Neogene Tree Trunk Fossils from the Meshgin Shahr Area, Northwest Iran

geosciences

Article Neogene Tree Trunk Fossils from the Meshgin Shahr Area, Northwest Iran

George E. Mustoe 1,* , Nasrollah Abbassi 2, Afsaneh Hosseini 3 and Yousef Mahdizadeh 4

1 Geology Department, Western Washington University, Bellingham, WA 98225, USA 2 Department of Geology, Faculty of Science, University of Zanjan, Zanjan, Iran; [email protected] 3 Department of Paleontology, Museum of Natural History and Genetic Resources, Pardisan Natural Park, Department of Environment of Iran, Tehran, Iran; [email protected] 4 Department of Environment of Iran, Administration of Ardabil Province, Be’sat Administrative Town, Ardabil, Iran; [email protected] * Correspondence: [email protected]

Received: 4 July 2020; Accepted: 20 July 2020; Published: 23 July 2020

Abstract: In 2016, an extensive fossil forest was discovered near Meshgin Shahr, northwest Iran. Silicified tree trunks occur in Miocene fluvial sediments and at multiple stratigraphic levels within a 27-m thick sequence of Pleistocene volcaniclastics. The Miocene trunks likely represent stream transport. Pleistocene examples originated during repeated eruptive events when volcaniclastic sediments buried a standing forest. The site, informally named Meshgin Shahr Fossil Forest, was registered in 2017 as a national natural monument by the Iranian Cultural, Handicraft and Tourism Organization. To date, 16 fossilized trunks have been found, all but one of them representing gymnosperms. The ancient coniferous forest was very different from modern forests in Iran and adjacent Azerbaijan, a result of climatic changes that were principally caused by the demise of the Paratethys Sea and by rain shadow effects caused by the uplift of the Alborz and Zagros mountain ranges. X-ray diffraction patterns reveal that woods from the fossil forest contain three types of silica: opal-CT, pure quartz, and a mixture of opal-CT and quartz. In addition, optical photomicrographs show the abundant presence of amorphous opal-A. Mineralogic variations occur among different fossil trees and within a single trunk. These silica polymorphs resulted from a combination of processes: silica minerals precipitated in multiple episodes under differing geochemical conditions and the diagenetic transformation of an opaline parent material.

Keywords: Caspian Sea; fossil wood; Iran; Mt. Sabalan; opal-A; opal-CT; paleobotany; Paratethys

1. Introduction In 2016, siliciﬁed tree trunks were discovered exposed on the ground surface in a remote location in the Ardabil province, northwest Iran. The site is located 100 km west of Ardabil, near the town of Sheikh Mohammadlu, as shown in Figure1. The site, informally named in this report as Meshgin Shahr Fossil Forest, was registered in 2017 as a national natural monument by Iranian Cultural, Handicraft and Tourism Organization. This report describes the results of a study sponsored by the Department of Paleontology, Museum of Natural History and Genetic Resources in Tehran. The goals were to evaluate the age and stratigraphic position of the fossil site, to identify the tree genera that are represented, and to determine the processes that caused the wood to become mineralized.

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Figure 1. Location map of tree trunk fossil site in the northwest Ardabil, NW. Figure 1. LocationLocation map map of of tree tree trunk fossil site in the northwest Ardabil, NW. 1.1. Site Description 1.1. Site Description The fossil logs are exposed on the surface of weathered sediment that that forms forms the the desert desert floor. ﬂoor. The fossil logs are exposed on the surface of weathered sediment that forms the desert floor. The majoritymajority ofof the the specimens specimens occur occur in Quaternaryin Quaternary volcaniclastic volcaniclastic sediments, sediments, but fossilbut fossil logs alsologs occur also The majority of the specimens occur in Quaternary volcaniclastic sediments, but fossil logs also inoccur underlying in underlying Miocene Miocene ﬂuvial sediments. fluvial sediments. Nearby hills Nearby are composed hills are of composed more durable of extrusivemore durable rocks occur in underlying Miocene fluvial sediments. Nearby hills are composed of more durable thatextrusive form arocks protective that form capping a protective layer, as capping shown inlayer, Figure as 2shown. Stratigraphy in Figure is 2. shown Stratigraphy in Figure is3 .shown in extrusiveFigure 3. rocks that form a protective capping layer, as shown in Figure 2. Stratigraphy is shown in Figure 3.

Figure 2. OutcropsOutcrops of of sedimentary sedimentary and and volcanic volcanic units units in the in thestudy study area. area. (A, B) ( AGeneral,B) General views views of fossil of fossilFiguresite, showing site, 2. Outcrops showing dark-colored of dark-colored sedimentary capping capping and layer volcanic layer(1) and (1) units li andght-colored in light-colored the study tuffite-containing area. tuﬃ (te-containingA, B) General fossil fossilviews wood wood of (2). fossil ( (2).C) (site,FossilC) Fossil showing trunk trunk 4, dark-colored sample 4, sample #T-5. #T-5. capping layer (1) and light-colored tuffite-containing fossil wood (2). (C) Fossil trunk 4, sample #T-5.

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Figure 3. Stratigraphic column showing position of individual fossil logs. Figure 3. Stratigraphic column showing position of individual fossil logs. Field investigations conducted in 2019 included geologic mapping and detailed surveying of the Field investigations conducted in 2019 included geologic mapping and detailed surveying of fossil trunks. Sixteen fossil trees were discovered, with 31 samples collected from the central, middle, the fossil trunks. Sixteen fossil trees were discovered, with 31 samples collected from the central, and outer zones of each trunk. Twenty-six specimens were chosen for detailed study, as shown in middle, and outer zones of each trunk. Twenty-six specimens were chosen for detailed study, as Table1. In addition, two specimens were collected from the sedimentary strata for palynologic analysis. shown in Table 1. In addition, two specimens were collected from the sedimentary strata for The geographic distribution of fossil logs is shown in Figure4. palynologic analysis. The geographic distribution of fossil logs is shown in Figure 4.

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Table 1. Location and Description of Fossil Tree Trunks.

Distance Sample Diameter Length Density Trunk Age above Basal Sample Position (m) (m) (g/cm3) Contact in Trunk 0 Miocene 0.60 1 5 T-26 outer - 1 Miocene 0.40 1 5 T-1 outer 2.17 2 Pleistocene 0.33 1 31 T-2 outer 2.03 T-3 outer 2.18 3 Pleistocene 0.45 1 31.3 T-4 middle 2.03 T-5 center 2.58 4 Pleistocene 1.2 1 T-6 middle 2.43 T-7 outer 2.29 33 T-8 center 2.09 5 Pleistocene 1 2.1 T-9 outer 2.30 6 Pleistocene 1.1 4 T-10 outer 2.42 T-11 center 2.07 T-12 outer 2.56 7 Pleistocene 0.6–0.25 10 38 T-13 outer 2.23 T-14 outer 2.07 T-15 center 2.31 8 Pleistocene 1.2 1 42 T-16 middle 2.35 T-17 outer 2.47 9 Pleistocene 0.3 0.5 T-18 outer 2.44 48 10 Pleistocene 0.3 0.3 T-19 middle 2.07 T-20 center 1.98 11 Pleistocene 1.4 1 T-21 middle 2.31 T-22 outer 2.28 T-23 center 2.32

12 Pleistocene 0.85 1 58 T-24 middle 2.23 T-25 outer 1.99 T-27 outer - 13 Pleistocene 0.7 1 T-28 middle - T-29 center - 14 Pleistocene 0.8 3.1 62 T-30 outer - 15 Pleistocene 0.6 0.5 58 T-31 outer - Geosciences 2020, 10, 283 5 of 29

Figure 4. Geographic distribution of fossil logs. (Left) Satellite map; (Right) geologic map. Faults are shown by red lines.

1.2. Geologic Setting and Paleoclimate One of the major events in the Mesozoic history of the Middle East was the appearance of the Tethys Sea, which separated the supercontinents Laurasia and Panegea [1]. The Tethys closed during the early Cenozoic when continental fragments (Africa and Arabia) converged with Eurasia. The term “Paratethys” describes a string of epicontinental basins that became separated from the main Tethys in the early Oligocene by the uplift of the Alpine–Caucasian mountain chain, as shown in Figure5[ 2–4]. The Cenozoic paleogeography was in a state of flux, with episodes when the Paratethys and the Mediterranean Sea were connected, alternating with periods of separation. Variations in global sea level combined with tectonic and orogenic events to produce regional climatic effects. The Paratethys epicontinental sea had a strong effect on Eurasian climate because it was a major source of atmospheric water vapor, affecting precipitation patterns. In addition, the high heat capacity of the water reduced the seasonal temperature variation. Shrinkage of the Paratethys in the Late Miocene caused the climate of Eurasia to shift from oceanic to continental, with colder winters and increased seasonality. The climate of the Arabian Peninsula became drier, and a subtropical desert developed in North Africa [5]. This climatic history provides a framework for understanding the environmental conditions that affected the Meshgin Shahr Fossil Forest. Fossil logs in Miocene strata came from a time when the Paratethys was experiencing fluctuations in connectedness with the Mediterranean, so the geographic conditions were variable. The most likely scenario is that these logs were transported from lowland forests that grew on the low elevation floodplain, becoming buried in fluvial sediments deposited by rivers that flowed into the Eastern Paratethys basin to form large deltas.

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Figure 5. Paleogeographic setting of the Meshgin Shahr fossil forest. (A) During much of the Mesozoic, Figure 5. Paleogeographic setting of the Meshgin Shahr fossil forest. (A) During much of the the Tethys seaway was located in the rift zone between Laurasia and Gondwana. (B) Early Cenozoic Mesozoic, the Tethys seaway was located in the rift zone between Laurasia and Gondwana. (B) Early closure of the Tethys was followed by the development of the Paratethys. Generalized map from Late Cenozoic closure of the Tethys was followed by the development of the Paratethys. Generalized map Miocene, 8.5–7.0 Ma, adapted from [4]. from Late Miocene, 8.5–7.0 Ma, adapted from [4]. The majority of the fossil logs were observed in Pleistocene volcaniclastic strata that were formed underThe plate majority tectonic of conditions the fossil that logs were were similar observed to the in modern Pleistocene era, but volcaniclastic at a time of episodicstrata that volcanic were eruptionsformed under and plate in a paleoclimate tectonic conditions that was that cooler were and simi wetterlar to thanthe modern the current era, but climate. at a time The of Paratethys episodic hadvolcanic receded, eruptions leaving and the in Black a paleoclimate Sea, Caspian that Sea, was and cooler Aral and Sea wetter as landlocked than the relicts. current Major climate. climatic The shiftsParatethys accompanied had receded, the cyclic leaving pattern the Black of glacial Sea, Caspian and interglacial Sea, and events.Aral Sea In as addition, landlocked global relicts. sea Major levels fluctuatedclimatic shifts as the accompanied result of freezing the cyclic and thawingpattern of of gl majoracial iceand sheets. interglacial At the events. peak of In the addition, last glaciations global 21,000–22,000sea levels fluctuated years ago,as the when result sea of freezing level was and 120 thawing m lower of thanmajor at ice present, sheets. theAt the Pesian peak Gulf of the was last a waterlessglaciations basin 21,000–22,000 [6]. In northern years ago, and westernwhen sea Iran, level the wa climates 120 m alternated lower than from at present, dry, cold the glacial Pesian events Gulf towas warm, a waterless moist basin interglacial [6]. In episodesnorthern and [7]. western Fossil logs Iran, in the Pleistocene climate alternated strata at Meshginfrom dry, Shahr cold glacial Fossil Forestevents presumablyto warm, moist came interglacial from a forest episodes that flourished [7]. Fossil during logs in an Pl interglacialeistocene strata episode. at Meshgin By the onset Shahr of theFossil Holocene, Forest presumably the region was came becoming from a forest increasingly that flourished arid, with during temperate an interglacial forests replaced episode. by steppeBy the vegetationonset of the [8 Holocene,–11]. the region was becoming increasingly arid, with temperate forests replaced by steppe vegetation [8–11]. 1.3. Regional Geology 1.3. Regional Geology The Meshgin Shahr Fossil Forest is located in a region that has been described both as the Alborz–AzarbaijanThe Meshgin Shahr geological Fossil zone Forest [12] andis located the Urumieh–DokhtarMagmatic in a region that has been Beltdescribed [13]. This both area as isthe in theAlborz–Azarbaijan magmatic arc zone geological of Arabia-Eurasia zone [12] and collision the Urumieh–DokhtarMagmatic belt [14,15]. The zone includes Belt numerous [13]. This small-scale area is in magmaticthe magmatic belts thatarc rangezone inof ageArabia-Eurasia from Cretaceous collision to Quaternary belt [14,15]. [16–19 The]. The zone tree includes fossils are numerous located in thesmall-scale Tarom–Sabalan–Arasbaran magmatic belts that (TSA) range zone,in age bounded from Cretaceous by the Aras, to Quaternary Astara, and [16–19]. Tabriz faults.The tree fossils are locatedIgneous in components the Tarom–Sabalan–Arasbaran of the TSA zone include (TSA) Late zone, Cretaceous–Paleogene bounded by the Aras, submarine Astara, volcanogenic and Tabriz rocksfaults. of intermediate-acidic composition and Cenozoic pyroclastic rocks with calc-alkaline and potassium-richIgneous components alkaline compositions of the TSA [18 ,20zone]. These incl Cenozoicude Late rocksCretaceous–Paleogene were produced in twosubmarine phases: (1)volcanogenic Eocene trachybasalts rocks of intermediate-acidic and trachyandesitic composition basalt, which and are overlainCenozoic by pyroclastic Eocene flysch rocks deposits, with andcalc-alkaline (2) Late and Miocene–Quaternary potassium-rich alkaline basic compositions to intermediate [18,20]. volcanic These Cenozoic rocks. rocks With were the exceptionproduced ofin two two phases: logs found (1) Eocene in Miocene trachybasalts sediments, and trachyandesitic the fossil wood basalt, described which in are this overlain report by occurs Eocene in Pliocene–Pleistoceneflysch deposits, and (2) tu ffLateaceous Miocene–Quaternary beds. These beds basic are overlainto intermediate by volcaniclastic volcanic rocks. conglomerates, With the ignimbrite,exception of and two olivine–pyroxenelogs found in Miocene basalt sediments, that erupted the fromfossil awood volcano described close to in thethis fossil report forest occurs area, in asPliocene–Pleistocene shown in Figure6. tuffaceous beds. These beds are overlain by volcaniclastic conglomerates, ignimbrite,Four rock and unitsolivine–pyroxene are exposed basalt in the that 20 hectares erupted offrom Meshgin a volcano Shahr close Fossil to the Forest. fossil forest In ascending area, as stratigraphicshown in Figure order 6. these are: Four rock units are exposed in the 20 hectares of Meshgin Shahr Fossil Forest. In ascending 1. Sedimentary rocks: Early Miocene strata outcrop in a small area in the south of the study area. stratigraphic order these are: These sediments are the oldest rock units in the studied area and include 30 m of polymictic 1. Sedimentary rocks: Early Miocene strata outcrop in a small area in the south of the study area. These sediments are the oldest rock units in the studied area and include 30 meters of

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well-rounded conglomerate, green to gray sandstone with tree trunk fossils, sandy marl and claystone with leaf fossils and gypsum veins. 2. Light-colored porphyritic dacitic tuﬀ containing 14 known in situ tree trunk fossils. Plagioclase, biotite and hornblende are main minerals and are 1–1.5 mm in size with ~3.5 mm lithic particles. 3. Dark ignimbrite tuﬃtes containing ~4 mm phenocrysts of plagioclase, biotite, hornblende, quartz, clinopyroxene and lithic particles. This unit contains no fossil wood. 4. Basalt porphyry containing ~2 mm clinopyroxene phenocrysts.

Figure 6. Geology map of fossil forest area includes Eocene–Quaternary rock units. Modiﬁed from [21].

1.4. Age of Fossil Wood The Meshgin Shahr Fossil Forest consists of two elements: trees that lived during the Early Miocene during a time of volcanic quiescence and a forest that ﬂourished during the volcanically active Pleistocene. Preservation of the fossil-bearing strata is related to protection provided by erosion-resistant capping layers of ignimbrite and basalt deposited during the Pleistocene. The age of the sedimentary strata is considered to be Early Miocene, based on stratigraphic correlation and palynology. The sediments lithologically resemble the Ziveh Formation in the Moghan area in the northern part of the TSA. These sediments are interpreted to have originated as 1 ﬂuvially-dominated delta deposits [22]. As discussed later, diverse palynomorphs in two samples collected from marls within this rock unit are consistent with a Miocene age. Radiometric dating provides constraints on the age of the logs that are preserved in Pleistocene volcaniclastic deposits. This volcanism was associated with Mt. Sabalan (4820 m altitude), the largest volcano in northwest Iran. Sabalan eruptions began in the Miocene, but the major volcanic events occurred from the Pliocene to Quaternary. Early eruptions produced calc-alkaline and calc-alkaline to shoshonitic lavas, transitioning to sodic-alkaline lavas in the late Miocene [23,24]. Protracted eruptive activity took place between 5.6 and 1.4 Ma, as evidenced by K–Ar dating, producing volcanic phases that are known as Paleo-Sabalan and Neo-Sabalan [25–27].The Neo-Sabalan rocks have dacitic Geosciences 2020, 10, 283 8 of 29 compositions with phenocrysts of plagioclase +amphibole alkali-feldspar quartz. These dacitic ± ± strata contain the majority of the known fossil logs. The ages of the log-bearing strata are estimated from high spatial resolution and sensitivity U–Pb geochronology using Secondary Ionization Mass Spectrometry, which yielded zircon ages from 545 to 149 ka for Neo-Sabalan strata (all ages are averages of multiple determinations per sample). U–Th zircon geochronology for selected Neo-Sabalan rocks agrees with the U–Pb ages, with the youngest zircon rims dating to ca. 110 ka [27].

2. Methods Petrographic thin sections were prepared for all fossil trees. All specimens were deposited in the Natural History and Genetic Resources Museum, Pardisan Natural Park, Department of Environment of Iran, Tehran, Iran, with numbers MMTT-437-456. Mineralogic studies were made using 25 specimens, representing 12 fossil trees, as shown in Table1. Specimen densities were measured using a Mettler analytical balance (Metler-Toldedo LLC, Columbus, OH, USA), equipped with a hydrostatic weighing device [28]. X-ray diffraction patterns were obtained with a Rigaku Geigerflex diffractometer using a Cu target X-ray tube to provide Ni-filtered Kα radiation. SEM images were made using a Tescan Vega3 SEM, using fractured specimens mounted on 1 cm aluminum stubs using epoxy adhesive, sputter-coated with PD to provide electrical continuity. Major element compositions were determined for SEM samples using an Oxford energy dispersive XRF spectrometer. Optical photomicrographs were made using a Zeiss petrographic microscope equipped (Carl Zeiss Microscopy, White Plains, NY, USA) with a 5-megapixel digital camera). All mineralogic data in this report were obtained using laboratory facilities at Western Washington University, Bellingham, WA, USA. Palynology preparation followed the methods of [29,30].

3. Results

3.1. Palynology of Miocene Sediments The ﬂuvial sediments at Meshgin Shahr Fossil Forest were found to contain an abundance of well-preserved palynomorphs. Pollen and spores comprise at least 40 morphotypes; selected examples are shown in Figure7. These microfossils are worthy of detailed study. For the purposes of this report, two characteristics are important: the palynomorphs are consistent with the Miocene age suggested by lithostratigraphy and botanical diversity is evidence of a complex plant community containing angiosperms, gymnosperms and ferns. The fossil woods provide evidence of trees, but the palynomorphs also record the existence of shrubs and herbaceous plants. Geosciences 2020, 10, 283 9 of 29 Geosciences 2020, 10, x FOR PEER REVIEW 9 of 30

Figure 7. Selected palynomorphs from Miocene marlmarl at Meshgin ShahrShahr site.site. Spores: ( A) Unidentified Unidentiﬁed fern spore;spore; ( B(B) )Lycopodiumsporites Lycopodiumsporites;(C; ()CCyathidites) Cyathidites;(D; )(PolypodiaceosporitesD) Polypodiaceosporites;(E) ;Verruciatosporites (E) Verruciatosporites;(F) Pteris; (F) sp.Pteris Angiosperm sp. Angiosperm pollen: (pollen:G) Malvaceae; (G) Malvaceae; (H) Pterocarya (H);( PterocaryaI) Caryapollenites; (I) Caryapollenites;(J) Graminidites; (J.) GymnospermGraminidites. pollen:Gymnosperm (K) Pinuspollenites pollen: (K);( PinuspollenitesL) Abietinaepollenites; (L) Abietinaepollenites;(M) Cedripollenites; (M;(N) Cedripollenites) Podocarpites;(; O(N) )Araucariacites Podocarpites.; 3.2. Taxonomy(O) Araucariacites of Fossil. Wood

3.2. TaxonomyIdentification of Fossil of silicified Wood woods in the Meshgin Shahr Fossil Forest is difficult for several reasons. One difficulty is that the tissues in many specimens have been compressed or distorted after burial, Identification of silicified woods in the Meshgin Shahr Fossil Forest is difficult for several obscuring cellular detail. In addition, all but one of the trunks represent gymnosperms, and these are reasons. One difficulty is that the tissues in many specimens have been compressed or distorted after challenging to identify at the genus level. burial, obscuring cellular detail. In addition, all but one of the trunks represent gymnosperms, and Gymnosperm woods can be readily discerned from angiosperms because of several characteristics: these are challenging to identify at the genus level. the absence of conductive vessels, the lack of fibers, and the presence of bordered pits. Resin ducts Gymnosperm woods can be readily discerned from angiosperms because of several are commonly present and vary in size and abundance among various taxa. The most reliable form characteristics: the absence of conductive vessels, the lack of fibers, and the presence of bordered of identification for genera comes from the morphology of cross-field pits, which are the apertures pits. Resin ducts are commonly present and vary in size and abundance among various taxa. The that occur on areas of contact between horizontal ray cells and adjacent vertical tracheids. Although most reliable form of identification for genera comes from the morphology of cross-field pits, which cross-field pits show some degree of intergradation, they can be divided into several categories that are the apertures that occur on areas of contact between horizontal ray cells and adjacent vertical reflect taxonomy at the genus level [31]. The difficulty comes from the scarcity of ray cells in most tracheids. Although cross-field pits show some degree of intergradation, they can be divided into gymnosperms. In approximately 50 thin sections prepared from the Meshgin Shahr Fossil Forest, only several categories that reflect taxonomy at the genus level [31]. The difficulty comes from the scarcity a few slides revealed cross-field pits. We had hoped to prepare additional slides to provide taxonomic of ray cells in most gymnosperms. In approximately 50 thin sections prepared from the Meshgin clarity, but the closure of university research labs because of the Covid-19 pandemic has precluded that Shahr Fossil Forest, only a few slides revealed cross-field pits. We had hoped to prepare additional possibility. Detailed taxonomic analysis is a future goal. Meanwhile, the general anatomical features slides to provide taxonomic clarity, but the closure of university research labs because of the provide information for understanding the ancient forest. For specimens in which the wood was not Covid-19 pandemic has precluded that possibility. Detailed taxonomic analysis is a future goal. degraded prior to fossilization or distorted during early diagenesis, the anatomical preservation of the Meanwhile, the general anatomical features provide information for understanding the ancient fossil wood is fairly good, as shown in Figures8 and9. forest. For specimens in which the wood was not degraded prior to fossilization or distorted during early diagenesis, the anatomical preservation of the fossil wood is fairly good, as shown in Figures 8 and 9.

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FigureFigure 8. Anatomical8. Anatomical characteristics characteristics ofof gymnospermgymnosperm wood wood from from Meshgin Meshgin Shahr Shahr Fossil Fossil Forest. Forest. (A)Transverse(A)Transverse view, view, Trunk Trunk 8, showing 8, showing distortion distortion and compression and compression of annual of annual growth growth rings. (rings.B) Transverse (B) view,Transverse Trunk 4, displaying view, Trunk gradual 4, displaying transition gradual between transition earlywood between and earlywood latewood. and (C latewood.) Tangential (C) view, TrunkTangential 12, uniseriate view, ray Trunk cells 12, seen uniseriate in cross ray section. cells seen (D )in Transverse cross section. view, (D) TrunkTransverse 12, showing view, Trunk resin 12, canals. showing resin canals. (E) Radial view, Trunk 7, with fusiform tracheids and horizontal ray cells. (F) (E) Radial view, Trunk 7, with fusiform tracheids and horizontal ray cells. (F) Radial view, Trunk 7, Radial view, Trunk 7, showing bordered pits typical of conifers. showing bordered pits typical of conifers. Only one fossil trunk (Trunk 10, represented by sample T-19) is an angiosperm, as shown in OnlyFigure one 9. This fossil ring-porous trunk (Trunk wood 10,exhibits represented a combination by sample of anatomical T-19) isfeatures an angiosperm, that includes as vessels shown in Figurewith9. This both ring-poroussimple and scalariform wood exhibits perforation a combination plates, and ofnarrow anatomical uniseriate features rays. The that 0.3 includes m diameter vessels with bothof the simple fossil log and is scalariforman indication perforation that it is from plates, a tree and rather narrow than uniseriatea shrub. Families rays. Thethat 0.3have m these diameter of thefeatures fossil loginclude is an the indication Araliaceae, that Euphorbiacea, it is from a Rhamnaceae tree rather thanand Sapindacedae. a shrub. Families However, that as have with these featuresthe includegymnosperm the Araliaceae, woods, the Euphorbiacea, taxonomic assignment Rhamnaceae of this and dicot Sapindacedae. requires detailed However, anatomical as with the gymnosperminformation woods, that theis not taxonomic presently assignment available. Necessary of this dicot data requires include detailed the anatomy anatomical of vessel/ray information that isparenchyma not presently pits, available.type of parenchyma Necessary dist dataribution include and ray the cellular anatomy composition. of vessel / ray parenchyma pits, type of parenchyma distribution and ray cellular composition.

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Figure 9. Anatomical features of angi angiospermosperm wood, trunk T-10. ( A) Ring porous structure as seen in transverse view.view. ( B()B Transverse) Transverse view view showing showin branchg branch attachment. attachment. (C) Radial (C) view. Radial Vessels view. segmented Vessels bysegmented tyloses canby tyloses be seen can in proﬁlebe seen view in prof (arrow),ile view with (arrow), horizontal with horizontal rays. (D) Tangentialrays. (D) Tangential view showing view raysshowing in cross rays section.in cross section. (E) Tangential (E) Tangential view of view epithelial of epithelial cells (parenchyma cells (parenchyma cells) surroundingcells) surrounding a pit. (aF )pit. Paranchyma (F) Paranchyma cells adjacent cells adjacent to vessel. to ( Gvessel.) Tangential (G) Tangential view of vessel view showing of vessel scalariform showing scalariform perforation plateperforation (arrow). plate (arrow).

To summarize,summarize, MioceneMiocene strata strata at at Meshgin Meshgin Shahr Shah Fossilr Fossil Forest Forest preserve preserve diverse diverse palynomorphs, palynomorphs, but thebut twothe two silicified silicified trunks trunks are fromare from gymnosperms. gymnosperms. Of theOf 14the known 14 known trunks trunks in Pleistocene in Pleistocene beds, beds, 13 are 13 gymnosperms,are gymnosperms, and and 1 is 1 an is angiosperm.an angiosperm. 3.3. Density Measurements 3.3. Density Measurements The measurement of density (specific gravity) provides a simple method for evaluating the The measurement of density (specific gravity) provides a simple method for evaluating the mineralogy of silicified wood specimens. Woods mineralized with opal have densities of 1.9–2.1 g/cm3, mineralogy of silicified wood specimens. Woods mineralized with opal have densities of 1.9–2.1 compared to 2.3–2.6 g/cm3 for woods mineralized with chalcedony or quartz [28]. Densities of g/cm3, compared to 2.3–2.6 g/cm3 for woods mineralized with chalcedony or quartz [28]. Densities of specimens from 12 trees from the Meshgin Shahr Fossil Forest show wide variation, as shown in specimens from 12 trees from the Meshgin Shahr Fossil Forest show wide variation, as shown in Table1. Densities of some specimens indicate an opal composition (T-2, T-4, T-8, T-11, T-14, T-19, T-20, Table 1. Densities of some specimens indicate an opal composition (T-2, T-4, T-8, T-11, T-14, T-19, T-25), but other specimens have higher densities that are indicative of quartz or chalcedony (T-5, T-6, T-20, T-25), but other specimens have higher densities that are indicative of quartz or chalcedony (T-5, T-6, T-10, T-12, T-17, and T-18). Many specimens have densities in the range between 2.1 and

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3 2.3T-10, g/cm T-12,3. These T-17, intermediate and T-18). Many densities specimens may be have the result densities of wood in the that range is not between completely 2.1 and mineralized, 2.3 g/cm . withThese void intermediate spaces that densities reduce maythe apparent be the result density. of wood Another that is possibility not completely is that mineralized, the wood contains with void a mixturespaces thatof reduceopal and the quartz/chalcedony. apparent density. Another Evidence possibility from XRD is that patterns the wood and contains SEM and a mixture optical of microscopyopal and quartz support/chalcedony. the latter Evidencehypothesis. from XRD patterns and SEM and optical microscopy support the latter hypothesis. 3.4. X-Ray Diffraction 3.4. X-ray Diffraction X-ray diffraction patterns are useful for the identification of minerals present in fossil wood, X-ray diffraction patterns are useful for the identification of minerals present in fossil wood, with certain limitations. One of these is the inability to discriminate between quartz and chalcedony, with certain limitations. One of these is the inability to discriminate between quartz and chalcedony, which produce similar diffraction patterns. Opal-CT is readily discernible, evidenced by diffuse which produce similar diffraction patterns. Opal-CT is readily discernible, evidenced by diffuse peaks characteristic of cristobalite/tridymite. However, amorphous opal-A is not recognizable in peaks characteristic of cristobalite/tridymite. However, amorphous opal-A is not recognizable in XRD XRD patterns. This imposes a serious limitation for opalized wood. If a specimen consists of pure patterns. This imposes a serious limitation for opalized wood. If a specimen consists of pure opal-A, opal-A, no diffraction peaks would be evident. However, the presence of small amounts of any no diffraction peaks would be evident. However, the presence of small amounts of any crystallized crystallized mineral would produce diffraction peaks, introducing analytical confusion. For mineral would produce diffraction peaks, introducing analytical confusion. For example, a specimen example, a specimen that contains mostly opal-A, and a small amount of opal-CT, would produce a that contains mostly opal-A, and a small amount of opal-CT, would produce a pattern that would likely pattern that would likely be interpreted as representing an opal-CT composition. Similarly, opal-A be interpreted as representing an opal-CT composition. Similarly, opal-A wood that contains a small wood that contains a small quartz-filled fracture would produce a pattern that would appear to quartz-filled fracture would produce a pattern that would appear to indicate a quartz composition. indicate a quartz composition. Although opal-A is not recognizable in XRD patterns, optical and Although opal-A is not recognizable in XRD patterns, optical and SEM microscopy indicates this SEM microscopy indicates this amorphous mineral is an important constituent of many Meshgin amorphous mineral is an important constituent of many Meshgin Shahr Fossil Forest specimens. Shahr Fossil Forest specimens. Specimens from the fossil forest are characterized by three XRD patterns that represent opal-CT, Specimens from the fossil forest are characterized by three XRD patterns that represent pure quartz, and a mixture of opal-CT and quartz, as shown in Figure 10. These patterns are valuable opal-CT, pure quartz, and a mixture of opal-CT and quartz, as shown in Figure 10. These patterns for two reasons. First, they indicate that the mineralogy of the silicified wood is variable, both among are valuable for two reasons. First, they indicate that the mineralogy of the silicified wood is different fossil trees and within a single trunk. Second, the XRD patterns confirm that silica minerals variable, both among different fossil trees and within a single trunk. Second, the XRD patterns are the primary agents of petrifaction. confirm that silica minerals are the primary agents of petrifaction.

Figure 10. X-ray diﬀraction patterns for three typical specimens: T-3 (trunk 3), T-5 (trunk 4), FigureT-20 (trunk 10. X-ray 11). diffraction Optical microscope patterns for images three oftypical specimens specimens: T-3 and T-3 T-20(trunk show 3), T-5 amorphous (trunk 4), opal-A T-20 (trunkas the predominant11). Optical microscope constituent, images but X-ray of specim patternsens only T-3 show and peaksT-20 show for minor amorphous crystalline opal-A component as the predominantopal-CT and quartz.constituent, but X-ray patterns only show peaks for minor crystalline component opal-CT and quartz.

3.5. Optical Microscopy

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3.5. Optical Microscopy

MagniﬁedGeosciences images 2020, 10, obtainedx FOR PEER REVIEW from30 µm thick sections provide much information about13 of 30 the mineral composition and fossilization process for the fossil wood. One of the most important assets is that, Magnified images obtained from 30µm thick sections provide much information about the unlike XRD,mineral polarized composition light and microscopy fossilization process can show for the the fossil presence wood. One of of opal-A. the most Theimportant amorphous assets nature of this materialis that, unlike causes XRD, opal-A polarized to belight isotropic, microscopy showing can show athe black presence ﬁeld of opal-A. of view The under amorphous crossed-polar illumination.nature In of contrast, this material weakly-crystalline causes opal-A to be opal-CT isotropic, shows showing low a birefringence,black field of view in contrastunder to the brighter birefringencecrossed-polar illumination. of quartz and In contrast, chalcedony. weakly-c Figurerystalline 11 showsopal-CT plain shows light low /polarizedbirefringence, light in pairs for contrast to the brighter birefringence of quartz and chalcedony. Figure 11 shows plain two specimenslight/polarized that appear light pairs to consist for two ofspecimens opal-A. that A thirdappear pairto consist shows of opal-A. opal-A A mineralizedthird pair shows wood from sample T-19opal-A (trunk mineralized 10), showing wood from excellent sample anatomicalT-19 (trunk 10), preservation showing excellent for thisanatomical angiosperm. preservation for this angiosperm.

Figure 11.FigureOptical 11. Optical photomicrographs photomicrographs of of specimens specimens that that contain contain opal-A opal-A as the asprimary the primary constituent. constituent. For photoFor pairs photo (A pairs,B) andA–B and (C, DC–),D the, the photo on on the the left is left an isillumi annated illuminated with ordinary with transmitted ordinary light transmitted light andand the the right right photo photo shows shows transmitte transmittedd light with light crossed with polarizers. crossed (A, polarizers. B): Specimen (T-2A, B(trunk): Specimen 2), T-2 transverse view showing opal-A with small amounts of birefringent quartz filling voids. (C, D): T-3 (trunk 2), transverse view showing opal-A with small amounts of birefringent quartz filling voids. (trunk 3), longitudinal view, showing isotropic opal-A as the only visible component. (E, F): (C,D): T-3Specimen (trunk 3),T-19, longitudinal showing transverse view, view showings under ordinary isotropic illumination. opal-A Image as the E shows only three visible pairs component.of (E,F): Specimenlatewood/earlywood T-19, showing annual transverse rings, and views abundant under vessels ordinary that are illumination. evidence that the Image wood ( Eis) showsan three pairs of latewood/earlywood annual rings, and abundant vessels that are evidence that the wood is an angiosperm. At higher magnification, image (F) shows that some vessels are partitioned by tyloses, which are present in heartwood rather than sapwood. This feature is consistent with the position of this sample in the fossil trunk (e.g., it was collected from a middle position rather than the outer zone). Under polarized light, specimen T-19 is isotropic, producing a black field of view similar to photo (D). Geosciences 2020, 10, x FOR PEER REVIEW 14 of 30

angiosperm. At higher magnification, image F shows that some vessels are partitioned by tyloses, which are present in heartwood rather than sapwood. This feature is consistent with the position of

Geosciencesthis sample2020, 10, in 283 the fossil trunk (e.g., it was collected from a middle position rather than the outer14 of 29 zone). Under polarized light, specimen T-19 is isotropic, producing a black field of view similar to photo D. Figure 12 shows specimens mineralized with opal-CT, and with quartz. These transverse views showFigure cellular 12 architecture. shows specimens Cell walls mineralized may owe with their opal-CT, dark brown and with color quartz. to relict These organic transverse matter. views show cellular architecture. Cell walls may owe their dark brown color to relict organic matter.

Figure 12. TransverseTransverse views views of of fossil fossil wood wood showing showing ordina ordinaryry transmitted transmitted light light images images on the on left the and left andpolarized polarized light lightimages images on the on right. the right.(A) Specimen (A) Specimen T-1 (trunk T-1 (trunk1). Birefringent 1). Birefringent opal-CT opal-CT fills cell ﬁlls lumen. cell lumen.(B) Specimen (B) Specimen T-12 (trunk T-12 7) (trunk is similar 7) is in similar appearance in appearance to T-1, but to in T-1, this but specimen in this specimen cell, lumina cell, contains lumina containsonly quartz. only The quartz. opal-CT The and opal-CT quartz and compositions quartz compositions are consistent are consistent with specimen with specimendensities (e.g., densities 2.56 (e.g.,g/cm3 2.56 for g/T-12,cm3 forcompared T-12, compared to 2.17 to g/cm 2.173 g /forcm 3T-1).Opal-CTfor T-1).Opal-CT and and quartz quartz identifications identiﬁcations are are also supported by XRD patterns.

Optical photomicrographsphotomicrographs show show that that fossil fossil wood wood specimens specimens have experienced have experienced multiple episodesmultiple ofepisodes mineralization. of mineralization. Episodic mineralization Episodic ismineralization supported by severalis supported structural by characteristics: several structural (A) cell luminacharacteristics: that contain (A) cell diﬀ erentlumina mineralogy that contain from different cell walls; mineralogy (B) intercellular from spacescell walls; and larger(B) intercellular voids that containspaces and minerals larger di voidsﬀerent that from contain the cell minerals lumen; different (C) fractures from that the contain cell lumen; late-stage (C) fractures mineral that precipitates. contain Figurelate-stage 13 showsmineral several precipitates. examples Figure of these 13 show characteristics.s several examples of these characteristics.

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Figure 13. A FigureEvidence 13. Evidence for for multiple multiple phases phases of of mineralization.mineralization. (A ( ) Polarized) Polarized light light view view of specimen of specimen T-10 T-10 (trunk(trunk 6), showing 6), showing quartz-filled quartz-filled fracture fracture in in quartz-mineralizedquartz-mineralized wood. wood. (B) (SpecimenB) Specimen T-7 (trunk T-7 (trunk 4) 4) containscontains quartz-filled quartz-filled fractures fractures in quartz-mineralizedin quartz-mineralized wood. wood. The The cross-cutting cross-cutting relationships among among the quartzthe veins quartz are veins evidence are evidence that fracturing that fracturing and fracture and fracture filling occurredfilling occurred as several as several episodes. episodes. (C) Specimen (C) T-2 (trunkSpecimen 2)is T-2 opalized (trunk 2) wood is opalized (density wood 2.03 (density g/cm3 )2.03 that g/cm contains3) that acontains void space a void having space having a peripheral a layerperipheral of botryoidal layer opal-CTof botryoidal with opal-CT quartz with crystals quartz filling crystals the filling interior the interior zone. zone. (D) Specimen(D) Specimen T-9 T-9 (log 5) has intermediate(log 5) has intermediate opal/quartz opal/quartz density density of 2.30 of g /2.30cm3 g/cm, consistent3, consistent with with optical optical photomicrographs photomicrographs that showthat isotropic show opalizedisotropic woodopalized containing wood containing a zone filled a zone with filled layers with of opal-CT layers andof opal-CT microcrystalline and microcrystalline quartz. For photo pairs C and D, the image on the left is ordinary transmitted light; quartz. For photo pairs (C,D), the image on the left is ordinary transmitted light; the right image is the right image is polarized light. polarized light.

3.6. Evidence3.6. Evidence from from Polished Polished Slabs Slabs When sawn surfaces are polished, megascopic features are evident. These images and provide a When sawn surfaces are polished, megascopic features are evident. These images and provide “big picture” view of structural characteristics. These polished surfaces reveal the deformation of a “big picture” view of structural characteristics. These polished surfaces reveal the deformation of grain patterns, the presence of decayed areas and open or mineralized fractures, and color variations, as shown in Figure 14. Geosciences 2020, 10, x FOR PEER REVIEW 16 of 30

Geosciences 2020, 10, 283 16 of 29 grain patterns, the presence of decayed areas and open or mineralized fractures, and color variations, as shown in Figure 14.

Figure 14. Specimen 25 (log 12). Density of 1.99 g/cm3 indicates opal composition. (A,B): Exterior surface shows a network of quartz-ﬁlled fractures. (C,D): polished interior surface shows large void 3 spacesFigure that 14. contain Specimen white 25 opaque (log 12). silica, Density presumed of 1.99 to beg/cm opal-CT.indicates The lightopal colorcomposition. of the exterior (A,B): relative Exterior tosurface the dark shows brown a network color of of the quartz-filled interior suggests fractures. the (C,D presence): polished of relict interior organic surface matter shows that large became void bleachedspaces that from contain exposure white to weathering. opaque silica, presumed to be opal-CT. The light color of the exterior relative to the dark brown color of the interior suggests the presence of relict organic matter that 3.7. Scanningbecame Electronbleached Microscopyfrom exposure to weathering. SEM images provide supporting evidence for the presence of opal-A as a major constituent in 3.7. Scanning Electron Microscopy many samples and conﬁrms opal-CT as a component, as shown in Figure 15. SEM images provide supporting evidence for the presence of opal-A as a major constituent in many samples and confirms opal-CT as a component, as shown in Figure 15.

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FigureFigure 15. SEM 15. SEM evidence evidence of opal of phases.opal phases. (A) Specimen(A) Specimen T-2 (trunkT-2 (trunk 2), oblique 2), oblique longitudinal longitudinal view view showing cells mineralizedshowing cells withmineralized vitreous with opal. vitreous (B) Specimen opal. (B) Specimen T-16 (trunk T-16 8). (trunk Longitudinal 8). Longitudinal view showsview shows cells that havecells been that mineralized have been with mineralize hydrousd with opal hydrous that contains opal that prominent contains shrinkageprominent cracks.shrinkage (C cracks.) Specimen (C) T-2 (trunkSpecimen 2), radial T-2 orientation (trunk 2), showingradial orientation bordered show pitsing (arrows). bordered For pits specimens (arrows). A–C, For siliciﬁcationspecimens A–C, is in the formsilicification of opal-A. ( Dis) in Specimen the form T-1 of (logopal-A. 1), longitudinal (D) Specimen view T-1 shows(log 1), cells longitudinal mineralized view with shows vitreous cells opal that containsmineralized shrinkage with vitreous cracks, opal with that exterior contains surfaces shrinkage covered cracks, by with crinkly exteri microcrystalsor surfaces covered of opal-CT. by crinkly microcrystals of opal-CT. Opal-A in the Meshgin Shahr Fossil Forest woods does not show the obvious spherical structure Opal-A in the Meshgin Shahr Fossil Forest woods does not show the obvious spherical that has been reported for this silica polymorph in some geologically young specimens, as shown structure that has been reported for this silica polymorph in some geologically young specimens, as in Figure 16A,B. In some samples, opal-A appears as a coalescence of microspheres, as shown in shown in Figure 16A,B. In some samples, opal-A appears as a coalescence of microspheres, as shown Figurein 16FigureC,D. 16C,D. In the In Meshgin the Meshgin Shahr Shahr specimens, specimens, isotropic isotropic opal opal typically typically has has homogeneous homogeneous texture and vitreousand luster,vitreous as luster, shown as in shown Figure in 17 FigureA, but 17A, athigh but at magniﬁcation, high magnification, some specimenssome specimens show show granular texturesgranular that aretextures suggestive that are ofsuggestive relict microspheres, of relict microspheres, as shown as in shown Figure in 17FigureB. 17B.

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Figure 16. Neogene fossil wood showing opal-A microspheres. (A) Miocene, Virgin Valley northern FigureFigure 16. 16.Neogene Neogene fossil fossil wood wood showing showing opal-A microspheres. microspheres. (A) ( AMiocene,) Miocene, Virgin Virgin Valley Valley northern northern Nevada, USA. (B) Pliocene, Hazen, central Nevada USA. (C, D) Bordered pits in Pinus contorta, Nevada,Nevada, USA. USA. (B )(B Pliocene,) Pliocene, Hazen,Hazen, central Nevada Nevada, USA. USA. (C, (DC), DBordered) Bordered pits pitsin Pinus in Pinus contorta contorta, , showing coalescence of opal-A microspheres, from modern hot springs at Yellowstone National showingshowing coalescence coalescence of opal-Aof opal-A microspheres, microspheres, from from modern modern hot hot springs springs atatYellowstone Yellowstone NationalNational Park, Park, Wyoming, USA. Wyoming,Park, Wyoming, USA. USA.

FigureFigure 17. 17.(A )(A) Opal-A, Opal-A, Meshgin Meshgin ShahrShahr specimen T-2 T-2 (trunk (trunk 2). 2). (B) ( BPresumed) Presumed relict relict opal-A opal-A texture texture in in Figure 17. (A) Opal-A, Meshgin Shahr specimen T-2 (trunk 2). (B) Presumed relict opal-A texture in quartz-mineralizedquartz-mineralized wood, wood, specimen specimen T-12T-12 (trunk 7). 7). quartz-mineralized wood, specimen T-12 (trunk 7). Opal-CTOpal-CT is readilyis readily identiﬁable identifiable inin XRD patterns patterns of of opalized opalized wood wood from from the Shahr the Shahr Fossil Fossil Forest, Forest, Opal-CT is readily identifiable in XRD patterns of opalized wood from the Shahr Fossil Forest, and the characteristic crystalline form is observable in SEM images, as shown in Figure 18, and in andand the the characteristic characteristic crystalline crystalline formform is observab observablele in in SEM SEM images, images, as shown as shown in Figure in Figure 18, and 18 ,in and in optical photomicrographs, as shown in Figure 13C,D. Lepispheres typically occur along the margins opticaloptical photomicrographs, photomicrographs, as as shown shown in in Figure Figure 13 13C,D.C,D. Lepispheres typically typically occur occur along along the the margins margins of of open voids, where the opal-CT has space to develop this morphology. These hemispherical openof voids, open wherevoids, thewhere opal-CT the opal-CT has space has tospace develop to develop this morphology. this morphology. These These hemispherical hemispherical structures are similar to features described from Sinter Island, Taupo Volcanic Field, New Zealand, which are interpreted as representing the early stage of transition of opal-A to opal-CT [32].

Geosciences 2020, 10, x FOR PEER REVIEW 19 of 30 Geosciences 2020, 10, 283 19 of 29 structures are similar to features described from Sinter Island, Taupo Volcanic Field, New Zealand, whichGeosciences are 2020interpreted, 10, x FOR as PEER representing REVIEW the early stage of transition of opal-A to opal-CT [32]. 19 of 30

structures are similar to features described from Sinter Island, Taupo Volcanic Field, New Zealand, which are interpreted as representing the early stage of transition of opal-A to opal-CT [32].

Figure 18.FigureOpal-CT 18. Opal-CT lepispheres lepispheres on on walls walls of of openopen spaces spaces in specimens in specimens from th frome Meshgin the Meshgin Shahr Fossil Shahr Fossil Forest. (AForest.) Specimen (A) Specimen T-2 (trunk2). T-2 (trunk2). (B ()B Specimen) Specimen T-3 T-3 (trunk (trunk 3 outer 3 outer zone). zone). (C) Specimen (C) Specimen T-4 (trunk 3 T-4 (trunk 3 middle zone).middle zone). Figure 18. Opal-CT lepispheres on walls of open spaces in specimens from the Meshgin Shahr Fossil FibrousForest. (chalcedonyA) Specimen isT-2 visible (trunk2). in some(B) Specimen SEM images, T-3 (trunk for example,3 outer zone). as shown (C) Specimen in Figure T-4 19C,D,(trunk 3 and Fibrouscrystalline chalcedonymiddle quartz zone). is isidentifiable visible in optical some phot SEMomicrographs, images, foras shown example, in Figure as shown13A,B, and in SEM Figure 19C,D, and crystallinephotos, quartzas shown is identiﬁablein Figure 19A,B. in opticalHowever, photomicrographs, for many specimens asthat shown show chalcedony/quartz in Figure 13A,B, and SEM photos, aspeaks shown Fibrousin XRD in Figurechalcedonypatterns, 19 orA,B. is which visible However, have in some density SEM for values manyimages, greater specimens for example, than 3.2 thatas g/cm shown show3, the in chalcedonyformFigure of 19C,D, the silica /andquartz peaks cannotcrystalline be determined quartz is identifiable with certainty. in optical photomicrographs, as shown3 in Figure 13A,B, and SEM in XRD patterns,photos, as or shown which in have Figure density 19A,B. valuesHowever, greater for many than specimens 3.2 g/cm that, theshow form chalcedony/quartz of the silica cannot be determinedpeaks with in certainty.XRD patterns, or which have density values greater than 3.2 g/cm3, the form of the silica cannot be determined with certainty.

Figure 19. Microscrystalline quartz and ﬁbrous chalcedony is present in some specimens, as indicated by SEM images. (A) Specimen T-16 (trunk 8) consists of fractured opal surrounded by microcrystalline quartz. This mixed composition is consistent with the intermediate opal/quartz density of 2.35 g/cm3. (B) Specimen 9 (trunk 5). Subhedral quartz crystals developed on fracture surface in opalized wood, explaining the intermediate opal/quartz density of 2.30 g/cm3.(C) Specimen T-12 (trunk 7). Chalcedony zones in quartz-mineralized wood (density 2.58 g/cm3). (D) Specimen T-6 (trunk 4). High-magniﬁcation view of chalcedony in wood having an intermediate opal/quartz density of 2.43 g/cm3. Geosciences 2020, 10, x FOR PEER REVIEW 20 of 30

Figure 19. Microscrystalline quartz and fibrous chalcedony is present in some specimens, as indicated by SEM images. (A) Specimen T-16 (trunk 8) consists of fractured opal surrounded by microcrystalline quartz. This mixed composition is consistent with the intermediate opal/quartz 3 Geosciences 2020density, 10, 283 of 2.35 g/cm . (B) Specimen 9 (trunk 5). Subhedral quartz crystals developed on fracture 20 of 29 surface in opalized wood, explaining the intermediate opal/quartz density of 2.30g/cm3. (C) Specimen T-12 (trunk 7). Chalcedony zones in quartz-mineralized wood (density 2.58g/cm3). (D) 3.8. Wood ColorSpecimen and PorosityT-6 (trunk 4). High-magnification view of chalcedony in wood having an intermediate opal/quartz density of 2.43 g/cm3. Most samples are brown in color on freshly exposed surfaces, much lighter on weathered exteriors. 3.8. Wood Color and Porosity This natural bleaching suggests that wood color may be caused by the presence of relict carbon. SEM/EDS analysesMost samples show thatare brown carbon in iscolor present on freshly in varying exposed amounts, surfaces, generally much lighter rather on lowweathered but probably suﬃcientexteriors. to be aThis signiﬁcant natural bleaching pigment, suggests as shown that wood in Figure color may20. Porositybe caused wasby the not presence evaluated of relict in detail, carbon. SEM/EDS analyses show that carbon is present in varying amounts, generally rather low but but during specimen preparation the absorption of water into the end grain of fossil wood was a probably sufficient to be a significant pigment, as shown in Figure 20. Porosity was not evaluated in noticeabledetail, characteristic, but during specimen presumably preparation caused the by absorption the incomplete of water mineralization into the end grain of of intercellular fossil wood spaces. This interpretationwas a noticeable is consistentcharacteristic, with presumably SEM observations, caused by the whereincomplete topographic mineralization cell of structure intercellular is visible only whenspaces. spaces This betweeninterpretation cells is remain consistent open. with SEM observations, where topographic cell structure is visible only when spaces between cells remain open.

Figure 20.FigureSEM 20./EDS SEM/EDS spectra spectra typically typically show show silicon silicon and and oxygen oxygen as as the the major major constituents,constituents, with with a a small peak representingsmall peak representing relict carbon. relict The carbon. Pd peak The isPd a peak spectral is a artifactspectral causedartifact caused by sputter-coating by sputter-coating specimens with thisspecimens metal to with provide this metal electrical to provide conductivity. electrical conductivity.

3.9. Minor3.9. AccessoryMinor Accessory Minerals Minerals Most specimensMost specimens contain contain silica mineralssilica minerals as the as only the observableonly observable constituent, constituent, but but in ain few a few specimens specimens high magnification reveals additional minerals in small amounts. These include gypsum high magniﬁcation reveals additional minerals in small amounts. These include gypsum and calcium and calcium phosphate (apatite or francolite). These minerals typically occur in cell lumen in phosphateopalized (apatite wood, or francolite). where they These provide minerals evidence typically of multiple occur episodes in cell lumen of mineralization in opalized during wood, where they provide evidence of multiple episodes of mineralization during diagenesis, as shown in Figure 21. Geosciencesdiagenesis, 2020 as, 10 shown, x FOR PEERin Figure REVIEW 21. 21 of 30

FigureFigure 21. Accessory 21. Accessory minerals minerals in in opalized opalized wood.wood. (A (A) )Specimen Specimen T-25 T-25 (trunk (trunk 12). Cell 12). walls Cell are walls are mineralizedmineralized with opal,with lumenopal, lumen contain contain crystalline crystalline gypsum. gypsum. (B ()B Specimen) Specimen T-9T-9 (trunk(trunk 5). Opalized wood wood with lumen filled with blocky crystals of calcium phosphate (apatite or francolite). with lumen ﬁlled with blocky crystals of calcium phosphate (apatite or francolite). 4. Discussion Fossil wood occurs at other sites in Iran, and the main importance of the Meshgin Shahr Fossil Forest is the abundance of large well-preserved logs. The only strata that have previously been reported to preserve intact tree trunks are coal-bearing beds in the Upper Triassic to Lower Jurassic (Norian–Batjocian) Shemshank Group in the Alborz Mountains of central Iran. These Mesozoic fossil trees have been identified as the form genera Prototaxoxylon, Metataxodioxylon, Xenoxylon, Protopinoxylon and Protosciadopitys[33–35]. The Meshgin Shahr Fossil Forest represents a much younger age and very different geologic setting. The woods preserved in Pleistocene presumably represent extant species, though perhaps not taxa that still survive in the region.

Geosciences 2020, 10, 283 21 of 29

4. Discussion Fossil wood occurs at other sites in Iran, and the main importance of the Meshgin Shahr Fossil Forest is the abundance of large well-preserved logs. The only strata that have previously been reported to preserve intact tree trunks are coal-bearing beds in the Upper Triassic to Lower Jurassic (Norian–Batjocian) Shemshank Group in the Alborz Mountains of central Iran. These Mesozoic fossil trees have been identiﬁed as the form genera Prototaxoxylon, Metataxodioxylon, Xenoxylon, Protopinoxylon and Protosciadopitys [33–35]. The Meshgin Shahr Fossil Forest represents a much younger age and very diﬀerent geologic setting. The woods preserved in Pleistocene presumably represent extant species, though perhaps not taxa that still survive in the region.

4.1. Paleoecology, Paleogeography and Paleoclimate Miocene Trunk 1 is from a conifer, but diverse palynomorphs suggest the existence of a mixed conifer/deciduous forest. The abundance of fern spores suggests this was perhaps a successional flora, where fern glades were interspersed with forest trees. Fossil pollen is not preserved in the Pleistocene volcaniclastic sediments, but the abundance of fossil wood provides evidence of the paleocology. With only one dicot specimen and 13 gymnosperms, the ancient forest was presumably predominately coniferous. The presence of well-developed annual rings indicates a seasonal climate. The dominance of conifers contrasts to modern temperate forests in northern Iran and adjacent Azerbaijan, where angiosperms predominate, with conifers comprising a minor element. In view of the young geologic age of the volcaniclastic strata at Meshgin Shahr Fossil Forest, the difference between the ancient forest and the modern treeless desert is the result of environmental change rather than botanical evolution. The dominant cause of botanical change was climate change that occurred at global and regional level. The cycle of glacial and interglacial episodes was linked to astronomical cycles (Milankovich cycles) that had worldwide effects. At a regional level, latitude plays an important role. In winter, large barometric pressure differences between equatorial and polar latitudes enhance circumpolar vortices and equatorial high-pressure cells. The result is westerly wind that increases precipitation in the middle latitudes. In summer, these patterns are reduced, causing a decline in precipitation [7]. It is, therefore, no surprise that fossil woods at Meshgin Shahr Fossil Forest show prominent earlywood/latewood rings. The question is why this Pleistocene coniferous forest disappeared. Eruptive events produced pyroclastic flows that may have decimated local forests, but the occurrence of fossil trunks at multiple stratigraphic levels is evidence of botanical recovery. Therefore, volcanism alone cannot be invoked as a source of forest extinction. This transition cannot be explained by plate tectonic motion because the continental position has not significantly changed since the mid-Pleistocene. Part of the explanation may be glacial fluctuations. The Meshgin Shahr forest presumably developed during the moist mild-temperature climate of an interglacial episode; the cool, dry conditions of a subsequent glacial phase may have created an unfavorable environment. The final disappearance of the coniferous forest was probably the result of increased aridity that resulted from orographic change. At present, about 75% of the total land area of Iran has arid or semi-arid climate with annual precipitation rates of ~350 mm to less than 50 mm. This aridity is caused by intense solar radiation and the transport of dry air masses by north-westerly to north-easterly winds. These effects are enhanced by the Alborz and Zagros mountain ranges, which prevent moisture-laden air masses from reaching the Iranian plateau. Conditions tend to be milder in northwest Iran because of the proximity of the Caspian Sea, but the present aridity of the Meshgin Shahr Fossil Forest area is ample evidence that the environment has greatly changed from that of the mid-Pleistocene. Modern forests exist in areas of northern Iran and adjacent Azerbaijan, but the botanical composition of these plant communities is very different from the ancient forests. The extant forests sometimes contain a few conifers, but the dominant components are angiosperms, particularly Fagus (Beech), Carpinus (Hornbeam), Fraxinus (Ash), Quercus (Oak)and a host of other broadleaved trees [36,37]. Geosciences 2020, 10, 283 22 of 29

What happened to descendants of the Pleistocene conifers at Meshgin Shahr? The present lack of detailed identiﬁcations is a hindrance for interpretation, but the general characteristics of the Meshgin Shahr fossil woods are reminiscent of the genera Pinus, Picea and Juniperus. The present ranges of extant members in the Middle East are geographically and ecologically diverse. The only native species of Pinus is P. eldarica (Afghan Pine), also knows a P. brutia var. eldarica (Turkish Pine). This tree is presently native to Armenia, Azerbaijan, Georgia, northern Iraq, and Turkey, where it grows from sea level to 1200 m, attaining heights of 20–35 m [38]. The region’s extant Picea is P. orientalis (Oriental Spruce = Caucasian spruce), ranging to the Caucasus Mountains of Russia, Georgia, Azerbijanm, Armenia and northeast Turkey. The Caucasian Spruce can also be found in Northern Iran, though its numbers have decreased due to deforestation. The tree is a large coniferous evergreen tree, typically growing to 30–45 m and with a trunk diameter of up to 1.5 m; much larger individuals are known. The dominant habitat is on moist, shaded slopes at elevations of 1000–2000 m [39]. The situation with Juniperus is more complex, because four species are known from Iran and neighboring countries, as well as a relative, Cupressus sempervirens (Mediterranean Cypress). These small coniferous evergreen trees and shrubs typically have wide geographic distribution. Indeed, J. communis has the largest geographical range of any woody plant, with a circumpolar distribution throughout the cool temperate Northern Hemisphere from the Arctic south in mountains to around 30◦N latitude in North America, Europe and Asia. Relict populations can be found in the Atlas Mountains of Africa [40]. In summary, the conifer-dominated Pleistocene forests typiﬁed by Meshgin Shahr have disappeared from the region, but individual members are represented by extant genera that persist in disparate environments.

4.2. Fossilization Process: Source of Silica Although silica is the most abundant constituent of the Earth’s crust, most silicate minerals have low solubility under near-surface conditions. Dissolved silica, necessary for the silicification of wood has two main sources: volcanic glass and feldspar. Volcanic glass has high solubility compared to crystalline silicate minerals, and many of the world’s fossil forests represent trees that were buried by volcanic ash [41,42]. Silica released from glassy matrix in basalt or other extrusive rocks may also result in wood petrifaction [43,44]. In sedimentary environments, the breakdown of feldspar can be an important source of dissolved silica [45]. At Meshgin Shahr Fossil Forest, silicified tree trunks are preserved within a 68 m thick stratigraphic succession where lithic tuffite comprises the upper 40 m. Eleven of the twelve tree trunks studied in this report came from this volcaniclastic zone, the exception being a single specimen from the underlying silty sandstone. The felsic tuffaceous strata are the likely source for dissolved silica in local groundwater. The matrix for lithic tuffite is primarily composed of felsic volcanic glass; phenocrysts include abundant plagioclase crystals, as shown in Figure 22.

4.3. Mineralization Sequence Mineralization sequence is not easy to interpret for the Meshgin Shahr fossil wood. One of the striking characteristics of silicified wood in the fossil forest is the presence of amorphous opal (opal-A) as a major component of many fossil tree trunks. Although amorphous hydrous silica is a common initial precipitate during silicification, opal-A is rarely observed in fossil wood. One explanation is the rapid rate of transformation of opal-A to opal-CT. Most opalized wood consists of opal-CT [46,47]. Opal-A mineralization of fossil wood is restricted to geologically young specimens, the most common examples being modern woods exposed to silica-rich hotspring water (e.g., [48]). Older examples include Miocene woods from northern Nevada, USA [49]. The presence of opal-A in the Meshgin Shahr Fossil Forest is consistent with the young Pliocene/Quaternary age. However, the digenetic evolution of the silicified wood is not fully clear. Geosciences 2020, 10, 283x FOR PEER REVIEW 23 of 30 29

Figure 22. Lithic tuffite. (A). Sample S8, from approx. 37 m stratigraphic level. (B). Magnified image Figure 22. Lithic tuffite. (A). Sample S8, from approx. 37 m stratigraphic level. (B). Magnified image showing quartz, feldspar, and pyroxene phenocrysts in tuffaceous matrix. (C). Sample S6, from 31 m showingstratigraphic quartz, level. feldspar, Plagioclase and phenocrystspyroxene phenocrysts in tuffaceous in matrix.tuffaceous Left matrix. photo: ( ordinaryC). Sample transmitted S6, from light.31m stratigraphicRight photo: transmittedlevel. Plagioclase polarized phenocrysts light. The in glassy tuffaceous tephra matrix. that comprises Left photo: the matrixordinary isisotropic. transmitted light. Right photo: transmitted polarized light. The glassy tephra that comprises the matrix is isotropic.The strongest evidence for the presence of opal-A comes from polarized light views of petrographic thin sections, where the isotropic optical properties of opalized woods are consistent with an opal-A 4.3.composition, Mineralization as shown Sequence in Figure 13. However, XRD patterns of the opaline specimens all show the presenceMineralization of opal-CT, sequence even in is samples not easy where to interpret the mineral for the was Meshgi not detectedn Shahr fossil by optical wood. microscopy. One of the Astriking possible characteristics explanation isof thatsilicifi opal-Aed wood is in thein the process fossil of forest undergoing is the apresence solid-state of transformationamorphous opal to (opal-A)opal-CT, as a reactiona major thatcomponent has not of proceeded many fossil to atree point trunks. where Although cristobalite amorphous/tridymite hydrous lattices silica are large is a commonenough to initial be microscopically precipitate during visible. Thesilicification, transformation opal-A of is opal-A rarely to opal-CTobserved is in likely fossil to occurwood. during One explanationthe aging of silicifiedis the rapid wood, rate based of transformation on evidence fromof op diagenesisal-A to opal-CT. of siliceous Most biogenicopalized sediments wood consists [50–57 of] opal-CTand siliceous [46,47]. hot Opal-A spring sintersmineralization [58–62]. of fossil wood is restricted to geologically young specimens, the mostQuartz-mineralized common examples specimens being modern probably woods represent exposed two tovery silica-rich different hotspring mechanisms. water (e.g., For some[48]). Oldersamples, examples SEM images include show Miocene quartz-mineralized woods from northern wood tissue Nevada, that is USA similar [49]. in The morphology presence toofspecimens opal-A in thethat Meshgin consist primarily Shahr Fossil of opal-A, Forest asis consistent shown in Figure with the 15. young This morphology Pliocene/Quaternary resembles age. the microspherical However, the digeneticstructure evolution of opal-A of as the an silici earlyfied phase wood inis not the fully precipitation clear. of silica in modern hot springs [31]. TheseThe similarities strongest support evidence the for widely the presence accepted hypothesisof opal-A comes that amorphous from polarized opal can light transform views of to petrographiccrystalline forms thin ofsections, silica. However, where the this isotropic hypothesis optical assumes properties that of opal-CT opaliz ised an woods intermediate are consistent phase withbetween an opal-Aopal-A andcomposition, chalcedony as/quartz. shown However, in Figure opal-CT, 13. However, chalcedony, XRD and patterns quartz of appear the opaline to have specimensalso developed all show as direct the presence precipitates of opal-CT, during multipleeven in samples episodes where when the fractures mineral and was other not opendetected spaces by wereoptical mineralized microscopy. by elementsA possible carried explanation by groundwater. is that opal-A is in the process of undergoing a solid-stateResearch transformation from other regions to opal-CT, provides a abundantreaction that examples has not of petrified proceeded wood to that a point was resulted where cristobalite/tridymitefrom multiple episodes lattices of mineral are large deposition enough [to49 ,be63 –microscopically69]. At Meshgin visible. Shahr The Fossil transformation Forest, mineral of opal-Afeatures to suggest opal-CT that is thelikely following to occur processes during werethe aging involved of silicified in wood wood, petrifaction: based on evidence from diagenesisStep 1—Initial of siliceous silicification: biogenic sediments Experimental [50–57] studies and siliceous microscopic hot sp examinationring sinters [58–62]. of silicified wood suggestQuartz-mineralized that the silicification specimens of wood probably typically beginsrepresent with two the very precipitation different of mechanisms. amorphous silica For insome the samples,cell wall, causedSEM images by the chemicalshow quartz-mineralized affinity of cellulose w andood lignin tissue for that silica is deliveredsimilar in by morphology the penetration to specimensof groundwater that consist [70]. Thisprimarily process of opal-A, is commonly as shown described in Figure as 15. “organic This morphology templating”. resembles This initial the microspherical structure of opal-A as an early phase in the precipitation of silica in modern hot

Geosciences 2020, 10, 283 24 of 29 silicification may leave larger spaces open. These open areas may include tracheid cell interiors (lumen), spaces between cells (intracellular spaces), conductive vessels, and voids created by decay or insect damage. Step 2—Later silicification: Subsequent mineralization episodes may result when minerals are precipitated within spaces left vacant during initial cell wall mineralization. For silica, the polymorph that is precipitated depends on chemical and physical factors. For example, high dissolved Si levels favor the rapid precipitation of amorphous silica (opal), while dilute Si solutions allow the gradual development of well-ordered lattices (chalcedony/quartz). Step 3—Fracture filling: Wood that has been partially mineralized is likely to become brittle, and post-burial physical stresses may produce open fractures. Fracturing may have been caused by shrinkage from loss of moisture, or by tectonic or structural forces that disrupted the semi-fossilized wood. At Meshgin Shahr Fossil Forest, many tree trunks show evidence of fracturing, visible in the field, as shown in Figure 13, and in collected specimens. Although some fractures remain unmineralized, commonly, these voids became filled with crystalline quartz, as shown in Figure 13A,B, and in some samples opal-CT is present as an initial layer, as shown in Figure 13C, or as an interlayering with chalcedony/quartz, as shown in Figure 13D. Step 4—Precipitation of non-silica minerals: The 25 specimens examined for this study contain silica minerals as the major cause of petrifaction, but in a few samples SEM images reveal the presence of other minerals, as shown in Figure 20. In specimen 9 (outer zone of trunk 9), blocky microcrystals of calcium phosphate fill some cell lumina. In Specimen 25 (outer zone of log 12), lumina contain gypsum crystals. In both specimens, the presence of these minerals is evidence that cell interiors (lumen) remained open after the cell walls had been mineralized with opal. Calcium phosphate has previously been reported as a major constituent in fossil woods that range in age from Carboniferous to Holocene (summarized by [69]), and as a minor constituent of Eocene carbonized wood in Eocene wood from California, USA [70]. The precipitation of calcium phosphate is favored by the presence of dissolved Ca and P, and low pH that favors the precipitation of Ca5(PO4)3. Silica precipitation requires higher pH. Gypsum is not well-known as a component of fossil wood. The presence of this mineral at Meshgin Shahr probably represents a time when groundwater contained dissolved sulfate, causing Ca to be precipitated as CaSO4 rather than as CaCO3 or Ca5(PO4)3. These variations are evidence that groundwater conditions changed during successive episodes of mineral deposition. Step 5—Diagenetic transformation: The final mineral composition of the fossil wood was presumably affected by silica transformations that included the conversion of opal-A to opal-CT, and the transformation of opal-CT to chalcedony/quartz.

4.4. Stratigraphic Considerations The Meshgin Shahr Fossil Forest preserves ancient trees than occur at multiple levels over approximately 50 m of stratigraphic section. These petrified trees show a range of mineral compositions, but these variations do not appear to be related to stratigraphic position. Density values, as shown in Table1, show that opalized woods occur at all stratigraphic levels within the tu ffaceous sediment. Quartz-mineralized wood is typically present as zones within logs that also contain opal. For example, in trunk 7, the central zone (specimen T-11) has a density of 2.07 g/cm3, typical of opal, but the outer zone contains material having a density of 2.56 g/cm3 (specimen T-12), representing pure quartz. A second sample from the outer zone (Specimen T-13) has an intermediate density of 2.23 g/cm3. Trunk 4 shows a reverse trend, with quartz-density wood in the central zone (specimen T-5), and lower density wood in the outer zone (T-7). These phenomena suggest that silica mineralization varied significantly within individual logs, regardless of their stratigraphic position. The driving forces for controlling mineral precipitations were probably changes in groundwater composition during diagenesis and changes in physical conditions (pH, eH, temperature) that affected both the precipitation of new minerals and phase changes within existing minerals. Geosciences 2020, 10, 283 25 of 29

4.5. Summary A reconstruction of the geologic history of Meshgin Shahr Fossil Forest is shown in Figure 23. The site preserves silicified tree trunks that date from two environments. Miocene fluvial strata contain diverse palynomorphs, and at least two conifer trunks. Abundant fossil logs occur at multiple levels in Pleistocene volcaniclastics that were produced by episodic eruptions of Mt. Sabalan. Our research provides a first look at the geology of the site, and a detailed consideration of the mineralogic processes that were responsible for wood silicification. In addition, we have made speculative interpretations regardingGeosciences 2020 paleoecology, 10, x FOR PEER and REVIEW paleoclimate. Hopefully, future investigations will provide detailed26 of 30 information on the paleobotanical characteristics of this important fossil site.

Figure 23. (A) Miocene floodplain forest. (B) By the middle Pleistocene, tilted sediments were eroded to Figure 23. (A) Miocene floodplain forest. (B) By the middle Pleistocene, tilted sediments were eroded produce a new land surface where forests flourished. (C) Episodic eruptions of Mt. Sabalam produced to produce a new land surface where forests flourished. (C) Episodic eruptions of Mt. Sabalam pyroclastic flows that buried local forests. (D) Later, deposition of basalt and ignimbrite provided a produced pyroclastic flows that buried local forests. (D) Later, deposition of basalt and ignimbrite capping layer. (E) Uplift and erosion combined to expose the Meshgin Shahr Fossil Forest. provided a capping layer. (E) Uplift and erosion combined to expose the Meshgin Shahr Fossil Forest.

Author Contributions: N.A. directed this study, conducted the field investigations, performed the initial microscopy of palynomorphs and fossil woods, and submitted the original research report. G.E.M. studied the mineralogy of fossil wood and wrote the first draft of this manuscript. N.A. and G.E.M. prepared the final version. A.H. organized specimens in the Museum of Natural History and Genetic Resources of Iran (MMTT), and she was supervisor of the original research project. Y.M. organized field studies, including providing a vehicle and dormitory, and assisted with field sampling.

Funding: Research by Abbassi was supported by the Department of Environment, Ardabil, Iran, and the Museum of Natural History and Genetic Resources of Iran (MMTT) in Tehran, Iran. Mustoe received no external funding.

Geosciences 2020, 10, 283 26 of 29

Author Contributions: N.A. directed this study, conducted the field investigations, performed the initial microscopy of palynomorphs and fossil woods, and submitted the original research report. G.E.M. studied the mineralogy of fossil wood and wrote the first draft of this manuscript. N.A. and G.E.M. prepared the final version. A.H. organized specimens in the Museum of Natural History and Genetic Resources of Iran (MMTT), and she was supervisor of the original research project. Y.M. organized field studies, including providing a vehicle and dormitory, and assisted with field sampling. All authors have read and agreed to the published version of the manuscript. Funding: Research by Abbassi was supported by the Department of Environment, Ardabil, Iran, and the Museum of Natural History and Genetic Resources of Iran (MMTT) in Tehran, Iran. Mustoe received no external funding. Acknowledgments: We thank the MMTT chairman, Mohammad Medadi and Zahra Orak, the vice manager of the MMTT Department of Paleontology, and the manager and personnel of the department. We also thank Javad Rabbani (University of Zanjan, Iran), Keith Richards (Amsterdam University, The Netherlands), Johannnes Martin Bouchal (Natural History Museum of Sweden) and Estella Leopold (University of Washington, USA) for their help with palynology. Theodore Dillhoff (Evolving Earth Foundation, USA), Elisabeth Wheeler (North Carolina State University, USA), Nareerat Boonchai (Florida State University, USA) and Marc Phillipe (University of Lyon, France) provided helpful advice regarding wood taxonomy. Two anonymous peer reviewers provided constructive suggestions for improving the manuscript. Conflicts of Interest: The authors declare no conflicts of interest.

References

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