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The James RossBasin located at the northeastern margin of the Antarctic Peninsula representsone of the best continuous expositions in the world for studying the K/Pgtransition. The whole K/Pg sequence(Marambio Group) is developed within a mid-shelf, deltaic setting in a back-arcbasin. Fossil plants were originally derived fromcontinental areas in the eastern margin of the Antarctic Peninsula and transportedto the shelf, thus providing one of the scarce documentation of paleocontinentalareas such high southern latitudes. Here, we synthetize our results basedon research allowed by the Instituto Antártico Argentino and Argentineangrants.The Santa Marta Formation, represents thebasal unit of the Marambio Group, being deposited during the Santonian-Campanian(e.g. Olivero 2012). Macrofossil plants have been collected at the upper BetaMember (mid Campanian). The macroflora is diverse and known by compressions ofleaves, seeds, wood, and twigs (Hayes et al. 2006; Iglesias 2016) as well as petrifiedwoods (Pujana et al. 2017, 2018). The fossil record includes cycads (Zamiaceae),conifers (Araucariaceae and Podocarpaceae), ferns (including Pteridaceae andSchizaeaceae?) and several angiosperms (including Atherospermataceae, Cunoniaceae,Lauraceae, Monimiaceae, Winteraceae). Permineralized specimens in marinecalcareous concretions have preserved vegetative and reproductive organs withdelicate tissues such as parenchymatic cells from leaf mesophyll and twig piths,with exquisite three-dimensional organic connections as well. Thepeel-technique has been used for anatomical studies, using petrographicsections, HCl dilution, and cellulose acetate sheets. The presence of framboidal pyrite and themicritic/sparitic cementation enabled very good preservation of the plant organsthat indicate fast and early diagenetic processes. Thiskind of preservation allows the fine study of leaf phylotaxis, disposition ofvascular traces and secretory channels, and the inner structure of fruit andseeds. Conifers are frequent in these concretions, consisting of leafy shootsand isolated leaves and seeds. So far we have differentiated almost four differentconifer foliage (Araucariaceae, Podocarpaceae, and Cupressaceae s.l.). Werecorded the first permineralized fern petioles and roots related toGleicheniaceae (in accordance with previous palynological studies). About angiosperms,so far there have been found two representatives of basal angiosperms of theANA grade (a seed of Nymphaeaceae and a stem of Chloranthaceae); as well as magnoliids,monocots, and eudicots (including the oldest record of reproductive structuresof Nothofagus). The information obtained from these peels provides plant anatomyfor precise taxonomy and better physiology, ecology, and reconstructions of theseplants; thus a better understanding of the continental paleo-biomes inAntarctica. The Snow Hill Island Formationrepresents the late Campanian infilling of the basin.Petrified woods of Laurales, Cunoniaceae, and two angiosperm incertae sedis were studied in detail (Pujanaet al. 2018 and a new wood of Illiciaceae? that is under study. Interestingly, this record encompasses a diversificationof Nothofagaceae pollen record with presence of three of the subgenerapollen-types.The Sandwich Bluff Member of the López de Bertodano Formation,located at Cape Lamb, Vega Island, is represented by a progradationalnear-shore marine environment. Although previous macrofloristic records wereknown, these were based only on broad conifer leaves assigned to Araucaria fibrosa. Previouspalynological analyses have also recorded Cyatheaceae, Dicksoniaceae,Gleicheniaceae, Lycophytes, Polypodiaceae, Schizaeaceae, Araucariaceae, Podocarpaceae,Proteaceae, Nothofagaceae, and several other angiosperms (Barreda et al. 2019).New collections have been found on a well preserved macroflora. A single pinnais assigned to the fern cf. Marattiopsisvodrazkae (Marattiaceae), formerly recorded for the earliest CampanianHidden Lake Formation. The iconic Australasian (also known in the Maastrichtianand Paleogene of Patagonia) extant genus Agathis(Araucariaceae) was identified based on fossil leaves. Among angiosperms, foursmall-sized (nanophyll) and dentate leaf morphotypes were identified. The angiospermleaves likely correspond to unrecorded species of the James Ross Basin. Amature fossil cupule (woody flower involucre) can be assigned to Nothofagaceae,together with new compression leaves.In the basin, the Paleoceneis represented by three marine sedimentary units: the upper units of the Lópezde Bertodano Formation, the Sobral Formation, and the Cross Valley-WimanFormation (CVWF).The upper part of the Lópezde Bertodano Formation (units 10 and 11) represents one of the best exposed sectionsrecording the K/Pg boundary in the world, including the first ~600 Ky of thePaleocene (Scasso et al. 2020). Few plant macrofossils were found, including Araucaria sp. branches with attached helicoidally arranged leaves. Concretionswith plants at a fish mortality horizon, 3 m above the K/Pg boundary, are understudy.The Sobral Formationrepresents the mid and upper Danian (C28 and C27 chrons) and still possessesscarce plant records with permineralized woods at its base.The uppermost Paleocene unit(CVWF) overlies the Sobral Formation above a high-relief erosive discordance.Contrary to the underlying units, the CVWF is characterized by the preservationof a rich flora of compressions and petrifications. It preserves leaves andseeds at its uppermost section (Bahía Pingüino Allomember) that has been datedas upper Paleocene (C25n, Thanetian) and corresponds to lagoon or protected bayfacies (Montes et al. 2019). Plant fossil records are known since Dusén (1908),who proposed 87 leaf taxa. Despite its Southern Hemisphere importance broadlycited in the scientific literature, only two papers have revised the plant typematerials, recognizing a richness reduction from the originally proposed taxa tothree fern species, two conifers, and twenty-five angiosperms (Cantrill et al.2011, Tossolini et al. 2020). New fieldworks has made it possible to identifythe nineteen previous taxa, recognized two more from Dusén (Mollinedia seymourensis and Phyllites sp. 14), and added seven newfindings: a fertile Dryopteridaceae, a filmy fern Hymenophyllaceae, and fiveangiosperm taxa, including Potamogetonaceae and Ripogonaceae. Unbiasedcollection allows measuring the relative abundance of the taphoflora; andindicates the dominance (35%) of ferns (Cladophlebisand Sphenopteris). The most commonangiosperm families are: Atherospermathaceae, Lauraceae, Moraceae?,Nothofagaceae, and Winteraceae. Araucariaceae are well represented, inagreement with wood studies.In summary, the Upper Cretaceousmacroflora is in accordance with an important floristic change at the lateCampanian-early Maastrichtian interval, which was supported by recentpalynological record (Barreda et al. 2019) in the Antarctic Peninsula. Thefloristic change can be associated to climatic shifting at the end of theCretaceous, although some Gondwanic plant families persisted and furtherdiversificated into the Antarctic Paleogene temperate forests (Scasso et al.2020). The new CVWF taphoflora overpasses the Paleocenerichness known for the Antarctic Peninsula, adds new elements, and allows to quantifythe relative proportion of taxa.Complete descriptions of these fossilplants and anatomical comparisons with living relative taxa are key tounderstand the continental environment where these plants lived, 84-57 millionyears ago, at such high latitudes of the Antarctic Peninsula; and to establishthe times for biogeographic connection between Patagonia and Australasia viaAntarctica.

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