Chorrillo Formation
The Chorrillo Formation is a Upper Cretaceous geological unit in the Austral-Magallanes Basin of southern Patagonia, Argentina, dating to the early Maastrichtian stage (approximately 72–70 million years ago) and renowned for its diverse fossil record spanning vertebrates, invertebrates, plants, and insects from the final stages of the Mesozoic era.[1] Outcropping primarily in the southwestern Santa Cruz Province, about 30 km southwest of El Calafate near Lago Argentino, the formation extends southward into Chile as the equivalent Dorotea Formation and represents a regressive sequence of continental deposits influenced by Andean orogeny.[1] It overlies the La Irene Formation and underlies the Maastrichtian Calafate Formation, with a thickness of around 250–500 meters composed mainly of intercalated greenish and reddish sandstones, conglomeratic beds, and mudstones indicative of braided and meandering fluvial systems, deltaic environments, and distal floodplains with swampy, low-energy settings.[1] Paleontologically, the Chorrillo Formation has yielded an exceptionally rich biota, including titanosaurian sauropods such as Nullotitan glaciaris, ornithopod dinosaurs like Isasicursor santacrucensis, and theropods including megaraptorids, alongside the neornithine bird Kookne yeutensis.[1] Recent discoveries have expanded the mammalian record to include the first South American Cretaceous monotreme Patagorhynchus pascuali (platypus relative), gondwanatherians such as Magallanodon baikashkenke and the new species Magallanodon terrwerr, meridiolestidans like Orretherium, and therians including Patagomaia chainko, highlighting a high diversity of Mesozoic mammals second only to the Los Alamitos Formation.[2] Aquatic and terrestrial reptiles are represented by mosasaur teeth, anurans (e.g., Calyptocephalellidae), turtles (Chelidae), and snakes (cf. Rionegrophis madtsoioides), while fish remains include teleosts and amiiforms.[1] Invertebrate fossils are diverse, featuring terrestrial and freshwater gastropods (e.g., Ampullariidae, Physidae, Bulimulidae) and bivalves (e.g., ostreids, mytilids), alongside the first Maastrichtian insect body fossils from the Southern Hemisphere, such as chironomid midges (Chironomidae), mayfly naiads (Ephemeroptera), and lepidopteran larvae and scales from organic-rich mudstones.[1][3] Plant remains include fossil woods (e.g., Podocarpoxylon dusenii), leaf impressions, and a palynoflora dominated by fern spores (Gleicheniaceae), gymnosperm pollen (Podocarpaceae), and angiosperms (e.g., Proteaceae), with key taxa like Peninsulapollis gilli supporting the early Maastrichtian age.[1] This assemblage, preserved in floodplain and fluvial contexts, offers critical insights into the ecosystems and biodiversity dynamics in southern Gondwana immediately preceding the Cretaceous–Paleogene mass extinction.Geological setting
Location and extent
The Chorrillo Formation is situated in the southwestern portion of Santa Cruz Province, southern Patagonia, Argentina, within the Austral-Magallanes Basin.[4] It outcrops along the southern margin of Lago Argentino, primarily on the high plateaus of the La Anita and Alta Vista farms, approximately 30 km southwest of El Calafate city. The formation's exposures are located south of the Centinela River and extend toward the international border with Chile near Hito Baguales 2.[1] The Chorrillo Formation forms a narrow band oriented northeast-southwest, with a maximum east-west width of about 2 km, and continuous outcrops spanning roughly 30 km in a southwest-northeast direction.[1][4] Its extent is restricted to the southern end of Lago Argentino, southeast of Lago Viedma and north of Lago Argentino, where it thins eastward due to erosion.[4] Southward, the formation correlates with and continues into the Dorotea Formation in Chile's Las Chinas River valley, reflecting a shared depositional history across the Andean foreland.[1] The unit reaches a maximum thickness of approximately 500 m near the Argentina-Chile border, decreasing progressively to the east.[4] This thickness represents a low-gradient fluvial succession dominated by fine-grained deposits, accumulated during the Maastrichtian stage of the Late Cretaceous.[4]Stratigraphy
The Chorrillo Formation represents a Maastrichtian continental sedimentary sequence within the Austral-Magallanes Basin in Santa Cruz Province, Patagonia, Argentina.[5] It attains a maximum thickness of approximately 500 meters in its western exposures, thinning eastward due to post-depositional erosion.[5][6] Lithologically, the formation is dominated by fine-grained deposits, including mudstones and siltstones, which comprise over 60% of the sequence and reflect floodplain accumulation.[6] These are intercalated with coarser channelized and lobate sandstone bodies, organized into five main architectural elements: complex sandy narrow-sheet channels (SS), complex gravelly narrow-sheet channels (GS), sandstone lobes (SL), thick fine-grained deposits (FG), and thin dark fine-grained deposits (DF).[5] Notable intervals include organic-rich, dark gray to black mudstone beds, such as a 2.5-meter-thick layer (Megafloral level 1) occurring about 215 meters above the base, which preserves exceptional fossil assemblages due to anoxic conditions.[6] Stratigraphically, the Chorrillo Formation unconformably overlies the Late Campanian to early Maastrichtian La Irene Formation, characterized by coarse-grained braided fluvial deposits.[5][6] It is in turn overlain, either conformably or with minor unconformity, by the Maastrichtian Calafate Formation, which records a transition to shallow marine environments.[5][6] No formal members or subdivisions have been defined, though studies have sampled distinct lower, middle, and upper levels for palynological and sedimentological analysis.[7]Age and depositional environment
Geochronology
The geochronology of the Chorrillo Formation is primarily constrained by biostratigraphic correlations using vertebrate fossils, palynomorph assemblages, and stratigraphic relationships with adjacent units, supplemented by indirect radiometric dates from laterally equivalent formations. The formation overlies the Maastrichtian La Irene Formation (which in turn overlies the Early Campanian Alta Vista Member of the Cerro Fortaleza Formation) and interfingers with the early Maastrichtian Cerro Cazador Formation, while it is unconformably overlain by the late Maastrichtian Calafate Formation. These relationships indicate deposition during a regressive episode spanning the Campanian-Maastrichtian boundary.[8] Biostratigraphy based on dinosaurian remains provides key age indicators. Titanosaur osteology, including the derived titanosaur Nullotitan glaciaris from the lower to middle sections, aligns with late Campanian to early Maastrichtian faunas elsewhere in Patagonia, such as those in the Anacleto and Allen formations. Ornithopod fossils, including the elasmarian Isasicursor santacrucensis from the upper third, and megaraptorid theropods further support an early Maastrichtian age for the main fossil-bearing horizons, as these taxa exhibit morphologies transitional between Campanian and Maastrichtian assemblages without the advanced saltasaurine titanosaurs or abelisaurids typical of later Maastrichtian units. Mammalian remains, such as the monotreme Patagorhynchus and gondwanatherian fragments, corroborate this timeframe, representing the southernmost Late Cretaceous records of these groups.[8] Palynostratigraphy reinforces the Maastrichtian assignment. Assemblages from the lower and middle sections include diagnostic gymnosperm and angiosperm pollen such as Peninsulapollis gillii and Proteacidites spp., which are restricted to the Late Cretaceous of Patagonia and correlate with the early Maastrichtian zones of the La Irene and Lefipán formations. Upper-level palynofloras, comprising over 100 taxa including Rhoipites minusculus and Trilites fasolae, show moderate diversity with humid-temperate affinities and no K-Pg boundary markers, suggesting a latest early to early late Maastrichtian age without reaching the boundary. These palynomorphs indicate floral stability across the formation, with minor shifts attributable to local depositional variations rather than major temporal turnover.[7][9] Absolute ages are inferred from radiometric dating of the coeval Dorotea Formation in the adjacent Magallanes-Austral Basin. U-Pb zircon dating of tuffaceous layers yields ages of 74.9 ± 0.5 Ma for the lower sections and 71.7 ± 1.2 Ma for the upper dinosaur-bearing levels, bracketing the Chorrillo Formation to approximately 75-71 Ma, spanning the late Campanian to early Maastrichtian. No direct radiometric dates exist for the Chorrillo Formation itself, but its lithostratigraphic and paleontological equivalence to the Dorotea Formation supports this calibration. Ongoing studies may refine these constraints with additional ash layers or magnetostratigraphy.Paleoenvironment
The Chorrillo Formation, exposed in the southern Patagonia region of Argentina within the Austral-Magallanes Basin, records a predominantly fluvial depositional environment characterized by a low-gradient floodplain system during the early Maastrichtian stage of the Late Cretaceous. The unit, approximately 500 meters thick, consists mainly of fine-grained sediments such as siltstones and mudstones, which make up over 60% of the succession, interspersed with subordinate sandstones representing channel fills and crevasse splays. These deposits overlie the coarser-grained, braided fluvial La Irene Formation and underlie the shallow marine Calafate Formation, indicating a progression from terrestrial to marginal marine settings in a syn-orogenic foreland basin context.[3][10] Paleosols preserved throughout the formation provide key insights into soil formation and landscape dynamics, revealing smectite-rich profiles with vertic and redoximorphic features. These include stacked hydromorphic, calcic, and argillic Vertisols, as well as Histosols, which formed in periodically waterlogged areas of the floodplain, with vertical stacking linked to channel avulsions and subtle topographic variations. Organic-rich, dark gray to black mudstones, often with diffuse lamination, represent low-energy, anoxic swamp or pond deposits in distal floodplain positions, supporting the interpretation of a wet, reducing environment conducive to organic preservation. The spatial distribution of these paleosols reflects proximal to distal gradients across the floodplain, from better-drained levee areas to waterlogged backswamps.[10][3] The paleoenvironment supported a diverse freshwater ecosystem, as evidenced by the co-occurrence of aquatic and terrestrial biota. Insect assemblages, including larval Chironomidae (midges) and naiads of Ephemeroptera (mayflies), along with remains of fishes, amphibians, turtles, and semiaquatic reptiles like snakes and crocodyliforms, indicate permanent or semi-permanent freshwater bodies integrated into the fluvial landscape. Floral elements, such as free-floating aquatics (e.g., Azolla spp. and Salviniaceae), floating-leaved plants (e.g., Nymphaeaceae), and emergent taxa (e.g., Marsileaceae), further confirm shallow, standing-water habitats amid a broader terrestrial vegetation of ferns, conifers (Araucariaceae, Podocarpaceae), and early angiosperms. Palynofloras from the upper levels, dominated by gymnosperm and angiosperm pollen (e.g., Peninsulapollis gillii, Rhoipites minusculus), reflect a moderately diverse vegetation adapted to fluvial influences with minor humidity fluctuations.[3][1] Paleoclimate proxies from paleosols and palynomorphs suggest temperate to warm conditions with seasonal humidity at paleolatitudes around 50°S, conducive to the observed biotic diversity. Abiotic indicators like vertic features and redoximorphic mottling in paleosols imply alternating wet-dry cycles driven by seasonal precipitation, while biotic signals from thermophilous flora and faunal tolerances point to mean annual temperatures above 15–20°C and sufficient moisture for swamp development. This setting hosted a complex food web, with detritivores and herbivores in aquatic zones sustaining higher trophic levels, including herbivorous dinosaurs like hadrosaurids and titanosaurs that roamed the floodplains. Subtle marine influences in upper strata, marked by mosasaur remains, hint at proximal connections to coastal areas, though the core environment remained continental-freshwater dominated.[10][3]History of research
Discovery and naming
The Chorrillo Formation was first recognized in the geological literature during the late 1930s, when the Italian-Argentine paleontologist Eduardo Feruglio described the strata as the "Estratos de Chorrillo" (Chorrillo Beds) in exposures south of Lago Argentino in southern Patagonia, Argentina.[11] Feruglio noted the presence of fossil wood logs and fragmentary dinosaur bones within these continental deposits, which he interpreted as part of the Upper Cretaceous sequence in the region, though he did not conduct extensive excavations at the time.[11] This initial mention, published in a collaborative work on Patagonian stratigraphy, marked the earliest documented reference to the unit's fossiliferous nature, but systematic study was limited due to the remote location and focus on broader regional mapping.[11] The first significant paleontological discovery occurred in 1980, when local resident Francisco Nullo identified a partial sauropod dinosaur skeleton on the Alta Vista farm, approximately 30 km southwest of El Calafate in Santa Cruz Province.[11] This find was reported to Argentine paleontologist José F. Bonaparte, who led an expedition in 1981 to collect a cervical vertebra from the site, confirming the presence of titanosaurian remains and highlighting the formation's potential for vertebrate fossils.[11] Bonaparte's work in the 1980s and 1990s further referenced these exposures, but detailed stratigraphic and paleontological investigations remained sparse until the early 2000s. The unit was formally established as the Chorrillo Formation in 2002 by geologist Héctor A. Arbe, who defined its lithostratigraphic boundaries within the Austral-Magallanes Basin as a Maastrichtian continental sequence characterized by fluvial and floodplain deposits.[11] This naming was refined and corroborated in 2006 by Francisco E. Nullo and colleagues, who integrated sedimentological data to delineate the formation's extent, thickness (up to approximately 500 m), and correlation with the equivalent Dorotea Formation across the border in Chile.[11] Prior to these formalizations, the strata had occasionally been referred to informally as "dinosaur-bearing strata" due to scattered reports of vertebrate fossils, emphasizing its role in Late Cretaceous paleontology.[12] Renewed interest in the 2010s, driven by expeditions from the Museo Argentino de Ciencias Naturales "Bernardino Rivadavia," led to the first comprehensive description of its diverse biota in 2019, building on the foundational naming and early discoveries.[1]Key studies and findings
The foundational paleontological study of the Chorrillo Formation was conducted by Novas et al. in 2019, providing the first formal descriptions of diverse fossils from this Upper Cretaceous unit in southern Patagonia, Argentina. This work documented a rich assemblage including the basal ornithopod dinosaur Isasicursor santacrucensis, the titanosaur Nullotitan glaciaris, indeterminate megaraptorid theropods, neornithine bird remains named Kookne yeutensis, fragmentary mammal vertebrae, large mosasaur teeth, chelid turtle material, anuran fossils, gastropod shells, fossil woods, and palynomorphs indicative of a cool, humid climate near the K-Pg boundary.[1] These findings highlighted a unique vertebrate fauna distinct from northern Patagonian assemblages, with an absence of abelisaurids, saltasaurines, and aeolosaurs, and emphasized the formation's role in understanding latest Cretaceous biodiversity in high-latitude Gondwana.[1] Subsequent research in 2021 by Coria et al. expanded on ornithischian diversity, describing hadrosaurid remains that correlated the Chorrillo Formation with the adjacent Chilean Dorotea Formation and identified four ornithischian taxa, including elasmarians and hadrosaurs, underscoring their underrepresentation in South American Late Cretaceous records compared to sauropods.[13] That year, Chimento et al. reported new gondwanatherian remains, including Magallanodon baikashkenke, further diversifying the mammalian record. In 2022, Rolando et al. reported the giant megaraptorid theropod Maip macrothorax, based on a partial skeleton from the formation, representing the largest known member of this clade (estimated 9-10 m long) and providing insights into theropod gigantism and megaraptorid anatomy near the end of the Cretaceous. The same year, Álvarez-Herrera et al. described Yatenavis ieujensis, a small enantiornithine bird from a partial humerus, marking one of the southernmost and youngest records of this extinct avian group and demonstrating their persistence and diversity in high-latitude Patagonia during the Maastrichtian.[14] Paleoenvironmental analyses advanced in 2023 with a study by Reguero et al. on the formation's paleosols, revealing smectite-rich soils with vertic and redoximorphic features that formed under a temperate-warm, seasonally humid climate, supporting a fluvial depositional system dominated by fine-grained sediments and indicating continentalization of the Austral Basin.[15] That year, Vera et al. analyzed a Maastrichtian insect assemblage of about 30 specimens, dominated by modern chironomid midges (e.g., Orthocladiinae, Diamesinae) alongside ephemeropterans and lepidopterans, suggesting that diverse, Cenozoic-like insect communities were established in southern South America prior to the K-Pg extinction and linked to angiosperm radiation.[6] Martin et al. described the first South American Cretaceous monotreme from the formation, a platypus relative, expanding the known diversity of Mesozoic mammals. Recent 2025 discoveries include new mammal remains reported by Chimento et al., comprising additional indeterminate therian vertebrae and fragments that enrich the sparse mammalian record from the formation and suggest a more diverse non-dinosaurian fauna in latest Cretaceous Patagonia.[2] Additionally, the description of the notosuchian crocodyliform Kostensuchus atrox by Lio et al., based on cranial and postcranial elements, indicates a terrestrial predator potentially adapted for hunting large prey like dinosaurs, further illustrating the complex carnivore guild in the Chorrillo Formation's ecosystem. These studies collectively underscore the formation's importance for reconstructing pre-K-Pg terrestrial biotas in southern high latitudes.Invertebrate paleobiota
Arthropods
The arthropod fossil record of the Chorrillo Formation is dominated by insects, representing the first documented body fossils of such organisms from the Maastrichtian stage in the Southern Hemisphere.[6] These remains, primarily recovered from palynological preparations of megafloral level 1, consist of approximately 30 well-preserved, non-deformed chitinous structures, including head capsules, wings, and scales, with sizes ranging from 10 to 250 µm.[6] The assemblage highlights a diverse insect community coexisting with late Cretaceous vertebrates and plants in a fluvial-lacustrine depositional environment. The identified taxa belong to three insect orders: Diptera, Ephemeroptera, and Lepidoptera. Within Diptera, the family Chironomidae is prominent, encompassing subfamilies such as Orthocladiinae, Diamesinae, and Tanypodinae, indicating that modern clades of these non-biting midges were already diversified and dominant by the late Cretaceous.[6] Ephemeroptera is represented by indeterminate mayfly remains, while Lepidoptera includes indeterminate moths and possible scale insects (Coelolepida indet.), suggesting aquatic and terrestrial habits among the group.[6] Indeterminate arthropod fragments with features such as compound eyes and spines are also present but not assignable to specific higher taxa, and may include non-insect arthropods.[6] This insect assemblage fills a critical gap in the global fossil record, bridging Early Cretaceous and Paleogene entomofaunas in Patagonia and providing insights into arthropod diversity immediately prior to the Cretaceous-Paleogene extinction event.[6] The preservation of delicate chitinous structures in fine-grained sediments underscores the formation's potential for conserving microfossils, with the chironomid dominance resembling patterns seen in younger Cenozoic deposits.[6]Molluscs
The molluscan paleobiota of the Chorrillo Formation is dominated by gastropods, with 229 specimens documented from multiple localities in Santa Cruz Province, Argentina, primarily representing freshwater and terrestrial habitats. These fossils, collected from mid- to upper levels of the formation, include casts and internal molds that indicate a diverse community adapted to fluvial and riparian environments during the Maastrichtian. No bivalves have been definitively attributed to the formation itself, though marine forms appear in transitional beds with the overlying Calafate Formation. The gastropod assemblage underscores the continental nature of the depositional system, with taxa suggesting temperate to subtropical, seasonally humid conditions conducive to malacofauna proliferation.[16][17] Initial discoveries reported 50 specimens from the Puma Cave locality (Locality 4), in the middle portion of the formation, comprising both aquatic and terrestrial forms. Subsequent analysis expanded the inventory to 229 studied specimens across sites such as Puma Cave, Monotreme, Megaraptorid, and Cuevas del Puma, revealing several new taxa and extending the biogeographic range of Stylommatophora and other groups.[16][17] Detailed taxonomic study has identified 13 gastropod taxa from the Chorrillo Formation, emphasizing endemic evolution in isolated southern Gondwanan ecosystems. The freshwater component is led by Tateidae, with Potamolithus fabiani n. sp. (holotype MPM-PI-21557; 191 casts) characterized by large, convex whorls up to 11.4 mm in height, distinguishing it as the southernmost and second-oldest Potamolithus species. Physidae are represented by Physidae gen. et sp. indet. (one elongated specimen, 30.9 mm high), ?Physella sp. (11.5 mm high, 4 whorls), and Stenophysa cf. wichmanni (casts up to 18.8 mm, fusiform shell), the latter providing the first Cretaceous record and southernmost extension for the genus. Terrestrial forms include Achatinidae (Allopeas agnolini n. sp., 15.5 mm high, six whorls; first Cretaceous Achatinidae; ?Leptinaria sp., small pyramidal 9.6 mm), Holospiridae (Coelostemma patagonica n. sp., columnar with 10 whorls, up to 18.5 mm; inaugural South American record), Bulimulidae (Bulimulus sp. 1, 13.6 mm; Bulimulidae gen. et sp. indet., 17.7 mm; ?Naesiotus sp., fusoid 15.5–16.7 mm; first South American Cretaceous record), and the new Charopidae Lilloiconcha novasi n. sp. (3.9 mm high, 5.5 whorls; first Cretaceous record), as well as Orthalicidae? no, wait Bulimulidae for Bocourtia. Bocourtia leonardodavincii n. sp. (large 26.2 mm shell; oldest and southernmost Bocourtia). These species exhibit morphological adaptations for humid, vegetated lowlands, co-occurring with dinosaurs, mammals, and insects.[17]| Taxon | Family | Habitat | Key Features | Significance | Specimens |
|---|---|---|---|---|---|
| Potamolithus fabiani n. sp. | Tateidae | Freshwater | 4.5 convex whorls, 11.4 mm height | Southernmost Potamolithus; second Cretaceous record | 191 casts (holotype MPM-PI-21557) |
| Physidae gen. et sp. indet. | Physidae | Freshwater | Elongated, 4 whorls, 30.9 mm height | Larger than related Paleogene forms | 1 (MPM-PI-23066) |
| ?Physella sp. | Physidae | Freshwater | Globose, 4 whorls, 11.5 mm height | Contributes to Physidae diversity | 1 (MPM-PI-23067) |
| Stenophysa cf. wichmanni | Physidae | Freshwater | Fusiform, 4 whorls, 14.0–18.8 mm height | First Cretaceous; southernmost record | Multiple casts (e.g., MPM-PI-21559) |
| Allopeas agnolini n. sp. | Achatinidae | Terrestrial | 6 whorls, 15.5 mm height | First Cretaceous Achatinidae | 1 (holotype MPM-PI-23332) |
| ?Leptinaria sp. | Achatinidae | Terrestrial | Pyramidal, 6 whorls, 9.6 mm height | Oldest Subulininae record | 2 casts (MPM-PI-23068) |
| Coelostemma patagonica n. sp. | Holospiridae | Terrestrial | Columnar, 10 whorls, 18.5 mm length | First South American Holospiridae | 2 (holotype MPM-PI-21562) |
| Bulimulus sp. 1 | Bulimulidae | Terrestrial | 5 whorls, 13.6 mm height | Southernmost; first Patagonian Cretaceous | 1 cast (MPM-PI-23072) |
| Bocourtia leonardodavincii n. sp. | Bulimulidae | Terrestrial | 2 whorls, 26.2 mm height | Oldest and southernmost Bocourtia | 1 (holotype MPM-PI-23077) |
| ?Naesiotus sp. | Bulimulidae | Terrestrial | Fusoid, 7–8 whorls, 15.5–16.7 mm height | First South American Cretaceous | 4 casts (e.g., MPM-PI-23074) |
| Bulimulidae gen. et sp. indet. | Bulimulidae | Terrestrial | 6 whorls, 17.7 mm height | Contributes to family diversity | 1 cast (MPM-PI-23078) |
| Lilloiconcha novasi n. sp. | Charopidae | Terrestrial | 5.5 whorls, 3.9 mm height | First Cretaceous Charopidae | 1 (holotype MPM-PI-23079) |