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Acrocanthosaurus

Acrocanthosaurus atokensis is a large-bodied carcharodontosaurid theropod dinosaur that lived during the Early Cretaceous epoch, approximately 115 to 100 million years ago, in what is now the southern United States of North America.^[1] Named for its exceptionally tall neural spines, which formed a sail-like ridge along its back—this carnivorous apex predator measured up to 11.5 meters in length and weighed approximately 6 tonnes.^[2]^[3]^[4] The genus was established in 1950 by J. Willis Stovall and Wann Langston Jr., based on two partial skeletons recovered from the Antlers Formation in Atoka County, Oklahoma, representing the first major theropod discovery from the Lower Cretaceous of eastern North America.^[5] Subsequent finds, including the most complete known specimen (NCSM 14345) with a 1.29-meter-long skull and much of the postcranium, were unearthed in the same formation in 1983 and formally described in 2000, providing critical insights into its anatomy.^[3] These fossils, primarily from the Aptian-Albian stages of the Trinity Group, reveal a robust build with powerful hindlimbs, reduced forelimbs, and a deep skull equipped with serrated teeth suited for dispatching large prey such as sauropods.^[1] Phylogenetically, Acrocanthosaurus is classified within the superfamily Allosauroidea and the family Carcharodontosauridae, as sister taxon to the African genus Eocarcharia, supporting evidence of faunal exchange between Laurasian landmasses during the Early Cretaceous via Beringian and North Atlantic connections.^[1] As North America's earliest known giant theropod, it dominated floodplain and riverine habitats, filling an ecological niche later occupied by tyrannosaurids, and its remains indicate rapid growth to adulthood over two to three decades.^[6]^[1] Recent discoveries, including a tibia from Maryland's Arundel Clay in 2023 and possible footprints exposed by floods near Austin, Texas, in July 2025 (likely attributable to Acrocanthosaurus), extend its known range northward and provide insights into its locomotion, underscoring its role as a key predator in pre-tyrannosaurid ecosystems.^[5]^[7]

Discovery and naming

Initial discoveries

The first remains of Acrocanthosaurus were reported in the spring of 1940 from Atoka County in southeastern Oklahoma, following accounts of large bones eroding from the local landscape. Paleontologists J. Willis Stovall, then director of the University of Oklahoma Museum, and his graduate student Wann Langston, Jr., investigated the site in April 1940 on the farm of Herman Arnold, with assistance from geologist C. N. Gould and a Works Progress Administration crew. Initial excavations uncovered fragments of a large theropod skeleton, including vertebrae with notably tall neural spines, embedded in the Lower Cretaceous Trinity sands approximately 120 feet below the formation's top.^[2] Further exploration between 1940 and 1941 revealed two partial skeletons at sites about 0.75 miles apart: the holotype from W. P. Cochran's land and the paratype from Arnold's property. These specimens, representing much of the axial and appendicular skeleton of an adult individual, were collected from a near-shore depositional environment marked by conglomerates and clay lenses within the Trinity Group, now correlated with the Antlers Formation. The bones' massive size initially suggested a large carnivorous dinosaur, though fragmentary nature delayed full analysis amid wartime constraints.^[2]^[8] In 1950, Stovall and Langston formally named the genus Acrocanthosaurus atokensis based on these neural spines, which extended up to twice the height of adjacent vertebrae. The generic name derives from the Greek words akros (high), akantha (spine or thorn), and sauros (lizard), emphasizing the distinctive elongated dorsal projections that likely supported a muscular ridge along the back. The specific epithet atokensis honors Atoka County, the type locality. This description established Acrocanthosaurus as a novel theropod taxon from the Early Cretaceous of North America.^[2]

Key specimens and nomenclature

The genus Acrocanthosaurus was established by J. Willis Stovall and Wann Langston Jr. in 1950 for the type and only valid species A. atokensis, named for the high neural spines ("acro" meaning high and "kanthos" meaning spine or thorn in Greek) and the type locality in Atoka County, Oklahoma ("atokensis" referring to Atoka).^[9] No other species have been recognized as valid within the genus, with proposed synonyms such as those conflating it with Saurophaganax maximus rejected based on diagnostic differences in vertebral and cranial morphology.^[10] The holotype specimen, OMNH 10146 (originally designated M.U.O. 8-0-S9), consists of a partial skeleton including a partial skull (braincase and posterior elements), several cervical, dorsal, and caudal vertebrae, nine tall neural spines, two chevrons, and fragments of the pelvis (partial ischium and pubis); it was collected from the Lower Cretaceous Antlers Formation (Trinity Group) in Atoka County, southeastern Oklahoma.^[9] The paratype, OMNH 10147 (originally M.U.O. 8-0-S8), is a second partial skeleton of comparable size, preserving additional vertebrae, ribs, and limb elements from the same formation and nearby locality in Atoka County.^[9] Together, the holotype and paratype represent approximately 25-35% completeness of the skeleton, providing the foundational material for the genus diagnosis.^[1] Key referred specimens include SMU 74646, a partial postcranial skeleton (including vertebrae, ribs, pelvis, and hindlimb elements but lacking most of the skull) from the Lower Cretaceous Twin Mountains Formation (Trinity Group) in Coryell County, Texas, which was referred to A. atokensis due to shared features such as elongated neural spines and allosauroid pelvic structure.^[11] Another significant referral is NCSM 14345 (nicknamed "Fran"), a near-complete skeleton exceeding 11.5 m in length, including the only known complete skull and much of the postcranium (representing over 50% completeness for that individual), collected from the Antlers Formation in McCurtain County, Oklahoma, and assigned to A. atokensis based on diagnostic traits like neural spines over 2.5 times the centrum height and pleurocoelous vertebral fossae.^[3] These specimens, combined with fragmentary referrals, indicate that about 70-80% of the Acrocanthosaurus skeleton is known across multiple individuals, though no single specimen exceeds 60% completeness.^[12]

Recent discoveries

In the early 2000s, a significant addition to the known material of Acrocanthosaurus atokensis came from the discovery of specimen NCSM 14345, a partial skeleton representing approximately 50% of an adult individual, recovered from the Antlers Formation in McCurtain County, Oklahoma.^[3] Unearthed in 1983 by amateur collectors Cephis Hall and Sid Love but formally described in 2000, this specimen provided the first near-complete adult skeleton of the taxon, offering enhanced insights into its overall anatomy and ontogenetic development beyond fragmentary historical material like the holotype OMNH 10146.^[3] More recently, in 2023, paleontologist Matthew T. Carrano described USNM 466054, a subadult partial postcranial skeleton—including a partial neural arch from dorsal vertebra 2 or 3, another anterior dorsal vertebra neural arch, two caudal vertebrae, the proximal end of a left femur, and the proximal end of a right tibia—from the Arundel Formation (Potomac Group) in Prince George's County, Maryland, marking the first definitive record of Acrocanthosaurus in eastern North America. This specimen, collected in the late 19th century but only recently analyzed in detail, exhibits diagnostic features such as anteroposteriorly short dorsal neural arches and reduced caudal transverse processes consistent with A. atokensis, confirming the presence of this large carcharodontosaurid in mid-Atlantic deposits dating to the Aptian-Albian stages.^[5] Additional referrals include isolated teeth and postcranial elements from the Cloverly Formation in Wyoming and Montana, with a key specimen being UM 20796—a partial skeleton including anterior dorsal vertebrae and other postcranial elements assigned to Acrocanthosaurus atokensis in 2012—alongside denticles-bearing teeth that match the compressiform, serrated morphology of the genus. These finds, primarily from the early Aptian, represent tentative but compelling evidence of the predator's presence in northern regions during the Early Cretaceous. In October 2025, a tooth attributed to Acrocanthosaurus was discovered by students during a field trip near Washington, D.C., further supporting its presence in eastern North American deposits.^[13] Collectively, these post-2000 discoveries expand the known geographic range of Acrocanthosaurus atokensis from its core southern U.S. localities (Oklahoma and Texas) northward into the northern mid-continent and eastward to the mid-Atlantic seaboard, indicating a broader continental distribution across Aptian-Albian North America than previously recognized.

Description

Overall size and build

Acrocanthosaurus was a large-bodied theropod dinosaur characterized by its impressive overall dimensions, with adult individuals estimated to reach lengths of 9 to 12 meters from snout to tail tip, and the holotype specimen (OMNH 10146) measuring approximately 11 meters.^[3] The height at the hips for mature specimens, such as NCSM 14345, stood at about 4 meters, contributing to its imposing stature as a bipedal predator.^[14] Mass estimates for adults typically range from 6 to 9 metric tons, derived from volumetric reconstructions and limb bone scaling methods that account for skeletal proportions and soft tissue assumptions. For the well-preserved specimen NCSM 14345, recent 2025 studies using advanced body size evolution models yield a mass of approximately 9 metric tons, with variations of 7-15% depending on neural spine soft tissue density (sail-backed versus hump-backed reconstructions); these models also indicate a posterior center of mass at approximately 6% of the glenoacetabular distance, suggesting a more upright limb posture compared to tyrannosaurids.^[15] The build of Acrocanthosaurus was robust and bipedal, adapted for terrestrial predation, with notably elongated neural spines along the vertebrae forming a prominent sail-like structure that may have supported musculature or thermoregulatory functions.^[1] Analysis of limb bone shafts in 2025 research reveals narrower cross-sections in larger individuals, suggesting a reduction in locomotor efficiency compared to smaller theropods, likely as an adaptation to their increased body mass.^[16] Sexual dimorphism has not been confirmed in Acrocanthosaurus, with observed size variations among specimens more plausibly attributed to ontogenetic differences across growth stages rather than gender-specific traits.^[15]

Skull

The skull of Acrocanthosaurus atokensis measured approximately 1.29 meters in length from the premaxilla to the quadrate, with a preorbital length of 85 centimeters and a height of 47 centimeters in front of the orbit.^[3] It exhibited a long, low, and narrow profile typical of allosauroids, featuring a narrow rostral tip and a flat, vertical facial surface anterior to the orbit, separated by a prominent ridge that contributed to its boxy overall appearance.^[3] The premaxilla housed four teeth and formed an acute angle where the two premaxillae met, influencing the orientation of the anterior dentition.^[3] Dentition consisted of 19 teeth in the upper jaw per side (four in the premaxilla and 15 in the maxilla), with the lower jaw bearing around 17 teeth in the dentary, resulting in 19–21 serrated, laterally compressed teeth per maxillary side across specimens.^[3]^[17] The teeth were blade-like with D- or J-shaped cross-sections at the base in mesial positions, featuring straight to slightly recurved crowns and carinae armed with fine, chisel-shaped denticles at a density of 12.5–17.5 per 5 millimeters.^[18]^[3] Premaxillary teeth were not D-shaped due to the acute premaxillary angle, but lateral teeth displayed pronounced lateral compression and braided enamel texture characteristic of allosauroids.^[3]^[18] Sensory structures included large, elongate external nares measuring 16 centimeters in length, bordered by the premaxilla and nasal bones, and an elongated, triangular antorbital fenestra 42 centimeters long that occupied about 63% of the preorbital height.^[3] The braincase was robust, with CT scans revealing fused frontal and parietal elements, cylindrical olfactory nerve openings, and a pituitary fossa indicative of advanced sensory processing.^[1] The quadrate, measuring 31 centimeters tall, was sturdy with a 28-millimeter quadrate foramen, medial and posterior pneumatic recesses, and strong articulation features supporting powerful jaw adduction.^[3]^[1] Additional cranial details come from the referred specimen SMU 74646, a partial skeleton that includes fragmentary rostral elements such as the right palatine, left ectopterygoid, partial left jugal, and posterior mandible, providing insights into palatal and jugal morphology consistent with the holotype.^[1]^[17] These elements confirm the presence of four premaxillary teeth and reinforce the dentition pattern observed in more complete skulls like NCSM 14345.^[17]

Postcranial skeleton

The postcranial skeleton of Acrocanthosaurus is distinguished by its axial elements, particularly the vertebrae bearing elongated neural spines. These spines are present from the mid-dorsal region through the sacral vertebrae and extend more than 2.5 times the height of the vertebral centra, creating a prominent dorsal sail-like structure along the back.^[3] Mid-caudal centra measure approximately 16 cm in length, 10 cm in width, and 11.5 cm in height, while distal caudal centra are smaller at about 12 cm long, 7.5 cm wide, and 7.5 cm high; neural spines in these regions are restricted to posterior positions and feature accessory transverse processes in some mid-caudal vertebrae.^[3] Cervical vertebrae exhibit triangular anterior processes on their spines and double ventral keels, contributing to the overall robust axial framework.^[3] The appendicular skeleton reflects Acrocanthosaurus' bipedal adaptations, with robust hindlimbs featuring a straight femur estimated at 1.28 m long and a tibia around 0.96 m.^[3] The forelimbs are markedly reduced, with the humerus measuring 37 cm, ulna 25.5 cm, and radius 22 cm, resulting in an arm length (humerus to digit II) of about 1.05 m; the manus is three-fingered, supported by robust metacarpals (e.g., metacarpal I at 6.2 cm, II at 11.6 cm, III at 8.9 cm).^[3] The pes includes a robust structure with metatarsal II at 41 cm and metatarsal III estimated at 44 cm, along with phalanges such as digit I at 15 cm and digit III elements at 16 cm and 11.5 cm.^[3] The pubis displays a boot-like distal expansion exceeding 30% of its total length, and the ischium features a similar boot-shaped distal end, with both elements having obturator openings as notches.^[3] The pectoral girdle comprises a scapula measuring 97 cm (curved length 103 cm) and a coracoid 36 cm long, with a pronounced notch between the acromial process and coracoid; in adults, these elements articulate closely, effectively fusing the girdle for stability.^[3] Gastralia are present, forming a ventral abdominal basket of rib-like bones.^[3] The ilium includes a hook-like ventral process creating a preacetabular notch.^[3] Pneumaticity is extensive throughout the postcranial skeleton, particularly in the presacral, sacral, and caudal vertebrae, where pleurocoelous fossae and foramina indicate air sac invasion; complex internal pneumatization occurs in posterior cervical and anterior dorsal centra, with subsidiary foramina in caudal neural spines.^[3] Ribs also show evidence of pneumatic features, supporting an efficient respiratory system.^[3]

Classification

Historical classifications

Upon its description in 1950, Acrocanthosaurus atokensis was classified within the family Allosauridae due to shared features in the skull, limb proportions, and overall build with the Late Jurassic predator Allosaurus fragilis.^[2] This placement reflected the limited theropod diversity known at the time and emphasized similarities in predatory adaptations, such as robust maxillary and dentary bones supporting serrated teeth. Throughout the 1950s and 1970s, subsequent studies reinforced this assignment to Allosauridae, though some analyses highlighted transitional traits, such as vertebral morphology, positioning it intermediately between allosaurids and more derived tyrannosaurids.^[19] In the 1980s, the distinctive tall neural spines along the dorsal vertebrae—reaching up to twice the height of the centra—prompted debate over potential affinities with spinosaurids, exemplified by Spinosaurus, whose elongated spines suggested a similar sail-like structure.^[20] This interpretation, popularized in semi-technical literature, was based primarily on the spines but was soon refuted by comparisons revealing fundamental differences in cranial structure, dentition, and limb morphology that aligned Acrocanthosaurus more closely with advanced tetanurans rather than the piscivorous spinosaurids.^[20] By the 1990s, Acrocanthosaurus was broadly recognized as a carnosaur within Allosauroidea, with emerging evidence from new specimens pointing to carcharodontosaurid affinities, as proposed by Currie and colleagues through detailed osteological comparisons emphasizing shared autapomorphies like finely serrated, laterally compressed teeth and elongated premaxillae.^[21] A notable taxonomic debate arose from similarities in tooth morphology with the newly described South American Giganotosaurus carolinii, leading to initial confusion over isolated North American teeth potentially belonging to either genus; this was resolved in the early 2000s via comprehensive phylogenetic and biogeographic analyses confirming Acrocanthosaurus as a distinct North American carcharodontosaurid, separate from its southern relatives.

Phylogenetic analyses

Phylogenetic analyses consistently position Acrocanthosaurus atokensis as a basal carcharodontosaurid within the larger clade Allosauroidea, a group of large-bodied theropod dinosaurs that includes other major predators like Allosaurus from the Late Jurassic.^[22] This placement is supported by cladistic analyses using morphological characters from the postcranial and cranial skeleton, recovering Acrocanthosaurus as part of Carcharodontosauridae based on shared derived traits such as fused interdental plates in the maxilla and a dorsomedially oriented femoral head. In a comprehensive 2024 analysis by Cau incorporating over 300 taxa and 1,944 characters, Acrocanthosaurus is resolved as the sister taxon to Eocarcharia dinops (from the Aptian of Niger), forming the Acrocanthosaurus + Eocarcharia clade and emphasizing its role near the base of carcharodontosaurid diversification.^[23] Key synapomorphies linking Acrocanthosaurus to carcharodontosaurids include elongated neural spines on the presacral vertebrae (forming a distinctive "sail" structure, though more pronounced as an autapomorphy), finely chisel-like denticles on the carinae of maxillary and dentary teeth, and a quadrate with a slender, elongate shaft and expanded pneumatic head.^[22] These features distinguish it from more derived carcharodontosaurids like Giganotosaurus while aligning it closely with basal forms, supporting its position as a transitional taxon in allosauroid evolution. Dated to the Early Cretaceous (Aptian–Albian stages, approximately 125–100 million years ago), Acrocanthosaurus bridges the Jurassic radiation of allosauroids (e.g., Allosauridae) and the more specialized carcharodontosaurids that dominated Gondwanan ecosystems in the mid-Cretaceous.^[22] Recent studies as of 2025 have reinforced this phylogenetic framework through body size analyses, showing Acrocanthosaurus exemplifying allosauroid trends toward gigantism with estimated masses of 5–6 tonnes, consistent with scaling patterns in basal carcharodontosaurids.^[24] Additionally, isolated elements from formations such as the Cedar Mountain Formation (Utah) exhibit morphological variations that hint at potential undescribed diversity among North American carcharodontosaurids. Earlier classifications occasionally misidentified it as a spinosaurid due to its prominent neural spines, but modern datasets firmly refute this.

Paleobiology

Growth and metabolism

Acrocanthosaurus exhibited rapid growth during its juvenile phase, with early ontogenetic rates estimated at approximately 144 kg per year based on bone histology from the juvenile specimen UM 20796.^[25] Bone sections from adult specimens, including the fibula of NCSM 14345 and tibia of OMNH 10146, reveal fibrolamellar bone tissue dominated by woven-parallel fibered matrix with densely packed primary osteons, indicating sustained high growth rates into maturity.^[25] Lines of arrested growth (LAGs) in these elements suggest skeletal maturity was reached in 24–28 years, with the 24 LAGs in NCSM 14345's fibula corresponding to an age of about 24 years at death and 28 LAGs in OMNH 10146's tibia indicating up to 28 years.^[25] Maximum longevity for Acrocanthosaurus is estimated at 25–30 years, as histological evidence from adults shows slowed but continued appositional growth beyond skeletal maturity, consistent with indeterminate growth patterns in large theropods.^[25] The recently described Maryland specimen USNM 466054, the smallest known individual, represents a subadult with incomplete neurocentral fusion in its vertebrae, supporting that full maturity occurred later in ontogeny. Metabolic inferences for Acrocanthosaurus point to an endothermic or mesothermic physiology, primarily evidenced by the presence of fibrolamellar bone in limb elements and ribs across ontogenetic stages, which correlates with elevated resting metabolic rates and efficient heat production in archosaurs.^[25] This tissue type, combined with annual growth marks, aligns with adaptations for high-energy demands in large-bodied theropods. A 2025 analysis of theropod size evolution highlights Acrocanthosaurus's narrower femoral shafts relative to body mass, suggesting biomechanical adaptations such as more upright limb postures to support its estimated 6.5–7.5 tonne mass while minimizing energetic costs of locomotion.^[15] Ontogenetic changes in Acrocanthosaurus included elongation of neural spines with age, as comparative vertebral proportions in juvenile UM 20796 show relatively shorter spines that approach adult heights in larger specimens like NCSM 14345.^[25] Juvenile skulls appear more robust proportionally, with broader snouts and deeper jaw adductor chambers inferred from scaling patterns in related carcharodontosaurids, though direct juvenile cranial material remains limited.^[25]

Sensory capabilities

The endocranial cast of Acrocanthosaurus atokensis, derived from CT scans of the braincase (OMNH 10146), reveals a total volume of approximately 123 cm³, with prominent olfactory bulbs and tracts measuring 6.6 cm in length and separated by a median septum. These structures occupy a substantial portion of the rostral endocranium, and the olfactory bulb ratio (greatest diameter of the olfactory bulb relative to the cerebral hemisphere) is 58.1%, indicating a well-developed sense of smell comparable to that in other basal theropods and exceeding ratios in many coelurosaurs. This expanded olfactory region, aligned with the forebrain, suggests Acrocanthosaurus relied on chemosensory detection for locating prey or carrion over distances, consistent with its role as a large apex predator. A 2024 re-analysis confirmed this updated endocast volume based on re-examination of the original CT data.^[26]^[27] The inner ear labyrinth, reconstructed from the same endocast, features three semicircular canals that are elongate and roughly orthogonal: the anterior canal angles posterodorsally, the posterior slopes posteroventrally, and the horizontal canal lies ventral to the floccular lobe. The orientation of the horizontal semicircular canal implies a preferred head posture approximately 25° below the horizontal, facilitating balance during locomotion and potentially aiding in scanning the environment from a slightly downturned position. The overall labyrinth morphology, similar to that in Carcharodontosaurus and Allosaurus, supports agile head movements for tracking dynamic prey.^[27] Visual processing appears moderate, with dorsomedially positioned optic lobes visible on the endocast but lacking the expansion seen in more derived theropods like tyrannosaurids. The skull's orbital configuration yields a binocular field of view estimated at around 20°, typical for allosauroids and sufficient for depth perception in predatory strikes but without specialized enhancements for stereopsis beyond this overlap. Auditory capabilities are inferred from the robust inner ear structures, including a vestibule connected to the semicircular canals, though specific cochlear duct details remain undocumented; as in other large non-coelurosaurian theropods, this likely emphasized sensitivity to low-frequency sounds for detecting distant rumbles or footsteps. Recent 2024 analyses of theropod endocasts, including Acrocanthosaurus (endocast volume ~123 ml, estimated brain mass 38–52 g), confirm plesiomorphic trends in carcharodontosaurids: relatively small brains dominated by olfactory and vestibular regions, with neuronal densities akin to extant reptiles rather than the elevated cognition in maniraptorans.^[26] These features align with a sensory profile optimized for ambush predation in forested Early Cretaceous environments, prioritizing olfaction and equilibrium over advanced visual or auditory acuity.^[26]

Locomotion and forelimb use

Acrocanthosaurus exhibited bipedal locomotion characterized by a pillar-like erect posture, with parasagittal limb movements restricted to a plane parallel to the body axis.^[28] This gait relied on robust hindlimbs for propulsion, but its large body size imposed limitations on performance. Biomechanical models estimate a top speed of approximately 25–30 km/h (7 m/s), based on sensitivity analyses of muscle leverage and body mass.^[28] Recent cross-sectional analyses of limb bone shafts indicate a decline in locomotor efficiency with increasing size, as narrower shafts in Acrocanthosaurus relative to smaller theropods reduced muscle attachment areas and overall agility.^[15] Potential trackway evidence supporting these locomotor traits comes from the Glen Rose Formation in Texas, where large theropod footprints exhibit stride lengths consistent with Acrocanthosaurus proportions, though direct attribution remains debated due to preservational ambiguities and co-occurring taxa. For stability during movement, the dinosaur's long, heavy tail counterbalanced the head and torso, positioning the center of gravity over the hips to facilitate turns and prevent toppling. The forelimbs of Acrocanthosaurus were reduced in length compared to the hindlimbs but retained significant strength, with the humerus capable of retracting posteriorly to a near-horizontal position (approximately 90° from vertical).^[29] This range of motion, exceeding that of tyrannosaurids but similar to early theropods like Dilophosaurus, suggests utility in prey manipulation—such as grasping struggling victims close to the body—or aiding in rising from a resting position.^[29] The manus featured three functional digits with strongly curved claws adapted for secure holding and impaling rather than slashing, enabling the forelimbs to secure smaller prey or assist in subduing larger ones after initial jaw engagement.^[29]

Feeding mechanics

Acrocanthosaurus possessed a robust skull adapted for powerful biting, with estimated bite forces ranging from approximately 8,000 N anteriorly to 17,000 N posteriorly, as determined through phylogenetically predicted physiological cross-sectional areas of jaw adductor muscles and static lever modeling. This posterior increase in force suggests capability for bone crushing during prey processing, distinguishing it from less forceful allosauroids. The analysis utilized a Bayesian phylogenetic predictive modeling framework trained on 59 extant and extinct archosaur species, incorporating skull width measurements of 480 mm for Acrocanthosaurus.^[30] The skull mechanics featured limited kinesis, constrained by robust ligaments and tight articulations, such as the tongue-in-groove joint between the quadratojugal and jugal, which enhanced structural stability during forceful bites. These adaptations minimized flexibility in the upper jaw while allowing efficient force transmission to the mandible. The teeth, numbering 17–19 curved and serrated structures per maxillary side, were specialized for slashing flesh rather than deep puncturing, facilitating rapid tissue separation in large prey.^[31] Jaw adductor muscles, particularly the large temporalis, formed a significant portion of the feeding apparatus, with phylogenetic reconstructions indicating substantial physiological cross-sectional areas contributing to overall bite power. No direct gut contents have been preserved, but feeding behavior is inferred from tooth marks on sauropod bones in coeval formations, supporting a strategy as an ambush predator relying on short bursts of speed to deliver devastating bites to sizable herbivores.^[30]

Reproduction and pathology

No direct fossil evidence, such as eggshells or nests, has been attributed to Acrocanthosaurus atokensis, leaving reproductive behaviors inferred from those of related theropod dinosaurs.^[32] Clutch sizes for non-avian theropods, including allosauroids, are estimated at 10–20 eggs per nest based on preserved clutches from taxa like Troodon and oviraptorids, suggesting Acrocanthosaurus may have produced similar numbers given its body size and phylogenetic position within Carcharodontosauridae.^[32] Possible nesting occurred in floodplain environments of the Antlers Formation, consistent with the depositional setting of known specimens, though no specific nesting sites are documented.^[33] Pathological evidence from Acrocanthosaurus fossils reveals instances of trauma and potential infection, providing insights into individual health and survival. In the referred specimen NCSM 14345 (nicknamed "Fran"), multiple healed fractures are evident, including several broken right ribs.^[34] Growth interruptions are recorded through lines of arrested growth (LAGs) in long bones, indicating periodic stress that slowed skeletal development. The holotype tibia (OMNH 10146) contains 11 LAGs, reflecting annual or seasonal pauses in growth potentially linked to environmental stressors.^[25] A juvenile specimen (UM 20796) preserves 4 LAGs in the femur alongside a healed crush injury on the right pubis, suggesting early-life trauma that the individual survived.^[25] Vertebral fusion anomalies, including incomplete neurocentral fusion in subadults transitioning to full fusion in adults, may indicate age-related changes or pathological spondylosis in older individuals.^[35] Recent analyses of subadult theropod pathologies, including those comparable to Acrocanthosaurus relatives, highlight elevated predation risks during ontogeny, with healed injuries often linked to failed attacks by larger conspecifics or competitors.^[36]

Paleoecology

Geological context

Acrocanthosaurus fossils date to the Early Cretaceous epoch, spanning the Aptian and Albian stages approximately 125 to 100 million years ago.^[37] The genus is primarily represented by specimens from the Antlers Formation, a unit of the Trinity Group exposed in southern Oklahoma and northern Texas, where the bulk of known material—including the type specimens—has been recovered. These deposits consist of sandstones, mudstones, and conglomerates formed in fluvial and deltaic environments along a coastal plain adjacent to the proto-Gulf of Mexico. Additional fossils occur in other Lower Cretaceous units correlated to the same temporal interval, including the Twin Mountains Formation in northern Texas, also part of the Trinity Group.^[6] Remains have further been identified from the Cloverly Formation in Wyoming, representing fluvial channel and overbank deposits in a semi-arid floodplain setting.^[25] A recent discovery in 2023 provided the first definitive eastern North American record from the Arundel Formation (Potomac Group) in Maryland, with an additional Acrocanthosaurus tooth uncovered at Dinosaur Park in October 2025.^[5]^[13] Taphonomic evidence indicates that most Acrocanthosaurus specimens are disarticulated, with bones often showing signs of abrasion and fragmentation attributable to transport within river systems before final burial in channel or floodplain sediments. The stratigraphic ages of these formations are constrained through biostratigraphic correlations, including palynomorph assemblages from the Trinity Group that align with the late Aptian to early Albian, falling within the Cretaceous Normal Superchron (C34n).

Habitat and fauna

Acrocanthosaurus inhabited Early Cretaceous paleoenvironments across what is now the southern and eastern United States, primarily within the Antlers Formation of Oklahoma and Texas, and the Arundel Formation (part of the Potomac Group) in Maryland. These settings comprised coastal floodplains, meandering river systems, and deltaic plains subject to periodic seasonal flooding from transgressing marine influences. Sedimentary evidence, including fine-grained sandstones, clays, and carbonaceous shales, points to low-energy depositional environments with occasional overbank flooding and proximity to brackish waters.^[38] The prevailing climate was subtropical and warm, with mean annual temperatures ranging from 20 to 30°C and relatively low precipitation levels, estimated at around 350–400 mm per year in the Antlers Formation, consistent with semi-arid conditions supporting wetland and riparian habitats. Recent analyses of phyllosilicates from the Antlers Formation indicate a shift toward wetter conditions during the Albian stage, with increased humidity facilitating lush vegetation growth. The eastern Arundel Formation represents swampy, humid lowlands near the proto-Atlantic coast, as indicated by sedimentary features, plant remains, and a higher moisture availability compared to western sites.^[39]^[40]^[41] Dominant flora included gymnosperms such as conifers (e.g., members of Podocarpaceae and Araucariaceae), ferns, and cycads, forming dense forests and understory cover along riverbanks and floodplains. Palynological studies from the Antlers and equivalent formations reveal a prevalence of these non-angiosperm plants, with early flowering plants present but not yet ecologically dominant, reflecting a transitional vegetation assemblage adapted to the warm, seasonally wet conditions.^[38] The associated fauna exhibited lower taxonomic diversity than Jurassic ecosystems, characterized by a mix of large herbivores, smaller dinosaurs, and aquatic vertebrates. Prominent herbivores included giant sauropods like Sauroposeidon proteles from the Antlers Formation and Paluxysaurus jonesi from correlative units in Texas, alongside ornithischians such as the iguanodontian Tenontosaurus tilletii. Smaller theropods like Deinonychus antirrhopus coexisted, while semi-aquatic groups comprised crocodylomorphs (e.g., goniopholids) and turtles (e.g., Basilemys). In the Arundel Formation, faunal assemblages similarly feature sauropod teeth attributable to Astrodon and isolated Acrocanthosaurus remains, alongside hybodont sharks and abundant crocodilian fossils, underscoring a riverine-deltaic community with limited marine incursions.^[42]

Ecological role

Acrocanthosaurus served as the apex predator in its Early Cretaceous North American ecosystems, occupying the top carnivore niche and exerting significant influence on community structure. As the largest theropod in its habitats, it was capable of preying on substantial herbivores, including ornithopods like Tenontosaurus and large sauropods such as Sauroposeidon, based on co-occurrence in formations like the Antlers and Cloverly. This predatory role is supported by the dominance of Acrocanthosaurus as the sole large-bodied theropod species in these assemblages, allowing it to target prey beyond the capabilities of smaller carnivores.^[25]^[5] Interactions with other predators likely involved competition for resources, particularly smaller prey items that could be contested by dromaeosaurids such as Deinonychus, which coexisted in the same environments and specialized in pack hunting of ornithopods. Acrocanthosaurus may have supplemented its diet through scavenging, opportunistically feeding on carcasses taken down by these smaller theropods or abandoned kills, thereby reducing direct confrontations while maximizing energy intake. Such dynamics highlight its position as a versatile top predator, capable of both active hunting and opportunistic behaviors to sustain its large body size.^[25] The scarcity of Acrocanthosaurus fossils—limited to only four major skeletal specimens and numerous isolated elements—suggests a low population density typical of large apex predators, enabling effective control over herbivore populations without overexploitation. This rarity underscores its role in maintaining ecological balance by preventing unchecked growth in prey species like Tenontosaurus, which formed a significant portion of the fauna. Recent findings, including the first definitive eastern North American specimen from Maryland's Arundel Clay, indicate a broad continental range spanning from the western interior to the Atlantic seaboard, potentially reflecting migratory patterns or a wider Laurasian distribution during the Aptian–Albian stages.^[14]^[5]

References

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Re-evaluation of a well-preserved skull of Acrocanthosaurus atokensis (NCSM 14345) provides new information regarding the palatal complex and inner surfaces of ...
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I Know Dino Podcast Show Notes: Acrocanthosaurus (Episode 35)
J. Willis Stovall and Juan Langston Jr. named the species in 1950, though the first fossils were discovered in the early 1940s. The holotype and paratype are ...<|separator|>
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ABSTRACT: Acrocanthosaurus, from the Trinity Group of Oklahoma and Texas, remains the only adequately known large theropod from the Lower Cretaceous of ...Missing: oil prospectors
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Nov 3, 2018 · OMNH 10146/MUO-8-0-S9. Hip height: ~2.39 metres; Shoulder height: ~2.27 metres · Total height: ~2.81 metres ; SMU 74646. Hip height: ~2.64 metres ...Missing: nomenclature | Show results with:nomenclature
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Paleontologists have discovered fragmentary Acro remains in Oklahoma, Texas, and Maryland. Because of these finds, and because similar climatic conditions ...
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Acrocanthosaurus atokensis Stovall et Langston from the Albian stage of Oklahoma occupies an intermediate position between the Allosauridae and Tyrannosauridae.
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Untitled Document - Paleofile.com
SMU 74646 (FWMSH 93B-9): Right palatine, left ectopterygoid, a partial left jugal, the caudal end of one mandible, 2 teeth, fragmentary cervical, dorsal ...
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Paleobiology and geographic range of the large-bodied Cretaceous ...
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[34]
New information on paleopathologies in non-avian theropod ...
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[36]
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