Although slightly smaller than
gigantic relatives like
Giganotosaurus,
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
6,177 kilograms (13,618 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 bones. 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.
Classification and Systematics
Acrocanthosaurus
is
classified in the
superfamily
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 some more recent studies.
Other scientists classify it as a member of the related family
Carcharodontosauridae.
Discovery
and Naming
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 private collectors, 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 formations
throughout the western United States have also been referred to
Acrocanthosaurus, but most
of these have been misidentified.
Paleobiology
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
joints 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
archosaurs. 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
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.
Brain and inner ear structure
In 2005, scientists reconstructed an
endocast (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 hemispheres,
more like a
crocodile than a
bird. This is in
keeping with the overall conservatism of non-coelurosaurian
theropod brains.
Acrocanthosaurus had large and bulbous
olfactory bulbs,
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.
The
Glen Rose Formation of
central Texas preserves many dinosaur footprints, including large,
three-toed theropod prints. The most famous of these
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, 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.
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 radiometrically,
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.
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