The Allen Formation is a geological formation in Argentina whose strata date back to the Late Cretaceous (middle Campanian to late Maastrichtian). Dinosaur remains are among the fossils that have been recovered from the formation.Weishampel, David B; et al. (2004). "Dinosaur distribution (Late Cretaceous, South America)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 600-604. . Indeterminate chelid remains and other vertebrates have also been discovered in this formation.
Description
Uliana and Dellapé defined the formation's stratotype in 1981 in the eastern area of the Bajo de Añelo, where the relation between the base and top is clearly exposed. The deposits are mostly clastic, interbedded with banks of limestone and layers of anhydrite, which were defined as continental and shallow marine facies associated with semiarid conditions.
The interpreted sedimentary paleoenvironments range from purely continental such as ephemeral lacustrine, aeolian and fluvial systems to coastal marine paleoenvironments with development of estuaries and tidal flats, followed by a lagoon sedimentary stage from marsh to sea with carbonate precipitation in an area protected from waves, ending with a retraction leading to the accumulation of evaporites.
Armas and Sánchez performed a detailed facies analysis of the formation in 2015, where the authors concluded the formation represents a hybrid coastal system of tidal flats, dominated by Atlantic ingressions, with a large storm influence in some areas linked to aeolian systems.
A study of pollen found in outcrops of the middle member of the Allen Formation supported a late Maastrichtian age for these layers.
Fossil content
Dinosaurs Dinosaur eggs are known from the formation.
Ornithischians
Ankylosaurs Genus Species Location Stratigraphic position Material Notes ImagesAnkylosauria Indet.IndeterminateArriagada FarmTwo teeth, five osteoderms, and a partial sacrumFossil remains possibly belonging to a parankylosaur distinct from PatagopeltaPatagopeltaP. cristata Salitral Moreno localityLowerMultiple specimens and fragmentary remains, including skull bones, vertebrae, limb bones and girdles, and osteoderms. A parankylosaur
Hadrosaurs Genus Species Location Stratigraphic position Material Notes ImagesBonapartesaurusB. rionegrensis Salitral Moreno and Islas Malvinas.Lower A partial skeleton. A austrokritosaurian saurolophine Kelumapusaura K. machi Cerro Matadero, Arriagada FarmUpper Partial skull, a cervical vertebrae, several sacrals, sternal rib and sternal plate, and multiple referred specimens found in a bonebed A austrokrytosaurian saurolophine Lapampasaurus L. cholinoi Islas MalvinasLower Elements of the axial and appendicular skeleton of a subadult individual A hadrosaurid hadrosaur Willinakaqe W. salitralensisSalitral Moreno site.LowerA right premaxilla. A saurolophine hadrosauridJuárez Valieri et al., 2010
Saurischians
Sauropods Genus Species Location Stratigraphic position Material Notes Images Aeolosaurus A. rionegrinus Lower A aeolosaurin lithostrotianWeishampel et al., 2004, p.604 BonatitanB.reigi"63.10 Provincia de Río Negro, Argentina; 4. Río Colorado Formation," in Weishampel, et al. (2004). Page 604. Lower Braincases, caudal vertebrae, and several limb elements A saltasaurin saltasaurine Menucocelsior M. arriagadaiLower Seventeen caudal vertebrae and several appendicular bones: a right humerus, a left fibula and some metapodial. A eutitanosaurian titanosaurRolando et al., 2022 Panamericansaurus P. schroederi "Five tail vertebrata, sacral vertebrae, left humerus and rib fragments" A aeolosaurin lithostrotian"Table 13.1," in Weishampel, et al., 2004, p.269 Pellegrinisaurus? P. powelliLower (if it is from the formation) "Dorsal and caudal vertebrae, partial femur" A opisthocoelocaudiine saltasaurid Rocasaurus R. munioziMiddle "Partial postcranial skeleton" A saltasaurin saltasaurine"Table 13.1," in Weishampel, et al., 2004, p.270 Titanosauria indeterminateMiddle Teeth and partial skeletonLacks diagnostic materials to be named.
Theropods Genus Species Location Stratigraphic position Material Notes Images Austroraptor A. cabazai Lower A fragmentary skeleton including parts of the skull, lower jaw, a few neck and torso vertebrae, some ribs, a humerus, and assorted bones from both legs A unenlagiine theropod BonapartenykusB. ultimusUpper A holotype consists of a mid-dorsal vertebra, both scapulocoracoids, left tibia and femur, left pubis articulated with the pubic peduncle of the ilium, the anterior blade of the left ilium, and two partially preserved eggs. A patagonykine alvarezsaurid Lamarqueavis L. australisLower "Right coracoid with damaged sternal and omal extremities, and lacking acrocoracoidal process" A cimolopterygid ornithuranAgnolin, 2010 Limenavis L. patagonicaLower "Partial forelimb" A ornithuran bird"Table 11.1," in Weishampel, et al., 2004, p.215 NieblaN. antiquaUpperBraincase, fragmentary jaw and teeth, relatively complete scapulocoracoid, dorsal ribs, and incomplete vertebrae. A brachyrostran abelisaurid Quilmesaurus Q. currieiUpper Femur and distal tibia A carnotaurin furileusaurian"Table 4.1," in Weishampel, et al., 2004, p.77
Pterosaurs Fragmentary fossils are known from the formation.
Genus Species Location Stratigraphic position Material Notes Images Aerotitan A. sudamericanus Bajo de Arriagada Upper Partial rostrum The first unambiguous azhdarchid pterosaur from South-America
Fish Genus Species Location Stratigraphic position Material Notes Images Atlantoceratodus A. patagonicus 3 tooth platesA ceratorontiform lungfish Belonostomus B. lamarquensis A aspidorhynchid fish Chondrichthyes Indet indeterminate 11 vertebral centra Diplomystidae Indet. indeterminate 4 incomplete pectoral spines Siluriformes Indet. indeterminate 5 incomplete pectoral spines Lepisosteidae Indet indeterminate 6 vertebral centra Teleostei Indet. indeterminate 10 isolated teeth cf. Percichthyidae Indet. indeterminate 19 fragmentary vertebrae
Squamata Genus Species Location Stratigraphic position Material Notes Images Alamitophis A. argentinus incomplete trunk vertebra A madtsoiid snake Madtsoiidae Indet. indeterminate incomplete trunk vertebraPaleoteiusP. lakui Bonapartenykus site on Arriagada Farm Upper A specimen consists of a partial skull and skeleton belonging to a single individual. A scincomorph lizard Patagoniophis P. parvus incomplete trunk vertebra A madtsoiid snake Pleurodonta Indet. indeterminate incomplete left maxilla ?Scincoidea Indet. indeterminate incomplete left maxilla Teiioidea Indet. indeterminate incomplete right maxilla
Turtles Genus Species Location Stratigraphic position Material Notes Images Iaremys I. batrachomorpha A chelid turtle Trapalcochelys T. sulcata A meiolaniform turtle Yaminuechelys Y. gasperinii A chelid turtle
Rhynchocephalia Genus Species Location Stratigraphic position Material Notes Images Lamarquesaurus L. cabazai Cerro Tortuga A neosphenodontian sphenodontianApesteguía & Rougier, 2007, p.3
Plesiosauria Genus Species Location Stratigraphic position Material Notes Images Kawanectes K. lafquenianum Quarry of the “Bentonitas Patagónicas” company Upper A partial skeleton consists of six cervical vertebrae, three dorsal vertebrae, three sacral vertebrae, nine caudal vertebrae, right femur, right humerus, ilium, one 216 mesopodial element, one caudal phalanx and one caudal rib . A weddellonectian elasmosaurid
Frogs Unnamed frogs belonging to the family Calyptocephalellidae and Leptodactylidae, and those with no family designation were also found. Genus Species Location Stratigraphic positionMaterial Notes Images Calyptocephalella C. satan Cerro Tortuga Lower A right incomplete maxilla.A calyptocephalellid frog KuruleufeniaK. xenopoidesCerro Tortuga, Bajo Santa Rosa, and Cerro Bonaparte Lower An almost complete sphenethmoid.A pipid frog
Mammals The mammal fauna of the Allen Formation is known from seven teeth, which document the presence of several species.Rougier et al., 2009 Genus Species Location Material Notes Images Mesungulatum M. lamarquensis Cerro Tortuga Two upper molars and a fragmentary lower molar A dryolestoid mammal Groebertherium G. stipanicici Cerro Tortuga One upper molar A dryolestoid mammal cf. Brandonia sp. Cerro Tortuga One lower molar A dryolestoid mammal Barberenia B. allenensis Cerro Tortuga One upper molariform A dryolestoid mammal Solanutherium S. walshi Cerro Tortuga A right lower molar and left dentary fragment. A meridiolestid cladotherian Trapalcotherium T. matuastensis Cerro Tortuga One first lower molar A feruliotheriid gondwanathere
Plants Genus Species Location Stratigraphic positionMaterial Notes Images PodocarpoxylonP. mazzonii Valcheta Petrified Forest, Río Negro Province. LowerFossil wood A conifer tree WintucycasW. stevensonii Salitral Ojo de Agua, Río Negro Province. LowerSilicificated cycad trunks An encephalartoid cycad
See also List of dinosaur-bearing rock formations Adamantina Formation La Colonia Formation Lecho Formation Los Alamitos Formation Los Llanos Formation Marília Formation
References
Bibliography
Further reading