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A Silicified Carboniferous Lycopsid Forest in the Colorado Rocky

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A Silicified Carboniferous Lycopsid Forest in the Colorado Rocky geosciences Article A Silicified Carboniferous Lycopsid Forest in the Colorado Rocky Mountains, USA Mike Viney 1 , Robert D. Hickey 2 and George E. Mustoe 3,* 1 College of Natural Sciences Education and Outreach Center, Colorado State University, Fort Collins, CO 80423, USA; [email protected] 2 Arkansas Headwaters Recreation Area State Park, Salida, CO 81201, USA; [email protected] 3 Geology Department, Western Washington University, Bellingham, WA 98225, USA * Correspondence: [email protected] Received: 16 November 2019; Accepted: 5 December 2019; Published: 7 December 2019 Abstract: The 1930 discovery of Carboniferous lycopsid fossils in south central Colorado resulted in the naming of a new species of scale tree, Lepidodendron johnsonii (=Lepidophloios johnsonii (Arnold) DiMichele). Cellular structures of L. johnsonii axes and periderm are preserved in silica—an unusual mode of fossil preservation for Pennsylvanian lycopsid plant remains. The early reports on the Trout Creek lycopsid fossils focused on taxonomic and paleobotanical aspects. Our 2019 reinvestigation of the locality produced many new specimens and a wealth of new data from a variety of analytical methods. Optical microscopy, X-ray diffraction, scanning electron microscopy, energy dispersive electron spectroscopy, determination of specific gravity, and Loss on Ignition provide details of mineralization. Cell walls are preserved with very small fine quartz particles, and cell lumina are filled with microcrystalline quartz. Some cell exteriors are encrusted with euhedral quartz crystals. These multiple forms of quartz are evidence that petrifaction involved several episodes of silicification. The dark color of the fossil wood and siliceous matrix appears to be caused by traces of dispersed carbon, but 500 ◦C Loss on Ignition reveals that the fossil wood preserves only very small amounts of the original organic matter. Keywords: Carboniferous; Colorado; fossil wood; Lepidodendron; Lepidophloios; lycopsid; paleobotany; silicification 1. Introduction Lycopsids evolved as large arborescent forms, attaining maximum size during the Carboniferous, when they became one of the dominant members of low elevation swamp forests (Figure1). Mature trees exceeded 30 m, with trunk diameters of 1 meter or more. Arborescent lycopsids disappeared by the close of the Paleozoic, but herbaceous forms persist to the modern era. Extant genera Lycopodium and Isoetes rarely exceed 0.75 m in height. The taxonomy of arborescent lycophytes merits explanation. Trunk tissues bear distinctive leaf scars that allow various genera to be identified (e.g., Lepidodendron, Pleuromeia, Lepidophloios). Underground rooting structures are known by the genus name Stigmaria. Dispersed leaves are identified as Lepidophylloides. Intact reproductive organs are assigned to the form genus Lepidostrobus or Flemingites; fertile leaves that make up these cone-like structures are named Lepidostrobophyllum. Geosciences 2019, 9, 510; doi:10.3390/geosciences9120510 www.mdpi.com/journal/geosciences Geosciences 2019, 9, 510 2 of 18 Geosciences 2019, 9, x FOR PEER REVIEW 2 of 19 FigureFigure 1. 1. CarboniferousCarboniferous lycopsids. lycopsids. (A) Lepidodendronobovatum (A) Lepidodendron obovatum. Drawing. Drawing adapted from adapted [1]. ( fromB) Coal [1]. swamp(B) Coal reconstructionswamp reconstruction showing showingarborescent arborescent lycopsids lycopsids as the largest as the trees, largest circa trees, 1890 circa etching, 1890 etching,public . domainpublic domain. DuringDuring the the summer summer of 1930, geologist J. HarlanHarlan Johnson discovereddiscovered remnantsremnants ofof ananearly early PennsylvanianPennsylvanian fossilforest fossil forest near near Trout Trout Creek Creek Pass, Pass, in in Chafee Chafee County County Colorado, Colorado, USA USA (Figure (Figure 22).). BotanicalBotanical remains remains consist consist of of nu numerousmerous fragments fragments of of fossilized fossilized LepidodendronLepidodendron johnsonii ( = LepidophloiosLepidophloios johnsoniijohnsonii (Arnold)(Arnold) DiMichelle) DiMichele) lycopsid trunks. trunks. No No othe otherr genera genera have have been been found found at at this this locality. locality. TheseThese fossils fossils are are evidence evidence of of ancient ancient trees trees that that had had diameters diameters as as large large as as 0.6 0.6 m. m. The The lycopsid lycopsid remains remains werewere studied studied byArnold by Arnold [2], [2 ],who who provided provided a detailed a detailed description description of ofthe the site site and and a careful a careful analysis analysis of theof theanatomy anatomy of ofthe the silicified silicified tissue. tissue. Arnold Arnold named named the the material material as as a anew new species, species, LepidodendronLepidodendron johnsonniijohnsonnii.. Subsequently,Subsequently, thethe Trout Trout Creek Creek Pass Pass fossils fossils were includedwere included in various in paleobotanyvarious paleobotany textbooks textbooks(e.g., [3,4]). (e.g., The [3,4]). taxon The has taxo sincen has been since renamed been renamed as Lepidophloios as Lepidophloios johnsonii johnsonii(Arnold) (Arnold) DiMichele DiMichelle [5]. [5]. Our decision to reevaluate the Trout Creek Pass fossil locality was based on several factors. Arnold’s original 1940 report [2] is rich in detail, but the investigations focused on specimens that showed exceptionally good cellular preservation, which provided a basis for taxonomic studies. Arnold believed that these trunks had been preserved in an upright position, an interpretation that is reflected in his reconstruction of the landscape, showing numerous standing trees. Our field studies revealed that the well-preserved stems illustrated by Arnold are relatively rare, and that the majority of specimens represent trunk fragments that were preserved in a horizontal position. Rather than originating as a standing forest, the paleoenvironment may instead have been a floodplain that preserved an abundance of fluvially transported wood fragments. A second reason for our study is that knowledge of the geologic origin of the Colorado Rocky Mountains has improved considerably since 1940, the later awareness of the importance of plate tectonics being a particularly relevant example. Geosciences 2019, 9, 510 3 of 18 Geosciences 2019, 9, x FOR PEER REVIEW 3 of 19 Figure 2. Location Map. OurFinally, decision although to reevaluate the quality ofthe the Trout original Creek taxonomic Pass fossil studies locality is excellent, was based the workon several was based factors. on Arnold’stransmitted original light microscopy.1940 report This[2] is methodrich in detail provides, but detailed the investigations anatomical focused information, on specimens but very littlethat showedevidence exceptionally to understand good the mineral cellular processes preservation, that caused which the provided wood to a become basis for fossilized. taxonomic This studies. issue is Arnoldimportant, believed because that the these Trout trunks Creek had fossil been lycopsid preserve woodd in is an mineralized upright position, with microcrystalline an interpretation quartz. that isLycopsid reflected stem in his and reconstruction root structures of arethe abundantlandscape, in showing other locations, numerous principally standing preserved trees. Our in field three studies ways: revealedas silt or sandthat the casts well-preserved (internal and external),stems illustrated permineralized by Arnold with are calcium relatively carbonate rare, and in that coal the balls, majority and as ofcompressions specimens /representimpressions trunk in shale fragments and concretions. that were preserved The Trout Creekin a horizontal fossil locality position. therefore Rather appears than originatingto represent as unusual a standing conditions forest, ofthe fossilization. paleoenvironment We have may used instead polarized have light been optical a floodplain microscopy, that preservedscanning electron an abundance microscopy, of fluvially X-ray ditransportedffraction, density wood fragments. determination, and 500 ◦C loss on ignition to investigateA second the reason mineralogy for our in detail.study is that knowledge of the geologic origin of the Colorado Rocky Mountains has improved considerably since 1940, the later awareness of the importance of plate 2. Site Description tectonics being a particularly important example. Finally,The lycopsid although fossil the study quality site of is the located original in thetaxonomic east central studies part is ofexcellent, Chaffee thework County, CO,was inbased the oncentral transmitted portion oflight the microscopy. Mosquito Range, This justmethod west ofprov U.S.ides Highway detailed 24 anatomical/285. The site information, is on Colorado but Statevery Trustlittle evidence Land, and to aunderstand permit is required the mineral for specimenprocesses collecting.that caused Weathered the wood specimensto become fossilized. ranging in This size issuefrom smallis important, hand samples because to the small Trout boulder Creek sizefossil can lycopsid be found wood lying is mineralized on the surface with throughout microcrystalline a large quartz.meadow Lycopsid area. Dry stem creek and beds root to structures the north ofare the abundant meadow in contain other locations, eroded silicified principally specimens. preserved in three ways: as silt or sand casts (internal and external), permineralized with calcium carbonate in coal balls, and as compressions/impressions in shale and concretions.
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