"Acracanthus" Langston, 1947 vide Czaplewski, Cifelli, & Langston, W.R., 1994 (nomen nudum)
Acrocanthosaurus (
carcharodontosaurid dinosaur that existed in what is now North America during the Aptian and early Albian stages of the Early Cretaceous, from 113 to 110 million years ago. Like most dinosaur genera, Acrocanthosaurus contains only a single species, A. atokensis. It had a continent-wide range, with fossil remains known from the U.S. states of Oklahoma, Texas, and Wyoming in the west, and Maryland
, which most likely supported a ridge of muscle over the animal's neck, back, and hips. Acrocanthosaurus was one of the largest theropods, with the largest known specimen reaching 11–11.5 meters (36–38 ft) in length and weighing approximately 4.4–6.6 metric tons (4.9–7.3 short tons). Large theropod footprints discovered in Texas may have been made by Acrocanthosaurus, although there is no direct association with skeletal remains.
Recent discoveries have elucidated many details of its anatomy, allowing for specialized studies focusing on its
ankylosaurs
.
Discovery and naming
Known parts of the Acrocanthosaurus specimens (to scale) as of 2015.
Acrocanthosaurus is named after its tall neural spines, from the
Atoka County in Oklahoma, where the original specimens were found. The name was coined in 1950 by American paleontologists J. Willis Stovall and Wann Langston Jr.[2] Langston had proposed the name "Acracanthus atokaensis" for the genus and species in his unpublished 1947 master'sthesis,[3][4] but the name was changed to Acrocanthosaurus atokensis for formal publication.[2]
The
Black Hills Institute in South Dakota, and is now housed at the North Carolina Museum of Natural Sciences in Raleigh. The specimen is the largest and includes the only known complete skull and forelimb. Skeletal elements of OMNH 10147 are almost the same size as comparable bones in NCSM 14345, indicating an animal of roughly the same size, while the holotype and SMU 74646 are significantly smaller.[6]
The presence of Acrocanthosaurus in the
UM 20796. The specimen, consisting of parts of two vertebrae, partial pubic bones, a femur, a partial fibula, and fragments, represents a juvenile animal. It came from a bonebed in the Bighorn Basin of north-central Wyoming, and was found near the shoulder blade of a Sauroposeidon. An assortment of other fragmentary theropod remains from the formation may also belong to Acrocanthosaurus, which may be the only large theropod in the Cloverly Formation.[7]
Unlike many other dinosaur genera, much less large theropods, Acrocanthosaurus inhabited both the western and eastern regions of the North American continent. The presence of the genus in the Arundel Formation of Maryland (roughly concurrent with the western formations) had long been suspected, with teeth almost identical to Acrocanthosaurus previously known from the formation.[8] In 2024, an incomplete theropod skeleton (USNM 466054) from the Arundel Formation was identified as that of a subadult Acrocanthosaurus, referred to as A. cf. atokensis, marking the first definitive record of the genus from eastern North America. This skeleton, the most completely known theropod specimen from the formation despite its fragmentary nature, had been previously identified as an ornithomimosaur until this study, and also represents the smallest known individual of the genus.[9]
Acrocanthosaurus may be known from less complete remains outside of Oklahoma, Texas, Wyoming, and Maryland. A tooth from southern
geologic formations throughout the western United States have also been referred to Acrocanthosaurus, but most of these have been misidentified;[12] there is, however, some disagreement with this assessment regarding fossils from the Cloverly Formation.[7]
Description
Size comparison of seven carcharodontosaurids (Acrocanthosaurus in light brown)
Acrocanthosaurus was among the largest theropods known to exist, with an estimated skull length of 1.23–1.29 m (4.0–4.2 ft) and body length of 11–11.5 m (36–38 ft) based on the largest known specimen (NCSM 14345).[13][6][14] Researchers have yielded body mass estimates for this specimen between 4.4 and 6.6 metric tons (4.9 and 7.3 short tons) based on various techniques.[13][15][16]
Skull
Skull diagram of NCSM 14345
The skull of Acrocanthosaurus, like most other
dentary (tooth-bearing lower jaw bone) was squared off at the front edge, as in Giganotosaurus, and shallow, while the rest of the jaw behind it became very deep. Acrocanthosaurus and Giganotosaurus shared a thick horizontal ridge on the outside surface of the surangular bone of the lower jaw, underneath the articulation with the skull.[6]
Postcranial skeleton
Skeletal diagram
The most notable feature of Acrocanthosaurus was its row of tall neural spines, located on the vertebrae of the neck, back, hips and upper tail, which could be more than 2.5 times the height of the vertebrae from which they extended.
dorsal (back) vertebrae had prominent depressions (pleurocoels) on the sides, while the caudal (tail) vertebrae bore smaller ones. This is more similar to carcharodontosaurids than to Allosaurus.[5]
Aside from its vertebrae, Acrocanthosaurus had a typical allosauroid skeleton. Acrocanthosaurus was bipedal, with a long, heavy tail counterbalancing the head and body, maintaining its
metatarsals,[5][6] suggesting that Acrocanthosaurus was not a fast runner.[19] Unsurprisingly, the hind leg bones of Acrocanthosaurus were proportionally more robust than its smaller relative Allosaurus. Its feet had four digits each, although as is typical for theropods, the first was much smaller than the rest and did not make contact with the ground.[5][6]
Classification and systematics
Life restoration
Acrocanthosaurus is
infraorder Tetanurae. This superfamily is characterized by paired ridges on the nasal and lacrimal bones on top of the snout and tall neural spines on the neck vertebrae, among other features.[17] It was originally placed in the family Allosauridae with Allosaurus,[2] an arrangement also supported by studies as late as 2000.[6] Most studies have found it to be a member of the related family Carcharodontosauridae.[17][20][21]
At the time of its discovery, Acrocanthosaurus and most other large theropods were known from only fragmentary remains, leading to highly variable classifications for this genus.
Alfred Sherwood Romer in 1956.[22] To other authors, the long spines on its vertebrae suggested a relationship with Spinosaurus.[23][24] This interpretation of Acrocanthosaurus as a spinosaurid persisted into the 1980s,[25] and was repeated in the semi-technical dinosaur books of the time.[26][27]
Tall spined vertebrae from the Early Cretaceous of
Becklespinax, separate from both Acrocanthosaurus and Altispinax.[30] It was still regarded as an Allosaurid by some researchers researches, and is still occasionally placed within that family.[31][32]
Acrocanthosaurus skull in multiple views
Most
cladistic analyses including Acrocanthosaurus have found it to be a carcharodontosaurid, usually in a basal position relative to Carcharodontosaurus of Africa and Giganotosaurus from South America.[17][5][33] It has often been considered the sister taxon to the equally basal Eocarcharia, also from Africa. Neovenator, discovered in England, is often considered an even more basal carcharodontosaurid, or as a basal member of a sister group called Neovenatoridae.[19][21] This suggests that the family originated in Europe and then dispersed into the southern continents (at the time united as the supercontinentGondwana). If Acrocanthosaurus was a carcharodontosaurid, then dispersal would also have occurred into North America.[5] All known carcharodontosaurids lived during the early-to-middle Cretaceous Period.[17]
In 2011, Oliver Rauhut named a new genus of theropod dinosaur from the Jurassic aged Tendaguru Formation in Tanzania named Veterupristisaurus and found it to be a sister taxon to Acrocanthosaurus, further supporting its position as a carcharodontosaurid.[34] The following cladogram after Novas et al., 2013, shows the placement of Acrocanthosaurus within Carcharodontosauridae.[35]
Harris lines were lost. If accounting for these lines, Acrocanthosaurus needed 18–24 years to be mature.[7]
Bite force
The bite force of Acrocanthosaurus was studied and compared with that of 33 other dinosaurs by Sakamoto et al. (2022). According to the results, its bite force at the anterior part of the jaws was 8,266 newtons, while the posterior bite force was estimated to be 16,894 newtons.[36]
Forelimb function
Like those of most other non-avian theropods, Acrocanthosaurus forelimbs did not make contact with the ground and were not used for locomotion; instead, they served a predatory function. The discovery of a complete forelimb (NCSM 14345) allowed the first analysis of the function and range of motion of the forelimb in Acrocanthosaurus.
The shoulder of Acrocanthosaurus was limited in its range of motion compared to that of humans. The arm could not swing in a complete circle, but could
carpals (wrist bones) fit together precisely, suggesting the presence of a large amount of cartilage in the wrist, which would have stiffened it. All of the digits were able to hyperextend (bend backward) until they nearly touched the wrist. When flexed, the middle digit would converge towards the first digit, while the third digit would twist inwards. The first digit of the hand bore the largest claw, which was permanently flexed so that it curved back towards the underside of the hand. Likewise, the middle claw may have been permanently flexed, while the third claw, also the smallest, was able to both flex and extend.[37] After determining the ranges of motion in the joints of the forelimb, the study went on to hypothesize about the predatory habits of Acrocanthosaurus. The forelimbs could not swing forward very far, unable even to scratch the animal's own neck. Therefore, they were not likely to have been used in the initial capture of prey and Acrocanthosaurus probably led with its mouth when hunting. On the other hand, the forelimbs were able to retract towards the body very strongly. Once prey had been seized in the jaws, the heavily muscled forelimbs may have retracted, holding the prey tightly against the body and preventing escape. As the prey animal attempted to pull away, it would only have been further impaled on the permanently flexed claws of the first two digits. The extreme hyperextensibility of the digits may have been an adaptation allowing Acrocanthosaurus to hold struggling prey without fear of dislocation. Once the prey was trapped against the body, Acrocanthosaurus may have dispatched it with its jaws. Another possibility is that Acrocanthosaurus held its prey in its jaws, while repeatedly retracting its forelimbs, tearing large gashes with its claws.[37]
Other less probable theories have suggested the forelimb range of motion being able to grasp onto the side of a sauropod and clinging on to topple the sauropods of smaller stature, though this is unlikely due to Acrocanthosaurus having a rather robust leg structure compared to other similarly structured theropods.
Brain and inner ear structure
Digital endocranial endocast of the braincase of specimen NCSM 14345
In 2005, scientists reconstructed an
computed tomography (CT scanning) to analyze the spaces within the holotype braincase (OMNH 10146). In life, much of this space would have been filled with the meninges and cerebrospinal fluid, in addition to the brain itself. However, the general features of the brain and cranial nerves could be determined from the endocast and compared to other theropods for which endocasts have been created. While the brain is similar to many theropods, it is most similar to that of allosauroids. It most resembles the brains of Carcharodontosaurus and Giganotosaurus rather than those of Allosaurus or Sinraptor, providing support for the hypothesis that Acrocanthosaurus was a carcharodontosaurid.[38]
The brain was slightly sigmoidal (S-shaped), without much expansion of the
lateral semicircular canal was parallel to the ground, as it usually is when an animal is in an alert posture.[38]
trackways was discovered along the Paluxy River in Dinosaur Valley State Park, a section of which is now on exhibit in the American Museum of Natural History in New York City,[40] although several other sites around the state have been described in the literature.[41][42] It is impossible to say what animal made the prints, since no fossil bones have been associated with the trackways. However, scientists have long considered it likely that the footprints belong to Acrocanthosaurus.[43] A 2001 study compared the Glen Rose footprints to the feet of various large theropods but could not confidently assign them to any particular genus. However, the study noted that the tracks were within the ranges of size and shape expected for Acrocanthosaurus. Because the Glen Rose Formation is close to the Antlers and Twin Mountains Formations in both geographical location and geological age, and the only large theropod known from those formations is Acrocanthosaurus, the study concluded that Acrocanthosaurus was most likely to have made the tracks.[44]
Digital fly-through over the Glen Rose trackway, reconstructed from photographs
The famous Glen Rose trackway on display in New York City includes theropod footprints belonging to several individuals which moved in the same direction as up to twelve
sauropod dinosaurs. The theropod prints are sometimes found on top of the sauropod footprints, indicating that they were formed later. This has been put forth as evidence that a small pack of Acrocanthosaurus was stalking a herd of sauropods.[40] While interesting and plausible, this hypothesis is difficult to prove and other explanations exist. For example, several solitary theropods may have moved through in the same direction at different times after the sauropods had passed, creating the appearance of a pack stalking its prey. The same can be said for the purported "herd" of sauropods, who also may or may not have been moving as a group.[45] At a point where it crosses the path of one of the sauropods, one of the theropod trackways is missing a footprint, which has been cited as evidence of an attack.[46] However, other scientists doubt the validity of this interpretation because the sauropod did not change gait, as would be expected if a large predator were hanging onto its side.[45]
Definite Acrocanthosaurus fossils have been found in the
stages of the Early Cretaceous has been located within the Glen Rose Formation of Texas, which may contain Acrocanthosaurus footprints and lies just above the Twin Mountains Formation. This indicates that the Twin Mountains Formation lies entirely within the Aptian stage, which lasted from 125 to 112 million years ago.[48] The Antlers Formation contains fossils of Deinonychus and Tenontosaurus, two dinosaur genera also found in the Cloverly Formation, which has been radiometrically dated to the Aptian and Albian stages, suggesting a similar age for the Antlers.[49] Therefore, Acrocanthosaurus most likely existed between 125 and 100 million years ago.[17]
During this time, the area preserved in the Twin Mountains and Antlers formations was a large
ornithopod like Tenontosaurus.[52] The smaller theropod Deinonychus also prowled the area but at 3 m (10 ft) in length, most likely provided only minimal competition, or even food, for Acrocanthosaurus.[49]
^Langston, Wann R. (1947). A new genus and species of Cretaceous theropod dinosaur from the Trinity of Atoka County, Oklahoma. Unpublished M.S. thesis. University of Oklahoma.
^Czaplewski, Nicholas J.; Cifelli, Richard L.; Langston, Wann R. Jr. (1994). "Catalog of type and figured fossil vertebrates. Oklahoma Museum of Natural History". Oklahoma Geological Survey Special Publication. 94 (1): 1–35.
^ abcdefHarris, Jerald D. (1998). "A reanalysis of Acrocanthosaurus atokensis, its phylogenetic status, and paleobiological implications, based on a new specimen from Texas". New Mexico Museum of Natural History and Science Bulletin. 13: 1–75.
^Ratkevich, Ronald P. (1998). "New Cretaceous brachiosaurid dinosaur, Sonorasaurus thompsoni gen. et sp. nov., from Arizona". Journal of the Arizona-Nevada Academy of Science. 31 (1): 71–82.
Kirkland, James I.
; Estep, J.W. (eds.). Lower and Middle Cretaceous Terrestrial Ecosystems. New Mexico Museum of Natural History and Science Bulletin 14. pp. 225–228.
^Olshevsky, George (1991). A Revision of the Parainfraclass Archosauria Cope, 1869, Excluding the Advanced Crocodylia. San Diego: Publications Requiring Research. p. 196pp.
^ abBird, Roland T. (1941). "A dinosaur walks into the museum". Natural History. 43: 254–261.
^Rogers, Jack V. (2002). "Theropod dinosaur trackways in the Lower Cretaceous (Albian) Glen Rose Formation, Kinney County, Texas". Texas Journal of Science. 54 (2): 133–142.
^Hawthorne, J. Michael; Bonem, Rena M.; Farlow, James O.; Jones, James O. (2002). "Ichnology, stratigraphy and paleoenvironment of the Boerne Lake Spillway dinosaur tracksite, south-central Texas". Texas Journal of Science. 54 (4): 309–324.
^Langston, Wann (1974). "Non-mammalian Comanchean tetrapods". Geoscience and Man. 3: 77–102.
^Molnar, R. E., 2001, Theropod paleopathology: a literature survey: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, p. 337–363.
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