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Abstract Detail


Paleobotanical Section

Strayer, Sandra [1], Tomescu, Alexandru [2].

The spores of Psilophyton dawsonii from the Early Devonian of Quebec (Canada).

Psilophyton dawsonii, an Early Devonian trimerophyte, is preserved in abundance by cellular permineralization in carbonate concretions found in strata around Gaspe Bay (Quebec,Canada). New specimens of P. dawsonii from Gaspe Bay allowed for documentation of spore morphology and wall structure in this species. Spores are round to subtriangular, 49-86 µm in diameter. The laesurae of the trilete mark are 90% of the spore radius, with thickened,darker areas extending from the center to 63% of the radius. The spore wall appears baculate-echinate to rugulate in different focal planes, in light microscopy. Electron microscopy reveals the presence of two layers. The inner layer is laevigate, ca. 0.5 µm thick, whereas the outer layer, absent from contact areas, is up to 1.6 µm thick and possesses elaborate ornamentation. The outer layer becomes easily detached from the inner layer on the distal side of the spore, where it is also more markedly sculpted, whereas on the proximal side it is well attached to the inner layer and its ornamentation has a lower profile. The outer layer is formed by superposition of up to 5-6 sublayers each of which is discontinuous,with each successive sublayer occupying less surface area than the underlying sublayer. The basal 1-2 sublayers form a rugulate pattern; rugulae are ca. 0.5 µm tall, 0.4 µm wide, separating pits 0.4-0.9 µm in diameter. The upper layers are stacked to form irregular units with a stepped pyramid shape, topped by echinules (0.14 µm tall, 0.07 µm wide). In situ spores are known from five other Psilophyton species. Our measurements of spore size are in accord with previously published values for the species. The species best characterized previously is P. forbesii. P.dawsonii spores are similar to those of P.forbesii (albeit slightly smaller on average) in the rugulate pattern and stacked elements seen in their outer layer. However, in P. dawsonii this type of organization is much more elaborate. Observation of P. dawsonii sporangia in various stages of dehiscence and preservation show that (1) within and between individual sporangia, dehisced and undehisced, all spores are similar and mature; (2) degraded sporangia contain spores that are darker and distorted, as compared to those in well preserved sporangia. These observations are consistent with the proposed ecology of Psilophyton, whose reproductive mode is thought to have been ruderal-type, with a high output of spores released in bursts from simultaneously maturing sporangia.

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

Keywords:
Psilophyton
spore
spore wall
sporangium
Devonian
fossil
trimerophyte
Trimerophytina.

Presentation Type: Oral Paper:Papers for Sections
Session: 6
Location: Union A/Hyatt
Date: Monday, July 9th, 2012
Time: 8:45 AM
Number: 6001
Abstract ID:728


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