Noasaurus

Noasaurus is a genus of small-bodied, carnivorous theropod dinosaur belonging to the clade Noasauridae within Abelisauroidea, known from an incomplete skeleton discovered in the Maastrichtian Lecho Formation of northwestern Argentina.^[1] The type species, Noasaurus leali, was named and described in 1980 by José F. Bonaparte and Jaime E. Powell based on holotype specimen PVL 4061, which includes elements such as a left maxilla with 13 alveoli, a right quadrate, a cervical neural arch, ribs, a manual phalanx and ungual, and a metatarsal II.^[1] Estimated at approximately 2 meters in length, N. leali represents one of the earliest recognized members of the noasaurid radiation, a group of gracile ceratosaurs that diversified in the Southern Hemisphere during the Late Cretaceous.^[1] The fossil was unearthed in 1975 during an expedition led by paleontologist José F. Bonaparte at El Brete, Salta Province, in sediments dated to around 70–69 million years ago, making it a key taxon for understanding Maastrichtian terrestrial ecosystems in northern Argentina.^[1] Initially interpreted as a coelurosaur, subsequent analyses have firmly placed Noasaurus within Ceratosauria, closely related to genera like Masiakasaurus, Velocisaurus, and Laevisuchus, highlighting a Gondwanan distribution of small abelisauroids.^[1] Notable anatomical features include ziphodont dentition with short, recurved teeth bearing asymmetrical denticles, a strongly arched quadrate, and a highly curved manual ungual lacking a flexor tubercle, which together suggest adaptations for opportunistic predation on small vertebrates, potentially including fish via gaffing with the claws.^[1] Despite its fragmentary preservation, Noasaurus has informed broader discussions on noasaurid diversity, manus evolution in abelisauroids, and the ecological roles of diminutive theropods in Late Cretaceous floodplains.^[1]

Taxonomy and discovery

Naming and type material

The genus Noasaurus was established by Argentine paleontologists José F. Bonaparte and Jaime E. Powell in 1980, based on fossils discovered in 1975 at the El Brete locality in northwestern Argentina. The generic name combines "noa," an abbreviation for noroeste argentino (northwestern Argentina, the type region), with the Greek saurus (lizard), thus meaning "northwestern Argentina lizard." The specific epithet leali honors Juan Carlos Leal, the technician who first identified the fossil-bearing site.^[2] The holotype specimen, cataloged as PVL 4061, comprises a fragmentary but diagnostic partial skeleton including a left maxilla bearing five teeth, a right quadrate, a mid-posterior cervical vertebral neural arch, a mid-cervical rib, a posterior cervical rib, an indeterminate postcervical vertebra, a manual phalanx (likely from digit III), two manual unguals (possibly from digits I of the left and right hands; originally misidentified as pedal claws), and a right metatarsal II.^[2] This material was formally named and briefly described in the original publication, Mémoires de la Société Géologique de France (Nouvelle Série 139: 19–28), marking Noasaurus leali as the first named theropod dinosaur from northern Argentina. The specimen is housed in the collections of the Instituto y Fundación Miguel Lillo, Universidad Nacional de Tucumán, in San Miguel de Tucumán, Argentina, where it remains the sole known material for the taxon despite its incomplete preservation, with some elements showing minor fractures and sediment infilling.^[2] A 2024 osteological redescription confirmed the diagnostic value of these bones while reinterpreting the manual elements as specialized for predatory grasping.^[2]

Geological context and excavation history

The fossils of Noasaurus leali were discovered at the El Brete locality (Estancia El Brete) in the Department of Candelaria, southern Salta Province, northwestern Argentina, at coordinates approximately 26°02′S 65°20′W.^[3] This site represents a significant bonebed yielding over 100 dinosaur elements, including theropod remains.^[3] The type specimen derives from the middle part of the Lecho Formation, about 22 meters above its base, within the Balbuena Subgroup of the Salta Group.^[3] The Lecho Formation comprises red beds, sandstones, and finer sediments indicative of fluvial and lacustrine depositional environments during the Late Cretaceous.^[4] It dates to the early to mid-Maastrichtian stage, approximately 71–69 million years ago.^[3] Fieldwork at El Brete began in late May 1975 under the leadership of paleontologist José F. Bonaparte, with the fossil locality initially identified by technician Juan Carlos Leal, to whom the species is dedicated.^[3] Excavations continued through August 1975 and extended into 1976–1977, recovering a partial theropod skeleton (holotype PVL 4061) amid challenges posed by the site's inaccessibility, the fragility of the bones, and incomplete documentation of the quarry's spatial distribution.^[3] Initial preparation occurred at the Instituto Miguel Lillo in Tucumán, where the material remains housed.^[3] The Maastrichtian age of the Lecho Formation at El Brete is established through its stratigraphic correlation within the Salta Group, radiometric dating of interbedded volcanic ashes yielding ages around 71–69 Ma, and the presence of index fossils such as the titanosaur Saltasaurus loricatus.^[3] No direct radiometric dating has been applied to the Noasaurus specimen itself.^[3]

Anatomy

Cranial anatomy

The cranial remains of Noasaurus leali are limited to a partial left maxilla bearing five teeth and a right quadrate, preserving key aspects of the snout and jaw articulation in this small ceratosaurian theropod.^[2] The maxilla is subtriangular in outline, measuring approximately 75 mm in length and 33 mm in height, with a well-developed antorbital fossa but lacking a promaxillary fenestra, a condition differing from many other ceratosaurs.^[2] It features a concave alveolar margin and a prominent dorsal ridge within the maxillary fossa, the latter representing a diagnostic apomorphy for the taxon.^[2] A shallow pneumatic depression is present on the lateral surface, indicating limited cranial pneumatization.^[2] The dentition of Noasaurus comprises 13 maxillary alveoli, with the five preserved teeth displaying ziphodont morphology: recurved crowns up to 20 mm high, labiolingually compressed with a lenticular cross-section, and fine distal denticles (20-25 per 5 mm) while mesial denticles are minute or absent.^[2] The teeth are procumbent and exhibit subtle fluting on the mesial surface, features suggestive of adaptations for grasping soft or slippery prey such as fish.^[2] Enamel texture is smooth overall, with theca-like spacing between teeth that may have facilitated prey manipulation.^[2] The right quadrate measures 43 mm in height and is characterized by a strongly arched shaft, a low and dorsoventrally compressed pterygoid flange, and a subcircular otic head for articulation with the braincase.^[2] It lacks a distinct quadrate foramen but possesses a large, well-delimited pneumatic depression posteriorly, supporting robust jaw adduction mechanics despite the overall lightweight cranium.^[2] No orbital process is preserved.^[2] Based on comparisons with related noasaurids like Masiakasaurus, the skull of Noasaurus is estimated to have been 15-20 cm long, consistent with its small body size of around 2 m when scaled to postcranial elements such as the femur.^[2] Recent revisions by Hendrickx et al. (2024) provide a detailed redescription of these elements, confirming noasaurid affinities through unique maxillary and quadrate morphologies, including the absence of certain fenestrae and the presence of apomorphic ridges and arches, though CT-scan analyses were not possible due to specimen restrictions.^[2]

Postcranial anatomy

The postcranial skeleton of Noasaurus leali is incompletely known from the holotype specimen PVL 4061, which preserves elements of the axial column, forelimb, and hindlimb, indicating a small-bodied theropod with a gracile build inferred from the slenderness of preserved bones.^[2] Overall size estimates place the animal at a total length of 1.7–2 meters and a body mass of 6–38 kg, with the lower end of the mass range more consistent with its slender proportions.^[2] The material suggests potential juvenile features, such as unfused neurocentral sutures in the cervical neural arch, though no histological analysis has confirmed ontogenetic stage.^[2] The axial skeleton includes a mid-posterior cervical neural arch (likely the 6th or 7th vertebra), mid- and posterior cervical ribs, and an indeterminate dorsal vertebra, all characterized by elongated centra and low neural spines typical of noasaurid theropods.^[2] The cervical neural arch measures 55.78 mm in length, 19.34 mm in height, and 27.33 mm in width, featuring hypertrophied epipophyses with distinct anterior and posterior prongs, as well as pneumatic fossae on the lateral surfaces of the postzygapophyses.^[2] The mid-cervical rib is 46.99 mm long and 17.9 mm high, with a short capitulum-tuberculum distance and pneumatic foramina, while the posterior cervical rib measures 24.47 mm in length and 37.46 mm in height, also exhibiting pneumatic features.^[2] The dorsal vertebra has a centrum 32.48 mm long, 20.64 mm high, and 14.73 mm wide, with a low neural spine and a postzygapophyseal centrodiapophyseal fossa containing subfossae indicative of pneumatization.^[2] No caudal vertebrae or chevrons are preserved, limiting direct inferences about tail flexibility.^[2] No elements of the pelvic girdle are preserved in the known material of N. leali.^[2] The hindlimb is represented solely by a partial right metatarsal II, measuring 115.56 mm in length with a proximal width of 14.92 mm, which is gracile and mediolaterally compressed, featuring an ovoid proximal end and a subquadrangular distal end with a deep collateral ligament sulcus.^[2] This element suggests cursorial adaptations consistent with the elongated lower leg proportions estimated for noasaurids, though direct measurements of the femur and tibia are unavailable.^[2] No pedal unguals are preserved.^[2] The forelimb includes a manual phalanx (likely digit III-1) and two manual unguals (one complete, one incomplete), reidentified as belonging to the manus rather than the pes in detailed comparative analyses.^[5] The manual phalanx measures 15.9 mm in length and 8.22 mm in height, being short, dorsoventrally compressed, and with a narrow ginglymoid distal articular surface.^[2] The complete manual ungual (likely digit I) is 36.66 mm long and 13.27 mm high, exhibiting a strongly falciform shape with a 90° curvature, a deep C-shaped cotyloid articular surface, and a V-shaped flexor fossa without a prominent tubercle; the incomplete counterpart is 25.75 mm long.^[2] These unguals show convergence with those of spinosaurids in their procumbent, raptorial form suited for prey manipulation.^[2]

Classification

Phylogenetic analyses

Phylogenetic analyses consistently position Noasaurus leali within Ceratosauria as a member of Noasauridae, a clade nested in Abelisauroidea, representing a basal abelisauroid theropod from the Late Cretaceous of Argentina.^[2] In modern cladistic studies, Noasaurus forms a polytomy with closely related taxa such as Masiakasaurus knopfleri from Madagascar, Laevisuchus indicus from India, and Velocisaurus unicuspis from Patagonia, highlighting a Southern Hemisphere radiation of small-bodied noasaurids during the Campanian–Maastrichtian.^[2] This placement is supported by matrix-based phylogenies that incorporate extensive character data, resolving Noasauridae as a monophyletic group sister to more derived abelisaurids like Carnotaurus and Majungasaurus. Key synapomorphies diagnosing Noasauridae, and thus supporting the inclusion of Noasaurus, include ziphodont dentition with elongated, recurved premaxillary teeth featuring fine denticles, reduced forelimbs with short, curved manual unguals lacking a prominent flexor tubercle but possessing a subtriangular flexor fossa, and specific quadrate features such as strong arching and pneumaticity with a dorsal ridge in the maxillary fossa.^[2] Character scoring in these analyses emphasizes cranial and dental traits, such as the pointed pre-antorbital body and asymmetrical tooth crowns, alongside postcranial elements like cervical neural arches with anterior epipophyseal prongs, which distinguish noasaurids from other ceratosaurs.^[2] These features are scored across large datasets to ensure diagnostic resolution for the clade. A comprehensive 2024 analysis by Hendrickx et al. utilized three modified datasets for cladistic evaluation: one with 246 characters and 54 taxa (from Baiano et al. 2023), another with 431 characters and 49 taxa (from Agnolín et al. 2022), and a third with 746 characters and 206 taxa (from Rauhut and Pol 2021), incorporating over 200 characters focused on theropod morphology.^[2] The results produced strict consensus trees with Noasaurus in a polytomy among core noasaurids, achieving moderate support in pruned topologies after removing unstable wildcard taxa, confirming its basal position within Noasauridae.^[2] This underscores the evolutionary implications of Noasaurus as a key taxon in the Gondwanan radiation of diminutive ceratosaurs, potentially involving miniaturization and niche specialization in Late Cretaceous ecosystems of South America, Africa, and Asia.^[2]

Historical interpretations

Upon its initial description in 1980, Noasaurus leali was classified by Jaime E. Powell and José F. Bonaparte as a coelurosaurian theropod within the newly erected family Noasauridae, based primarily on the misidentification of its strongly curved manual ungual as a pedal "sickle claw" suggestive of dromaeosaurid affinities. This placement aligned it with small-bodied carnivores like those in Coeluridae or Compsognathidae, reflecting the limited theropod material available at the time and an emphasis on cursorial predatory adaptations.^[3] In the 1990s, Bonaparte revised this interpretation, reassigning Noasaurus to Ceratosauria as an early abelisauroid, positioning Noasauridae as a sister group to Abelisauridae within a Gondwanan radiation of short-armed theropods. This shift was driven by comparative analyses of fragmentary South American specimens, highlighting shared primitive traits like elongated cervical vertebrae and emphasizing its distinction from northern hemisphere coelurosaurs.^[6] The early 2000s saw further refinement with Wilson et al. (2003) providing a phylogenetic definition for Noasauridae as the most inclusive clade containing Noasaurus leali but excluding Carnotaurus sastrei, incorporating the recently described Masiakasaurus knopfleri from Madagascar as a close relative and potential congener.^[7] This work solidified Noasaurus as the type genus of a clade of small, gracile ceratosaurs, though Masiakasaurus was later recognized as distinct based on cranial differences.^[3] Prior to 2020, Noasaurus was frequently considered a junior synonym of the contemporaneous Velocisaurus unicus from the same Argentine formations, due to overlapping pedal morphology and stratigraphic proximity, while broader debates oscillated between spinosauroid (tetanuran) and ceratosaur affinities, with some analyses favoring the former based on inferred aquatic or piscivorous traits in related taxa.^[8] These uncertainties stemmed from incomplete specimens and varying character codings in early cladograms.^[6] By the 2010s, multiple phylogenetic studies confirmed Noasaurus' placement within Noasauridae, supported by shared maxillary features such as a dorsal ridge on the antorbital fossa, resolving it as a basal abelisauroid ceratosaur rather than a coelurosaur or spinosauroid. A 2024 analysis further resolved lingering polytomies, affirming its close ties to Velocisaurus and other Gondwanan noasaurids.^[3]

Paleoecology

Diet and predatory behavior

Noasaurus leali is inferred to have been an opportunistic carnivore that primarily targeted small prey, including possibly fish and small tetrapods, based on its ziphodont dentition characterized by labiolingually compressed, recurved crowns with denticulated carinae suited for piercing and holding slippery or agile animals. The small size of the teeth (crown heights around 5-6 mm) and their morphology, including minute mesial denticles and larger distal denticles (DSDI ≈1.3-1.44), further support a diet focused on grasping rather than crushing larger prey.^[3] The strongly curved manual unguals, with an approximately 90° arch and a deep C-shaped articular surface, likely functioned to gaff or manipulate small prey, such as impaling fish or securing small vertebrates during capture, analogous to adaptations seen in other small theropods but distinct in the noasaurid context. Jaw mechanics, inferred from the apneumatic quadrate with subcircular ectocondyle, indicate a relatively weak bite force consistent with the dinosaur's small body size (≈2 m length), enabling rapid closure for dispatching diminutive targets rather than overpowering larger ones. Tooth wear is minimal, with no prominent facets suggesting routine hard-object feeding, though one lingual wear surface on a maxillary tooth hints at occasional contact with resistant materials.^[3] As a likely solitary mesopredator, Noasaurus occupied a niche in riparian or aquatic-adjacent environments of the Late Cretaceous Lecho Formation, where it could exploit small vertebrates while avoiding direct competition with larger abelisauroid predators through specialization on elusive, smaller prey items. This behavioral strategy aligns with its anatomical adaptations for precise, opportunistic hunting rather than pack-based or confrontational predation.^[3]

Locomotion and niche

Noasaurus leali exhibited skeletal adaptations consistent with bipedal, cursorial locomotion, characterized by long and slender hindlimbs that supported agile, terrestrial movement. The gracile build of the hindlimb elements, including a long metatarsal II, indicates a lightweight construction optimized for running rather than sustained sprinting. Reduced forelimbs further emphasize reliance on hindlimb propulsion, distinguishing it from quadrupedal or heavily forelimb-dependent theropods. In its ecological niche, the small body size of Noasaurus (estimated at under 2.5 meters in length) positioned it as a mesopredator, capable of exploiting smaller prey in understory habitats or near aquatic margins, thereby partitioning resources from larger abelisaurid carnivores that dominated apex roles. This agile, lightly built form likely facilitated pursuits in varied terrains of the Late Cretaceous Lecho Formation, avoiding direct competition with bulkier predators. The holotype specimen of Noasaurus represents an ontogenetically immature individual, as evidenced by the open neurocentral sutures and the strongly arched quadrate. Such subadult proportions suggest potential shifts in locomotor efficiency or niche occupancy during growth, with adults possibly achieving greater cursorial prowess.

Paleoenvironment

Lecho Formation depositional setting

The Lecho Formation represents Maastrichtian red beds within the Balbuena Subgroup of the Salta Group, attaining thicknesses of approximately 300–500 m across the Salta Basin in northwestern Argentina. Composed mainly of fine- to coarse-grained sandstones interbedded with mudstones and shales, the formation records fluvial-lacustrine and aeolian depositional environments during the post-rift sag phase of the basin's evolution. Fining-upward cycles and large-scale cross-bedding in the sandstones reflect sediment transport by meandering rivers with seasonal flow regimes, transitioning to wind-dominated aeolian dunes in more arid intervals, indicative of a dynamic, low-energy continental setting that periodically supported trackway preservation and accumulation of fish remains in quieter lacustrine facies.^[9] Paleoclimatic conditions during deposition were tropical to subtropical, with humid phases punctuated by dry seasons, as inferred from the red-bed coloration due to iron oxidation and the presence of evaporitic features like gypsum nodules in associated shales. Paleosols developed in overbank mudstones further suggest periodic soil formation under warm, seasonally variable precipitation, promoting vegetation in floodplain areas while allowing evaporation in shallow lakes. These climatic signals align with broader Late Cretaceous patterns in the region, where rift-related thermal subsidence facilitated stable basin filling without major tectonic disruptions. Tectonically, the Lecho Formation accumulated in the Salta rift basin, an intracontinental structure initiated in the Early Cretaceous and transitioning to thermal subsidence by the Maastrichtian, with sediments derived from eroding rift shoulders and proximal highlands. This setting preceded the Cenozoic Andean orogeny, during which basin inversion incorporated the Lecho deposits into the retroarc foreland system, with rising Andean ranges later influencing regional sediment routing and uplift. The low-relief depositional landscape contributed to a taphonomic bias favoring fragmentary preservation, as weathering in the exposed, oxidized red beds preferentially destroyed delicate elements while conserving robust bones, as seen in the incomplete skeleton of Noasaurus leali.^[10]

Associated biota

The Lecho Formation preserves a diverse assemblage of vertebrates and invertebrates from the Maastrichtian stage of the Late Cretaceous in northwestern Argentina, reflecting a continental ecosystem with fluvial and lacustrine influences. Dinosaur remains dominate the known vertebrate record, featuring herbivorous titanosaurs such as Saltasaurus loricatus as the principal large-bodied herbivores, alongside small theropods that filled predatory roles.^[11] Non-dinosaurian vertebrates include enantiornithine birds, represented by tarsometatarsi comparable to cf. Neuquenornis and other isolated elements indicating arboreal or ground-foraging avifauna. Invertebrates are evidenced by ostracods and gastropods, which inhabited freshwater environments.^[12] The overall diversity patterns show low theropod representation, limited to small-bodied forms, with no large carnivores documented, implying specialized niches for agile predators amid abundant herbivorous prey. Noasaurus likely functioned as a nimble, small-scale hunter within this community. Knowledge gaps persist due to limited excavation at key sites like El Brete; no new Noasaurus material has emerged.^[13]