Meraxes

Meraxes is a genus of large carcharodontosaurid theropod dinosaur that lived during the Late Cretaceous period in Patagonia, Argentina.^[1] The type and only known species is Meraxes gigas, represented by a nearly complete skeleton (specimen MMCh-PV 65) discovered in 2012 and excavated between 2012 and 2014 from the Huincul Formation in Las Campanas Canyon, Neuquén Province.^[1] This specimen includes a well-preserved skull measuring approximately 127 cm in length, as well as forelimbs, vertebrae, and other postcranial elements, making it one of the most complete carcharodontosaurid skeletons known from the Southern Hemisphere.^[1] The dinosaur was a massive carnivore, with an estimated body mass of 4,263 kg (ranging from 3,196 to 5,331 kg based on volumetric models), suggesting a total length of around 11 meters from snout to tail.^[1] M. gigas is notable for its extremely reduced forelimbs, which are only about 47% the length of the femur and feature a robust humerus with an ovoid fossa, a trait that evolved convergently with the short-armed tyrannosaurids like Tyrannosaurus rex.^[1] Phylogenetic analysis places Meraxes as a basal member of the Giganotosaurini tribe within Carcharodontosauridae, highlighting evolutionary trends in arm reduction among large predatory theropods during the Cenomanian-Turonian stages (approximately 95–90 million years ago).^[1] The Huincul Formation, where the fossil was found 13 meters above the underlying Candeleros Formation, represents a fluvial environment conducive to the preservation of such large-bodied apex predators.^[1]

Discovery and Naming

Discovery

The holotype specimen of Meraxes gigas (MMCh-PV 65), a nearly complete skeleton including the skull, was discovered in 2012 in a lag deposit within the Huincul Formation at Las Campanas Canyon, approximately 25 km southwest of Villa El Chocón in Neuquén Province, Argentina.^[1] The site lies about 13 m above the contact with the underlying Candeleros Formation, dating to the late Cenomanian to Turonian stages of the Late Cretaceous, roughly 95 million years ago.^[1] Excavations were conducted over four field seasons from 2012 to 2014 by a team led by paleontologists Juan I. Canale, Sebastián Apesteguía, and Pablo A. Gallina from institutions including the Museo Paleontológico Egidio Feruglio and the Universidad Maimónides.^[1] Overburden was removed using jackhammers and rock saws, with bones extracted via picks, chisels, and plaster jackets; consolidants such as Paraloid B-72 were applied during preparation to stabilize the fragile material.^[1] The specimen, representing an adult individual approximately 11 meters long, was housed at the Museo Municipal “Ernesto Bachmann” in Villa El Chocón.^[1] The genus and species name Meraxes gigas was formally established in a 2022 description by Canale and colleagues, honoring Meraxes, a dragon from George R.R. Martin's A Song of Ice and Fire series, combined with the Greek gigas meaning "giant," reflecting the dinosaur's large size.^[1] This find represents one of the most complete carcharodontosaurid skeletons from South America, enhancing understanding of theropod diversity in the Neuquén Basin prior to the group's decline.^[1]

Etymology

The genus name Meraxes honors a dragon from George R.R. Martin's fantasy series A Song of Ice and Fire, the basis for the television series Game of Thrones. This choice reflects the dinosaur's imposing presence as a large carnivorous theropod.^[1] The specific epithet gigas derives from the Greek word γίγας (gigas), meaning "giant," in reference to the species' substantial size, estimated at over 10 meters in length and approximately 4 metric tons in mass. This naming convention underscores the animal's status as one of the largest known carcharodontosaurids.^[1]

Description

Overall Morphology

Meraxes gigas was a large-bodied carcharodontosaurid theropod, with an estimated body mass of 4,263 kg based on volumetric modeling of the holotype skeleton.^[1] The specimen represents an adult individual, indicated by the co-ossification of its five sacral vertebrae and the presence of multiple growth marks in the dorsal ribs, which exhibit a mix of primary and secondary bone tissue with approximately eight observable lines of arrested growth.^[1] The overall skeletal completeness is exceptional for a giant carcharodontosaurid, preserving much of the skull, axial column, pectoral and pelvic girdles, fore- and hindlimbs, and partial tail.^[1] The skull of Meraxes gigas measures 127 cm in length and is characterized by a long, low profile with profusely ornamented dermal bones, including vertical furrows and ridges on the maxilla, extensive rugosities and rounded bumps on the nasals, and pronounced ridges on the lacrimal.^[1] Additional features include a robust, laterally projecting brow horn on the postorbital bone, a wide parietal skull table, and a supraoccipital with a conspicuous midline knob.^[1] These traits contribute to a robust cranial structure adapted for predatory function.^[1] The axial skeleton includes fused neural spines on the first two and subsequent pairs of anterior caudal vertebrae, which are pneumatic and possess hyposphene-hypantrum articulations for enhanced stability.^[1] The pectoral girdle comprises a long, robust, and gently curved scapula paired with a coracoid featuring a rounded outline and a posteroventral process.^[1] In the forelimb, the humerus is stout with expanded proximal and distal ends and a deep ovoid fossa on the posterior surface, while the ulna is short and robust with a block-shaped olecranon process measuring 27% of total ulnar length; metacarpals II and III are robust with expanded articular ends.^[1] The pelvic girdle features a trapezoidal iliac blade, an enclosed pubic foramen, and a straight ischial shaft.^[1] The hindlimb includes a femur with an upturned head and a minimum shaft circumference of 452 mm, a tibia with a subrectangular, anterodorsally projecting cnemial crest, and a pes in which metatarsal III is the longest; the largest pedal ungual is on digit II-3, and the astragalus is hourglass-shaped with a shallow ascending process groove.^[1] These elements reflect a bipedal stance with powerful propulsion from the hindlimbs.^[1]

Forelimb Reduction

Meraxes gigas exhibits pronounced forelimb reduction, a trait characteristic of several large theropod lineages, with the preserved holotype specimen providing near-complete forelimbs for detailed analysis. The forelimb length measures approximately 47% of the femur length, rendering the arms disproportionately short relative to the hindlimbs and body size. This reduction is evident in the robust yet compact humerus, a short and stout ulna featuring a prominent olecranon process (comprising 27% of the ulna's total length), and robust metacarpals II and III, which suggest strong musculature despite the diminutive overall proportions.^[2] This morphology represents an instance of convergent evolution, as similar forelimb proportions—converging around a forelimb-to-femur ratio of 0.4—appear independently in tyrannosaurids (such as Tarbosaurus), abelisaurids (like Carnotaurus, though less extreme), and other carcharodontosaurids, including Meraxes itself within its family. The reduction likely correlates with allometric scaling tied to the dinosaur's massive skull and overall body size, where increasing head dimensions impose selective pressures that favor minimized forelimbs, possibly constrained by developmental limits in the pectoral girdle. Fossil evidence from the Huincul Formation indicates that Meraxes, at around 11 meters in length and over 4 metric tons, prioritized powerful jaws for predation over forelimb utility.^[2]^[3] The functional role of these reduced forelimbs remains speculative but is inferred from muscle attachment scars and comparative anatomy. Large insertion sites on the humerus and scapula suggest well-developed shoulder and arm muscles, implying the limbs retained some utility beyond vestigiality, such as assisting in rising from a reclined position or potentially in reproductive behaviors like grasping during mating. They were unlikely involved in hunting, given the dinosaur's enormous head equipped for bone-crushing bites. This pattern of forelimb diminution across multiple theropod clades underscores a shared evolutionary response to gigantism and cranial specialization in apex predators.^[4]^[2]

Classification and Phylogeny

Taxonomic Position

Meraxes gigas is classified within the domain Eukarya, kingdom Animalia, phylum Chordata, class Reptilia, clade Dinosauria, order Saurischia, suborder Theropoda, clade Tetanurae, superfamily Allosauroidea, family Carcharodontosauridae, subfamily Carcharodontosaurinae, and tribe Giganotosaurini.^[1] This placement positions it among the large-bodied carnivorous theropods that dominated Southern Hemisphere ecosystems during the Late Cretaceous.^[1] Phylogenetic analyses based on 175 cranial and postcranial characters from a modified dataset consistently recover Meraxes gigas as the basalmost member of the Giganotosaurini tribe within derived Carcharodontosauridae.^[1] It forms a clade with other massive South American carcharodontosaurids, including Giganotosaurus carolinii and Mapusaurus roseae, sharing synapomorphies such as a centrally widened nasal bone and a tongue-like parietal projection.^[1] This positioning highlights Meraxes as a representative of the peak diversity achieved by Carcharodontosauridae prior to their decline in the Turonian-Coniacian stages.^[1] The taxonomic assignment underscores convergent evolutionary trends in forelimb reduction observed across multiple theropod lineages, with Meraxes exhibiting a humerus-to-femur length ratio of approximately 0.47, comparable to that in tyrannosaurids and abelisaurids.^[1] Such features distinguish Giganotosaurini from earlier allosauroids while affirming their role as apex predators in Gondwanan faunas.^[1]

Evolutionary Significance

Meraxes gigas, a carcharodontosaurid theropod from the Cenomanian stage of the Late Cretaceous, holds significant evolutionary importance as it illuminates patterns of morphological evolution among large-bodied carnivorous dinosaurs.^[1] Its discovery provides a well-preserved specimen that refines the phylogenetic relationships within Carcharodontosauridae, positioning Meraxes as a close relative of Giganotosaurus carolinii and contributing to a more robust understanding of allosauroid diversity in the Southern Hemisphere.^[1] This placement underscores the biogeographic distribution of carcharodontosaurids during the mid-Cretaceous, highlighting their dominance in Gondwanan ecosystems before the rise of tyrannosaurids in the Northern Hemisphere.^[1] A key evolutionary insight from Meraxes is the documentation of convergent forelimb reduction in megapredatory theropods, a trait independently evolved across multiple lineages despite phylogenetic distance.^[1] In Meraxes, the forelimbs have a length approximately 47% that of the femur, or about 5% of the total body length of around 11 meters—mirroring the diminutive arms of tyrannosaurids like Tyrannosaurus rex, which reduced their forelimbs to similar proportions through distinct developmental pathways.^[1] This convergence suggests that extreme body size, rather than shared ecology or predation strategy, drove the loss of functional forelimbs, as carcharodontosaurids like Meraxes relied on powerful hindlimbs and massive skulls for hunting, with forelimb vestigiality emerging as a byproduct of hypermorphic growth patterns.^[1] Furthermore, the nearly complete cranium of Meraxes enables refined estimates of skull dimensions in related taxa, such as Giganotosaurus, projecting a length of approximately 1.62 meters (range 1.58–1.69 meters) among the longest for non-avian theropods.^[1] These findings collectively advance models of theropod gigantism and limb evolution, demonstrating how independent selective pressures could yield analogous adaptations in disparate clades.^[1]

Growth and Paleobiology

Osteohistology

Osteohistological analysis of the holotype specimen (MMCh-PV 65) of Meraxes gigas was conducted using thin sections from the mid-shaft of the right femur, the fibula, a dorsal rib, and fragments of a gastralium to assess growth patterns, age at death, and bone microstructure.^[1] These samples revealed fibrolamellar bone tissue typical of fast-growing dinosaurs, with variations in vascularization and remodeling across elements.^[1] The femur exhibited 24 lines of arrested growth (LAGs) in the primary cortex and four additional LAGs within the External Fundamental System (EFS), indicating skeletal maturity at death.^[1] Its cortex displayed a laminar to reticular vascular pattern, consistent with rapid juvenile growth transitioning to slower deposition in adulthood.^[1] In contrast, the fibula showed extensive remodeling with 3–4 generations of secondary osteons and an EFS, but lacked discernible LAGs due to Haversian bone replacement.^[1] The dorsal rib preserved approximately eight LAGs, some doubled, though taphonomic damage and remodeling obscured precise counting.^[1] The gastralium fragment was dominated by secondary tissue with an EFS, suggesting maturity but limited growth history data.^[1] Based on the femoral LAG count and accounting for potential resorption, the individual died at an estimated age of 39–53 years, representing the oldest known non-avian theropod.^[1]^[5] This longevity implies a hypermorphic growth strategy, with a prolonged period of active somatic growth compared to other allosauroids, potentially linked to the species' large body size.^[1]

Inferred Behavior

Meraxes gigas, as a large-bodied carcharodontosaurid theropod, is inferred to have been an apex predator in its Late Cretaceous Patagonian ecosystem, primarily targeting large herbivorous dinosaurs such as titanosaurs and rebbachisaurids based on its massive skull dimensions and dentition suited for inflicting deep, slashing wounds.^[1] The ziphodont teeth, characterized by finely serrated, recurved edges, suggest a feeding strategy involving powerful bites to cause hemorrhage and debilitation in prey, similar to other carcharodontosaurids, rather than bone-crushing.^[1] The prominent rugosities and ornamentation on the facial bones, including the nasal, lacrimal, and postorbital regions, indicate a potential role in social signaling, possibly for intra-specific displays related to mating or dominance hierarchies, reflecting accelerated evolutionary rates in cranial traits among late-surviving carcharodontosaurids.^[1] This ornamentation, combined with the dinosaur's estimated body mass exceeding 4 tons, supports inferences of complex social behaviors, though direct evidence from trackways or multiple associated specimens is lacking.^[1] Despite significant forelimb reduction—with the humerus shorter than the radius and ulna—the preserved elements show robust construction and large muscle attachment scars, particularly on the scapula and humerus, implying retained functionality beyond predation, such as aiding in rising from a prone position or possibly in reproductive behaviors like grasping during mating.^[1] The forelimb-to-femur length ratio of approximately 0.47 underscores biomechanical adaptations for bipedal locomotion, where the reduced arms did not compromise stability but may have minimized energetic costs in a gigantothermic predator.^[1] Osteohistological analysis of the holotype reveals a prolonged growth trajectory, with the individual estimated at 39–53 years old at death and skeletal maturity reached around 35–49 years, indicating a life history strategy of extended ontogeny rather than accelerated growth rates, potentially allowing for larger adult sizes and influencing behavioral patterns like delayed reproduction or territoriality over a long lifespan.^[1] This hypermorphic growth, evidenced by 24 annual lines of arrested growth in the femur alongside fibrolamellar bone tissue, aligns with inferences of a relatively low metabolic pace in adulthood, consistent with ambush or opportunistic hunting rather than sustained pursuit.^[1]

Paleoenvironment

Geological Context

Meraxes gigas was discovered in the Huincul Formation of the Neuquén Group, a Upper Cretaceous sedimentary sequence exposed in the Neuquén Basin of northern Patagonia, Argentina.^[2] The specimen (MMCh-PV 65) was recovered from outcrops in Las Campanas Canyon, approximately 25 km southwest of Villa El Chocón in Neuquén Province.^[2] The Huincul Formation, part of the Río Limay Subgroup, overlies the Candeleros Formation conformably and is overlain by the Cerro Lisandro Formation.^[6] It consists primarily of sandstones, siltstones, and mudstones, representing fluvial depositional systems that transitioned from braided to meandering river environments.^[6] The bones of M. gigas were found in an overbank mudstone deposit, overlain by a ~2 m thick sandstone layer interpreted as the result of a high-energy flooding event, located about 13 m above the contact with the underlying Candeleros Formation.^[2] Stratigraphically, the Huincul Formation records deposition during a phase of tectonic quiescence in the Andean foreland basin system, following Albian forebulge migration and preceding renewed orogenic loading.^[6] This interval corresponds to an overfilled basin stage without significant orogenic influence, allowing for the accumulation of continental fluvial sediments under a warm, greenhouse climate typical of the Late Cretaceous.^[6] The formation's age is constrained to the late Cenomanian–early Turonian stages, approximately 95–93 million years ago, based on biostratigraphic correlations with ammonites and other fossils from the Neuquén Group.^[6]

Associated Fauna

The Huincul Formation, from which Meraxes gigas was recovered, preserves a rich vertebrate assemblage indicative of a fluvial to floodplain environment in northern Patagonia during the late Cenomanian to early Turonian.^[2] Among theropod dinosaurs, Meraxes coexisted with other large carcharodontosaurids, including Mapusaurus roseae, a similarly sized predator known from multiple individuals suggesting gregarious behavior.^[2] Abelisaurids were also prominent, represented by mid-sized taxa such as Skorpiovenator bustingorryi, which featured a robust skull adapted for bone-crushing bites, and smaller indeterminate forms.^[7] Megaraptorans like Aoniraptor libertatem added to the carnivorous diversity, with elongated manual claws suited for prey manipulation.^[8] Additionally, basal coelurosaurs and possible paravians indicate a broader spectrum of smaller theropods.^[9] Sauropod remains dominate the megafaunal record, reflecting abundant herbivorous prey for apex predators like Meraxes. Titanosaurs include the enormous Argentinosaurus huinculensis, one of the largest known dinosaurs with estimated lengths exceeding 30 meters, and more recent discoveries such as Chucarosaurus diripienda, a colossal form with a long cervical series, and Sidersaura marae, a rebbachisaurid sauropod described in 2024.^[2]^[10]^[11] Rebbachisaurids, specialized for low browsing, are exemplified by Cathartesaura anaerobica, featuring a deep thoracic cavity and modified caudal vertebrae, and other fragmentary diplodocoids. Ornithischians were previously rare, but recent discoveries include the basal ornithopod Chakisaurus nekul described in 2024.^[12]^[13] Beyond dinosaurs, the fauna encompasses aquatic and semi-aquatic vertebrates adapted to the formation's ephemeral river systems. Osteichthyan fishes include semionotids and dipnoans, with robust tooth plates for crushing. Chelid turtles, including indeterminate remains, suggest riparian habitats. Crocodyliforms are represented by isolated teeth and osteoderms, likely notosuchians similar to those in coeval Patagonian units, indicating opportunistic predators in wetland margins. This assemblage highlights a dynamic ecosystem with large-bodied herbivores supporting multiple guilds of carnivores, including Meraxes as a top-tier ambush hunter.^[2]