New fossil assemblage with amber, plants and vertebrates from the lower Cenomanian near Châtellerault (Vienne, western France)

– A large number of fossil-rich beds have been located from over 30 km along the Tours-Poitiers High Speed Line (LGV) during earthworks prior to its construction, and in particular amber was collected from Scorbé-Clairvaux (locality of La Bergeonneau) to the north of Poitiers. The paper describes also amber pieces from Châtellerault (locality of La Désirée) discovered during the development of sewage treatment plant along the Vienne river. Lower Cenomanian shelly sandstones and siltstones of Scorbé-Clairvaux contain rare amber pieces associated with seed plants ( Frenelopsis sp., Nehvizdya sp., and angiosperm seeds) and a diversi ﬁ ed fauna, composed of micro-remains of 27 taxa, comprising elasmobranchs ( Haimirichia amonensis Cappetta and Case, 1975, Protolama sp. and Squalicorax sp.), actinopterygians ( Enchodus sp. and Pycnodontidae), reptiles including vertebrae the marine snake Simoliophis rochebrunei Sauvage, 1880, some rare helochelydrid plates ( cf. Plastremys ), teeth of three crocodilian families (Atoposauridae, Goniopholididae and Bernissartiidae) and an undetermined dinosaurian long bone fragment.


Introduction
Amber-bearing deposits in the Vienne and more widely in the Poitou area have been little documented in the literature. Le Touzé de Longuemar (1866) has first reported amber pieces from the Cenomanian "green sandstones" near Chincé and Parigny, southwest of Châtellerault. Lacroix (1910) mentioned that "honey yellow resin was collected from the lignite at the base of the sandstones of Chéneché" near Marigny-Brizay. In their geological maps of Mirebeau, Cariou and Joubert (1989a, b) suggested that this amber was probably collected from lignitic claystones, fine glauconitic sands and sandstones, which are present in this area. In the 1990s, during some landscaping projects in the locality of "La Bourdillière", Saint Cyr, amateur paleontologists collected wood samples and rare altered amber pieces, today lost, from 8-m-deep lignitic claystones (M. Barkat, pers. com.). Valentin et al. (2014) has reported a few amber grains from the lower Cenomanian fossil-rich assemblage of Jaunay-Clan-Ormeau-Saint Denis; Part of the town of Jaunay-Clan is now called Jaunay-Marigny. They were collected during the LGV earthworks, and come from plant levels 10, 14 and 16 of Polette et al. (2019).
We report herein two other nearby amber localities with very similar geological settings in Châtellerault and Scorbé-Clairvaux. The latter locality has also yielded a diversified fauna and flora, with some taxa typically Cenomanian in age. Although the vertebrate material is fragmentary, its description provides further information concerning the poorly known fauna from the Cenomanian of France. We provide a preliminary survey of the paleobotanical and paleontological assemblages. The Cenomanian in the westernmost margin of the Basin of Paris has been surveyed by Le Touzé de Longuemar (1866) for the geological map of the Vienne department. It consists of a continuous depositional series up to the end of Turonian, and forms an isolated hill surrounded by the valley of La Pallu to the South and by the cuesta of Envigne to the North. Paleontological excavations during the High Speed Line "LGV SEA-Sud Europe Atlantic" railroad earthwork around La Pallu conducted by the research association Palaios provided us with a thorough knowledge of the Cenomanian series, which unconformably overlies the Upper Jurassic limestones. The lowermost Cenomanian beds consist of fine gray lignitic claystones containing numerous plants megafossils and millimetric amber pieces (locality of Jaunay-Clan-Ormeaux-Saint Denis, Valentin et al., 2014;Polette et al., 2019). Two new localities (Figs. 1A and 1B), Scorbé-Clairvaux-La Bergenonneau (SCB) and Châtellerault-La Désirée (CDR) corresponding to the base of this Cenomanian series yielded different fossil beds with diverse plant and animal assemblages ( Fig. 2 and Tab. 1). The series continues up into green sandstones, already described by Mathieu (1960Mathieu ( , 1968, and progressively reaches up into 30 m in thickness in Colombiers; they contain rich fossil wood and oyster shells (Exogyra colomba Lamarck, 1819), which are dated as middle Cenomanian in age (Robin et al., 2018;Boura et al., 2019). They are overlaid by upper Cenomanian marlstones, locally called "Chalk tufa", which bear Calycoceras Hyatt, 1900, by lower Turonian beds with Mammites nodosoides Schlüter, 1871 and Lewesiceras peramplum Mantell 1822, and by flint breccia deposits containing a few plant megafossil inclusions (Gomez et al., 2018).

Scorbé-Clairvaux-La Bergeonneau (SCB)
This fossil locality (46°48'06.75" N, 0°23'18.78" E) was discovered by one of us (X.V.) in 2013 at about 1.5 km of the town Scorbé-Clairvaux, in a place called "La Bergeonneau" on the side of the LGV earthwork (excavation side no PK 69900). The beds mainly consist of fine, green quartz sands, sometimes brown to yellow, glauconitic with consolidated calcareous sandstones, locally called "Grison". This facies is comparable to the "sands of Vierzon" of Alcaydé et al. (1976), which correspond to the base of the Cenomanian and well exposed from Marigny Brizay to Châtellerault Boura et al. (2019). The amber grains were collected from a lenticular, dark, clayey bed at the base of the section (Fig. 2), which also plant mesofossils preserved as wood and cuticle, and in particular the conifer Frenelopsis Schenk, 1869 that this genus has been already reported from a drill core sample of Monts-sur-Guesnes (Lecointre and Carpentier, 1939). The second amber-bearing bed is richer, and it is interspersed with a blue marly limestones containing the marine mollusk genus Astartes Sowerby, 1816 and the acanthoceratid ammonite Mantelliceras lymense Spath, 1926, which is notably known from the lower Cenomanian glauconitic Chalk Formation of the Sarthe (Kennedy in Morel, 2015). The upper part of the section consists of clayey sandstones with cross-stratified sandy strips, containing numerous coarse bioclastic elements forming a lumachelle with the oysters Ceratostreon flabellatum Goldfuss, 1837 and the Rhynchostreon suborbiculatum Lamarck, 1801 associated with vertebrate remains of crocodile, marine snake, turtle and numerous selacians (Fig. 2).

Châtellerault-La Désirée (CHD)
During work on a sewage treatment plant in Châtellerault in 2002, one of us (J.M.B.) collected millimetric amber pieces and decimetric charred wood fragments from lignitic claystones of La Désirée locality (46°49'57.38" N, 0°32'31.09" E). The stratigraphy could not be established because the fossil bed is in the river. A single unsuccessful borehole was attempted at the station's flow. The occurrence of similar gray ferruginous claystones underlying the Cenomanian green sandstones has been reported by Le Touzé de Longuemar (1866) in the locality of La Duranderie near Châtellerault (Fig. 1A).

In the field
Micro-remains were directly picked up on the prospection surfaces after the rains. Additionally, softened rocks directly collected from the bed in the field were screen-washed with water, and then cleaned with formic acid (10%). Specimens mostly consisting of isolated teeth and bones and teeth were sorted out under a Leica stereomicroscope.

Amber
Amber samples were prepared following the procedure described by Saint Martin and Saint Martin (2018) for the study of micro-inclusions. Photographs were taken with a camera Canon PowerShot A620 mounted on a Zeiss Axioscope 40 light microscope using the Zeiss Axiovision acquisition software. Some samples either coated or uncoated were mounted on aluminum stubs. They were observed with a Hitachi SU3500 scanning electron microscope (SEM).

Plants
Fossil-plant-bearing claystones were soaked into a plastic box of tap water for a day, and then the resulting mud was washed through a 0.1-mm-mesh sieve. Plant fossil fragments were dried naturally, and then dry sieved with sieves of 1-mm and 0.5-mm meshes. Specimens were sorted under a stereomicroscope Olympus SZX 10, and photographed with the same mounted with a Canon EOS 60D. Large handrock specimens were photographed with a Canon 100-mm macrolens.
All studied specimens which are catalogued UP/SCB are housed and curated by the "Centre de Valorisation des collections de l'Université de Poitiers (CVCU)". All amber pieces consist of fragments of originally larger blocks, and exhibit a wide range of shapes. They are translucent and red-colored (Fig. 3A). They are mostly centimetric. They show numerous narrow fractures (Figs. 3B and 3C), and are furrowed by yellow veinlets forming a sort of powdery network on the outer surface (Fig. 3A). In section they show a yellow color, and they are actually characterized by narrow fracturing lines and dense networks of filamentous appearance (Figs. 3D and 3E).

Châtellerault-La Désirée (CHD)
The amber grains really differ from those of Scorbé-Clairvaux in different aspects (Fig. 4A). Translucent grains usually exhibit an orange-reddish color, and a whitish crust around can be observed (Fig. 4A). Others are either rather dark brown or completely opaque. They are millimetric to centimetric. When they have the whole original size or mostly, they show the usual outer surface shrinkage cracks (Fig. 4B). Examination under the light microscope shows that the dark-brownish amber grains contain generally large bullous inclusions and/or very dense filamentous networks

Plants
The most abundant plant fossils is by far a conifer, Frenelopsis, which is a Cretaceous representative of the extinct family Cheirolepidiaceae. There are a few large, several-timesbranched shoots (Figs. 5A and 5B), but most specimens are isolated, three-leafed, sheathing whorls (Fig. 5C). They are mostly less than 10 mm long and 3 mm wide, and even numerous narrow specimens are less than 0.5 mm wide. Some specimens show a single branching arising through the leafy sheath (Fig. 5D), which is typical of branching in the vegetative leafy stems (Barral et al., 2019). A few others show three branchings per whorl (Figs. 5E-5H), which are probably where male cones are borne (Daviero et al., 2001;Gomez et al., 2002). Such tiny cones are very rare, being less than 4 mm long and wide (Figs. 5I and 5J). They are currently in preparation in order to isolate the pollen grains that may confirm affinities with the Cheirolepidiaceae.  Far less abundant are fragments of elliptic leaves of up to 15 mm wide showing about ten unbranched veins (Fig. 5K). These leaves are amphistomatic; on the adaxial surface stomata are randomly distributed (Fig. 5L), whereas on the abaxial surface stomatal bands alternate with non-stomatal bands along the veins (Fig. 5M Similar leaves are known as Nehvizdya Hlu stík, 1977 (e.g., Gomez et al., 2000). The most abundant seeds are about 4 mm long and 1 mm wide. They show a thin tegument and an apical micropyle (Fig. 5N). The constant absence of hilum suggests that they may probably belong to Coniferophytes rather than to Angiosperms.

Vertebrates
The rare vertebrates from the uppermost sandstones of Scorbé-Clairvaux-La Bergeonneau consist of millimetric to centimetric teeth, vertebrae, scales and bone fragments of 27 taxa, which mainly belong to selachians. Many of the genera identified are well-known and commonly reported from the Cenomanian localities of Charentes (e.g., Vullo et al., 2005;Vullo et al., 2007;Rage et al., 2016). They represent a mixed, terrestrial and marine, ecological assemblage (Tab. 1).

Paleoecological and paleoenvironmental insights
The ambers of SCB and CHD deposited in paralic basins with inputs from both the continent and the sea based on sedimentological and paleontological settings (Guelorget and Perthuisot, 1992). These tidally-influenced, temporally and  spatially submerged zones corresponded to either outer estuary or lagoon marine environments. The lower Cenomanian lense of lignitic claystones in SCB that bear amber, wood and leaves of the coastal mangrove-like Frenelopsis is overlaid by a decimetric brackish bed containing the euryhaline cardiid genus Protocardia Beyrich, 1845(Fürsich, 1994Néraudeau et al., 2013). Then, after a short episode of marine carbonate deposition (bed with Mantelliceras lymense), yellow and green sands with fine, detrital, transported grains set up and progressively pass to coarse bioclastic sandstones with abundant marine oysters Ceratostreon flabellatum and Rhynchostreon subordiculatum, which are typical of shallow, nearshore environments (Videt and Platel, 2005). These fossil vertebrate beds are in some cases associated with fine-grained sands to lumachelles. Based on thousands of animal microfossils collected, terrestrial components are rare compared to the abundance of marine fish and margino-littoral reptiles.
These microfossils are well-preserved and mainly composed of small individuals. They were transported and deposited at short distance. Overall, and taking particularly into account taphonomic processes, such fossil deposits are often related to lagoonal environments (e.g., Vullo et al., 2003). The paralic environments are often places of intense life activity (Hogarth, 1999), which is fed by both marine and terrestrial inputs.

Comparisons with other amber localities from the Cenomanian of western France
The amber grains from Châtellerault are mostly small, rarely droplet-shaped, and range from translucent to opaque, which are very similar to those reported from several Cenomanian localities of western France such as in Sarthe (Breton and Tostain, 2005;Breton, 2007;Girard et al., 2013), Dordogne (Saint Martin et al., 2013a), and Mayenne . These characteristics also prevail for ambers from different ages and origins, such as those from the Turonian of Dordogne  and the Albian-Cenomanian and Santonian of the south-eastern France (Saint Martin et al., 2012, 2013bFrau et al., 2020;Saint Martin et al., 2020). As in these latter ambers, the opaque appearance of certain amber grains from Châtellerault results from the colonization of the resin by networks of filamentous microorganisms like sheathed bacteria (Saint Martin and Saint Martin, 2018) and actinomyctes or a high density of bubbles. In contrast, the amber grains from Scorbé-Clairvaux show singular characteristics, especially with the veinlet network and the constant translucent appearance, which were not recognized in any other Cretaceous ambers. The microinclusion content also appears unique with only rare fungal mycelia (Saint Martin et al., work in progress).
The question of the botanical origin of amber during the Cretaceous and of its unicity per locality has been often debated. The majority of authors consider that the main producers of resin were conifers, but the main families concerned changed through the Cretaceous (for a review see Tab. 1 of Seyfullah et al., 2018). It is noteworthy that it was the time period when the Cheirolepidiaceae became extinct worldwide, the Araucariaceae decreased and are nowadays restricted into the Southern Hemisphere, whilst the Cupressaceae and to a lesser extent the Pinaceae increased. Only a few studies of Cretaceous materials indicated that there were more than one producer genus per locality during the Cretaceous. For instance Menor-Salván et al. (2010) suggested that both the Cheirolepidiaceae Frenelopsis and other undetermined botanical sources could produce resin that fossilized into a large quantity of amber from the Albian and Cenomanian of Cantabria, Spain. Moreover, Gomez et al. (2018) showed that amber solid bodies and ducts were preserved inside the mesophyll of at least five taxa (the conifers Frenelopsis and Arctopitys Bose and Manum, 1990, the two ginkgoes Nehvizdya and Pseudotorellia Florin, 1936 and one probable cycad) from the same Albian locality of El Soplao, Cantabria, Spain. So these observations suggest that resin production was, actually, a widespread response to abiotic and biotic constraints in Cretaceous gymnosperms. Frenelopsis and Nehvizdya are the two main taxa in terms of cuticle fragments in the plant fossil assemblage of Scorbé-Clairvaux, but further analysis is needed to determine whether the amber of this locality might originate from one, two or more botanical sources.