Abstract Detail


El-Abdallah, Samar [1], Tomescu, Alexandru [2].

Constructing a whole plant concept for an Early Devonian zosterophyll of the Beartooth Butte Formation (Wyoming).

Detailed reconstructions in the form of whole-plant concepts of Early Devonian plants are rare despite their evolutionary importance. While this interval was witness to the evolution of key land plant characters, Early Devonian fossil plant assemblages often consist of heavily fragmented material due to taphonomic processes. However, establishment of objective taxonomic schemes and inclusion of fossils in phylogenetic analyses require whole-plant concepts. The Beartooth Butte Formation of Wyoming is the only Early Devonian formation (specifically Lochkovian-Pragian, c. 415-409 Ma ago) in western North America that hosts abundant plant fossils, providing insight on Early Devonian floristics and biogeography. Some of the plant types present in this unit provide opportunities for detailed reconstructions of fossil species. We analyzed >600 fragmentary specimens (the largest 12cm long) representing a new type of zosterophyll, recovered at the Cottonwood Canyon locality of the Beartooth Butte Formation, in view of constructing a whole-plant concept for this plant. The new zosterophyll has smooth, slightly sinuous axes up to 6mm wide that branch every 10-40mm, but sometimes as closely as 6.8mm or as far apart as 57mm. The axes have low taper coefficient (~.02). At the base of plants axes radiate from a central point and the rhizomatous ones show K-branching. In more distal portions branching is quasi-isotomous and subaxillary tubercles are absent. Branches diverge at ~40° angles but curve in apical direction at the base becoming parallel with the main axis. The axes have circinate apices and dormant branch buds, relatively frequent, also exhibit different degrees of circination. The epidermal cells have central papillae and radiating ridges of wall material. The reniform bivalvate sporangia, up to 3.5 x 2.3mm, have thin, equal, unornamented valves. They are sparse, usually isolated, occasionally subopposite. The new zosterophyll is similar to the genera Oricilla and Tarella (in its smooth axes, branching morphology, sporangia, and absence of subaxillary tubercles) but does conform exactly either of them. Oricilla does not produce dormant branch buds and has sporangia abundantly distributed along the axes. While Tarella possesses dormant buds, their widely ranging morphologies are distinct from those of the new zosterophyll. Notably, K-branching has not been observed in Oricilla and Tarella. These differences justify erection of a new taxon, which adds to the diversity of the Cottonwood Canyon flora and of zosterophylls, in general. The relative paucity of sporangia among specimens of this new zosterophyll suggests a clonal growth strategy, similar to the lycopsid Sengelia described from the same layers. Of the >14 plant types identified at Cottonwood Canyon, the new zosterophyll is only the second to be reconstructed with a whole-plant concept. Very few other zosterophylls have been reconstructed as whole plants using detailed morphometric studies, which continues to frustrate conclusive taxonomic comparisons and phylogenetic analyses. Interestingly, detailed work on this new zosterophyll revealed the presence at Cottonwood Canyon of at least two other zosterophyll types unidentified previously. Detailed characterization of these plants will expand the known diversity of Early Devonian plant communities and provide additional data points for future phylogenic analyses.

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

whole-plant concept

Presentation Type: Oral Paper
Number: PB1001
Abstract ID:505
Candidate for Awards:Isabel Cookson Award,Maynard F. Moseley Award

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