Quilmesaurus

Quilmesaurus is an extinct genus of carnivorous abelisaurid theropod dinosaur from the Late Cretaceous period of Patagonia, Argentina.^[1] Known only from a partial right hindlimb consisting of the distal end of a femur and a complete tibia (holotype specimen MPCA-PV 100), it represents a subadult individual estimated to have measured approximately 5.3 meters in total body length.^[2]^[3] The remains were discovered in the Allen Formation at Salitral Ojo de Agua, Río Negro Province, dating to the middle Campanian to lower Maastrichtian stages (roughly 83 to 70 million years ago).^[1]^[2] Named in 2001 by paleontologist Rodolfo A. Coria, the genus honors the indigenous Quilmes people of northwestern Argentina, while the species Q. curriei recognizes Canadian paleontologist Philip J. Currie for his contributions to theropod research.^[2] Initially described as a basal tetanuran theropod, Quilmesaurus was later reclassified within Abelisauridae based on diagnostic features such as a well-developed mediodistal femoral crest and an expanded cnemial crest on the tibia.^[2]^[1] Some analyses have suggested a closer affinity to the subfamily Carnotaurinae, though its fragmentary nature has led to debates regarding its taxonomic validity, with early proposals considering it a nomen vanum due to a lack of unique autapomorphies.^[2] Despite this, recent phylogenetic studies continue to recognize it as a valid abelisaurid, highlighting its role as one of the youngest known large theropods from Patagonia before the end-Cretaceous extinction.^[4] Bone microstructure analysis indicates rapid growth rates typical of abelisaurids, with dense vascularization and remodeling suggesting the holotype had not yet reached full adult size.^[3] As a medium-sized predator, Quilmesaurus likely inhabited coastal environments and preyed on smaller dinosaurs and other vertebrates in a diverse Late Cretaceous ecosystem that included titanosaurs and other theropods.^[1]

Discovery

History of research

The single known specimen of Quilmesaurus curriei, cataloged as MPCA-PV 100, consists of the distal end of a right femur and a complete right tibia from a subadult individual, collected from the Allen Formation at Salitral Ojo de Agua in Río Negro Province, Patagonia, Argentina.^[2] The specimen is housed at the Museo Provincial "Carlos Ameghino" in Cipolletti, Río Negro Province.^[2] The taxon was first described and named by Rodolfo A. Coria in 2001 in the volume Mesozoic Vertebrate Life: New Research Inspired by the Paleontology of Philip J. Currie, where it was classified as a basal tetanuran theropod, potentially related to allosauroids such as Giganotosaurus. Coria noted the fragmentary nature of the material but highlighted features like the robust tibia as indicative of a large-bodied predator. Early re-evaluations began in 2002, when Alexander W. A. Kellner and Diogenes de Almeida Campos suggested affinities with abelisaurids based on shared traits in the tibia with Pycnonemosaurus nevesi, proposing a possible placement within Abelisauria. This interpretation was expanded in a 2007 study by Rubén D. Juárez Valieri, Lucas E. Fiorelli, and Lilian E. Cruz in the Revista del Museo Argentino de Ciencias Naturales, which conducted a comparative analysis and confirmed Quilmesaurus as an abelisaurid, likely within the subfamily Carnotaurinae, due to characteristics such as the hatchet-shaped cnemial crest on the tibia and the overall robust hindlimb morphology.^[5] However, the authors concluded that the specimen lacks sufficient autapomorphies to distinguish it from other abelisaurids, rendering Quilmesaurus curriei a potential nomen vanum.^[5] Subsequent research has continued to explore its phylogenetic position and biological attributes. In broader theropod reviews, such as Ezcurra and Novas (2016), Quilmesaurus is retained as a valid abelisaurid from the Campanian–Maastrichtian of Patagonia, contributing to understanding late Gondwanan theropod diversity.^[6] In the 2020s, discussions on its validity persist in phylogenetic analyses, where it is often scored as an abelisaurid but sometimes treated as a chimera or insufficiently diagnostic; for instance, a 2020 study by Baiano et al. on South American abelisaurids includes it in cladistic matrices supporting its placement near Carnotaurus,^[7] while a 2024 analysis by Baiano et al. includes Quilmesaurus but identifies it as an unstable taxon due to incompleteness, leading to polytomies in the ceratosaur phylogeny.^[8] Additionally, histological analyses by Baiano and Cerda (2017) examined the bone microstructure of the tibia, revealing rapid growth rates consistent with other abelisaurids and reinforcing its taxonomic assignment despite the limited material.^[9]

Naming and holotype

The genus Quilmesaurus was erected by Rodolfo A. Coria in 2001, with the name combining a reference to the Quilmes, an indigenous people of northwestern Argentina, and the Ancient Greek sauros for "lizard."^[10] The specific epithet curriei honors Philip J. Currie, a prominent Canadian paleontologist known for his extensive work on theropod dinosaurs, including abelisauroids.^[10] The holotype, cataloged as MPCA-PV 100 and housed in the Museo Provincial Carlos Ameghino in Cipolletti, Río Negro Province, Argentina, comprises the distal portion of a right femur and a complete right tibia from a subadult individual.^[10] In the original description, Coria diagnosed the taxon based on several features of the hindlimb, including a well-developed mediodistal crest on the femur, a robust tibia with a pronounced fibular crest, a hook-shaped cnemial crest on the tibia, a lateral malleolus approximately twice the size of the medial malleolus, and an asymmetrical distal tibial articulation; these were interpreted as autapomorphies supporting a basal tetanuran placement at the time.^[11]^[10] Subsequent analysis by Juárez Valieri et al. in 2007 revised this diagnosis, determining that none of the proposed traits are uniquely autapomorphic to Quilmesaurus curriei but instead represent synapomorphies of larger clades such as Ceratosauria or Abelisauridae (e.g., the pronounced fibular crest and overall tibial robustness are widespread among abelisauroids, while the reduced proximal fibula—though not preserved in the holotype—is a common abelisaurid feature inferred from comparisons).^[10] This led to the exclusion of those characters from the species' unique diagnosis, rendering the taxon potentially a nomen vanum pending discovery of additional material to confirm its distinctiveness.^[10]

Description

Preserved anatomy

The known skeletal remains of Quilmesaurus curriei are limited to the holotype specimen (MPCA-PV 100), consisting of the distal end of the right femur and a complete right tibia, providing insight into the lower leg structure of this abelisaurid theropod.^[2] The distal femur features a strongly developed mediodistal crest that is quadrangular in anterior view, with the medial condyle larger than the lateral one, which is shorter but thicker; a shallow and wide extensor groove is present between the condyles.^[2] The tibia measures approximately 529 mm in length, featuring a robust shaft and a markedly expanded cnemial crest that is hook-shaped and distally directed, as well as an asymmetrical distal end with the medial malleolus positioned more proximally and the lateral malleolus more expanded.^[12]^[2] Overall body size is estimated at approximately 5.3 meters in length for this subadult individual, based on comparisons with other abelisaurids.^[2] Pathological modifications are evident on the tibia but are addressed separately.

Pathological features

The holotype specimen of Quilmesaurus curriei (MPCA-Pv 100) preserves evidence of pathology in the right tibia, manifesting as a marked variation in the microstructural arrangement of the outer cortex, characterized by radial fibrolamellar bone tissue with altered vascularization patterns. This abnormality, detected through histological analysis, indicates bone remodeling in response to an underlying condition, likely a stress injury or infectious process affecting the weight-bearing limb.^[13]^[9] The pathological tissue lacks external signs of deformation due to taphonomic factors, but the abrupt shift from typical parallel-fibered bone to densely vascularized radial tissue in the outermost compacta suggests a healed response to trauma or disease, comparable to hypertrophic osteopathy or osteopetrosis observed in other theropods like Allosaurus. Possible etiologies include systemic infection or chronic stress, though definitive identification remains tentative without additional specimens.^[13]^[14] Histological evidence of cyclical growth marks and remodeling demonstrates that the subadult individual survived the pathology without reaching somatic maturity, with no apparent long-term compromise to mobility, as the bone maintained structural integrity for locomotion. This case represents the first documented theropod paleopathology from the Allen Formation, providing insights into individual health resilience in Late Cretaceous abelisaurids.^[13]^[9]

Classification

Initial interpretations

In its original description, Quilmesaurus curriei was interpreted as a basal tetanuran theropod of uncertain affinities, tentatively placed near the carcharodontosaurid Giganotosaurus carolinii due to a longitudinal groove on the tibial shaft.^[2] This placement emphasized the taxon's "very peculiar" morphology, with the preserved hindlimb showing a slender fibula reduced distally in a manner reminiscent of avian-like coelurosaurs such as ornithomimids and troodontids. The partial metatarsals further suggested an arctometatarsal condition in the pes, a feature commonly associated with those gracile coelurosaur groups, supporting the initial view of a non-ceratosaurian theropod.^[2] The material's limitations—comprising only the distal femur, complete tibia and fibula, and incomplete metatarsals—contributed to this provisional classification, as the fragmentary remains lacked sufficient autapomorphies for a more definitive position within Theropoda.^[2] Exclusion from Abelisauridae stemmed from the absence of characteristic robust hindlimb features, including a distally expanded cnemial crest and symmetrical distal tibial malleoli, which contrast with the stocky tibiae of abelisaurids like Carnotaurus sastrei. These traits underscored the perceived gracility of Quilmesaurus, aligning it more closely with basal tetanurans rather than derived ceratosaurs.^[2]

Current phylogenetic placement

Quilmesaurus curriei is recognized as a member of Abelisauridae, a clade within the ceratosaurian theropods, based on a comprehensive phylogenetic analysis that reclassified it from its original assignment as a basal tetanuran. This placement was first supported by Carrano and Sampson (2008), who incorporated the holotype into their dataset and recovered it within Abelisauridae due to shared derived features such as a reduced fibula relative to the tibia.^[15] Within Abelisauridae, Quilmesaurus is positioned as a relatively basal member or potential sister taxon to Majungasaurinae, as indicated in analyses from the late 2000s to 2014 that exclude it from more derived lineages including Carnotaurus. For instance, phylogenetic trees in studies like those by Pol and Rauhut (2012) and Tortosa et al. (2014) depict Quilmesaurus branching early within the family, emphasizing its reliance on hindlimb morphology for scoring.^[16]^[17] More recent analyses, such as Pol et al. (2024), continue to place it within Abelisauridae, though as an unstable taxon due to its fragmentary nature.^[8] However, the taxon's validity has been questioned due to its fragmentary remains—limited to partial hindlimb bones—and the absence of unique autapomorphies that distinguish it from other abelisaurids. Juárez Valieri et al. (2016) affirmed the abelisaurid affinity through comparisons highlighting traits like a well-developed mediodistal crest on the femur and an asymmetrical distal tibia but ultimately deemed Quilmesaurus a nomen vanum, as its diagnostic features are not exclusive.^[5] No additional specimens have been reported since the 2001 holotype, with ongoing placements depending on comparative morphology from coeval Allen Formation abelisaurids such as Aucasaurus.^[2]

Paleoecology

Geological setting

The Allen Formation represents the basal unit of the Malargüe Group within the Neuquén Basin of northern Patagonia, Argentina, and spans the Late Cretaceous (middle Campanian to early Maastrichtian stages), approximately 83 to 70 million years ago. This formation records a transition from continental to marginal marine conditions during the first Atlantic ingression into the basin, with its strata reflecting dynamic sedimentation influenced by tectonic subsidence and eustatic sea-level rise.^[18] The holotype specimen of Quilmesaurus curriei was discovered at the Salitral Ojo de Agua locality in Río Negro Province, roughly 40 km south of General Roca city. This site lies within exposures of the formation's lower sections, where vertebrate fossils are relatively common amid the broader Patagonian Cretaceous record.^[2] Lithologically, the Allen Formation comprises reddish to yellowish fine- to medium-grained sandstones interbedded with mudstones, pelites, and minor conglomerates, often exhibiting cross-bedding and heterolithic lamination. These sediments indicate a depositional environment dominated by fluvial channels and lacustrine settings in the lower units, punctuated by seasonal flooding events that promoted sediment aggradation in low-gradient plains.^[18] Age constraints for the formation derive from magnetostratigraphic correlations and biostratigraphic ties to regional sections, placing the Quilmesaurus-bearing levels in the late Campanian, around 80 to 75 million years ago. Taphonomic evidence suggests that fossils, including theropod remains, were preserved in channel lag deposits within fluvial sandstones, implying rapid burial during high-energy riverine events that minimized post-mortem transport and weathering.^[19]

Faunal associations

The Allen Formation of Patagonia preserves a diverse Late Cretaceous vertebrate assemblage that co-occurred with Quilmesaurus curriei, providing insights into its ecological context. Among dinosaurs, multiple titanosaur genera dominated the herbivorous niche, including Bonatitan reigi, Rocasaurus muniozi, and Aeolosaurus sp., representing medium- to large-bodied sauropods that likely formed the primary prey base in floodplain environments.^[20]^[21] Ornithischian herbivores included hadrosaurs such as Bonapartesaurus rionegrensis, indicating a mix of browsing and grazing strategies among medium-sized dinosaurs. Other theropods contributed to carnivorous diversity, with Austroraptor cabazai (a large dromaeosaurid), an unnamed alvarezsaurid, indeterminate tetanurans, and additional abelisaurids represented by isolated teeth and bones, suggesting a range of predatory sizes and behaviors.^[22]^[23] Non-dinosaurian vertebrates further enriched the ecosystem, reflecting a mix of terrestrial, freshwater, and marginal marine influences. Aquatic and semi-aquatic taxa included chelid turtles, madtsoiid snakes, pipid and leptodactylid anurans, and osteichthyan fish such as diplomystids (catfish), lepisosteids (gars), percichthyids (perch-like), and dipnoans (lungfish) from lacustrine deposits. Reptilian components featured sphenodonts (rhynchocephalians, marking their first upper Late Cretaceous record in Patagonia) and elasmosaurid plesiosaurs, the latter indicating episodic marine incursions. Early neornithine birds, including Lamarqueavis australis and isolated coracoids, represent the avian component. In 2024, new mammalian fossils, including meridiolestidans, were described from the formation, indicating a diverse small vertebrate component.^[24]^[20] As a medium-sized abelisaurid estimated at 5–6 meters in length and approximately 0.7 metric tons in mass, Quilmesaurus likely occupied the role of an apex predator or scavenger, targeting medium-sized herbivores like hadrosaurs and juvenile titanosaurs in a floodplain setting with fluvial and lacustrine features. Associated theropod teeth and fragmentary tracks indicate a diverse carnivorous guild, implying competition for resources among abelisaurids, dromaeosaurids, and smaller coelurosaurs. Although no direct evidence of interactions involving Quilmesaurus exists, the broader formation assemblage underscores a vibrant Patagonian biota during the Campanian–Maastrichtian, characterized by high sauropod abundance and emerging avian and mammalian elements.^[23]^[21]