Matsunaga, Kelly K.S. , Tomescu, Alexandru M.F. .
Early Devonian Drepanophycus from the Beartooth Butte Formation of Wyoming: a focus on rooting structures.
The Beartooth Butte Formation hosts the only extensive Early Devonian flora known from western North America. Plants are preserved at two localities in northern Wyoming (Cottonwood Canyon and Beartooth Butte) as coalified or oxidized compressions and impressions, in dolomitic channel fill deposits interpreted as estuarine or fluvial sediments. The age of deposits, based on spore and fish biostratigraphy, is Late Lochkovian - Early Pragian at Cottonwood Canyon and middle-late Emsian at Beartooth Butte. Several taxa have been reported from the Beartooth Butte Formation (Psilophyton, Gosslingia, Rebuchia, Hostinella, BrĂ¶ggeria), but additional plant diversity awaits taxonomic treatment and formal publication. Among these, Drepanophycus, a taxonomically problematic basal lycophyte and one of the oldest known plants exhibiting stem-leaf-root organography, is among the most abundant plants in the Beartooth Butte Formation. At Cottonwood Canyon, Drepanophycus is found both as large isolated shoots, seemingly parautochthonous in massive and otherwise fossil-poor beds, and forming in situ preserved dense mats of shoots in 10-15 cm thick layers. The fossil material comprises shoot systems preserving details of branching, leaf morphology, and rooting structures. Leafy stems range from 8.0-26.0 mm in diameter and exhibit K-/H-type dichotomous branching. Preservation of the stele is common and vascular tissues are frequently the only type of tissue preserved in highly oxidized specimens. Phyllotaxis is helical, with lax gyres comprising ca. 6 leaves each; leaves are deltoid and vascularized. When laterally compressed, leaves are 2.5-5.0 mm long and 2.5-6.0 mm wide at the base; leaf length is generally proportional to stem size. Roots form small tufts that arise at irregular intervals along leafless axes, which are narrower (3.5-6.5 mm) than the leafy shoots. The roots are rarely >1 mm thick and have very thin steles. Within the tufts, roots exhibit dense isotomous branching proximally and less frequent branching distally. The genus Drepanophycus ranges in age from the Pragian into the Late Devonian. Dated in the Late Lochkovian - Early Pragian, the Cottonwood Canyon Drepanophycus is among the oldest records of the genus, along with fossils reported from Bathurst Island. Rooting structures are known as early as the Lochkovian, represented by simple, unbranched, negatively gravitropic axes produced by K-/H-branching in several zosterophylls. However, the oldest true roots are seen in Drepanophycus. In this context, the early age and frequent in situ preservation of the Cottonwood Canyon material can inform studies on early root evolution.
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1 - Humboldt State University, Department of 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
Location: Melrose/Riverside Hilton
Date: Monday, July 29th, 2013
Time: 8:15 AM
Candidate for Awards:Isabel Cookson Award,Maynard F. Moseley Award