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Colston, Courtney [1], Landaw, Kevin [2], Tomescu, Alexandru [3].

An early snapshot of plant-herbivore interactions from the Early Devonian of Gaspé (Québec, Canada).

The Lower Devonian (Emsian) Battery Point Formation of the Gaspé Peninsula (Québec, Canada) hosts rich assemblages of compressed and permineralized fossils. Many of these fossils represent trimerophytes, a plexus of early tracheophytes that form the base of the euphyllophyte clade. We describe a new trimerophyte from the Battery Point Formation, preserved by calcium carbonate permineralization and studied in serial sections using the cellulose acetate peel technique. The new plant has slender axes (2.5 mm thick) with a centrarch primary xylem that consists of tracheids (up to 30 μm diameter) with Psilophyton-type pitting. In the inner cortex, groups of 2-10 fibers with thick secondary walls form a discontinuous layer around the xylem. The sclerenchymatous outer cortex forms a jagged outline around the axis. This is due to numerous spine-like protrusions that have decurrent bases, which form short ridges along the axis. The spines fall into two morphological types. One type consists of shorter (0.9-1.9 mm), sturdier, barb-shaped spines with apically curved tips. The second type consists of longer (1.4-2.2 mm), straighter spines with stouter bases that diverge obliquely from the axis at relatively sharp angles. Some of the longer spines occur in pairs with laterally adjacent bases. The two types of spines show no regular positional pattern of arrangement along the axis or with respect to each other. When traced in serial cross sections, the spines demonstrate sharp, finely tapering tips. Anatomically, both spine types have a three cell-thick outer layer of sclerenchyma with thick secondary walls that is continuous with the outer cortex of the axis. Internally, they contain cells with thin, dark, potentially highly lignified walls. The anatomy of the new plant, especially its centrarch primary xylem, sclerenchymatous outer cortex, and characteristic tracheids, places it in the genus Psilophyton. As the best-characterized trimerophyte, Psilophyton occupies a key position in the euphyllophyte fossil record. Compared to previously described Psilophyton species, the new plant differs in possessing consistently arranged fibers in the inner cortex and two morphologically distinct spine types, which indicate that it may represent a new species in this genus. Importantly, the anatomy and morphology of the spines of this new plant are consistent with roles in anti-herbivore defense. Furthermore, the presence of two morphologically distinct types of spines suggests that plant-animal interactions had reached a previously unrecognized level of complexity by 400 million years ago, in the Early Devonian.

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1 - Humboldt State University, Biological Sciences, Arcata, CA, 95521, USA
2 - Humboldt State University, Biological Sciences, Arcata, CA, 95521
3 - Humboldt State University, Department Of Biological Sciences, 1 Harpst Street, Arcata, CA, 95521, United States

plant-animal interactions
plant defense
early tracheophytes.

Presentation Type: Oral Paper
Session: PL3, Paleobotany: Honoring Fran Hueber - Session 1
Location: /
Date: Tuesday, July 20th, 2021
Time: 11:45 AM(EDT)
Number: PL3005
Abstract ID:396
Candidate for Awards:Isabel Cookson Award,Maynard F. Moseley Award

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