The Chorrillo Formation, also called the Chorillo Formation,Río Leona at Fossilworks.org is a Late Cretaceous (Maastrichtian age) geological formation in southern Patagonia, Argentina. The formation is more than thick and underlies the Calafate Formation and rests on top of the La Irene Formation. It constitutes a significant uppermost Cretaceous continental sedimentary sequence within the Magallanes Basin, located in Santa Cruz Province, Argentina, approximately southwest of El Calafate. Initial investigations in the 1990s laid the groundwork, with detailed paleontological studies emerging since 2019, revealing a rich biotic assemblage. The formation correlates with the lower Dorotea Formation in Chile, sharing dinosaurian ecotypes, while both are clearly different from other northern Patagonian Maastrichtian units (e.g., Allen Formation, Los Alamitos Formation), which are dominated by saltasaurine sauropods and abelisaurids, suggesting potential temporal or environmental distinctions.
Geology The Chorrillo Formation is embedded within the foreland Magallanes Basin, shaped by Late Cretaceous Andean orogeny driven by subduction along the Pacific margin. Tectonic subsidence, coupled with sediment supply from the rising Patagonian Andes, facilitated deposition. Lithologically, the unit comprises fine- to coarse-grained clastics: mudstones and siltstones dominate floodplain facies, often organic-rich and pedogenically modified; sandstones, arkosic to lithic, form channel fills with cross-bedding; and conglomerates appear as basal lags. Clay mineralogy is dominated by smectite, with minor illite and kaolinite, reflecting volcanic ash alteration. Sedimentologically, it records a meandering fluvial system on a distal floodplain, featuring fining-upward channel sequences, crevasse splays, and overbank deposits. High-frequency paleosol stacking indicates local hydrologic and topographic variability, while low-frequency patterns suggest avulsion-driven progradation. Regional ties extend to the Dorotea Formation in Chile, reflecting basin-wide tectonic influences.
Paleoenvironment
Paleoenvironmental reconstructions indicate a temperate and humid subtropical climate with marked seasonality in rainfall. Mean annual temperatures were 12.1-14.5 °C with mean annual precipitation of 929–1321 mm, inferred from paleosols, clay mineralogy, and biotic indicators. Depositional settings encompass low-energy fluvial floodplains with paludal and lacustrine elements, marked by poor drainage, hydromorphic paleosols (e.g., vertisols, histosols), and redoximorphic features such as mottling and iron nodules, suggesting fluctuating water tables and episodic anoxia. Aquatic communities feature Nymphaeaceae, Salviniaceae, Salviniaceae, and Zygnemataceae, indicative of freshwater ponds and swamps, while terrestrial elements include conifers, ferns, and angiosperms in riparian zones. Floral phytozones, derived from palynological data, comprise five assemblages across the section, exhibiting quantitative variations in taxa proportions without major turnover: lower zones are fern-dominated (e.g., Cyatheaceae), transitioning to angiosperm-rich upper zones (e.g., Proteaceae, Nothofagidites), attributed to subtle humidity fluctuations or habitat gradients relative to fluvial channels. These patterns align with Maastrichtian assemblages in southern Gondwanan basins (e.g., La Anita, Lefipán Formation), reflecting angiosperm diversification and Weddellian Province expansion.
Fossils
Arthropoda
Decapoda Genus Species Locality Material Notes Images Astacidea indet. Indeterminate Locality 4 MPM 21516, three gastroliths Previously identified as crushing fish teeth
Insects Genus Species Locality Material Notes Images Coelolepida indet. Indeterminate La Anita farm MPM-Pal 21835-3:W34/1; MPM-Pal 21835-23:W55/0; MPM-Pal 21835-22:R52/2; MPM-Pal 21835-20:Z47/3; MPM-Pal 21835-22: G55/0; MPM-Pal 2183-21: E42/3 A glossatan lepidopteran Diamesinae indet. "Morphotype 3" La Anita farm MPM-Pal 21835-9:E34/4; MPM-Pal 21835-19: V34/4 A chironomid cematoceran. Similar to the living genus Paraheptagyia. Ephemeroptera indet. Indeterminate La Anita farm MPM-Pal 21835-9:W37/1 A mayfly Lepidoptera indet. Indeterminate La Anita farm MPM-Pal 21835-10:X30/2; MPM-Pal 21835-7:S27/2 Orthocladiinae indet. "Morphotype 1" La Anita farm MPM-Pal 21835-24: M31/0; MPM-Pal 21835-3: P46/4 A chironomid nematoceran "Morphotype 2" La Anita farm MPM-Pal 21835-25: V26/3 "Morphotype 5" La Anita farm MPM-Pal 21835-9: Q58/3 It resembles modern Cricotopus Tanypodinae indet. "Morphotype 4" La Anita farm MPM-Pal 21835-2:Y40/3 A chironomid nematoceran
Molluscs
Gastropods Genus Species Locality Material Notes Images Allopeas A. agnolini Locality 4Shells An achatinid gastropod Bocourtia B. leonardodavinciiLocality 4Shells A bulimulid gastropod Coelostemma C. patagonicaLocality 4Shells A holospirid gastropod Holospira H. sp.Locality 4ShellsAn urocoptidae gastropodLeptinariaCf.L. sp.Locality 4ShellsAn achatinid gastropod Lilloiconcha L. novasiLocality 4Shells A charopid gastropodNaesiotusCf.N. sp.Locality 4ShellsA bulimulid gastropod Physa P. sp.Locality 4ShellsA physidae gastropodPhysellaCf.P. sp.Locality 4ShellsA physidae gastropod Pleuroceridae indet. IndeterminateLocality 4ShellsIncertade sedis Pomacea P. sp.Locality 4ShellsAn ampullariidae gastropod Potamolithus P. fabianiLocality 4Shells A tateid gastropod Stenophysa S. sp.Locality 4ShellsA physidae gastropod
Bivalves
Genus Species Locality Material Notes ImagesChlamys"C." salamancaLocality 5ShellsA pectinid bivalveCubitostreaC. ameghinoiLocality 5ShellsAn ostreid bivalve Gryphaeostrea G. callophylaLocality 5Shells A gryphaeid bivalve
Fishes Genus Species Locality Material Notes Images Lamniformes indet. IndeterminateLocality 5 Isolated upper lateral toothSimilar to Cretalamna Vidalamiinae indet. IndeterminateLocality 2 Isolated tooth An amiid fish
Amphibians Genus Species Locality Material Notes Images Anura indet. IndeterminateLocality 4 MPM 21518, isolated tibiofibula Calyptocephalella C. gayiLocality 2 MPM-PV-22844, proximal right radioulna; MPM-PV-22845, left proximal end of tibiofibula; MPM-PV-22846, proximal half of urostyle.A calyptocephalellid frogC. sp.Magallanodon Sitemaxillae MPM-PV-22841 and MPM-PV-22842; ilium MPM-PV-22843A species very small in size, distinct from C. gayi. Calyptocephalellidae indet. IndeterminateLocality 4 MPM 21519, distal end of right humerus with eroded distal margin Pipoidea indet. Indeterminate Locality 2 Distal end of right humerusResembles, yet is different from, Xenopus
Plesiosaurs Genus Species Locality Material Notes Images Elasmosauridae indet. IndeterminateLocality 5Incomplete cervical centrum and the lateral half of a very fragmentary dorsal vertebraMay belong to the Calafate Formation
Lepidosaurs
Mosasaurs Genus Species Locality Material Notes Images Mosasauridae indet. IndeterminateLocality 5Seven teethIts presence suggest marine influences
Rhynchocephalians Genus Species Locality Material Notes Images Notosphenos N. finisterreMonotreme siteFragmentary jawbone with two complete teeth and one partial tooth A sphenodontid rhynchocephalian
Snakes Genus Species Locality Material Notes ImagesAlethinophidia indet.IndeterminateLocality 4MPM 21522, a vertebraAnilioidea indet.IndeterminateLocality 2MPM-PV-22847, a vertebraConiophiidae indet.IndeterminateMonotreme siteTwo partial vertebraeShares some features with ConiophisMadtsoiidae indet.IndeterminateMonotreme site; Green Valley siteTwo partial vertebrae Rionegrophis R. madtsoioidesLocality 2 A partial vertebra A madtsoiid snake
Turtles Genus Species Locality Material Notes Images Chelidae indet. IndeterminateLocality 2Shell fragments (two peripheral and one costal plates)Largest plates suggest a maximum carapace length of 40 cm. Hydromedusinae indet. Indeterminate Localities 2, 4, 5 Shell fragments (left peripheral 3 or 4, bridge peripherals, right hypoplastron, buttress, and plastral fragment)Similar to Hydromedusa Meiolaniformes indet. IndeterminateLocality 4 Distal end of a left humerusSouthernmost record for the clade in South America
Archosaurs
Crocodylomorphs Genus Species Locality Material Notes Images Kostensuchus K. atrox Estancia La Anita Skull, jaws, and incomplete postcranial skeleton A large-bodied peirosaurid
Ornithischians Genus Species Locality Material Notes Images Hadrosauridae indet. IndeterminateLa Anita Farm Two incomplete caudal centra Isasicursor I. santacrucensisLocality 5 Multiple specimens, including teeth, several vertebrae, shoulder blades, and limbs An elasmarian ornithopodOrnithopoda indet.IndeterminateLa Anita FarmProximal end of a femur A small ornithopodParankylosauria indet.IndeterminateLa Anita FarmFive fragmentary caudal vertebrae, incomplete ribs, two incomplete humeri, and nine osteoderms
Sauropods Genus Species Locality Material Notes ImagesFusioolithusF. sp.Locality 2, Puma cave, or Locality 4EggshellsFusioolithidae ootaxonLithostrotia indet.IndeterminateRío LeonaMACN 5205 (3 middle–posterior caudal centra) Nullotitan N. glaciaris Estancia Alta Vista, Localities 1, 2, 4, and 5 A partial skeleton consisting of a third cervical vertebra, tail vertebrae, a neck rib, ribs, a left shoulder blade, the ends of a right thigh bone, a right shin, a right calf bone, and a right ankle bone A lithostrotian titanosaur Titanosauria indet. IndeterminateLocality 2 and 4 Seven isolated teeth
Theropods Genus Species Locality Material NotesEnantiornithes indet.IndeterminateLocality 2, and 4 Distal fragment of a pedal phalanx and two isolated ungual pedal phalanges Kank K. australis Monotreme site Fragmentary remains, including vertebrae, teeth, and pedal phalanges An unenlagiid theropod Kookne K. yeutensisLocality 2 An incomplete right coracoid, lacking the sternal end and the proximal end damaged An ornithuran bird similar to Anseriformes Maip M. macrothorax La Anita Farm Axis, several dorsal and caudal vertebrae, cervical and dorsal ribs, gastralia, a left coracoid, fragmentary scapula, partial right pubis, and partial metatarsal A large megaraptorid theropod Megaraptoridae indet. Indeterminate La Anita Farm Isolated teeth Noasauridae indet. IndeterminateLocality 4 Isolated right pedal phalanx IV-2?Similar to Velocisaurus and VespersaurusPrismatoolithidae indet.IndeterminateLocality 4EggshellsTheropod eggshells Yatenavis Y. ieujensis Estancia La Anita Distal right humerus An enantiornithean bird
Mammals Genus Species Locality Material Notes ImagesMagallanodon M. baikashkenke Río de Las Chinas Valley and La Anita FarmTentative first upper molariform, anterior end of a left dentary bearing part of its gliriform incisor, and upper mesial incisor from the right side A ferugliotheriid gondwanathere M. terrwerrLocality 2MPM-PV-23088, isolated right lower molariform; MPM-PV-23089, isolated right lower molariform A ferugliotheriid gondwanathere Mammalia indet. IndeterminateLocality 4 Two isolated caudal vertebrae Characters shared with mammals include having a long and gracile tail Meridiolestida indet. IndeterminateMonotreme siteMPM-PV-21707, two fragments of a right dentarySmall taxon similar to Reigitherium Mesungulatoidea indet. IndeterminateLa Anita FarmMPM-PV-21708, fragment of left dentary with five alveoli corresponding with three teeth. Larger than any known Mesozoic meridiolestidan OrretheriumO. tzeiMonotreme siteMPM-PV-23514, left upper second molariform. A member of Meridiolestida belonging to the family Mesungulatidae Patagorhynchus P. pascuali Monotreme site Lower right molar attached to a fragment of the dentary A ornithorhynchid monotreme Patagomaia P. chainko La Anita Farm Partial left acetabulum and ischium; MPM-PV-23515, one fragment of thoracic vertebra, posterior half of one caudal vertebra and 38 indeterminate bones. A large therian or gondwanathere similar to Adalatherium
Flora
Algae