A Silicified Carboniferous Lycopsid Forest in the Colorado Rocky

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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.

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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.

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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. The Trout Creek fossil locality therefore appears to represent unusual conditions of fossilization. We have used polarized light optical microscopy, scanning electron microscopy, X-ray diffraction, density determination, and 500 °C loss on ignition to investigate the mineralogy in detail.

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2. Site Description The lycopsid fossil study site is located in the east central part of Chaffee County, CO, in the central portion of the Mosquito Range, just west of U.S. Highway 24/285. The site is on Colorado State Trust Land, and a permit is required for specimen collecting. Weathered specimens ranging in size fromGeosciences small2019 hand, 9, 510 samples to small boulder size can be found lying on the surface throughout a 4large of 18 meadow area. Dry creek beds to the north of the meadow contain eroded silicified specimens.

3. Geologic Geologic Setting Setting The Trout Creek fossil locality provides a glimpse into the complex geologic history of the Rocky Rocky Mountain region.region. WeWe are are discussing discussing the the tectonic tectonic history history of the of site the in site detail in todetail explain to explain how the how remains the remainsof a lowland of a lowland swamp cameswamp to became preserved to be preserved in a high in mountain a high mountain meadow meadow (Figure3 ).(Figure 3).

Figure 3. Trout Creek lycopsid fossil site. ( A). Bill Evans examines the fossil specimens that occur as surface material.material. ( B()B Irregular) Irregular shaped shaped specimen specimen showing showing layers oflayers compressions of compressions/impressions/impressions exhibiting exhibitingleaf scars oriented leaf scars in orient diﬀerented in directions. different directions.

The TroutTrout Creek Creek fossil fossil locality locality occurs occurs in an in expansive an expansive wildflower wildflower meadow meadow that lies that at an lies elevation at an elevationof 2800 m of (9200 2800 ft)m (9200 in the ft) southern in the southern Rocky Rocky Mountains. Mountains. This settingThis setting is markedly is markedly different different from from the thepaleogeographic paleogeographic conditions conditions that that existed existed when when the th lycopside lycopsid forest forest was was alive. alive. OneOne importantimportant factor involves global tectonics [[6].6]. During During the the early early Paleozoic, breakup of the supercontinent Pannotia resulted in smaller continents that included included La Laurentiaurentia (composed (composed of of North North America America and and Greenland), Greenland), Gondwana (South America, Africa, Antarctica, Antarctica, India, India, and and Australia), Australia), Baltica Baltica (Europe) (Europe) and and Siberia. Siberia. The edges ofof thesethese newnew continents continents were were typically typically passive passive marginal marginal basins, basins, which, which, accompanied accompanied by seaby sealevel level rise, rise, caused caused continental continental interiors interiors to be to flooded be flooded by shallow by shallow seas. seas. This global global geography geography set set the the stage stage for for the the development development of the of theTrout Trout Creek Creek fossil fossil forest. forest. An Animportant important step step was was thethe collision collision between between Gondwana Gondwana and and Laurentia Laurentia during during the the middle middle Carboniferous Carboniferous to early Permian, Permian, causing extensive orogeny in No Northrth America (Figure 44).). InIn the early Pennsylvanian, the onset ofof upliftuplift causedcaused marine marine regression, regression, leaving leavin behindg behind the the shale, shale, sandstone, sandstone, and and limestone limestone of the of theBelden Belden and Glenand EyrieGlen FormationsEyrie Formations of Central of Cent and Easternral and Colorado,Eastern Colorado, respectively. respectively. These sediments These sedimentsare exposed are in exposed the southern in the Mosquitosouthern Mosquito Range at TroutRange Creekat Trout Pass. Creek This Pass. sea This level sea retreat level marked retreat markedthe end ofthe more end thanof more 200 than million 200 years million of marineyears of sedimentation marine sedimentation in Colorado. in Colorado. The newly The emerged newly emergedlowlands lowlands provided aprovided favorable a habitatfavorable for coastalhabitat swamps,for coastal including swamps, the including Trout Creek the site.Trout A fewCreek. other A fewPennsylvanian other Pennsylvanian plant sites plant have sites been have reported been fromreported central from Colorado, central Colorado, but these occurrencesbut these occurrences primarily primarilyconsist of foliageconsist impressionsof foliage impressions [7,8]. [7,8]. Early Pennsylvanian regional regional uplift uplift led led to to the rise of the Ancestral Rocky Mountains (ARM), which comprisedcomprised two two island island massifs: massifs: Frontrangia, Frontrangia, in what in is nowwhat central is now Colorado, central and Colorado, Uncompaghria and Uncompaghriato the southwest. to the The southwest. bedrock source The bedrock for these source ranges for included these ranges Precambrian included rocks, Precambrian and Paleozoic rocks, andlimestone Paleozoic and limestone dolomites. and Although dolomites. the ARM Although rose tothe elevations ARM rose of to approximately elevations of 3000approximately m (10,000 ft)3000 by 300 Ma, within 50 Ma the mountain range had been largely obliterated by erosion, producing extensive alluvial fans that extended into adjacent floodplains. By the close of the Permian, the ARM had been almost completely eroded, their bedrock foundation now buried by their own debris. In the late Paleozoic, the Laurentia/Gondwana contact zone lay close to the equator, with tropical and semitropical climate-favored development of lush plant growth in the coastal lowlands, producing extensive coal Geosciences 2019, 9, x FOR PEER REVIEW 5 of 19

m (10,000 ft) by 300 Ma, within 50 Ma the mountain range had been largely obliterated by erosion, producing extensive alluvial fans that extended into adjacent floodplains. By the close of the Permian, Geosciencesthe ARM2019, had9, 510 been almost completely eroded, their bedrock foundation now buried by their own 5 of 18 debris. In the late Paleozoic, the Laurentia/Gondwana contact zone lay close to the equator, with tropical and semitropical climate-favored development of lush plant growth in the coastal lowlands, swamps.producing In the extensive late Paleozoic coal swamps. Era, these In the inland late Paleozoic seas retreated, Era, these and inland regions seas that retreated, had once and received regions thick limestonethat had reef once deposits received were thick nowlimestone sites reef of deposition deposits were for now ﬂuvial sites sandof deposition and shale, for fluvial with organicsand and debris locallyshale, abundant. with organic debris locally abundant.

FigureFigure 4. The4. The collision collision of of the the supercontinentssupercontinents Laurasia Laurasia and and Gondwana Gondwana resulted resulted in the in rise the of rise the of the AppalachianAppalachian Mountains, Mountains, and and the the appearanceappearance of of the the Ancestral Ancestral Rocky Rocky Mountains Mountains as offshore as oﬀshore islands. islands. ThisThis illustration illustration represents represents Pennsylvanian Pennsylvanian tectonic positions positions at atapproximately approximately 300 300Ma. Ma.

TheThe present present Rocky Rocky Mountains Mountains areare a result of of the the Laramide Laramide Orogeny, Orogeny, when when late lateCretaceous Cretaceous and and Paleocene faulting brought crystalline rocks to the surface to form tilt-blocks and anticlinal ridges [9]. Paleocene faulting brought crystalline rocks to the surface to form tilt-blocks and anticlinal ridges [9]. Erosion of these highlands led to the accumulation of Eocene sediments in adjacent basins. Lesser Erosion of these highlands led to the accumulation of Eocene sediments in adjacent basins. Lesser uplift uplift events continued during the Miocene and Oligocene, accompanied by pulses of sedimentation. eventsDuring continued this period, during North the America Miocene became and recognizab Oligocene,le as accompanied a discrete continent, by pulses with ofrotation sedimentation. and Duringnorthward this period, transport North (Figure America 5). A becamedetailed recognizabledescription of asthe a regional discrete geology continent, is provided with rotation in a and northwardsupplementary transport material. (Figure 5). A detailed description of the regional geology is provided in the supplementaryGeosciences 2019, 9, x material. FOR PEER REVIEW 6 of 19

Figure 5. MapMap series series showing showing relative relative latitude latitude of oflycopsid lycopsid fossil fossil forest forest over over time. time. Figure Figure adapted adapted from from[10]. [10].

This tectonic history explains how the Trout Creek fossil forest originated as a lowland swamp at low latitude, and why the plant remains are now found in a high alpine meadow far to the north of the original location. This complex geologic history can be understood in more detail from the strata exposed in the present vicinity of the Trout Creek Pass. The Trout Creek fossil site lies within a thick sequence of Paleozoic sedimentary rocks that overlay Proterozoic granitic basement rocks. The generalized stratigraphy is shown in Figure 6. The regional structure was deformed, primarily as a result of the late Cretaceous to early Tertiary Laramide Orogeny. Individual Paleozoic Formations are typically separated by unconformities, evidence that they were produced by discrete episodes of sedimentation that were separated by prolonged intervals of non-deposition. The Belden Shale Formation, which contains the lycopsid fossils, is poorly exposed in the study area because of the susceptibility of the sediment to weathering. The rock consists of dark gray, brownish gray, and black shale. The general thickness of the strata is estimated to be 260 m (850’), but the Belden Shale reaches a maximum thickness of 520 m (1700’) to the north of the fossil forest site. The formation has a transitional contact with the overlying Minturn Formation, which consists of sandstone and shale, with lesser amounts of siltstone, limestone, and dolomite. The combination of clastic and carbonate beds in the Belden Shale and Minturn Formations are evidence of Pennsylvanian transgression and regression associated with the tectonic unification of Laurentia and Gondwana. The Belden Shale Formation contains marine invertebrate fossils and terrestrial plants, occurrences that suggest that the lycopsid forest occupied a coastal lowland.

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This tectonic history explains how the Trout Creek fossil forest originated as a lowland swamp at low latitude, and why the plant remains are now found in a high alpine meadow far to the north of the original location. This complex geologic history can be understood in more detail from the strata exposed in the present vicinity of the Trout Creek Pass. The Trout Creek fossil site lies within a thick sequence of Paleozoic sedimentary rocks that overlay Proterozoic granitic basement rocks. The generalized stratigraphy is shown in Figure6. The regional structure was deformed, primarily as a result of the late Cretaceous to early Tertiary Laramide Orogeny. Individual Paleozoic Formations are typically separated by unconformities, evidence that they were produced by discrete episodes of sedimentation that were separated by prolonged intervals of non-deposition. The Belden Shale Formation, which contains the lycopsid fossils, is poorly exposed in the study area because of the susceptibility of the sediment to weathering. The rock consists of dark gray, brownish gray, and black shale. The general thickness of the strata is estimated to be 260 m (850’), but the Belden Shale reaches a maximum thickness of 520 m (1700’) to the north of the fossil forest site. The formation has a transitional contact with the overlying Minturn Formation, which consists of sandstone and shale, with lesser amounts of siltstone, limestone, and dolomite. The combination of clastic and carbonate beds in the Belden Shale and Minturn Formations are evidence of Pennsylvanian transgression and regression associated with the tectonic uniﬁcation of Laurentia and Gondwana. The Belden Shale Formation contains marine invertebrate fossils and terrestrial plants, occurrences that suggestGeosciences that 2019 the, 9, x lycopsid FOR PEER REVIEW forest occupied a coastal lowland. 7 of 19

Figure 6.FigureGeneralized 6. Generalized stratigraphy. stratigraphy. A A detailed detailed description of ofthe the regional regional geology geology is provided is provided in a in a separateseparate data ﬁle. data file.

4. Previous Research Lycopsids were amongthe first large trees in the Earth’s history, preceded only by the Late Devonian Archaeopteris. They were the tallest members of Carboniferous coal swamps. Lycopsid remains are abundantly preserved as casts and in in calcareous coal balls, where fossilized organs include wood, rootlets, and foliage. Because of their evolutionary importance, lycopsid fossil have been the subject of considerable research. These studies have covered a vast scale, a span that ranges across entire fossil forests [11,12], individual trunks [13], root networks [14,15], and spores [16]. Arborescent lycopsids were well known to paleontologists by the late 19th century (e.g., [17–19]), but the 1930 discovery of the Trout Creek fossil locality captured attention because of the silica mineralization, a contrast to the calcareous preservations that had previously been studied. In addition, the excellent anatomical preservation provided an opportunity to better understand the cellular architecture. The site was first mentioned in print by Johnson in 1932 [20]; in 1940, Arnold [2] produced a monograph that is notable for its detailed description of geological as well as paleobotanical characteristics of the Trout Creek fossil locality. For our report, we are not providing a detailed summary of the anatomical characteristics of the silicified tissues. Instead, readers are

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4. Previous Research Lycopsids were among the first large trees in the Earth’s history, preceded only by the Late Devonian Archaeopteris. They were the tallest members of Carboniferous coal swamps. Lycopsid remains are abundantly preserved as casts and in in calcareous coal balls, where fossilized organs include wood, rootlets, and foliage. Because of their evolutionary importance, lycopsid fossil have been the subject of considerable research. These studies have covered a vast scale, a span that ranges across entire fossil forests [11,12], individual trunks [13], root networks [14,15], and spores [16]. Arborescent lycopsids were well known to paleontologists by the late 19th century (e.g., [17–19]), but the 1930 discovery of the Trout Creek fossil locality captured attention because of the silica mineralization, a contrast to the calcareous preservations that had previously been studied. In addition, the excellent anatomical preservation provided an opportunity to better understand the cellular architecture. The site was first mentioned in print by Johnson in 1932 [20]; in 1940, Arnold [2] produced a monograph that is notable for its detailed description of geological as well as paleobotanical characteristics of the Trout Geosciences 2019, 9, x FOR PEER REVIEW 8 of 19 Creek fossil locality. For our report, we are not providing a detailed summary of the anatomical referredcharacteristics to the oforiginal the silicified description tissues. [2] and Instead, two readerslater summaries are referred [3,4]. to General the original characteristics description are [2] reviewedand two laterin this summaries paper as a [ 3foundation,4]. General for characteristics understanding are the reviewed microscopic in this images paper and as a discussions foundation forof fossilizationunderstanding processes. the microscopic images and discussions of fossilization processes. InIn 1938, 1938, Gould Gould and and Johnson Johnson visited visited the the site site to to examine examine and and collect collect fossil fossil material material [3]. [3]. Fossils Fossils were were foundfound to consistconsist ofof portions portions of silicifiedof silicified trunks trunks and numerousand numerous shale fragmentsshale fragments that preserved that preserved imprints imprintsof Lepidodendron of Lepidodendronand Lepidophloios and Lepidophloios. Johnson. reported Johnson thatreported most ofthat the most cell structureof the cell breaks structure down breaks when downtreated when with treated hydrofluoric with hydrofluoric acid, evidence acid, that evidence these silicified that these tissues silicified preserved tissues very preserved little relict very organic little relictmatter. organic As a result,matter. cellulose As a result, acetate cellulose peels, acetate such as peel ares, used such for as studying are used coalfor studying balls, were coal found balls, to were have foundlittle value.to have Arnold little value. used detailedArnold observationsused detailed of observations thin sections of representing thin sections axes representing to describe axes a new to describespecies ofa new lycopsid, speciesLepidodendron of lycopsid, Lepidodendron johnsonii, named johnsonii in honor, named of J. in Harlan honor of Johnson, J. Harlan who Johnson, announced who announcedthe discovery the ofdiscovery the Trout of Creek the Trout fossil Creek deposit fossil in 1932. deposit The in mineralized 1932. The mi stemneralized section stem collected section by collectedArnold consists by Arnold of a consists pith area, of primarya pith area, xylem, primary secondary xylem, xylem, secondary and somexylem, cortex and some tissue. cortex The phloem, tissue. Theouter phloem, cortex outer tissue cortex and periderm tissue and are periderm not present are no int this present specimen in this (Figure specimen7). Like (Figure most 7). axes Like found most axesby Arnold found atby the Arnold site, theat the pith site, area the is filledpith area with is debris filled and with structures debris and he identifiedstructuresas heStigmaria identifiedroots. as StigmariaArnold interpreted roots. Arnold the interpreted presence of theStigmaria presenceroots of inStigmaria the pith roots as evidence in the pith that as the evidence roots of lycopsidthat the rootstrees of grew lycopsid around trees and grew through around dead and trunks through that were dead buried trunks within that were the soil. buried We didwithin not observethe soil. rootsWe didin specimensnot observe that rootsin we collected. specimens The that poor we collected. preservation The ofpoor the preservation pith and cortex of the are pith evidence and cortex of rapid are evidencedecay of of these rapid poorly decay lignified of these tissuespoorlylignified following tissues the death following of the the lycopsid. death of Eventhe lycopsid. the well-lignified Even the well-lignifiedperiderm often periderm exhibits of someten exhibits decay. some decay.

FigureFigure 7. 7. LycopsidLycopsid stem stem anatomy. anatomy. (A (A) )Transverse Transverse view view of of lycopsid lycopsid stem, stem, showing showing a a small small central central axis axis composedcomposed of of pith, pith, primary primary xylem, xylem, and and secondary secondary xylem xylem (wood). (wood). The The small small central central axis axis is is surrounded surrounded by a thick outer bark composed of inner and outer cortex enveloped by fibrous periderm. The periderm provided the structural stability for the lycopsid tree. (B) Central axis of L. johnsonii specimen collected by Arnold, now at Colorado School of Mines Museum of Geology, Golden, CO USA. The cortex, which originally comprised up to 80–90% of the trunk volume, is not preserved. The primary xylem ring is partially preserved, whereas the secondary xylem ring is intact. Arrows denote circular structures that Arnold interpreted as lycophyterootlets (Stigmaria) that invaded the decayed pith zone prior to fossilization.

5. Materials and Methods Scanning electron photomicrographs were obtained using a Vega3 SEM equipped with an Oxford X-ray detector running AzTek software. Optical photomicrographs were taken using a five megapixel digital microscope camera mounted on a Zeiss petrographic microscope. X-ray diffraction patterns are from a Rigaxu Geigerflex diffractometer, using nickel-filtered Cu Kα radiation. Loss on ignition measurements were made by measuring the weight loss of approximately 5 g of powdered fossil wood after heating for eight hours at 500 °C in an electric laboratory furnace. Samples were heated overnight at 100 °C to remove adsorbed moisture prior to LOI determination.

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by a thick outer bark composed of inner and outer cortex enveloped by ﬁbrous periderm. The periderm provided the structural stability for the lycopsid tree. (B) Central axis of L. johnsonii specimen collected by Arnold, now at Colorado School of Mines Museum of Geology, Golden, CO USA. The cortex, which originally comprised up to 80–90% of the trunk volume, is not preserved. The primary xylem ring is partially preserved, whereas the secondary xylem ring is intact. Arrows denote circular structures that Arnold interpreted as lycophyte rootlets (Stigmaria) that invaded the decayed pith zone prior to fossilization.

5. Materials and Methods Scanning electron photomicrographs were obtained using a Vega3 SEM equipped with an Oxford X-ray detector running AzTek software. Optical photomicrographs were taken using a five megapixel digital microscope camera mounted on a Zeiss petrographic microscope. X-ray diffraction patterns are from a Rigaxu Geigerflex diffractometer, using nickel-filtered Cu Kα radiation. Loss on ignition measurements were made by measuring the weight loss of approximately 5 g of powdered fossil wood after heating for eight hours at 500 ◦C in an electric laboratory furnace. Samples were heated overnight at 100 ◦C to remove adsorbed moisture prior to LOI determination.

6. Results

1. Anatomically well-preserved lycopsid stems and stem fragments (Figure8A). These are the materials that have been described by previous investigators [2–4,20]. The cortex, which originally comprised up to 80–90% of the trunk volume, is not preserved. Only the lignified cells, making up the small xylem ring of the central axes and portions of the outer periderm, are well-preserved in silica; 2. Wood fragments that show evidence of gravitational settling. This material consists of silicified wood that owes its dark color to relict organic matter, and specimens that have been bleached to a light color (Figure8C,D). These specimens contain mineralized tissue fragments that occupy only a portion of the original volume of the wood, as indicated by the outer cast surface. Thin sections show that the remaining space is typically filled with microcrystalline quartz that precipitated from Si-bearing groundwater, as evidenced by the absence of cellular remains in these regions; 3. Mixtures of microcrystalline quartz and coalified organic matter, commonly showing intense deformation (Figure8D,E).

The casts do not contain visible clastic sediment, suggesting that, during diagenesis, the outer cast surface remained as an intact barrier that prevented the inﬂux of solid material from the surrounding environment. Geosciences 2019, 9, x FOR PEER REVIEW 9 of 19

6. Results

6.1. Mineral Features Specimens at the Trout Creek fossil locality typically consist of lycopsid trunk fragments whose exterior surfaces preserve the texture of thestem’s leaf-scarred outer layer (Figure 3B). The interiors of these specimens are variable, but the fossils typically represent a sedimentary matrix that preserves a wood-shaped space, enclosing an internal filling that consistsof three basic types: 1. Anatomically well-preserved lycopsid stems and stem fragments (Figure 8A). These are the materials that have been described by previous investigators [2–4,20]. The cortex, which originally comprised up to 80–90% of the trunk volume, is not preserved. Only the lignified cells, making up the small xylem ring of the central axes and portions of the outer periderm, are well- preserved in silica; 2. Wood fragments that show evidence of gravitational settling. This material consists of silicified wood that owes its dark color to relict organic matter, and specimens that have been bleached to a light color (Figure 8C,D). These specimens contain mineralized tissue fragments that occupy only a portion of the original volume of the wood, as indicated by the outer cast surface. Thin sections show that the remaining space is typically filled with microcrystalline quartz that precipitated from Si-bearing groundwater, as evidenced by the absence of cellular remains in these regions; 3. Mixtures of microcrystalline quartz and coalified organic matter, commonly showing intense deformation (Figure 8D,E). The casts do not contain visible clastic sediment, suggesting that, during diagenesis, the outer Geosciencescast surface2019, 9, 510remained as an intact barrier that prevented the influx of solid material from the 9 of 18 surrounding environment.

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FigureFigure 8. Examples 8. Examples of various of various types types of preservation of preservation of fossil of fossil lycopsid lycopsid wood. wood. (A) Small(A) Small central central lycopsid axis withlycopsidaxis secondary with xylem.secondary (B) xylem. Transverse (B) Transverse view of tracheidsview of tracheids in secondary in secondary xylem. xylem. (C) Layers(C)Layers of bark compressionof bark compression/impressions/impressions associated associated with fragmented with fragmented pieces of pieces silicified of silicified wood orwood fiber or oriented fiber in differentoriented directions, in different naturally directions, bleached, naturally and bleach showinged, and signs showing of gravitational signs of gravitational settling. (settling.D) Fragmented (D) Fragmented pieces of lycopsid tissue that have settled to the bottom of a space created when the pieces of lycopsid tissue that have settled to the bottom of a space created when the original wood mass original wood mass deteriorated. (E,F) Photomicrographs of thin sections of carbonaceous sediment deteriorated. (E,F) Photomicrographs of thin sections of carbonaceous sediment created by intense created by intense deformation of coalified lycopsid wood. Dark, carbon-rich layers are interspersed deformationwith thin ofbands coalified of crystalline lycopsid quartz. wood. Dark, carbon-rich layers are interspersed with thin bands of crystalline quartz. 6.2. Optical Microscopy 6.2. Optical Microscopy Specimens show abundant evidence of quartz mineralization. Specimens that show a well- Specimenspreserved cell show structure abundant contain a evidencemicrocrystalline of quartz quartz mineralization.filling cell lumina (Figure Specimens 9). Cell walls that are show a well-preservedtypically opaque cell structure in polarized contain light aviews. microcrystalline There are three quartz possible filling explanations cell lumina for (Figure this. One9). is Cell that walls are typicallythe cell walls opaque are composed in polarized of relict light organic views. matter. There areThis three interpretation possible explanationsconflicts with loss for this.on ignition One is that the celldata walls that reveal are composed that relict ofcarbon relict has organic low abundance matter. Thisin the interpretation fossil wood (Table conflicts 1). A second with loss possibility on ignition data thatis that reveal the cell that walls relict are carbon mineralized has low with abundance amorphous in opal the (opal-A). fossil wood The (TableLower1 Pennsylvanian). A second possibility age of is that thethe celldeposit walls makes are mineralizedan opaline composition with amorphous unlikely, opal because (opal-A). opal-A The transforms Lower Pennsylvanian relatively rapidly age to of the depositopal-CT, makes and an opalinefrom there composition to quartz; opalized unlikely, wood because is common opal-A in transforms Cenozoic relativelyformations, rapidly but virtually to opal-CT, unknown in older deposits [21]. We prefer the third interpretation, which is that the cell walls appear and from there to quartz; opalized wood is common in Cenozoic formations, but virtually unknown to be isotropic because they consist of aggregates of extremely small quartz particles. Because in olderbirefringence deposits is [21 related]. We to prefer specimen the thickness, third interpretation, in 30 micron whichthick petrographic is that the slides, cell walls the cell appear walls to be isotropicappear because dark under they consistpolarized of light aggregates because of the extremely individual small quartz quartz particles particles. are so small Because that birefringence they do is relatednot show to specimen interference thickness, colors. in This 30 micron interpretati thickon petrographic is supported slides, by SEM the cell images walls that appear show dark the under microstructure of cell walls (Figure 10). The dark color of the cell walls is not caused by traces of amorphous compounds such as iron and manganese, based on the absence of characteristic peaks for these elements in SEM/XRF spectra. The thin section views show that chalcedony and crystalline quartz fill larger spaces within the fossil wood, including rot pockets and open fractures (Figure 11).

Geosciences 2019, 9, 510 10 of 18 polarized light because the individual quartz particles are so small that they do not show interference colors. This interpretation is supported by SEM images that show the microstructure of cell walls (Figure 10). Geosciences 2019, 9, x FOR PEER REVIEW 11 of 19

Figure 9. Optical photomicrographs of lycopsid wood. In each pair, the photo on the left is made with FigureFigure 9. Optical 9. Optical photomicrographs photomicrographs of of lycopsid lycopsid wood.wood. In In each each pair, pair, the the ph photooto on onthe theleft is left made is made with with ordinary transmitted light; the right photo shows the polarized light view. Birefringence colors in ordinaryordinary transmitted transmitted light; light; the the right right photo photo showsshows the polarized polarized ligh lightt view. view. Birefringence Birefringence colors colors in in polarized images are indicative of quartz/chalcedony. (A) Transverse view of the fiber-like cells that polarized images are indicative of quartz/chalcedony. (A) Transverse view of the ﬁber-like cells that form periderm. Dark matter at the center of these cells and oriented in a radial direction was form periderm. Dark matter at the center of these cells and oriented in a radial direction was interpreted interpreted by Arnold as innermost wall material that had detached during degradation. (B) by ArnoldTransverse as innermost view of tracheids wall material in secondary that had xylem detached (C) Tangential during view degradation. of secondary (B) xylem, Transverse showing view of tracheidsscalariform in secondary tracheids. xylem (C) Tangential view of secondary xylem, showing scalariform tracheids.

FigureFigure 10. SEM 10. SEM images images showing showing oblique oblique transverse transverse view view ofof peridermperiderm cells. cells. (A (A) )Cell Cell walls walls stand stand out out in highin topographic high topographic relief. relief. (B) Close-up (B) Close-up views views show show cell cell walls walls have have been been replaced replaced by by an an aggregate aggregate of very very small quartz grains. smallvery quartz small grains. quartz grains.

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The dark color of the cell walls is not caused by traces of amorphous compounds such as iron and manganese, based on the absence of characteristic peaks for these elements in SEM/XRF spectra. The thin section views show that chalcedony and crystalline quartz ﬁll larger spaces within the fossil wood, including rot pockets and open fractures (Figure 11). Geosciences 2019, 9, x FOR PEER REVIEW 12 of 19

FigureFigure 11. Optical11. Optical photomicrographs photomicrographs of 30 of micron 30 micron thick slicesthick ofslices periderm of periderm tissue, transversetissue, transverse orientation. Imagesorientation. on the left Images are ordinaryon the left transmitted are ordinary light; transmitted right images light; right are polarized images are light polarized views. light (A) views. Degraded (A) Degraded tissue where individual cells are filled with chalcedony/quartz. Voids created by decay tissue where individual cells are filled with chalcedony/quartz. Voids created by decay have been filled have been filled with microcrystalline quartz. (B,C) Veinlets cross-cut cell layers. Each veinlet consists with microcrystalline quartz. (B,C) Veinlets cross-cut cell layers. Each veinlet consists of thin margins of thin margins of chalcedony with microcrystalline quartz filling the interior zone. of chalcedony with microcrystalline quartz filling the interior zone.

6.3. Density and 500 °C Loss onTable Ignition 1. Density and 550◦C Loss on Ignition.

The density of silicified wood can be used to3 evaluate the mineralogy. Woods mineralized with Sample Density g/cm Weight Loss after 550 ◦C opal have densities of 1.9–2.1 g/cm3, compared to 2.3–2.6 g/cm3 for wood mineralized with chalcedony or quartz [22]. These CL-1measurements are 2.42only valid for samples that 0.55% are solidly mineralized; open spaces reduce the measuredCL-2 density. 2.54 0.57% CL-3 2.32 0.82 Most Colorado lycopsid specimens have densities of 2.42–2.59 g/cm3, indicative of CL-4 2.46 1.23% quartz/chalcedony (TableCL-5 1). The exceptions 2.56 are CL-3, and CL-6, where 0.83% slight porosities explain the reduced density. CL-6 2.31 0.72% CL-8 2.59 0.76% CL-A1Table 1. Density 2.54 and 550°C Loss on Ignition. 1.56% L-BLKSample Density 2.58 g/cm3 Weight Loss after 1.69% 550 °C CL-1 2.42 0.55% CL-2 2.54 0.57% CL-3 2.32 0.82

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6.3. Density and 500 ◦C Loss on Ignition The density of siliciﬁed wood can be used to evaluate the mineralogy. Woods mineralized with opal have densities of 1.9–2.1 g/cm3, compared to 2.3–2.6 g/cm3 for wood mineralized with chalcedony or quartzGeosciences [22 2019]. These, 9, x FOR measurements PEER REVIEW are only valid for samples that are solidly mineralized; open13 of spaces19 reduce the measured density. CL-4 2.46 1.23% 3 Most Colorado lycopsidCL-5 specimens 2.56 have densities of 0.83% 2.42–2.59 g/cm , indicative of quartz/ chalcedony (Table1). The exceptionsCL-6 are2.31 CL-3, and CL-6, 0.72% where slight porosities explain the reduced density. CL-8 2.59 0.76% CL-A1 2.54 1.56% 6.4. Scanning Electron MicroscopyL-BLK 2.58 1.69%

6.4.SEM Scanning images Electron provide Microscopy topographic information for specimen areas that are not solidly mineralized. Figure 10 shows a specimen surface where the cell walls of periderm stand in relief compared to the SEM images provide topographic information for specimen areas that are not solidly quartz-filled lumina. At high magnification, the cell walls can be seen to consist of a randomly oriented mineralized. Figure 10 shows a specimen surface where the cell walls of periderm stand in relief aggregatecompared of veryto the small quartz-filled silica particles, lumina. At the high composition magnification, evidenced the cell walls by EDS can spectra.be seen to The consist particulate of a texturerandomly suggests oriented the aggregate possibility of very that small the cell silica walls particles, were the initially composition mineralized evidenced with by EDS opal spectra. that has transformedThe particulate to quartz texture during suggests diagenesis. the possibility The apparent that the cell isotropic walls were character initially of mineralized cell walls viewed with opal under polarizedthat has light transformed is presumably to quartz caused during by the diagenesis. small grain The size, apparent where isotropic individual character quartz of particles cell walls are so smallviewed that petrographic under polarized thin light sections is presumably do not display caused the by the normal small quartz grain size, birefringence where individual colors thatquartz would be expectedparticles are for so a 30 small micron that onpetrographic a thick layer. thin sections do not display the normal quartz birefringence colorsLongitudinal that would SEM be expected images clearly for a 30 show micron the on microcrystalline a thick layer. quartz that has replaced wood tissue (Figure 12Longitudinal). Cell interiors SEM (lumina)images clearly contain show solid the crystalline microcrystalline masses, quartz with euhedral that has crystalsreplaced encrusting wood outertissue cell surfaces.(Figure 12). The Cell sizes interiors of individual (lumina) crystalscontain solid arevariable, crystalline ranging masses, from with lesseuhedral than onecrystals micron encrusting outer cell surfaces. The sizes of individual crystals are variable, ranging from less than one to more than five microns. The larger crystals commonly developed on a substrate of small crystals micron to more than five microns. The larger crystals commonly developed on a substrate of small (e.g., Figure 12C), evidence that quartz precipitation occurred in several episodes. crystals (e.g., Figure 12C), evidence that quartz precipitation occurred in several episodes.

FigureFigure 12. SEM12. SEM views views of periderm of periderm cells cells showing showing silica silica mineralization. mineralization. (A ,(BA), ObliqueB) Oblique longitudinal longitudinal views showviews cells show are solidlycells are replaced solidly replaced by quartz, by quarts, with some with some intracellular intracellular spaces spaces remaining remaining empty. empty. (C ()C High) High magnification shows that cells are encrusted with euhedral quartz crystals. (D) Smooth-surfaced magniﬁcation shows that cells are encrusted with euhedral quartz crystals. (D) Smooth-surfaced cells cells are enclosed by a matrix of granular quartz crystals. are enclosed by a matrix of granular quartz crystals. 6.5. Macrophotography The largest quartz crystals are located in open spaces created by wood decay (Figure 13).

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6.5. Macrophotography

GeosciencesThe largest 2019, 9, x quartz FOR PEER crystals REVIEW are located in open spaces created by wood decay (Figure 13). 14 of 19 Geosciences 2019, 9, x FOR PEER REVIEW 14 of 19

Figure 13. Euhedral quartz crystals in cavities created as decayed spaces in periderm tissue. (A) Decay Figure 13. Euhedral quartz crystals in cavities created as decayed spaces in periderm tissue. (A) Decay Figurespace 13.containsEuhedral a geode-like quartz crystals lining, in cavitieswith the created central as space decayed remaining spaces in empty. periderm (B) tissue. Quartz (A )crystals, Decay space contains a geode-like lining, with the central space remaining empty. (B) Quartz crystals, spaceformed contains on spaces a geode-like ina rotted lining,area, include with thedouble-terminated central space remaining habits. empty. (B) Quartz crystals, formedformed on spaces on spaces ina rotted in a rotted area, area,include include double-terminated double-terminated habits. habits. 6.6. X-Ray Diffraction Evidence 6.6. X-Ray6.6. X-Ray Diffraction Diﬀraction Evidence Evidence Microcrystalline quartz and chalcedony are the only minerals observed by optical microscopy MicrocrystallineMicrocrystalline quartz quartz and and chalcedony chalcedony are arethe theonly only minerals minerals observed observed by by optical optical microscopy microscopy of of thin sections. X-ray diffraction patterns (XRD) (Figure 14) likewise show quartz/chalcedony as the of thinthin sections. sections. X-ray X-ray diffraction diﬀraction patterns patterns (XRD) (XRD) (Figure (Figure 14)14) likewise likewise show quartzquartz/chalcedony/chalcedony asas thethe only only detectable constituents (the two silica polymorphs are indistinguishable by XRD). onlydetectable detectable constituents constituents (the (the two two silica silica polymorphs polymorphs are are indistinguishable indistinguishable by by XRD). XRD).

Figure 14. X-ray diffraction pattern for silicified lycopsid wood. All diffraction peaks are produced by Figure 14. X-ray diffraction pattern for silicified lycopsid wood. All diffraction peaks are produced Figurequartz 14. X-ray/chalcedony. diffraction pattern for silicified lycopsid wood. All diffraction peaks are produced by quartz/chalcedony. by quartz/chalcedony. 6.7. SEM/EDS Data 6.7. SEM/EDS Data X-ray fluorescence spectra show SiO2 as the primary constituent of Colorado lycopsid wood. The X-ray fluorescence spectra show SiO2 as the primary constituent of Colorado lycopsid wood. The amount of relict carbon is variable, being highest in dark-colored specimens (Figure 15). amount of relict carbon is variable, being highest in dark-colored specimens (Figure 15).

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6.7. SEM/EDS Data

X-ray ﬂuorescence spectra show SiO2 as the primary constituent of Colorado lycopsid wood. TheGeosciences amount 2019 of, relict 9, x FOR carbon PEER REVIEW is variable, being highest in dark-colored specimens (Figure 15).15 of 19

FigureFigure 15. 15.SEM SEM/EDS/EDS spectra spectra of of lycopsid lycopsid wood. wood. (A (A) Light-colored) Light-colored specimen specimen contains contains aa smallsmall amountamount of relictof relict carbon. carbon. (B) Dark-colored(B) Dark-colored wood wood contains contains high high levels levels of of relict relict carbon.carbon. Pd Pd peaks peaks are are from from metal metal sputter-coatedsputter-coated on on the the specimens specimens to to provide provide electrical electrical conductivity.conductivity.

7. Discussion7. Discussion

7.1.7.1. Mineralogy Mineralogy InIn 1982, 1982, C.L. C.L. Stein Stein used used thirty-three thirty-three samples samples ofof silicifiedsilicified wood represen representingting different different locations locations andand ages ages to to study study the the occurrence occurrence of of silica silica in in petrifiedpetrified wood.wood. The published published study study [23] [23 included] included three three photosphotos of of a specimena specimen of ofLepidodendron Lepidodendron johnsonii johnsonii((== Lepidophloios johnsonii johnsonii (Arnold)(Arnold) DiMichelle) DiMichele) from from thethe Trout Trout Creek Creek locality. locality. The The caption caption for for a a 110X 110X imageimage describesdescribes “poor “poor preservation preservation of of scalariform scalariform tracheidstracheids by by quartz”. quartz”. Two Two higher higher magnification magnification viewsviews show euhedral quartz quartz crystals, crystals, similar similar to tothe the imagesimages we we show show in in Figure Figure 12 12.. Stein Stein [ 23[23]] made made thethe generalizationgeneralization that that quartz quartz in in silicified silicified wood wood was was typicallytypically a resulta result of of diagenetic diagenetic transformation transformation from an an opaline opaline precursor. precursor. This This opal opal A A→ opalopal CT CT → → → quartzquartz transformation transformation sequence sequence is is well-documented well-documented forfor the diagenesis of of siliceous siliceous marine marine sediments sediments andand for for siliceous siliceous hot hot spring spring sinter, sinter but, evidencebut evidence from from fossil fossil wood wood is less is clear. less Thisclear. sequence This sequence is certainly is a possibilitycertainly a forpossibility some fossil for some wood fossil occurrences, wood occurre butnces, many but investigations many investigations [24–28] [24–28] have shown have shown that the that the fossilization of wood may involve multiple episodes of mineralization, and that quartz may fossilization of wood may involve multiple episodes of mineralization, and that quartz may occur as a occur as a primary precipitate, akin to the formation of crystalline quartz in veins and geodes. primary precipitate, akin to the formation of crystalline quartz in veins and geodes. The petrography of Colorado lycopsid wood is indicative of multiple episodes of silica The petrography of Colorado lycopsid wood is indicative of multiple episodes of silica mineralization. The first step appears to have been the precipitation of amorphous silica at cell walls mineralization. The first step appears to have been the precipitation of amorphous silica at cell walls via via organic templating, when lignin and cellulose provided active sites for the attachment of organicmonosilicic templating, acid whenmolecules lignin from and groundwater cellulose provided [29]. activeMost of sites the for original the attachment organic ofmatter monosilicic was acideventually molecules lost, from and groundwater the amorphous [29]. Mostsilica of (opal) the original was eventually organic matter transformed was eventually to quartz lost, during and the amorphousdiagenesis. silica This (opal) transformation was eventually is supported transformed by the to quartz absence during of opal diagenesis. wood in ThisMesozoic transformation deposits, is supportedcompared by to the the absence abundance of opal of wood opal wood in Mesozoic in Cenozo deposits,ic deposits. compared A second to the step abundance in the petrifaction of opal wood in Cenozoicprocess was deposits. the deposition A second of step chalcedony/quartz in the petrifaction in processthe cell waslumina. the depositionThe coarse ofcrystallinity chalcedony of/quartz this in thefilling cell material lumina. (Figure The coarse 10B,C) crystallinity suggests that of this this silica filling was material a direct (Figureprecipitate, 10B,C) not a suggests transformation that this of an amorphous precursor phase. Relatively large quartz crystals occur in rot pockets, fracture zones, and some intercellular spaces, suggesting that the final phases of mineralization occurred during a later time, when lowered concentrations of dissolved silica allowed well-ordered lattices to develop in larger spaces within the wood (Figure 11).

Geosciences 2019, 9, 510 15 of 18 silica was a direct precipitate, not a transformation of an amorphous precursor phase. Relatively large quartz crystals occur in rot pockets, fracture zones, and some intercellular spaces, suggesting that the final phases of mineralization occurred during a later time, when lowered concentrations of dissolved silica allowed well-ordered lattices to develop in larger spaces within the wood (Figure 11). Because of the Pennsylvanian age and the complex tectonic history of the region, the source of the dissolved silica is a matter of speculation. As a general rule, silicification of wood occurs from alkaline conditions, where silica is dissolved from siliceous biogenic sediment, glass-rich volcanics, or feldspar-rich host materials. Airborne dust has been proposed as a source of silica for chert associated with Paleozoic carbonates [30,31]. This eolian deposition is assumed to have been related to the increasing aridity. The dissolution of quartz was favored by the very small grain size, which produced a relatively large surface area, and the abrasion of grain surfaces that produced amorphous silica [31]. The coastal lowland swamp forests that are represented by the Trout Creek fossil site may not have been associated with the delivery of eolian dust, but extensive silicification of the Belden Shale host sediment makes evident that dissolved silica was readily available. The underlying Lower Mississippian Leadville Limestone is also locally silicified. As noted earlier, fossil specimens are predominantly angular wood fragments rather than intact stumps or logs. The external form of these specimens commonly preserve bark impressions (Figure3) but the internal structure is often chaotic; degraded wood fragments lie in a horizontal layer at the bottom of a chamber that is filled with fine-grained quartz (Figure8C,D).

7.2. Paleoenvironment During the early Pennsylvanian, the Ancestral Rocky Mountains were at a much lower latitude than today (Figure4), indicative of a warm, relatively seasonless climate. The low elevation environment favored the existence of extensive swamps, where lycopsids were the dominant trees. The coastal setting is evidenced by the transgressive/regressive nature of the Carboniferous sediments; the Belden Shale includes strata that contain marine invertebrates, in addition to continental sediments that preserve leaf and wood fossils. Botanical aspects of lycopsid forests have received varied interpretations. Arborescent lycopsids may have had growth histories different from modern trees. Lycopsid trunks were characterized by secondary xylem that provided efficient water transport, but the thick secondary cortex (periderm) provided the structural support necessary to support large tree height. Some investigators have assumed that arborescent lycopsids were fast-growing [32–34], with growth rates perhaps 20 times faster than modern angiosperms and with maximum lifespans of 10–15 years [33]. An alternate interpretation is that the growth rates of arborescent lycopsids were very slow, with lifetimes of centuries [35]. The Trout Creek lycopsid fossil locality does not provide evidence to resolve this controversy, but the site does provide some clues with regards to the paleoenvironment. In other Pennsylvanian coal swamps, Lepidodendron plants were commonly destroyed by floods or storms, followed by recolonization by a new generation of trees [36]. This model may account for the fossils found at the Colorado site. Arnold’s original reconstruction [2] depicted many standing trees, as well as intact fallen trunks, in swampy lowland fed by sluggish streams that carried fine sediment (Figure 16). However, we observed that the most abundant specimens are angular wood fragments, preserved as casts that contain degraded fragments that settled as horizontal layers prior to silicification. This taphonomy suggests that the ancient swamp contained fallen trees, commonly experienced rapid decay, and that fragments were perhaps fluvially transported. These characteristics suggest an environment that was more chaotic than the calm scene portrayed in Arnold’s sketch. The most chaotic possibility is the scenario proposed in 2015 by Thomas and Cleal [36], where Carboniferous swamp forest deposits were interpreted to have resulted from catastrophic storm events where cyclone-strength winds caused extensive damage to vegetation, accompanied by flooding with sediment-laden water. In this situation, the fossiliferous strata may represent a very brief depositional episode, regardless of the age of the forest trees. Geosciences 2019, 9, 510 16 of 18 Geosciences 2019, 9, x FOR PEER REVIEW 17 of 19

FigureFigure 16. 16. Arnold’sArnold’s 1940 1940 reconstruction reconstruction of of th thee Trout Trout Creek lycops lycopsidid forest [2]. [2].

8.8. Conclusions Conclusions SilicifiedSilicified plantplant materialmaterial making making up up the the Trout Trout Creek Creek lycopsid lycopsid fossil fossil locality locality in the in Mosquito the Mosquito Range Rangeof south-central of south-central Colorado Colorado represents represents an unusual an unusual mode mode of preservation of preservation for a Pennsylvanianfor a Pennsylvanian aged ageddeposit. deposit. A single A single scientific scientific study study of the siteof the in 1940site in included 1940 included the naming the naming of a new of species a newof species lycopsid of lycopsidtree, Lepidodendron tree, Lepidodendron johnsonii ( =johnsoniiLepidophloios (= Lepidophloios johnsonii (Arnold) johnsonii DiMichele). (Arnold) DiMichlle). Newly collected Newly specimens collected specimensprovided an provided opportunity an opportunity to investigate to the investigate mineralization the mineralization sequence of silicified sequence plant of fossilssilicified from plant this fossilsunusual from site. this unusual site. TheThe Trout Creek lycopsidlycopsid fossilfossil localitylocality is is inferred inferred to to represent represent a a costal costal lowland lowland swamp swamp growing growing at atlow low latitudes latitudes during during the Pennsylvanianthe Pennsylvanian subperiod. subpe Subsequentriod. Subsequent plate movements, plate movements, mountain mountain building, building,weathering weathering and erosion and placed erosion the Trout placed Creek the lycopsid Trout Creek fossil sitelycopsid in its present-day fossil site in mountain its present-day meadow. mountainRemnants meadow. of the ancient Remnants forest of are the limited ancient toforest fragmental are limited fossil to plantfragmental remains, fossil of plant which remains, lycopsids of whichare the lycopsids only recognized are the only component. recognized The component. pith and most The ofpith the and cortex most of of the the buried cortexstems of the becameburied stemsdecayed. became Only decayed. sections Only of small sections central of small axes central and some axes peridermand some remainedperiderm remained to become to silicified. become silicified.The mineralization The mineralization of fossil woodof fossil at wood the Trout at the Creek Trout lycopsid Creek lycopsid locality locality occurred occurred in multiple in multiple stages. stages.The anatomically The anatomically preserved preserved cell structure cell structure provides evidenceprovides ofevidence a templating of a templating process, where process, amorphous where amorphoussilica precipitated silica precipitated on cell walls. on cell This walls. silica This was silica later was transformed later transformed to fine-grained to fine-grained quartz duringquartz duringdiagenesis. diagenesis. Layers Layers of larger of euhedral larger euhedral quartz crystals,quartz crystals, growing growing over small over quartz small crystals quartz alongcrystals cell along wall cellsurfaces, wall surfaces, represent represent further episodes further ofepisodes mineralization, of mineralization, when dissolved when silicadissolved concentrations silica concentrations were lower. wereThe largest lower. euhedralThe largest quartz euhedral crystals quartz are crystals found in are decayed found in spaces, decayed representing spaces, representing the final episode the final of episodesilicification. of silicification. Loss on Ignition Loss on values Ignition indicate values that indicate most of that the most original of the organic original matter organic was matter lost during was lostfossilization, during fossilization, but X-ray fluorescence but X-ray fluorescence spectra show sp aectra relationship show a relationship between relict between carbon relict content carbon and contentspecimen and color, specimen with higher color, carbonwith higher values ca inrbon dark-colored values in dark-colored specimens. specimens.

SupplementarySupplementary Materials: TheThe following following are are available available online at www.mdpi.com/xxx/s1http://www.mdpi.com/2076-3263. /9/12/510/s1. Author Contributions:Contributions: Conceptualization,Conceptualization, M.V.;M.V.; Investigation, Investigation, M.V., M.V., G.E.M. G.M. and and R.D.H.; R.H.; Methodology,Methodology, G.E.M.; G.M.; WritingWriting—original – original draft, G.E.M.;G.M.; Writing Writing—review – review & &editing, editing, M.V., M.V., G.M G.E.M. and andR.H. R.D.H.

Funding:Funding: ThisThis research research received received no no external funding. Acknowledgments: Specimens for this study were collected under the provisions of Colorado State Land Board Acknowledgements: Specimens for this study were collected under the provisions of Colorado State Land Board Department of Natural Resources Temporary Access Permit Application No. 112723. We thank William DiMichele Departmentfor his help forof improvingNatural Resources the ﬁnal draft,Temporary and for Access the constructive Permit Application suggestions ofNo. two 112723. anonymous We thank reviewers. William DiMichelle for his help for improving the final draft, and for the constructive suggestions of two anonymous reviewers.

Conflicts of Interest: The authors declare no conflicts of interest.

Geosciences 2019, 9, 510 17 of 18

Conﬂicts of Interest: The authors declare no conﬂicts of interest.

References

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