Acrocanthosaurus
Encyclopedia
Acrocanthosaurus is a genus
of theropod dinosaur
that existed in what is now North America
during the Aptian
and early Albian
stages of the Early Cretaceous
. Like most dinosaur genera, Acrocanthosaurus contains only a single species, A. atokensis. Its fossil
remains are found mainly in the U.S.
states of Oklahoma
, Texas
, and Arkansas
, although teeth attributed to Acrocanthosaurus have been found as far east as Maryland
.
Acrocanthosaurus was a bipedal predator. As the name suggests, it is best known for the high neural spines
on many of its vertebrae, which most likely supported a ridge of muscle over the animal's neck, back and hips. Acrocanthosaurus was one of the largest theropods, approaching 12 meters (40 ft) in length, and weighing up to 6–7 metric tons
(6.5–7.5 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 brain
structure and forelimb function. Acrocanthosaurus was the largest theropod in its ecosystem
and likely an apex predator
which possibly preyed on large sauropods and ornithopod
s.
, Acrocanthosaurus was still among the largest theropods ever to exist. The longest known individual measured 11.5 meters (38 ft) from snout to tail tip and weighed an estimated 6,000–7,000 kilograms (13,000–15,000 lb). Its skull
alone was nearly 1.3 meters (4.3 ft) in length.
The skull of Acrocanthosaurus, like most other allosauroids, was long, low and narrow. The weight-reducing opening in front of the eye socket (antorbital fenestra
) was quite large, more than a quarter of the length of the skull and two-thirds of its height. The outside surface of the maxilla
(upper jaw bone) and the upper surface of the nasal bone
on the roof of the snout were not nearly as rough-textured as those of Giganotosaurus
or Carcharodontosaurus
. Long, low ridges arose from the nasal bones, running along each side of the snout from the nostril back to the eye, where they continued onto the lacrimal bone
s. This is a characteristic feature of all allosauroids. Unlike Allosaurus
, there was no prominent crest on the lacrimal bone in front of the eye. The lacrimal and postorbital
bones met to form a thick brow over the eye, as seen in carcharodontosaurids and the unrelated abelisaurids. Nineteen curved, serrated teeth lined each side of the upper jaw, but a tooth count for the lower jaw has not been published. Acrocanthosaurus teeth were wider than those of Carcharodontosaurus and did not have the wrinkled texture that characterized the carcharodontosaurids. The 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.
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. Other dinosaurs also had high spines on the back, sometimes much higher than those of Acrocanthosaurus. For instance, the unrelated Spinosaurus
had spines nearly 2 meters (6.5 ft) tall, about 11 times taller than the bodies of its vertebrae. Rather than supporting a skin 'sail' as seen in Spinosaurus, the lower spines of Acrocanthosaurus had attachments for powerful muscles like those of modern bison
, probably forming a tall, thick ridge down its back. The function of the spines remains unknown, although they may have been involved in communication
, fat
storage, or temperature control
. All of its cervical
(neck) and 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.
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 center of gravity
over its hips. Its forelimbs were relatively shorter and more robust than those of Allosaurus but were otherwise similar: each hand bore three clawed digits. Unlike many smaller fast-running
dinosaurs, its femur
was longer than its tibia
and metatarsals, suggesting that Acrocanthosaurus was not a fast runner. 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.
Allosauroidea within the 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. It was originally placed in the family Allosauridae with Allosaurus, an arrangement also supported by studies as late as 2000. Most studies have found it to be a member of the related family Carcharodontosauridae.
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. J. Willis Stovall
and Wann Langston, Jr.
first assigned it to the "Antrodemidae," the equivalent of Allosaurid
ae, but it was transferred to the taxonomic wastebasket Megalosaurid
ae by Alfred Sherwood Romer in 1956. To other authors, the long spines on its vertebrae suggested a relationship with Spinosaurus
. This interpretation of Acrocanthosaurus as a spinosaurid
persisted into the 1980s, and was repeated in the semi-technical dinosaur books of the time.
Tall spined vertebrae from the Early Cretaceous of England
were once considered to be very similar to those of Acrocanthosaurus, and in 1988 Gregory S. Paul
named them as a second species of the genus, A. altispinax. These bones were originally assigned to Altispinax
, an English theropod otherwise known only from teeth, and this assignment led to at least one author proposing that Altispinax itself was a synonym of Acrocanthosaurus. These vertebrae were later assigned to the new genus Becklespinax
, separate from both Acrocanthosaurus and Altispinax.
Most cladistic analyses including Acrocanthosaurus have found it to be a carcharodontosaurid, usually in a basal
position relative to the Africa
n Carcharodontosaurus and Giganotosaurus from South America
. 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
. This suggests that the family originated in Europe
and then dispersed
into the southern continents (at the time united as the supercontinent
Gondwana
). If Acrocanthosaurus was a carcharodontosaurid, then dispersal would also have occurred into North America. All known carcharodontosaurids lived during the early-to-middle Cretaceous Period.
ɑκρɑ/akra ('high'), ɑκɑνθɑ/akantha ('thorn' or 'spine') and σɑʊρος/sauros ('lizard'). There is one named species
(A. atokensis), which is named after 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. Langston had proposed the name "Acracanthus atokaensis" for the genus and species in his unpublished 1947 master's
thesis
, but the name was changed to Acrocanthosaurus atokensis for formal publication.
The holotype
and paratype
(OMNH
10146 and OMNH 10147), described at the same time in 1950, consist of two partial skeletons and a piece of skull material from the Antlers Formation
in Oklahoma. Two much more complete specimens were described in the 1990s. The first (SMU
74646) is a partial skeleton, missing most of the skull, recovered from the Twin Mountains Formation
of Texas and currently part of the Fort Worth Museum of Science and History
collection. An even more complete skeleton (NCSM
14345, nicknamed 'Fran') was recovered from the Antlers Formation of Oklahoma by Cephis Hall and Sid Love, prepared by the Black Hills Institute in South Dakota
, and is now housed at the North Carolina Museum of Natural Sciences
in Raleigh
. This 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.
Acrocanthosaurus may be known from less complete remains outside of Oklahoma and Texas. A tooth from southern Arizona
has been referred to the genus, and matching tooth marks have been found in sauropod bones from the same area. Several teeth from the Arundel Formation
of Maryland have been described as almost identical to those of Acrocanthosaurus and may represent an eastern representative of the genus. Many other teeth and bones from various geologic formation
s throughout the western United States have also been referred to Acrocanthosaurus, but most of these have been misidentified.
s could move without dislocating. In many of the joints, the bones did not fit together exactly, indicating the presence of a considerable amount of cartilage
in the joints, as is seen in many living archosaur
s. Among other findings, the study suggested that, in a resting position, the forelimbs would have hung from the shoulders with the humerus
angled backwards slightly, the elbow bent, and the claws facing medially (inwards).
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 retract
(swing backwards) 109°
from the vertical, so that the humerus could actually be angled slightly upwards. Protraction
(swinging forwards) was limited to only 24° past the vertical. The arm was unable to reach a vertical position when adducting
(swinging downwards), but could abduct
(swing upwards) to 9° above horizontal. Movement at the elbow was also limited compared to humans, with a total range of motion of only 57°. The arm could not completely extend
(straighten), nor could it flex
(bend) very far, with the humerus unable even to form a right angle
with the forearm. The radius
and ulna
(forearm bones) locked together so that there was no possibility of pronation
or supination
(twisting) as in human forearms.
None of the 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 backwards) 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.
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.
(replica) of an Acrocanthosaurus cranial cavity
using 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.
The brain was slightly sigmoidal (S-shaped), without much expansion of the cerebral hemisphere
s, more like a crocodile
than a bird
. This is in keeping with the overall conservatism of non-coelurosauria
n theropod brains. Acrocanthosaurus had large and bulbous olfactory bulb
s, indicating a good sense of smell
. Reconstructing the semicircular canals of the ear, which control balance, shows that the head was held at a 25° angle below horizontal. This was determined by orienting the endocast so that the lateral semicircular canal was parallel to the ground, as it usually is when an animal is in an alert posture.
of central Texas preserves many dinosaur footprints, including large, three-toed theropod prints. The most famous of these trackway
s 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
, although several other sites around the state have been described in the literature. 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. 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.
The famous Glen Rose trackway on display in New York 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. 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. 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. 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.
shows light exostotic material on the squamosal
. The neural spine of the eleventh vertebra was fractured and healed while the neural spine of its third tail vertebra had an unusual hook-like structure.
, but scientists have used biostratigraphy
to estimate their age. Based on changes in ammonite
taxa
, the boundary between the Aptian
and Albian
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. The Antlers Formation contains fossils of Deinonychus
and Tenontosaurus
, two dinosaur genera also found in the Cloverly Formation
of Montana
, which has been radiometrically dated to the Aptian and Albian stages, suggesting a similar age for the Antlers. Therefore Acrocanthosaurus most likely existed between 125 and 100 million years ago.
During this time, the area preserved in the Twin Mountains and Antlers formations was a large floodplain
that drained into a shallow inland sea
. A few million years later, this sea would expand to the north, becoming the Western Interior Seaway
and dividing North America in two for nearly the entire Late Cretaceous
. The Glen Rose Formation represents a coastal environment, with possible Acrocanthosaurus tracks preserved in mudflats along the ancient shoreline. As Acrocanthosaurus was a large predator, it is expected that it had an extensive home range and lived in many different environments in the area. Potential prey animals include sauropods like Paluxysaurus
or possibly even the enormous Sauroposeidon
, as well as large ornithopods like Tenontosaurus. The smaller theropod Deinonychus also prowled the area but at 3 meters (10 ft) in length, most likely provided only minimal competition for Acrocanthosaurus.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of theropod dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
that existed in what is now North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
during the Aptian
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
and early Albian
Albian
The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous epoch/series. Its approximate time range is 112.0 ± 1.0 Ma to 99.6 ± 0.9 Ma...
stages of the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
. Like most dinosaur genera, Acrocanthosaurus contains only a single species, A. atokensis. Its fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
remains are found mainly in the U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
states of Oklahoma
Oklahoma
Oklahoma is a state located in the South Central region of the United States of America. With an estimated 3,751,351 residents as of the 2010 census and a land area of 68,667 square miles , Oklahoma is the 28th most populous and 20th-largest state...
, Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
, and Arkansas
Arkansas
Arkansas is a state located in the southern region of the United States. Its name is an Algonquian name of the Quapaw Indians. Arkansas shares borders with six states , and its eastern border is largely defined by the Mississippi River...
, although teeth attributed to Acrocanthosaurus have been found as far east as Maryland
Maryland
Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...
.
Acrocanthosaurus was a bipedal predator. As the name suggests, it is best known for the high neural spines
Spinous process
The spinous process of a vertebra is directed backward and downward from the junction of the laminae , and serves for the attachment of muscles and ligaments. In animals without an erect stance, the process points upward and may slant forward or backward...
on many of its vertebrae, which most likely supported a ridge of muscle over the animal's neck, back and hips. Acrocanthosaurus was one of the largest theropods, approaching 12 meters (40 ft) in length, and weighing up to 6–7 metric tons
Tonne
The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...
(6.5–7.5 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 brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
structure and forelimb function. Acrocanthosaurus was the largest theropod in its ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
and likely an apex predator
Apex predator
Apex predators are predators that have no predators of their own, residing at the top of their food chain. Zoologists define predation as the killing and consumption of another organism...
which possibly preyed on large sauropods and ornithopod
Ornithopod
Ornithopods or members of the clade Ornithopoda are a group of ornithischian dinosaurs that started out as small, bipedal running grazers, and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world, and dominated the North American...
s.
Description
Although slightly smaller than colossal relatives like GiganotosaurusGiganotosaurus
Giganotosaurus is a genus of carcharodontosaurid dinosaur that lived around 97 million years ago during the early Cenomanian stage of the Late Cretaceous Period. It included some of the largest known terrestrial carnivores, slightly larger than the largest Tyrannosaurus, but smaller than the...
, Acrocanthosaurus was still among the largest theropods ever to exist. The longest known individual measured 11.5 meters (38 ft) from snout to tail tip and weighed an estimated 6,000–7,000 kilograms (13,000–15,000 lb). Its skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
alone was nearly 1.3 meters (4.3 ft) in length.
The skull of Acrocanthosaurus, like most other allosauroids, was long, low and narrow. The weight-reducing opening in front of the eye socket (antorbital fenestra
Antorbital fenestra
An antorbital fenestra is an opening in the skull, in front of the eye sockets. This skull formation first appeared in archosaurs during the Triassic Period. Living birds today possess antorbital fenestrae, but the feature has been lost in modern crocodilians...
) was quite large, more than a quarter of the length of the skull and two-thirds of its height. The outside surface of the maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
(upper jaw bone) and the upper surface of the nasal bone
Nasal bone
The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face, and form, by their junction, "the bridge" of the nose.Each has two surfaces and four borders....
on the roof of the snout were not nearly as rough-textured as those of Giganotosaurus
Giganotosaurus
Giganotosaurus is a genus of carcharodontosaurid dinosaur that lived around 97 million years ago during the early Cenomanian stage of the Late Cretaceous Period. It included some of the largest known terrestrial carnivores, slightly larger than the largest Tyrannosaurus, but smaller than the...
or Carcharodontosaurus
Carcharodontosaurus
Carcharodontosaurus was a gigantic carnivorous carcharodontosaurid dinosaur that lived around 100 to 93 million years ago, during the late Albian to early Cenomanian stages of the mid-Cretaceous Period...
. Long, low ridges arose from the nasal bones, running along each side of the snout from the nostril back to the eye, where they continued onto the lacrimal bone
Lacrimal bone
The lacrimal bone, the smallest and most fragile bone of the face, is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders.-Lateral or orbital surface:...
s. This is a characteristic feature of all allosauroids. Unlike Allosaurus
Allosaurus
Allosaurus is a genus of large theropod dinosaur that lived 155 to 150 million years ago during the late Jurassic period . The name Allosaurus means "different lizard". It is derived from the Greek /allos and /sauros...
, there was no prominent crest on the lacrimal bone in front of the eye. The lacrimal and postorbital
Postorbital
The postorbital is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some vertebrates, the postorbital is fused with the postfrontal...
bones met to form a thick brow over the eye, as seen in carcharodontosaurids and the unrelated abelisaurids. Nineteen curved, serrated teeth lined each side of the upper jaw, but a tooth count for the lower jaw has not been published. Acrocanthosaurus teeth were wider than those of Carcharodontosaurus and did not have the wrinkled texture that characterized the carcharodontosaurids. The 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.
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. Other dinosaurs also had high spines on the back, sometimes much higher than those of Acrocanthosaurus. For instance, the unrelated Spinosaurus
Spinosaurus
Spinosaurus is a genus of theropod dinosaur which lived in what is now North Africa, from the lower Albian to lower Cenomanian stages of the Cretaceous period, about 112 to 97 million years ago. This genus was first known from Egyptian remains discovered in 1912 and described by German...
had spines nearly 2 meters (6.5 ft) tall, about 11 times taller than the bodies of its vertebrae. Rather than supporting a skin 'sail' as seen in Spinosaurus, the lower spines of Acrocanthosaurus had attachments for powerful muscles like those of modern bison
Bison
Members of the genus Bison are large, even-toed ungulates within the subfamily Bovinae. Two extant and four extinct species are recognized...
, probably forming a tall, thick ridge down its back. The function of the spines remains unknown, although they may have been involved in communication
Animal communication
Animal communication is any behavior on the part of one animal that has an effect on the current or future behaviour of another animal. The study of animal communication, is sometimes called Zoosemiotics has played an important part in the...
, fat
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...
storage, or temperature control
Thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...
. All of its cervical
Cervical vertebrae
In vertebrates, cervical vertebrae are those vertebrae immediately inferior to the skull.Thoracic vertebrae in all mammalian species are defined as those vertebrae that also carry a pair of ribs, and lie caudal to the cervical vertebrae. Further caudally follow the lumbar vertebrae, which also...
(neck) and dorsal
Dorsum (biology)
In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...
(back) vertebrae had prominent depressions (pleurocoels) on the sides, while the caudal
Tail
The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds...
(tail) vertebrae bore smaller ones. This is more similar to carcharodontosaurids than to Allosaurus.
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 center of gravity
Center of gravity
In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as the center of gravity...
over its hips. Its forelimbs were relatively shorter and more robust than those of Allosaurus but were otherwise similar: each hand bore three clawed digits. Unlike many smaller fast-running
Cursorial
Cursorial is a biological term that describes an organism as being adapted specifically to run. It is typically used in conjunction with an animal's feeding habits or another important adaptation. For example, a horse can be considered a "cursorial grazer", while a wolf may be considered a...
dinosaurs, its femur
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...
was longer than its tibia
Tibia
The tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
and metatarsals, suggesting that Acrocanthosaurus was not a fast runner. 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.
Classification and systematics
Acrocanthosaurus is classified in the superfamilyTaxonomic rank
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...
Allosauroidea within the infraorder Tetanurae
Tetanurae
Tetanurae, or "stiff tails", is a clade that includes most theropod dinosaurs, as well as birds. Tetanurans first appear during the early or middle Jurassic Period.-Definition:...
. 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. It was originally placed in the family Allosauridae with Allosaurus, an arrangement also supported by studies as late as 2000. Most studies have found it to be a member of the related family Carcharodontosauridae.
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. J. Willis Stovall
J. Willis Stovall
John Willis Stovall was a paleontologist at the University of Oklahoma. Along with his student Wann Langston, Jr., he named the theropod dinosaur Acrocanthosaurus in 1950. Most of his research centered on the Cimarron Valley region of extreme northwestern Oklahoma.-Selected publication:*Stovall,...
and Wann Langston, Jr.
Wann Langston, Jr.
Wann Langston, Jr. is an American paleontologist and professor at the University of Texas at Austin. He has worked on a number of different reptiles and amphibians in his long career, beginning with the 1950 description of the theropod dinosaur Acrocanthosaurus...
first assigned it to the "Antrodemidae," the equivalent of Allosaurid
Allosaurid
Allosauridae was a family of medium to large sized carnivorous theropod dinosaurs. Members included Saurophaganax, Allosaurus, and the poorly known Epanterias, which could well be a specimen of Allosaurus...
ae, but it was transferred to the taxonomic wastebasket Megalosaurid
Megalosaurid
Megalosauridae was a family of relatively primitive tetanuran theropod dinosaurs, order Saurischia. They were small-to-large carnivores with sharp teeth and three claws on each hand. Some members of this group were Megalosaurus, Eustreptospondylus, Streptospondylus and Torvosaurus...
ae by Alfred Sherwood Romer in 1956. To other authors, the long spines on its vertebrae suggested a relationship with Spinosaurus
Spinosaurus
Spinosaurus is a genus of theropod dinosaur which lived in what is now North Africa, from the lower Albian to lower Cenomanian stages of the Cretaceous period, about 112 to 97 million years ago. This genus was first known from Egyptian remains discovered in 1912 and described by German...
. This interpretation of Acrocanthosaurus as a spinosaurid
Spinosauridae
Spinosauridae is a family of specialised theropod dinosaurs. Members of this family were large, bipedal predators with elongated, crocodile-like skulls, sporting conical teeth with no or only very tiny serrations. The front dentary teeth fanned out, giving the animal a characteristic look...
persisted into the 1980s, and was repeated in the semi-technical dinosaur books of the time.
Tall spined vertebrae from the Early Cretaceous of England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
were once considered to be very similar to those of Acrocanthosaurus, and in 1988 Gregory S. Paul
Gregory S. Paul
Gregory Scott Paul is a freelance researcher, author and illustrator who works in paleontology, and more recently has examined sociology and theology. He is best known for his work and research on theropod dinosaurs and his detailed illustrations, both live and skeletal...
named them as a second species of the genus, A. altispinax. These bones were originally assigned to Altispinax
Altispinax
Altispinax is a genus of theropod dinosaur from the Early Cretaceous Obernkirchen Sandstein of Niedersachsen, Germany. Material has also been referred to it from the Hastings Beds , Purbeck Beds , and Lower Greensand of England, and the Weald Clay of England and Belgium, but this material...
, an English theropod otherwise known only from teeth, and this assignment led to at least one author proposing that Altispinax itself was a synonym of Acrocanthosaurus. These vertebrae were later assigned to the new genus Becklespinax
Becklespinax
Becklespinax is a genus of large theropod dinosaur based on a type specimen of three tall-spined vertebrae found in 1884 in Sussex, England by the fossil collector Samuel H. Beckles. The fossils are believed to have been from the Early Cretaceous....
, separate from both Acrocanthosaurus and Altispinax.
Most cladistic analyses including Acrocanthosaurus have found it to be a carcharodontosaurid, usually in a basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
position relative to the Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
n Carcharodontosaurus and Giganotosaurus from South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
. It has often been considered the sister taxon to the equally-basal Eocarcharia
Eocarcharia
Eocarcharia is a genus of carcharodontosaurid theropod dinosaur from the Early Cretaceous Elrhaz Formation that lived in the Sahara 112 million years ago, in what today is the country of Niger. It was discovered in 2000 on an expedition led by University of Chicago paleontologist Paul Sereno. The...
, also from Africa. Neovenator
Neovenator
Neovenator is a genus of allosauroid dinosaur. Since its discovery on the Isle of Wight, UK, it has become one of the best-known large carnivorous dinosaurs in Europe. Neovenator was at first considered possibly a new species of Megalosaurus. It measured approximately 7.5 meters in length,...
, discovered in England, is often considered an even more basal carcharodontosaurid, or as a basal member of a sister group called Neovenatoridae
Neovenatoridae
Neovenatoridae is a family of large carnivorous dinosaurs. The group is a branch of the allosauroids, a large group of carnosaurs that also includes the sinraptorids, carcharodontosaurids, and allosaurids...
. This suggests that the family originated in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and then dispersed
Biological dispersal
Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...
into the southern continents (at the time united as the supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
). If Acrocanthosaurus was a carcharodontosaurid, then dispersal would also have occurred into North America. All known carcharodontosaurids lived during the early-to-middle Cretaceous Period.
Discovery and naming
Acrocanthosaurus is named for its tall neural spines, from the GreekAncient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
ɑκρɑ/akra ('high'), ɑκɑνθɑ/akantha ('thorn' or 'spine') and σɑʊρος/sauros ('lizard'). There is one named species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
(A. atokensis), which is named after Atoka County in Oklahoma
Oklahoma
Oklahoma is a state located in the South Central region of the United States of America. With an estimated 3,751,351 residents as of the 2010 census and a land area of 68,667 square miles , Oklahoma is the 28th most populous and 20th-largest state...
, where the original specimens were found. The name was coined in 1950 by American paleontologists J. Willis Stovall and Wann Langston, Jr. Langston had proposed the name "Acracanthus atokaensis" for the genus and species in his unpublished 1947 master's
Master's degree
A master's is an academic degree granted to individuals who have undergone study demonstrating a mastery or high-order overview of a specific field of study or area of professional practice...
thesis
Thesis
A dissertation or thesis is a document submitted in support of candidature for an academic degree or professional qualification presenting the author's research and findings...
, but the name was changed to Acrocanthosaurus atokensis for formal publication.
The holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
and paratype
Paratype
Paratype is a technical term used in the scientific naming of species and other taxa of organisms. The exact meaning of the term paratype when it is used in zoology is not the same as the meaning when it is used in botany...
(OMNH
Sam Noble Oklahoma Museum of Natural History
The Sam Noble Oklahoma Museum of Natural History is a natural history museum in Norman, Oklahoma, operated by the University of Oklahoma. It is currently housed in a building on Chautauqua Avenue that opened on May 1, 2000. The museum's exhibits include a Native American gallery and collections of...
10146 and OMNH 10147), described at the same time in 1950, consist of two partial skeletons and a piece of skull material from the Antlers Formation
Antlers Formation
The Antlers Formation occurs as a band of strata from Arkansas through southern Oklahoma into northeastern Texas. It consists of 150 m of silty to sandy mudstones, fine- to coarse grained sandstones that are poorly to moderately sorted and cemented with clay or calcium carbonate...
in Oklahoma. Two much more complete specimens were described in the 1990s. The first (SMU
Southern Methodist University
Southern Methodist University is a private university in Dallas, Texas, United States. Founded in 1911 by the Methodist Episcopal Church, South, SMU operates campuses in Dallas, Plano, and Taos, New Mexico. SMU is owned by the South Central Jurisdiction of the United Methodist Church...
74646) is a partial skeleton, missing most of the skull, recovered from the Twin Mountains Formation
Twin Mountains Formation
The Twin Mountains Formation, also known as the Twin Mountain Formation, is a sedimentary rock formation, within the Trinity Group, found in Texas of the United States of America. It is a terrestrial formation of Aptian age , and is notable for its dinosaur fossils...
of Texas and currently part of the Fort Worth Museum of Science and History
Fort Worth Museum of Science and History
The Fort Worth Museum of Science and History is located on 1600 Gendy Street, Fort Worth, Texas 76107 in the city's Cultural District. It was opened in 1945 as the Fort Worth Children's Museum and moved to its current location in 1954. In 1968, the museum adopted its current name...
collection. An even more complete skeleton (NCSM
North Carolina Museum of Natural Sciences
The North Carolina Museum of Natural Sciences is located in Raleigh, North Carolina. This museum is the oldest established museum in North Carolina and the largest museum of its kind in the Southeast...
14345, nicknamed 'Fran') was recovered from the Antlers Formation of Oklahoma by Cephis Hall and Sid Love, prepared by the Black Hills Institute in South Dakota
South Dakota
South Dakota is a state located in the Midwestern region of the United States. It is named after the Lakota and Dakota Sioux American Indian tribes. Once a part of Dakota Territory, South Dakota became a state on November 2, 1889. The state has an area of and an estimated population of just over...
, and is now housed at the North Carolina Museum of Natural Sciences
North Carolina Museum of Natural Sciences
The North Carolina Museum of Natural Sciences is located in Raleigh, North Carolina. This museum is the oldest established museum in North Carolina and the largest museum of its kind in the Southeast...
in Raleigh
Raleigh, North Carolina
Raleigh is the capital and the second largest city in the state of North Carolina as well as the seat of Wake County. Raleigh is known as the "City of Oaks" for its many oak trees. According to the U.S. Census Bureau, the city's 2010 population was 403,892, over an area of , making Raleigh...
. This 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.
Acrocanthosaurus may be known from less complete remains outside of Oklahoma and Texas. A tooth from southern Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
has been referred to the genus, and matching tooth marks have been found in sauropod bones from the same area. Several teeth from the Arundel Formation
Arundel Formation
The Arundel Formation, also known as the Arundel Clay, is a clay-rich sedimentary rock formation, within the Potomac Group, found in Maryland of the United States of America. It is of Aptian age . This rock unit had been economically important as a source of iron ore, but is now more notable for...
of Maryland have been described as almost identical to those of Acrocanthosaurus and may represent an eastern representative of the genus. Many other teeth and bones from various geologic formation
Geologic formation
A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties...
s throughout the western United States have also been referred to Acrocanthosaurus, but most of these have been misidentified.
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 study examined the bone surfaces which would have articulated with other bones to determine how far the jointJoint
A joint is the location at which two or more bones make contact. They are constructed to allow movement and provide mechanical support, and are classified structurally and functionally.-Classification:...
s could move without dislocating. In many of the joints, the bones did not fit together exactly, indicating the presence of a considerable amount of cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
in the joints, as is seen in many living archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
s. Among other findings, the study suggested that, in a resting position, the forelimbs would have hung from the shoulders with the humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
angled backwards slightly, the elbow bent, and the claws facing medially (inwards).
Retraction (kinesiology)
Retraction is the anatomical term of motion for posterior movement of the arms at the shoulders. It is the opposite of protraction. Major muscles involved include: * Rhomboid major muscle* Rhomboid minor muscle* Trapezius muscle-External links:* *...
(swing backwards) 109°
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
from the vertical, so that the humerus could actually be angled slightly upwards. Protraction
Protraction
Protraction is the anatomical term of motion for anterior movement of an anatomical part of the body forward . This can include the shoulders, jaw, pelvis, etc.It is the opposite of retraction....
(swinging forwards) was limited to only 24° past the vertical. The arm was unable to reach a vertical position when adducting
Adduction
Adduction is a movement which brings a part of the anatomy closer to the middle sagittal plane of the body. It is opposed to abduction.-Upper limb:* of arm at shoulder ** Subscapularis** Teres major** Pectoralis major** Infraspinatus...
(swinging downwards), but could abduct
Abduction (kinesiology)
Abduction, in functional anatomy, is a movement which draws a limb away from the median plane of the body. It is thus opposed to adduction.-Upper limb:* of arm at shoulder ** Supraspinatus** Deltoid* of hand at wrist...
(swing upwards) to 9° above horizontal. Movement at the elbow was also limited compared to humans, with a total range of motion of only 57°. The arm could not completely extend
Extension (kinesiology)
In kinesiology, extension is a movement of a joint that results in increased angle between two bones or body surfaces at a joint. Extension usually results in straightening of the bones or body surfaces involved. For example, extension is produced by extending the flexed elbow. Straightening of...
(straighten), nor could it flex
Flexion
In anatomy, flexion is a position that is made possible by the joint angle decreasing. The skeletal and muscular systems work together to move the joint into a "flexed" position. For example the elbow is flexed when the hand is brought closer to the shoulder...
(bend) very far, with the humerus unable even to form a right angle
Right angle
In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles...
with the forearm. The radius
Radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...
and ulna
Ulna
The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...
(forearm bones) locked together so that there was no possibility of pronation
Pronation
In anatomy, pronation is a rotational movement of the forearm at the radioulnar joint, or of the foot at the subtalar and talocalcaneonavicular joints. For the forearm, when standing in the anatomical position, pronation will move the palm of the hand from an anterior-facing position to a...
or supination
Supination
Supination is a position of either the forearm or foot; in the forearm when the palm faces anteriorly, or faces up . Supination in the foot occurs when a person appears "bow-legged" with their weight supported primarily on the anterior of their feet.The hand is supine in the anatomical position...
(twisting) as in human forearms.
After determining the ranges of motion in the joints of the forelimb, the study went on to hypothesize
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
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.
Brain and inner ear structure
In 2005, scientists reconstructed an endocastEndocast
An endocast is the internal cast of a hollow object, often specifically used for an endocasts of the cranial vault. Endocasts can be man-made for examining the properties of a hollow, inaccessible space, or occur naturally through fossilisation....
(replica) of an Acrocanthosaurus cranial cavity
Cranial cavity
The cranial cavity, or intracranial space, is the space formed inside the skull. The brain occupies the cranial cavity, which is lined by the meninges and which contains cerebrospinal fluid to cushion blows....
using computed tomography
Computed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...
(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
Meninges
The meninges is the system of membranes which envelopes the central nervous system. The meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater. The primary function of the meninges and of the cerebrospinal fluid is to protect the central nervous system.-Dura...
and cerebrospinal fluid
Cerebrospinal fluid
Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...
, in addition to the brain itself. However, the general features of the brain and cranial nerves
Cranial nerves
Cranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of 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
Sinraptor
Sinraptor is a genus of theropod dinosaur from the Late Jurassic. The name Sinraptor comes from the Latin prefix "Sino", meaning Chinese, and "Raptor" meaning thief. The specific name dongi honours Dong Zhiming...
, providing support for the hypothesis that Acrocanthosaurus was a carcharodontosaurid.
The brain was slightly sigmoidal (S-shaped), without much expansion of the cerebral hemisphere
Cerebral hemisphere
A cerebral hemisphere is one of the two regions of the eutherian brain that are delineated by the median plane, . The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter called the cerebral cortex that is...
s, more like a crocodile
Crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...
than a bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
. This is in keeping with the overall conservatism of non-coelurosauria
Coelurosauria
Coelurosauria is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. In the past, it was used to refer to all small theropods, although this classification has been abolished...
n theropod brains. Acrocanthosaurus had large and bulbous olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...
s, indicating a good sense of smell
Olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
. Reconstructing the semicircular canals of the ear, which control balance, shows that the head was held at a 25° angle below horizontal. This was determined by orienting the endocast so that the lateral semicircular canal was parallel to the ground, as it usually is when an animal is in an alert posture.
Possible footprints
The Glen Rose FormationGlen Rose Formation
The Glen Rose Formation is a shallow marine to shoreline geological formation from the lower Cretaceous period exposed over a large area from South Central to North Central Texas...
of central Texas preserves many dinosaur footprints, including large, three-toed theropod prints. The most famous of these trackway
Fossil trackway
A fossil trackway is a type of trace fossil, a trackway made by an organism. Many fossil trackways were made by dinosaurs, early tetrapods, and other quadrupeds and bipeds on land...
s was discovered along the Paluxy River
Paluxy River
The Paluxy River is a river in the U.S. state of Texas. It is a tributary of the Brazos River. It is formed by the convergence of the North Paluxy River and the South Paluxy River near Bluff Dale, Texas in Erath County and flows a distance of before joining the Brazos just to the east of Glen...
in Dinosaur Valley State Park
Dinosaur Valley State Park
- History :Dinosaur Valley State Park, located just northwest of Glen Rose in Somervell County, is a scenic park set astride the Paluxy River. The land for the park was acquired from private owners under the State Parks Bonds Program during 1968 and opened to the public in 1972...
, a section of which is now on exhibit in the American Museum of Natural History
American Museum of Natural History
The American Museum of Natural History , located on the Upper West Side of Manhattan in New York City, United States, is one of the largest and most celebrated museums in the world...
in New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
, although several other sites around the state have been described in the literature. 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. 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.
The famous Glen Rose trackway on display in New York 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
Pack hunter
A pack hunter is a predator belonging to the animal kingdom, which has evolved to hunt its prey by working together with other members of its species. Normally, such animals are closely related. The most commonly known pack hunter is the Gray Wolf, the ancestor of all breeds of domesticated dogs...
of Acrocanthosaurus was stalking a herd of sauropods. 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. 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. 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.
Pathology
The skull of the Acrocanthosaurus atokensis holotypeHolotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
shows light exostotic material on the squamosal
Squamosal
The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...
. The neural spine of the eleventh vertebra was fractured and healed while the neural spine of its third tail vertebra had an unusual hook-like structure.
Paleoecology
Definite Acrocanthosaurus fossils have been found in the Twin Mountains Formation of northern Texas and the Antlers Formation of southern Oklahoma. These geological formations have not been dated radiometricallyRadiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
, but scientists have used biostratigraphy
Biostratigraphy
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period...
to estimate their age. Based on changes in ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
taxa
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
, the boundary between the Aptian
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
and Albian
Albian
The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous epoch/series. Its approximate time range is 112.0 ± 1.0 Ma to 99.6 ± 0.9 Ma...
stages
Faunal stage
In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.Rock...
of the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the 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. The Antlers Formation contains fossils of Deinonychus
Deinonychus
Deinonychus was a genus of carnivorous dromaeosaurid dinosaur. There is one described species, Deinonychus antirrhopus. This 3.4 meter long dinosaur lived during the early Cretaceous Period, about 115–108 million years ago . Fossils have been recovered from the U.S...
and Tenontosaurus
Tenontosaurus
Tenontosaurus is a genus of medium- to large-sized ornithopod dinosaur. The genus is known from the late Aptian to Albian ages of the middle Cretaceous period sediments of western North America, dating between 115 to 108 million years ago...
, two dinosaur genera also found in the Cloverly Formation
Cloverly Formation
The Cloverly Formation are Lower Cretaceous strata located in Montana and Wyoming, in the western United States. The term now includes strata that had formerly been called the Dakota Formation in central and southern Wyoming.-Members:...
of Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
, which has been radiometrically dated to the Aptian and Albian stages, suggesting a similar age for the Antlers. Therefore Acrocanthosaurus most likely existed between 125 and 100 million years ago.
During this time, the area preserved in the Twin Mountains and Antlers formations was a large floodplain
Floodplain
A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...
that drained into a shallow inland sea
Epeiric Sea
An epeiric sea is a shallow sea that extends over part of a continent.Epeiric seas are usually associated with the marine transgressions of the geologic past, which have variously been due to either global eustatic sea level changes, local tectonic deformation, or both, and are occasionally...
. A few million years later, this sea would expand to the north, becoming the Western Interior Seaway
Western Interior Seaway
The Western Interior Seaway, also called the Cretaceous Seaway, the Niobraran Sea, and the North American Inland Sea, was a huge inland sea that split the continent of North America into two halves, Laramidia and Appalachia, during most of the mid- and late-Cretaceous Period...
and dividing North America in two for nearly the entire Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
. The Glen Rose Formation represents a coastal environment, with possible Acrocanthosaurus tracks preserved in mudflats along the ancient shoreline. As Acrocanthosaurus was a large predator, it is expected that it had an extensive home range and lived in many different environments in the area. Potential prey animals include sauropods like Paluxysaurus
Paluxysaurus
Paluxysaurus is a genus of basal titanosauriform sauropod dinosaur from the late Aptian or early Albian-age Lower Cretaceous Twin Mountains Formation of Hood County, Texas, USA...
or possibly even the enormous Sauroposeidon
Sauroposeidon
Sauroposeidon is a genus of sauropod dinosaur known from four neck vertebrae that were found in the southeastern portion of the US state of Oklahoma. The fossils were found in rocks dating to the Early Cretaceous, a period when the sauropods of North America had diminished in both size and...
, as well as large ornithopods like Tenontosaurus. The smaller theropod Deinonychus also prowled the area but at 3 meters (10 ft) in length, most likely provided only minimal competition for Acrocanthosaurus.
External links
- Acrocanthosaurus FAQ from the North Carolina Museum of Natural Sciences.
- Acrocanthosaurus entry at The Theropod Database.
- Acrocanthosaurus skeletal images at The Grave Yard.
- History of "Fran" (NCSM 14345)
- The Glen Rose Trackway at the American Museum of Natural History.
- Life restoration of Acrocanthosaurus atokensis by paleoartist Julius T. Csotonyi.