Matsunaga, Kelly K.S. , Tomescu, Alexandru M.F. .
A whole-plant concept for an Early Devonian (Lochkovian-Pragian) lycophyte from the Beartooth Butte Formation (Wyoming).
The Beartooth Butte Formation contains the only Early Devonian flora known from western North America. The most prominent component of this flora is a drepanophycalean lycophyte, which is among the oldest representatives of the lineage. We present a whole-plant concept for this lycophyte based on morphometric analysis of >400 stem fragments and >110 rooting axes. The lycophyte is comprised of leafy stems up to 28mm thick, with narrow steles and a broad cortex that preserves little organic material, indicating aerenchymatous anatomy. Deltoid leaves are borne in loose gyres (8-9/gyre). Leaf length (1-6mm) shows positive correlation with stem width; leaf density is negatively correlated with stem width, with highest leaf densities on young stems. Infrequently occurring sporangia are cauline and sessile, 2.5-3.5mm in diameter. The plant exclusively produces K-branches, which yield leafy stems and smaller (2-8mm thick) leafless root-bearing axes. The 111 K-branches observed along a combined stem length totaling 19.7m give an average density of one branch/18cm; however, direct measurements show successive branches spaced 5-8cm apart. K-branches are either: (1) complete, with both branches developed; (2) incomplete, with one branch fully developed and the other forming a leafy bud; or (3) lateral buds. Rooting axes run parallel to leafy stems or vertically penetrate subjacent layers, producing lateral root tufts on consistently horizontal planes; the rooting axes, and not the roots, appear to be the primary foraging organ. The taphonomy and branching architecture of this plant indicate a mat-forming, prostrate habit. The disproportionately narrow steles and broad aerenchymatous cortex (occupying up to 99% of stem volume) suggest the shoot system was not self-supporting and only terminal shoots may have been erect, and is consistent with adaptation to intervals of submergence. Successive stem mats preserved in situ indicate multiple episodes of colonization and burial, consistent with high flood incidence. Nearly half the total stem length exhibits colonization by encrusting microconchid tubeworms, whose size distributions indicate extended intervals of recruitment, suggesting prolonged submergence episodes. Although microconchid colonization increases with stem age, consistent with longer submergence times for older stems, absence of microconchids from 35% total length of the oldest stems indicates that the plants were not growing in an aquatic habitat. The paucity of sporangia coupled with frequent reiterative branching and mat-forming habit indicate that clonality was the primary mode of propagation. Taken together, these data on morphology, architecture, growth habit, and ecology make this plant potentially the best-understood early lycophyte to date.
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1 - Humboldt State University, Biological Sciences, 1 Harpst St., Arcata, CA, 95521, United States
2 - Humboldt State University, Department Of Biological Sciences, 1 Harpst Street, Arcata, CA, 95521, USA
Presentation Type: Oral Paper:Papers for Sections
Date: Monday, July 28th, 2014
Time: 8:30 AM
Candidate for Awards:Isabel Cookson Award,Maynard F. Moseley Award