compressed laterally and features radial ruptures representing rays (probably uniseriate) which, due to their parenchymatous nature, provided the weakest structural planes that cleaved during diagenetic compression. Together, these features are indicative of secondary growth. Irrespective of its exact taxonomic affinities, anatomy and age place this plant at the base of the euphyllohyte clade, demonstrating that secondary growth had evolved prior to the divergence of cladoxyllaleans and progymnosperms. The Gaspé plant demonstrates another independent origin of secondary growth, pushing the earliest emergence of this developmental feature down into the Early Devonian.

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1 - Humboldt State University, Department of Biological Sciences, 1 Harpst St., Arcata, CA, 95521, USA
2 - Humboldt State University, Department of Biological Sciences, 1 Harpst Street, Arcata, CA, 95521, USA

Keywords:
fossil
Devonian
Trimerophyte
secondary growth
Psilophyton
Euphyllophyte.

Presentation Type: Oral Paper:Papers for Sections
Session: 06
Location: Lindell C/Chase Park Plaza
Date: Monday, July 11th, 2011
Time: 9:00 AM
Number: 06002
Abstract ID:491


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