Carnufex

Carnufex is an extinct genus of basal crocodylomorph, a group of archosaurian reptiles closely related to the ancestors of modern crocodilians, that inhabited North America during the Late Triassic epoch approximately 231 million years ago.^[1] The genus is represented by a single species, Carnufex carolinensis, commonly known as the "Carolina Butcher" due to its discovery site and predatory adaptations, and is notable as one of the earliest and largest known members of Crocodylomorpha, reaching an estimated body length of about 3 meters with a lightly built, rostrally elongate skull exceeding 50 cm in length.^[1]^[2] Fossils of C. carolinensis were first described in 2015 based on the holotype specimen NCSM 21558, a partial skeleton including elements of the skull, vertebrae, and limbs recovered from the Carnian-age Pekin Formation in Chatham County, North Carolina.^[1] The skull features a mosaic of primitive traits, including a large subtriangular antorbital fenestra measuring approximately 14 cm long by 6 cm high, thin laminar bone (about 1 mm thick) in the maxilla and jugal, and serrated, blade-like teeth suited for tearing flesh, indicating a carnivorous diet focused on smaller reptiles and early synapsids.^[2] Postcranial elements, such as a humerus roughly 21 cm long—less than half the skull's length—suggest reduced forelimbs and a predominantly bipedal or semi-erect posture, similar to that of contemporaneous rauisuchids and early dinosaurs.^[2] Phylogenetic analyses position Carnufex as a basal crocodylomorph within Pseudosuchia, outside the more derived crocodyliform lineage leading to modern crocodiles, and highlight its role in the diverse guild of top-tier carnivores during the recovery phase following the end-Permian mass extinction.^[1] This period saw Carnufex coexisting with early theropod dinosaurs like Coelophysis, contributing to a "predator pile-up" where multiple large-bodied archosaurs competed for apex niches in a recovering ecosystem.^[1] As a terrestrial predator in a subtropical floodplain environment, Carnufex exemplifies the early evolutionary experimentation among crocodylomorphs, blending crocodile-like cranial features with more upright, dinosaurian locomotor traits before the group's later specialization toward semiaquatic lifestyles.^[2]^[1]

Discovery and naming

Discovery

The holotype specimen of Carnufex carolinensis (NCSM 21558) was collected in 2003 from NCSM locality NCPALEO 1902, a former brick quarry in southeastern Chatham County, North Carolina, USA, within the Upper Triassic Pekin Formation of the Newark Supergroup.^[2] The fossils were recovered from red conglomerate layers deposited in a fluvial environment on private land with landowner permission.^[2] This locality has proven productive for Late Triassic archosaur remains, including multiple taxa, though the Carnufex material represents a disarticulated partial skeleton of a single subadult individual.^[2] The holotype includes elements of the partial skull—such as the right dentigerous premaxilla, left maxilla, left lacrimal, left jugal, left articular, right angular, and an isolated maxillary tooth—along with postcranial bones comprising an atlas intercentrum, a cervical neural arch, a dorsal neural arch, a cervical rib, several dorsal ribs, a gastralium, and a left humerus.^[1] A referred specimen, NCSM 21623, consists of the distal portion and shaft of a right humerus collected from the same locality, representing a smaller individual and suggesting intraspecific size variation.^[2] Carnufex carolinensis was formally named and described in 2015 by Lindsay E. Zanno, Vincent E. Ataydin, and Susan M. Drymala in the journal Scientific Reports, based on these specimens from the Carnian-stage Pekin Formation (approximately 231 million years old).^[1] An in-depth osteological study, including detailed descriptions of the cranial and postcranial anatomy, was published in 2016 by Zanno, Drymala, Sterling J. Nesbitt, and Vincent P. Schneider in PLOS ONE.^[2] Specimen preparation involved mechanical removal of matrix and consolidation, with the elements showing excellent three-dimensional preservation and minimal distortion.^[2] For analysis, high-resolution 3D surface scans were obtained using a Creaform EXAscan scanner at 0.050 mm resolution, enabling digital reconstruction of the skull in Autodesk Maya 2014 software to visualize the composite morphology.^[1]

Etymology

The genus name Carnufex is derived from the Latin carnifex, meaning "butcher" or "executioner," reflecting the animal's inferred role as a formidable predator.^[3] The term carnifex in ancient Roman usage denoted a public executioner or torturer, often evoking imagery of a merciless figure responsible for carrying out capital punishments. This choice highlights the taxon's predatory morphology and its dominance in the Late Triassic ecosystems of North Carolina, where the fossils were discovered.^[4] The species epithet carolinensis is a Latinized form meaning "from Carolina," directly referencing the North Carolina locality of the type specimen from the Pekin Formation.^[3] Together, the binomial Carnufex carolinensis combines these elements to evoke the "Carolina Butcher," an informal nickname emphasizing the creature's status as a top carnivore in its prehistoric habitat.^[4] The naming underscores the paleontological significance of the find in highlighting early crocodylomorph diversity before the rise of large dinosaurs.^[1]

Description

Skull

The skull of Carnufex carolinensis exhibits an elongated rostrum that is dorsoventrally deep, with the holotype preserving a partial cranium estimated at a minimum length exceeding 50 cm. This lightly built structure features thin laminar bones, typically 1–2 mm thick, and an overall narrow profile adapted for predatory function. The external surfaces of dermal bones such as the premaxilla, maxilla, lacrimal, and jugal display prominent ornamentation, including fine irregular patterns and rugose sub-circular pits, which likely enhanced structural integrity under stress.^[1]^[2] A defining cranial feature is the enlarged antorbital fenestra, which is subtriangular in outline with an anteroposterior length of approximately 14 cm and a dorsoventral height of about 6 cm, yielding a length-to-height ratio of roughly 2.3; this fenestra is hypertrophied relative to other early crocodylomorphs and bordered by a shallow antorbital fossa. The robust jugal bone is triradiate, contributing to the ventral margins of the orbit and infratemporal fenestra, and bears a prominent lateral boss that forms a horn-like projection; its posterior process is bifurcated, with a small posterodorsal flange. The quadrate, though incompletely preserved, shows a broad articular surface for the lower jaw, supporting efficient force transmission during feeding. Large supratemporal fenestrae, formed by elements including the parietal and squamosal, accommodated expansive jaw adductor musculature, facilitating powerful occlusion.^[1]^[2] Dentition is ziphodont and heterodont, with six premaxillary alveoli housing slender, conical, recurved teeth that bear fine serrations at a density of 8–9 per mm; these transition posteriorly to at least 15 maxillary alveoli containing laterally compressed, blade-like teeth with coarser serrations (4–5 per mm) and slight recurvature. This arrangement suggests specialization for slicing flesh, with evidence of thecodont replacement patterns akin to those in basal crocodylomorphs, including resorption pits on unerupted crowns. The skull's proportions, including the elongate rostrum and reduced cranial robustness compared to later crocodylians, indicate adaptations suited to a more terrestrial predatory niche.^[1]^[2]

Postcranium

The postcranium of Carnufex carolinensis is incompletely known, primarily from the holotype (NCSM 21558), which preserves elements of the axial skeleton and a humerus, supplemented by a referred isolated humerus (NCSM 21623). These elements reveal a robust yet gracile build adapted for terrestrial locomotion and body support in an early crocodylomorph.^[1]^[2] The axial skeleton features complex neural arches in the cervical and dorsal vertebrae, characterized by elongated prezygapophyses that likely enhanced neck flexibility and muscular support for predatory maneuvers. The preserved cervical neural arch is unfused to its centrum, exhibiting nine laminae (ridge-like structures) and twelve fossae (depressions), with a tall neural spine at least twice the height of the arch and inclined approximately 30° posteriorly; this configuration suggests strong ligamentous attachments and dorsiflexion capability. Dorsal neural arches have shorter, more gently inclined spines (20–30°) and reduced complexity, with fewer laminae and three fossae per side, contributing to torso rigidity. Although only fragments are preserved, the material indicates 10–12 cervical vertebrae, implying a relatively short neck compared to later crocodylomorphs. The atlas intercentrum is crescent-shaped, with an inferred occipital condyle width of 27 mm. Ribs are slender and interlocking, with cervical ribs bearing two capitular heads oriented at 90° to the shaft for stable articulation; these broad structures supported torso stability during movement. Gastralia are present as elongate elements with an ovate cross-section, reinforcing a rigid abdominal wall akin to that in basal pseudosuchians.^[2] The appendicular skeleton includes a short, robust humerus measuring 20.8 cm in length in the holotype, with a prominent deltopectoral crest indicating powerful forelimb retraction and potential for digging or restraining prey. The proximal end is four times the shaft width, while the distal end expands transversely to three times the shaft width, featuring an ectepicondylar groove and supinator process for enhanced mobility. This humerus represents less than 50% of the minimum estimated skull length (50 cm), underscoring reduced forelimbs relative to the cranium. No direct hindlimb elements are preserved, but femoral length is estimated at 35–44 cm based on crocodylomorph limb ratios, exceeding humeral length with a straight shaft that implies facultative bipedality and efficient terrestrial gait.^[1]^[2] The pelvic girdle is not directly preserved, but phylogenetic analysis of basal crocodylomorphs, including Carnufex, supports an ilium with a supracetabular crest for anchoring strong hip musculature and facilitating powerful hindlimb propulsion. Partial pubis and ischium configurations are inferred to form a closed acetabulum, similar to that in theropod dinosaurs, promoting a stable hip joint for bipedal or quadrupedal locomotion; the ischium-ilium and pubis-ilium articulations are separated by a non-articular concave surface, a derived crocodylomorph trait.^[2] Overall, the postcranium reflects a body length of approximately 3 m in the immature holotype; limb ratios, with elongate hindlimbs exceeding forelimbs, emphasize adaptations for agile, terrestrial predation over aquatic lifestyles. The preserved elements derive from a skeletally immature individual, suggesting potential for larger adult size.^[1]

Classification

Phylogenetic position

Carnufex is a monotypic genus known only from the type species Carnufex carolinensis, classified within Pseudosuchia as a basal member of Crocodylomorpha and positioned outside more derived clades such as Sphenosuchia.^[1]^[5] A comprehensive phylogenetic analysis by Zanno et al. (2015) incorporating 79 archosaur taxa and 413 morphological characters recovered C. carolinensis as the sister taxon to all other crocodylomorphs, forming an unresolved polytomy with the taxon CM 73372 at the base of Crocodylomorpha.^[1] This placement is supported by three unambiguous synapomorphies: the presence of a subnarial gap, an elongate lacrimal that reaches the ventral margin of the orbit, and the loss of fin-like hyposphen-hypantrum articulations in the axial skeleton.^[1] The analysis highlights Carnufex's mosaic morphology, including crocodylomorph features alongside plesiomorphic pseudosuchian traits such as a bulbous longitudinal ridge on the maxilla and a non-tapering dorsal process of the maxilla, which are shared with rauisuchids.^[1] Subsequent analyses, such as that of Brownstein (2016), which utilized a dataset of 41 pseudosuchian taxa and 251 characters expanded from Nesbitt (2011), consistently resolve Carnufex as an early-diverging crocodylomorph, though in a polytomy with Redondavenator quayensis and CM 73372 rather than as a direct sister to the broader clade.^[5] Key diagnostic traits supporting this classification include a combination of crocodylomorph synapomorphies, such as a hypertrophied subtriangular antorbital fenestra (approximately 14 cm long by 6 cm high) with a secondary opening in the antorbital fossa and ziphodont dentition featuring serrated, laterally compressed crowns, alongside retained pseudosuchian features like relatively reduced but robust forelimbs (humerus length about 21 cm, less than half the estimated skull length).^[5]^[1]

Evolutionary significance

Carnufex carolinensis represents a pivotal taxon in the early evolution of crocodylomorphs, filling a critical morphological gap between the robust, quadrupedal body plans of primitive pseudosuchians like rauisuchids and the lighter, more cursorial bauplans of derived crocodylomorphs.^[1] Its mosaic anatomy, including an elongate ascending process of the maxilla and laminar cranial elements, illustrates transitional features that appeared around 231 million years ago during the Late Carnian, prior to the major diversification of smaller-bodied crocodylomorphs.^[2] This intermediate form suggests that key crocodylomorph innovations, such as enhanced cranial lightness and reduced forelimb proportions, evolved in larger-bodied lineages before the shift to diminutive sizes in later taxa.^[1] In terms of body size evolution, Carnufex stands out as one of the largest known early crocodylomorphs, with adult estimates reaching approximately 3 meters in length based on femoral proportions, enabling it to occupy top predator niches in Late Triassic terrestrial ecosystems.^[2] This substantial size, exceeding that of contemporaneous early theropod dinosaurs, underscores how crocodylomorphs initially competed as apex predators before the radiation of dinosaurs diminished their dominance in such roles by the Late Triassic.^[1] Such large-bodied forms highlight an early phase of crocodylomorph diversification where pseudosuchians maintained predatory prominence amid rising dinosaur diversity. Dietary and locomotor adaptations in Carnufex further illuminate evolutionary shifts toward hypercarnivory and terrestrial predation, distinct from the aquatic specializations seen in later crocodylians. Its ziphodont dentition—characterized by serrated, blade-like teeth—indicates a specialization for slicing flesh, positioning it as a hypercarnivorous hunter of moderately sized vertebrates.^[2] Limb proportions, with reduced forelimbs relative to robust hindlimbs, suggest a semi-bipedal or facultatively quadrupedal gait suited for agile terrestrial pursuit, contrasting with the more upright, bipedal tendencies in some early crocodylomorphs and foreshadowing locomotor refinements in the group.^[1] However, gaps in the fossil record persist; the incomplete subadult holotype limits precise reconstructions of adult gait or full ontogenetic variation, and no evidence exists for sexual dimorphism or post-subadult growth patterns, hindering deeper insights into intraspecific evolutionary processes.^[2]

Paleoecology

Geological setting

The Pekin Formation forms part of the Chatham Group within the Deep River Basin of North Carolina, comprising the lowermost unit of this Late Triassic sequence in the Newark Supergroup.^[6] This formation is assigned to the Carnian stage, with an estimated age of approximately 231 million years based on paleomagnetostratigraphic correlations to dated sections in the broader Newark Supergroup.^[1] The Deep River Basin itself developed as one of several half-graben rift basins during the early stages of Pangean rifting, with sedimentation influenced by tectonic subsidence and fault-controlled depocenters that directed fluvial input from surrounding highlands.^[6] Lithologically, the Pekin Formation is dominated by red beds of mudstone, sandstone, and conglomerate, reflecting deposition in a fluvial-alluvial plain system with seasonal rivers traversing a semi-arid to subtropical landscape at a paleolatitude of approximately 5°N.^[7] These sediments accumulated in channel, floodplain, and alluvial fan settings, where coarser conglomerates and sandstones represent proximal fan and river channel fills, while finer mudstones indicate overbank and low-energy floodplain environments. The overall depositional regime points to episodic sediment transport in a dynamic rift valley, with sediment provenance largely from local Appalachian uplands eroded during extensional tectonics.^[8] The paleoclimate during Pekin Formation deposition was characterized by a humid subtropical regime punctuated by wetter phases linked to the Carnian Pluvial Event around 234–232 Ma, manifesting as increased humidity and rainfall that supported vegetation growth.^[9] Evidence for this includes abundant paleosols developed on floodplain surfaces, indicating soil formation under seasonally wet conditions, as well as fossil wood and plant remains preserved in fine-grained overbank layers, suggesting periodic fluvial inundation and biomass accumulation in a warming, equatorial setting. Taphonomic preservation in the Pekin Formation favored low-energy depositional contexts, with Carnufex fossils occurring in mudstone-dominated overbank deposits that facilitated rapid burial and minimal transport of skeletal elements. These settings likely represent floodplain aggradation during flood events, where disarticulated bones from multiple individuals accumulated, potentially at aggregation or mortality sites influenced by seasonal water availability in the rift basin.

Ecological role

Carnufex carolinensis occupied the role of a top-tier terrestrial predator within the Late Triassic ecosystem of the Pekin Formation, likely functioning as an apex or mesocarnivore that targeted medium-sized herbivores.^[1] Its estimated body length of approximately 3 meters and robust build positioned it as the largest known carnivore in this assemblage, preying on taxa such as traversodontid cynodonts and dicynodonts, including forms comparable to Placerias.^[1]^[10] The ziphodont dentition of Carnufex, characterized by serrated, recurved, and blade-like teeth, supports a hypercarnivorous diet focused on tearing flesh from vertebrate prey.^[2] This predator shared its fluvial habitat with a diverse array of contemporaneous fauna, including aetosaurians such as Coahomasuchus and Gorgetosuchus, early phytosaurs, and basal theropod dinosaurs.^[1]^[11] In the broader Norian assemblages of the Late Triassic, similar ecosystems featured early dinosaurs like coelophysoids and herrerasaurs, highlighting the dynamic biotic community in which Carnufex existed.^[1] These interactions underscore a complex food web where Carnufex contributed to the regulation of herbivore populations, such as dicynodonts and aetosaurs, through predation pressure.^[1] Terrestrial adaptations of Carnufex, including bipedal locomotion and a slender skull suited for agile hunting, suggest niche partitioning with other large carnivores, particularly rauisuchids, in competing for vertebrate prey across Late Triassic landscapes.^[1]^[2] As part of the pseudosuchian radiation, Carnufex exemplified the high diversity of large-bodied crocodylomorphs that dominated top predator guilds before the end-Triassic extinction event, which selectively eliminated many such competitors and facilitated the rise of dinosaurian dominance in subsequent ecosystems.^[1]