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Encyclopedia Largest prehistoric animals Vol.1 Vertebrates part3 Dinosaurs ch.1 Theropods - Early zauropod hunters
Автор: valentint
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Последна промяна: 23.05.2019 00:08
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Последна промяна: 23.05.2019 00:08
Torvosaurus
One of the biggest fields of study regarding Torvosaurus is its supposed size and possible claim to being the largest known Jurassic era theropod dinosaur. Though known from only partial remains, fossils from the Morrison Formation have been compared to other better preserved theropod genera of the time to yield estimates of around nine meters long. However further fossil material, namely a maxilla (upper jaw bone) has been found to be larger than equivalent remains in North America, and when the North American remains are scaled up for comparison a new estimate of up to eleven meters long come to light. If eleven meters is correct then Torvosaurus would certainly be one of the largest predators in North America during the Jurassic, and be even larger than the smaller estimate for Saurophaganax (a theropod estimated between ten and thirteen meters long).
Because of its potential large size, Torvosaurus has been speculated by some to have been a scavenger that used its larger size to intimidate and scare off smaller theropods such as Marshosaurus, Ceratosaurus and even Allosaurus from their kills. However this large size would have given Torvosaurus an advantage in dealing with larger prey types that would have been too slow to run away even from a theropod the size of Torvosaurus. Potential prey for Torvosaurus could have included ornithopods like Camptosaurus and plated dinosaurs like Stegosaurus. Sauropods such as Diplodocus, Apatosaurus, Brachiosaurus and Camarasaurus are also known to have been quite numerous, and smaller juveniles of these genera in particular would have been vulnerable to predators such as Torvosaurus.
Another large theropod called Edmarka is known to have stalked North America during the Jurassic, though there is speculation that Edmarka might actually be the same dinosaur as Torvosaurus, an idea that has increased in popularity in recent years.
Back in the year 2000, new theropod material was recovered from Portugal, and in a subsequent description Octбvio Mateus and Miguel Telles Antunes assigned these fossils as Torvosaurus sp. Later in 2006, the fossils from the Lourinhг Formation were assigned to the Torvosaurus type species T. tanneri. By 2012 another study by Carrano et al went to the point that the Torvosaurus fossils could be defined further than just a generic species, and in 2013 eggs with fossilised embryos inside them were assigned to Torvosaurus. Finally in 2014 description by Hendrickx and Mateus the Portuguese fossils were recognised as a species entirely separate from the type species which today is only known from North America. The Portuguese Torvosaurus are now credited as belonging to Torvosaurus gurneyi, a species named in honour of James Gurney, who is best known for his work creating the Dinotopia series of illustrated books.
It might seem strange that two species of the same dinosaur could co-exist upon what are now separate continents, but it is important to remember that in the late Jurassic North America and Europe were closer and still connected and that land animals could literally walk between the two. Torvosaurus is also not the only one either, with species of other dinosaur genera which are often described as North American such as Allosaurus, Ceratosaurus and Stegosaurus also being known from the Portuguese Lourinhг Formation. In fact many researchers often point out that in terms of the types of animals, the Lourinhг Formation is analogous to the famous Morrison Formation of the United States. Other dinosaur genera that Torvosaurus gurneyi may have come into contact with include the stegosaurs Miragaia and Dacentrurus, the sauropods Lusotitan and Lourinhasaurus, the theropod Lourinhanosaurus, as well as ornithopods and possibly even ankylosaurs.
Torvosaurus gurneyi was initially described as being about eleven meters long, but revisions to this estimate have already been made which has seen estimates reduced to around the ten meter mark. This would still make Torvosaurus gurneyi one of the largest theropod dinosaurs known to have lived in Europe, and still larger than many of the earlier European theropods such as Megalosaurus, Dubreuillosaurus and Poekilopleuron and Magnosaurus.
Yangchuanosaurus
Yangchuanosaurus is often likened to fulfilling the same ecological niche as Allosaurus, except in Asia instead of North America. Not only does Yangchuanosaurus have a similar morphology to Allosaurus it also had access to similar prey items such as stegosaurs and sauropods. Yangchuanosaurus had a characteristic growth on top of its nose as well as smaller horns and ridges, and also possessed a tail that made up half of its total length.
The first specimen of Yangchuanosaurus was discovered by a construction worker working upon the construction of a dam in Sichuan Province.This revealed an eight meter individual. In 1983 however the second specimen and species of Yangchuanosaurus was discovered from the same formation, and this time the remains revealed an individual estimated to be just under eleven meters long.Another specimen, assigned to the new species Y. magnus, was even larger, with a skull length of 1.11 m (3.6 ft).It may have been up to 10.8 m (35.4 ft) long, and weighed as much as 4 metric tons.There was a bony ridge on its nose and multiple hornlets and ridges, similar to Ceratosaurus.
This gives Yangchuanosaurus another similarity to Allosaurus as it has an average length of eight meters with some individuals suggesting up to eleven meters as well.
Yangchuanosaurus was a large, powerful meat-eater. It walked on two large, muscular legs, had short arms, a strong, short neck, a big head with powerful jaws, and large, serrated teeth. It had a long, massive tail that was about half of its length. Its arms were short. The first digit of its foot was a small dewclaw. The three outer toes were used to bear weight and each was equipped with a large claw.
Acrocanthosaurus
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Acrocanthosaurus ("high-spined lizard") 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 Wyoming, 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 11.5 m (38 ft) in length, and weighing up to 6.2 tonnes (6.8 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 ornithopods.
Although slightly smaller than gigantic relatives like Giganotosaurus, Acrocanthosaurus was still among the largest theropods ever to exist. The largest known specimen (NCSM 14345) is estimated to have measured 11.5 m (38 ft)from snout to tail tip and weighed 5.7 t (6.3 short tons)to 6.2 t (6.8 short tons),though Bates et al. (2009) noted that a weight of 7 t (7.7 short tons) is not impossible for this specimen.Its skull alone was nearly 1.3 m (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. 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.
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 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 Allosauridae, but it was transferred to the taxonomic wastebasket Megalosauridae 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 African 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.
The holotype and paratype (OMNH 10146 and OMNH 10147), discovered in the early 1940s and 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.
The presence of Acrocanthosaurus in the Cloverly Formation was established in 2012 with the description of another partial skeleton, UM 20796. This 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.
Acrocanthosaurus may be known from less complete remains outside of Oklahoma, Texas, and Wyoming. 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;there is, however, some disagreement with this assessment regarding fossils from the Cloverly Formation.
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 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.
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 City,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 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.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.
Definite Acrocanthosaurus fossils have been found in the Twin Mountains Formation of northern Texas, the Antlers Formation of southern Oklahoma, and the Cloverly Formation of north-central Wyoming. 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, 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 Astrodonor 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.
One of the biggest fields of study regarding Torvosaurus is its supposed size and possible claim to being the largest known Jurassic era theropod dinosaur. Though known from only partial remains, fossils from the Morrison Formation have been compared to other better preserved theropod genera of the time to yield estimates of around nine meters long. However further fossil material, namely a maxilla (upper jaw bone) has been found to be larger than equivalent remains in North America, and when the North American remains are scaled up for comparison a new estimate of up to eleven meters long come to light. If eleven meters is correct then Torvosaurus would certainly be one of the largest predators in North America during the Jurassic, and be even larger than the smaller estimate for Saurophaganax (a theropod estimated between ten and thirteen meters long).
Because of its potential large size, Torvosaurus has been speculated by some to have been a scavenger that used its larger size to intimidate and scare off smaller theropods such as Marshosaurus, Ceratosaurus and even Allosaurus from their kills. However this large size would have given Torvosaurus an advantage in dealing with larger prey types that would have been too slow to run away even from a theropod the size of Torvosaurus. Potential prey for Torvosaurus could have included ornithopods like Camptosaurus and plated dinosaurs like Stegosaurus. Sauropods such as Diplodocus, Apatosaurus, Brachiosaurus and Camarasaurus are also known to have been quite numerous, and smaller juveniles of these genera in particular would have been vulnerable to predators such as Torvosaurus.
Another large theropod called Edmarka is known to have stalked North America during the Jurassic, though there is speculation that Edmarka might actually be the same dinosaur as Torvosaurus, an idea that has increased in popularity in recent years.
Back in the year 2000, new theropod material was recovered from Portugal, and in a subsequent description Octбvio Mateus and Miguel Telles Antunes assigned these fossils as Torvosaurus sp. Later in 2006, the fossils from the Lourinhг Formation were assigned to the Torvosaurus type species T. tanneri. By 2012 another study by Carrano et al went to the point that the Torvosaurus fossils could be defined further than just a generic species, and in 2013 eggs with fossilised embryos inside them were assigned to Torvosaurus. Finally in 2014 description by Hendrickx and Mateus the Portuguese fossils were recognised as a species entirely separate from the type species which today is only known from North America. The Portuguese Torvosaurus are now credited as belonging to Torvosaurus gurneyi, a species named in honour of James Gurney, who is best known for his work creating the Dinotopia series of illustrated books.
It might seem strange that two species of the same dinosaur could co-exist upon what are now separate continents, but it is important to remember that in the late Jurassic North America and Europe were closer and still connected and that land animals could literally walk between the two. Torvosaurus is also not the only one either, with species of other dinosaur genera which are often described as North American such as Allosaurus, Ceratosaurus and Stegosaurus also being known from the Portuguese Lourinhг Formation. In fact many researchers often point out that in terms of the types of animals, the Lourinhг Formation is analogous to the famous Morrison Formation of the United States. Other dinosaur genera that Torvosaurus gurneyi may have come into contact with include the stegosaurs Miragaia and Dacentrurus, the sauropods Lusotitan and Lourinhasaurus, the theropod Lourinhanosaurus, as well as ornithopods and possibly even ankylosaurs.
Torvosaurus gurneyi was initially described as being about eleven meters long, but revisions to this estimate have already been made which has seen estimates reduced to around the ten meter mark. This would still make Torvosaurus gurneyi one of the largest theropod dinosaurs known to have lived in Europe, and still larger than many of the earlier European theropods such as Megalosaurus, Dubreuillosaurus and Poekilopleuron and Magnosaurus.
Yangchuanosaurus
Yangchuanosaurus is often likened to fulfilling the same ecological niche as Allosaurus, except in Asia instead of North America. Not only does Yangchuanosaurus have a similar morphology to Allosaurus it also had access to similar prey items such as stegosaurs and sauropods. Yangchuanosaurus had a characteristic growth on top of its nose as well as smaller horns and ridges, and also possessed a tail that made up half of its total length.
The first specimen of Yangchuanosaurus was discovered by a construction worker working upon the construction of a dam in Sichuan Province.This revealed an eight meter individual. In 1983 however the second specimen and species of Yangchuanosaurus was discovered from the same formation, and this time the remains revealed an individual estimated to be just under eleven meters long.Another specimen, assigned to the new species Y. magnus, was even larger, with a skull length of 1.11 m (3.6 ft).It may have been up to 10.8 m (35.4 ft) long, and weighed as much as 4 metric tons.There was a bony ridge on its nose and multiple hornlets and ridges, similar to Ceratosaurus.
This gives Yangchuanosaurus another similarity to Allosaurus as it has an average length of eight meters with some individuals suggesting up to eleven meters as well.
Yangchuanosaurus was a large, powerful meat-eater. It walked on two large, muscular legs, had short arms, a strong, short neck, a big head with powerful jaws, and large, serrated teeth. It had a long, massive tail that was about half of its length. Its arms were short. The first digit of its foot was a small dewclaw. The three outer toes were used to bear weight and each was equipped with a large claw.
Acrocanthosaurus
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Acrocanthosaurus ("high-spined lizard") 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 Wyoming, 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 11.5 m (38 ft) in length, and weighing up to 6.2 tonnes (6.8 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 ornithopods.
Although slightly smaller than gigantic relatives like Giganotosaurus, Acrocanthosaurus was still among the largest theropods ever to exist. The largest known specimen (NCSM 14345) is estimated to have measured 11.5 m (38 ft)from snout to tail tip and weighed 5.7 t (6.3 short tons)to 6.2 t (6.8 short tons),though Bates et al. (2009) noted that a weight of 7 t (7.7 short tons) is not impossible for this specimen.Its skull alone was nearly 1.3 m (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. 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.
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 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 Allosauridae, but it was transferred to the taxonomic wastebasket Megalosauridae 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 African 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.
The holotype and paratype (OMNH 10146 and OMNH 10147), discovered in the early 1940s and 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.
The presence of Acrocanthosaurus in the Cloverly Formation was established in 2012 with the description of another partial skeleton, UM 20796. This 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.
Acrocanthosaurus may be known from less complete remains outside of Oklahoma, Texas, and Wyoming. 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;there is, however, some disagreement with this assessment regarding fossils from the Cloverly Formation.
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 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.
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 City,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 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.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.
Definite Acrocanthosaurus fossils have been found in the Twin Mountains Formation of northern Texas, the Antlers Formation of southern Oklahoma, and the Cloverly Formation of north-central Wyoming. 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, 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 Astrodonor 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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