Conifers from the Cenomanian amber of Fouras (Charente-Maritime, western France)

– Fossil inclusions of arthropods and microorganisms are abundant in the Cretaceous amber from western France, but plant meso- or macroremains are scarce. Preserved remains are mostly tiny, very fragmented, and indeterminable. Only one amber locality in the Charente department has already provided conifer remains. Here, we report the ﬁ rst plant mesoremains ensnared in Cenomanian amber from Fouras – Bois Vert, in the Charente-Maritime department. They consist of three well-preserved leafy axes and one cone of Cheirolepidiacean conifers. Based on the helical arrangement of rhomboidal, longer than wide, and highly adpressed leaves, leafy axes are ascribed to the genus Pagiophyllum . The ovoid cone bears more than 15 imbricate, helically arranged, scale-like leaves and is ascribed to Classostrobus sp. Although Cretaceous ﬂ ora is abundant in lignitic clay from the Charentes region, Pagiophyllum is reported for the ﬁ rst time in Albian-Cenomanian deposits from this area. Xerophytic features of Pagiophyllum further support a harsh and instable coastal environment seasonally exposed to hot, dry conditions during the mid-Cretaceous in the Charentes region. mais les méso- et macrorestes végétaux sont quant à eux rares. Les restes préservés sont essentiellement minuscules, très fragmentés et indéterminables. Jusqu ’ ici, seule une localité du département de la Charente a livré de l ’ ambre contenant des conifères. Ici, nous présentons les premiers mésorestes végétaux connus dans l ’ ambre cénomanien de Fouras – Bois Vert, dans le département de la Charente-Maritime. Il s ’ agit de trois axes feuillés et d ’ un cône de conifère Cheirolepidiaceae bien préservés. Les axes feuillés sont attribués au genre Pagiophyllum sur la base de l ’ arrangement hélicoïdal de feuilles qui sont rhomboïdales, plus longues que larges et très plaquées sur les axes. Le cône est ovoïde, constitué de plus de 15 écailles arrangées de manière spiralée et est attribué à Classostrobus sp. Bien que les ﬂ ores crétacées soient abondantes dans les argiles ligniteuses de la région des Charentes, Pagiophyllum est pour la première fois signalé dans les dépôts albo-cénomaniens de ce secteur. Les caractéristiques xérophytiques de Pagiophyllum sont des arguments pour soutenir que les ﬂ ores du Crétacé moyen de Charente-Maritime étaient adaptées à des milieux côtiers saisonnièrement exposés à de fortes températures.


Geological setting
The pieces of amber containing the plant remains described herein were discovered in Cretaceous deposits from the "Plage de la Vierge" at Fouras -Bois Vert in the Charente-Maritime department (western France). Regionally, the upper Albian-Cenomanian series are divided into seven units, A to G (Néraudeau and Moreau, 1989;Moreau, 1996;Néraudeau et al., 1997). At Fouras -Bois Vert, the deposits are exposed from subunit B1 to subunit B3 which are early Cenomanian in age based on the presence of the foraminifera Orbitolina (Conicorbitulina) conica d'Archiac in the three subunits (Néraudeau et al., 2003). Subunit B1 consists of up to 5 m of fossiliferous silty to sandy limestone yielding rudists (Ichthyosarcolites triangularis Desmarest), oysters (Rhynchostreon suborbiculatum Lamarck) and the foraminifera Orbitolina plana concava Lamarck. Subunit B2 corresponds to up to 5 m of marl and glauconitic sand. Basally, this subunit (B2ms) consists of laminated grey to lignitic clay yielding abundant wood fragments (sometimes large trunks of up to 3 m long), amber and vertebrate remains (Néraudeau et al., 2003;Perrichot, 2005;Vullo et al., 2005). The wood was mainly ascribed to coniferous taxa such as Agathoxylon gardoniense Crié, Brachyoxylon Hollick & E. Jeffrey and Podocarpoxylon Gothan whereas a single specimen was ascribed to the Ginkgoalean wood Ginkgoxylon Saporta. The surface of trunks often bears marine oysters (Acutostrea lingularis Lamarck, Gyrostrea cf. delettrei Coquand) and xylophagous bivalve borings (Teredolites Leymerie). Amber from B2ms yielded various insect inclusions (Perrichot et al., 2010), among which occur some of the oldest known ants and trogossitid beetles (Perrichot et al., 2008b;Peris et al., 2014). The vertebrate remains were ascribed to terrestrial groups, such as dinosaurs (Iguanodontids; Néraudeau et al., 2003), pterosaurs (Vullo and Néraudeau, 2009) and turtles (Solemys de Lapparent de Broin & Murelaga; Vullo et al., 2010b), as well as marine ones such as the snake Simoliophis rochebrunei Sauvage and the shark Cretodus semiplicatus Agassiz (Vullo et al., , 2011. Upper part of the subunit B2 corresponds to sandstone with accumulation of oysters (Rhynchostreon suborbiculatum, Ceratostreon flabellatum Goldfuss). The subunit B3 consists of 6-8 m of limestone alternating with silty to sandy marl.

Material and methods
The pieces of amber were all collected in the subunit B2 (layer B2ms). The fossiliferous amber from Fouras -Bois Vert is mostly translucent and yellow to orange (Figs. 1A and 1B). Plant remains consist of three leafy axes and one conifer cone, that were originally preserved in two amber pieces with numerous arthropod inclusions: the piece IGR.FRS-1 contained 13 hexapods; and the piece IGR.FRS-7 contained 57 arthropods (see detailed list in Cockx et al., 2016: Tab. 1). For an optimal study of each fossil inclusion, these pieces were then cut in smaller fragments using scalpel blades, and each fragment was polished using silicon carbide papers on a Buehler Metaserv 3000 polisher. The conifer remains are preserved in three dimensions and show an exquisite preservation of the cuticle including stomatal apparatuses. Photographs were taken with a BMS digital USB camera (5 megapixels) mounted on a stereomicroscope Leica 125. The specimens presented in this paper are housed at the Geological Department and Museum of the University of Rennes 1.
Description. Leafy axes are straight (Figs. 1C, 1D, 1G, 1H) to slightly curved (Figs. 1E and 1F). The largest specimen is 10 mm long. Shoots are narrow and up to 1.5 mm in diameter. Leaves are persistent, helically arranged (phyllotaxy 3/8), highly adpressed and imbricated (Figs. 1C-1H). Leaves are scale-like, rhomboidal, longer than wide, and keeled on abaxial side (Figs. 1C-1J). They are 1.7-2.5 mm long and 0.6-0.8 mm wide. Apically, leaves display a free part which is up to 0.8 mm long. The abaxial surface of leaves is convex whereas adaxial surface is concave. The leaf margin is entire and the shape of the leaf apex is quite pointed. Surface of leaves locally shows the outlines of epidermal cells. On the abaxial surface, stomatal apparatuses are arranged in longitudinal rows that converge toward the leaf apex (Figs. 1I and 1J). Stomatal apparatuses on the abaxial surface are 50-70 mm long and 35-45 mm wide. Stomatal rows are not sunken but guard cells of individual stomata are sunken in a pit. Pit apertures are mainly oval, and sometimes circular. Their orientation is quite stable, mainly oriented transversally to the leaf margin. However, they are sometimes oriented obliquely to leaf margin. Subsidiary cells form a thick and well-marked rim around the pit apertures.
Abaxial surface of leaves partially shows the shape of ordinary epidermal cells which are rectangular and form longitudinal rows, that are oriented parallel to the leaf axis.
Remarks. The gross morphology of Pagiophyllum is close to those of the leafy axis Brachyphyllum Brongn. (e.g. straight twigs having needles helically arranged and small leaves). Harris (1979) highlighted the difficulty to distinguish both genera and proposed that the "sole distinction of Pagiophyllum from Brachyphyllum is its longer free leaf". He specified in the emended diagnoses of Brachyphyllum and Pagiophyllum "leaf composed of a basal cushion tapering into a small free part, length of free part (upper surface beyond leaf cushion) or total height of leaf and cushion (outward from shoot) less than width of leaf cushion" for the first, whereas "leaf about as broad as its basal cushion, length exceeding width of cushion" for the second. Brachyphyllum was reported from many Cretaceous palaeobotanical sites of western France (e.g. Coquand, 1860;Gomez et al., 2008;Moreau et al., 2014a). It differs from Pagiophyllum by its leaves that are as long as wide, with a shorter free part strongly appressed to the stem. Following Harris (1979), the specimens from Fouras can be confidently ascribed to Pagiophyllum.
Remarks. Inner structures of the cone are not visible. We may notice that tiny male cones ascribed to Classostrobus sp. and showing a gross morphology close to the specimen from Fouras -Bois Vert were discovered in Cenomanian lignitic clay from several areas of Charente-Maritime (e.g. Aix Island, Puy-Puy, Perrichot, 2005). Pagiophyllum and Classostrobus often co-occur in Mesozoic plant beds (Thévenard, 1993;Van Konijnenburg-Van Cittert, 1987). However, stomatal apparatuses being not visible, the cone from Fouras -Bois Vert cannot be confidently linked with the leafy axis Pagiophyllum described below.
Conifers with xerophytic characters have already been found among the Cretaceous coastal flora of western France (e. g. Brachyphyllum, Frenelopsis, and Glenrosa; Gomez et al., 2008;Moreau et al., 2015). According to these studies, these plants were adapted to withstand intense sunlight and coastal environments exposed to desiccant conditions coupled with saline sea water, and dry conditions. Pagiophyllum clearly shows fleshy shoots, small leaves pressed against the axis, thick cuticle, and sunken stomata apparatuses with subsidiary cells forming a thick rim. These features are those of a flora in a tropical climate with contrasted seasons cyclically dry (Thévenard et al., 2003). In accordance with this hypothesis, Néraudeau et al. (2003) interpreted the amber-bearing bed from Fouras as a paralic deposit showing both marine and terrestrial imputs. Complementing previous palaeobotanical analyses (e.g., Gomez et al., 2008;Moreau et al., 2015), the xerophytic features of Pagiophyllum are new arguments to support that Albian-Cenomanian coastal floras of Charente-Maritime were well-adapted to paralic environments seasonally exposed to hot, dry conditions. The Cretaceous flora from western France is mostly preserved as impressions and compressions with or without cuticle in clayey and lignitic deposits. By contrast with these highly compressed fossil plants, the Pagiophyllum leafy axes and the cone from the Fouras amber are preserved in three dimensions. Moreau et al. (2017aMoreau et al. ( , 2017bMoreau et al. ( , 2017c demonstrated that some specimens can exquisitely preserved most of the tissues, including epidermis, palisade parenchyma, spongy parenchyma, transfusion tracheids and vascular bundles. However, the inner structures of the specimens from Fouras cannot be explored using classic light microcopy. Although we cannot ensure such preservation, future investigations using 3D imaging technique such as synchrotron tomography should be appropriate to explore the histology of these specimens from Fouras.