The Marnes d'Auzas Formation (Auzas Marls Formation) is a geological Formation in southwestern France whose strata date back to the Late Maastrichtian, approximately 67.5 to 66 million years ago. It consists primarily of marls with some interbeds of sandstones and limestones. These sediments correspond to varied paleoenvironments where lagoons, coastal marshes and more continental environments (lakes, rivers and alluvial plains) coexist and/or follow one another. The Marnes d’Auzas Formation has yielded numerous fossils of aquatic and terrestrial invertebrates and vertebrates, including dinosaurs.
Location
The Marnes d’Auzas Formation outcrops in the Aurignac, Saint-Martory and Plagne anticlines in the central part of the Haute-Garonne department (Petites Pyrénées), as well as in the Montfa anticline in Ariège department (Plantaurel massif) in Occitania region. The Marnes d’Auzas Formation conformably overlies the Nankin limestone Formation in the Aurignac anticline, the Calcarenites du Jadet Formation in the Saint-Martory and Plagne anticlines, and the Grès de Labarre Formation in the Montfa anticline. The Marnes d’Auzas Formation is conformably overlain by the Lower Paleocene (Danian) Aurignac Formation.
Lithostratigraphy and paleoenvironment The Marnes d’Auzas Formation is composed mainly of grey to multi-coloured clayey marls (ochre, green, red, or wine-coloured). The marls contain locally several limestone levels (calcarenites, sandy limestones, marly limestones, chalky limestones) and sandstone beds, sometimes cross-bedded. In the type locality of the formation, Bilotte assigns it a thickness of . Other publications indicate a thickness of or only about .
The Marnes d’Auzas Formation corresponds to sediments whose depositional environment evolved from the paralic domain (coastal lagoons, tidal marsh, tidal muddy channels, mudflats) at the base of the formation, towards a more continental domain (freshwater marshes and lakes, alluvial plain, fluvial channels) in its upper part. The Marnes d’Auzas Formation was deposited on the west coast of the former Ibero-Armorican island, which included a large part of France and Spain.
The rocks of the Marnes d’Auzas Formation and the underlying Ausseing Group attest the alternation of shallow marine sediments and coastal continental deposits (estuarine and fluvial) corresponding to the fluctuations in sea level and the variations in the coastline during the upper Maastrichtian in this part of the Ibero-Armorican island.
Age The Marnes d’Auzas Formation is dated to the late Maastrichtian on the basis of its paleontological content, which notably combines marine and continental microfauna and microflora characteristic of this age, but also thanks to the age of the underlying geological formations, and on correlations with the open-ocean deposits of the blue marls of Saint-Loup and yellow marly limestones of Gensac, two western lateral equivalents of the Marnes d’Auzas, which yielded ammonites and foraminifera characteristic of the late Maastrichtian. Furthermore, at Larcan, the yellow marly limestones of Gensac Formation yielded remains of the hadrosaur Canardia garonnensis (also known in the Marnes d’Auzas Formation) in a stratum located less than one meter below the iridium level marking the Cretaceous–Paleogene boundary (and therefore between the Maastrichtian and the Danian).
Paleontology
The Marnes d’Auzas Formation contains a wide variety of fossils related to the diversity of marginal-littoral and continental paleoenvironments that meet and/or succeed each other. The formation has yielded numerous foraminifera, ostracods, bivalves, gastropods and charophytes characteristic of different environments. Vertebrate remains are also abundant, including numerous dinosaur bones, among which the lambeosaurine hadrosaurids predominate.These dinosaurs are represented only by juvenile specimens. According to Yves Laurent, this could be explained by a taphonomic bias due to particuliar transport conditions that would have resulted in the deposition of small materials.
Vertebrates of the Marnes d'Auzas Formation
Fish Fish of the Marnes d'Auzas FormationGenusSpeciesLocationStratigraphic positionMaterialNotesImages Alestidae indet.IndeterminateTricouté 4MiddleA lower toothA characiform fishChondrichthyes indet.IndeterminateMarsoulasTeethCoupateziaC. fallax PeyrecaveTop of LowerA toothA dasyatid stingray. Specimen originally assigned to an undetermined species of Coupatezia closely resembling C. fallax. Subsequently, Cappetta and Corral assigned this tooth to this species. Lepisosteidae indet. IndeterminateCassagnau 1 and 2MiddleScales, vertebraeA garcf. Lepisosteuscf. L. sp.Cassagnau 1 and 2MiddleTeethA garPalaeogaleusP. sp.PeyrecaveTop of LowerTwo teethA triakid sharkParatrygonorrhinaP. amblysodaPeyrecaveTop of LowerTeethA guitarfish. The only tooth figured in 1997 was first attributed to Rhinobatos sp., then to P. amblysoda in 2007.PhyllodontidaeIndeterminateCassagnau 1MiddleTeethcf. Pycnoduscf. P. sp.Mérigon 2UpperTeethRhombodusR. binkhorstiPeyrecaveTop of LowerTeethA stingray of the extinct family RhombodontidaeSparidae indet.IndeterminateCassagnau 1 and MarsoulasMiddleTeeth
Amphibians Amphibians of the Marnes d'Auzas FormationGenusSpeciesLocationStratigraphic positionMaterialNotesImages Albanerpetontidae indet. IndeterminateCassagnau 1MiddleMDE-Cas1-156, a dentary fragmentAnura indet.IndeterminatePeyrecaveTop of LowerFragments of long bone, scapula, and possibly vertebrae and maxillae (cited but not described)Urodela indet. IndeterminatePeyrecaveTop of LowerUnspecified (cited but not described)
Squamates Squamates of the Marnes d'Auzas FormationGenusSpeciesLocationStratigraphic positionMaterialNotesImages Amphisbaenia? indet.IndeterminateTricouté 1LowerA vertebraTeiidae indet.IndeterminatePeyrecaveTop of LowerToothTeiiodea indet.IndeterminateTricouté 4MiddleA partial dentaryA possible barbatteiid?=Varanoidea indet.IndeterminateCassagnau 2MiddleMDE-Ca2-124, a dorsal vertebraA probable freshwater mosasauroid
Turtles Turtles of the Marnes d'Auzas FormationGenusSpeciesLocationStratigraphic positionMaterialNotesImages ?Foxemys?F. sp.Cassagnau 2MiddleMDE-Cas2-212, right epiplastron + entoplastronA bothremydid turtleIberoccitanemysI. atlanticumCassagnau 2MiddleMDE-Cas2-259, an almost complete shellA bothremydid turtle. This specimen, first assigned to a new species of the genus Elochelys (E. convenarum), was later referred to the new genus Iberoccitanemys with the new combination I. convenarum. This species was subsequently synonymyzed with Polysternon atlanticum (which does not belong to the genus Polysternon) under the new combination I. atlanticum.Pleurodira indet.IndeterminateCassagnau 1 and 2, Mérigon 1Middle and UpperDermal plates, girdle and limb elements
Crocodilians Crocodilians of the Marnes d'Auzas FormationGenusSpeciesLocationStratigraphic positionMaterialNotesImages Crocodyliformes indet. IndeterminateAuzas, Cassagnau 1 and 2, Mérigon 1, Tricouté 1 and 3, PeyrecaveLower to UpperTeeth (all sites) vertebrae (Cas 1 and Cas 2) MDE-Cas2-263, 264, osteoderms MDE-Cas2-30, articulated articular and surangular MDE-Cas2-141, one chevron MDE-Ca2-51, 224, two left scapulaeEusuchia indet. IndeterminateCassagnau 2MiddleMDE-Cas2-229, dorsal (thoracic?) vertebra ; MDE-Cas2-31, 218, dorsal (lumbar?) vertebrae ; MDE-Cas-32, caudal vertebraVertebrae possibly belonging to ThoracosaurusMusturzabalsuchus? M sp.?Cassagnau 1 and 2MiddleTeethA probable allodaposuchid eusuchianThoracosaurusT. neocesariensis?Cassagnau 1 and 2MiddleMDE-Cas1-199, almost complete skull lacking mandible Teeth MDE-Cas2-262, left humerus MDE-Cas2-33, 202, two cervical vertebrae MDE-Cas1-167 and MDE-Cas2-126, osteodermsSkull initially assigned to Thoracosaurus neocesariensis. According to Christopher Brochu, this specimen belongs to a taxon disctinct from T. neocesariensis. However, this material has not been re-studied since.
Dinosauria Dinosauria of the Marnes d'Auzas FormationGenusSpeciesLocationStratigraphic positionMaterialNotesImages Abelisauridae indet. IndeterminateCassagnau 1MiddleMDE-Cas1-210, a tooth highInitially referred to a Theropoda indet., this tooth has been reinterpreted as belonging to an indeterminate abelisaurid.CanardiaC. garonnensisTricouté 3LowerMDE-Ma3-25, an isolated dentary tooth crown MDE-Ma3-26, a portion of the dental battery of the right mandible MDE-Ma3-16, holotype right maxilla MDE-Ma3-15, a partial left maxilla MDE-Ma3-17, a left quadrate MDE-Ma3-18, a right prefrontal MDE-Ma3-19, a partial left surangular MDE-Ma3-28, an articular MDE-Ma3-29, a partial right postorbital MDE-Ma3-30, a partial right pterygoid MDE-Ma3-21 and MDE-Ma3-12, two partial left scapulae MDE-Ma3-24, an incomplete left sternal plate MDE-Ma3-20, a left humerus MDE-Ma3-23, a partial right pubisA lambeosaurine hadrosaurid. Prieto-Márquez et al. initially assigned Canardia to a clade of basal lambeosaurines, the aralosaurines. Later, Longrich et al. classified Canardia and all European and North African lambeosaurines into a more derived clade, the arenysaurines. Alternatively, Ramírez-Velasco did not recognize either the aralosaurine or the arenysaurine clades and positioned Canardia as the sister taxon to the clade Corythosauria. Dromaeosauridae indet. IndeterminateCassagnau 1 and 2MiddleMDE-Cas1-92, tooth) MDE-Cas2-28, tooth (the largest tooth high and wide) MDE-Cas2-232Enantiornithes? indet. IndeterminateCassagnau 1 MiddleMDE-Cas1-152, a proximal end of a scapulaAn enantiornithean birdEuronychodon?E. sp.MarsoulasMiddleMDE-Mar-03, toothHadrosauridae indet. IndeterminateAuzasMDE-Auz1-01, a proximal humerus ; a distal femurIndeterminateMérigon 1UpperDentary, toothIndeterminatePeyrecaveTop of LowerFragmentary teethIndeterminateTricouté 2Lower MDE-Ma2-01, fragment of dentary dental battery including four teeth caudal vertebrae tibiae (including MDE-Ma1-02, a long specimen) MDE-Ma1-03, a right metatarsal IV The teeth possibly belong to a lambeosaurineLambeosaurinae indet. IndeterminateCassagnau 1MiddleMDE-Cas1-200, proximal region of a left ischium MDE-Cas1-14, MDE-Cas1-215 et 216, three ungual phalanges MDE-Cas1-03, maxillary tooth crown three dentary tooth crowns (including MDE-Cas1-01 et MDE-Cas1-11)IndeterminateCassagnau 2MiddleMDE-Cas2-138 and MDE-Cas2-248, a predentary and a complete left dentary belonging to a single individual MDE-Cas2-02, a partial left dentary MDE-Cas2-11, a set of dentary tooth crowns MDE-Cas2-223, an isolated dentary tooth crown MDE-Cas2-207, a distal end of left humerus MDE-Cas2-23 and MDE-Cas2-249, tibiae MDE-Cas2-250, a right fibula An articulated partial postcranial skeleton, several ribs (including MDE-Cas2-260), a chevron (MDE-Cas-2-239), and centra of caudal vertebrae were also found.Remains first attributed to an indeterminate Euhadrosauria then to an indeterminate lambeosaurine. The pubis of the Cassagnau 2 hadrosaurid exhibits typical lambeosaurine characteristics and in particular shows similarities with the pubis of Parasaurolophus cyrtocristatus, Corythosaurus casuarius, Magnapaulia laticaudus, and, among Ibero-Armorican lambeosaurines, with Arenysaurus. The presence of accessory ridges on at least some dentary teeth from this locality also suggests that they belong to a lambeosaurine. Since the maxilla is not preserved in the Cassagnau 2 lambeosaurine, it is not known whether the material from this site belongs to Canardia (known both in older and younger strata of the same region) or to another lambeosaurine. IndeterminateTricouté 1Lower MDE-Ma1-01, dentary tooth crown ; another tooth is cited and figured by Gheerbrant et al.MDE-Ma1-01 was first attributed to an indeterminate hadrosaurid, then to an indeterminate lambeosaurine.Titanosauria indet. IndeterminateCassagnau 1 and 2MiddleMDE-Cas1-10, tooth MDE-Cas2-26, tooth MDE-Cas2-27, tooth