Dilophosaurus wetherilli
Dilophosaurus wetherilli (Gr. "John Wetherill's two-crested lizard") is the only known species of the extinct genus Dilophosaurus a dilophosaurid theropod dinosaur that lived in the early Jurassic period, approximately 200 to 190 million years ago, between the Sinemurian and Pliensbachian, in what is now North America. The species Dilophosaurus sinensis, previously considered to belong to this genus, has been reclassified as belonging to the more modern synonym of the genus Sinosaurus, as S. sinensis.
At about 7 meters in length, with a weight of about 400 kilograms, Dilophosaurus was one of the first large predatory dinosaurs, although smaller than some later theropods. He was slim and lightly built, and the skull was proportionately large but delicate. The muzzle was narrow and the upper jaw had a gap from the nostril. It had a pair of longitudinal plate-like ridges on its skull, similar to a twin-crested cassowary. The jaw was thin and delicate at the front, but deep at the back. The teeth were long, curved, thin, and compressed to the sides. Those in the lower jaw were much smaller than those in the upper jaw. Most of the teeth had teeth on their front and back edges. The neck was long, and its vertebrae were hollow, and very light. The arms were powerful, with a long, slender arm bone. The hands had four fingers: the first was short but strong and had a large claw, the next two fingers were longer and thinner with smaller claws, and the fourth was vestigial. The thigh bone was massive, the feet strong, and the toes with large claws.
Paleontologists considered that Dilophosaurus could be a basal genus that generated clades such as ceratosaurs and tetanurans. Other paleontologists think, on the contrary, that in reality this genus is nothing more than a large coelophysid. The first specimens were described in 1954, but it was not until a decade later that the genus received its current name. Dilophosaurus is one of the oldest known Jurassic theropods, but also one of the least understood. Today it is considered a member of the family Dilophosauridae along with Dracovenator, a group located between the coelophysids and later theropods. Dilophosaurus would have been active and bipedal, and may have hunted large animals and may have also fed on smaller animals and fish. The function of the crests is unknown, they were too weak for battle, but they may have been used in display, such as species recognition and sexual selection. It may have grown rapidly, reaching a growth rate of 30 to 35 kilograms per year at the beginning of its life. The holotype specimen had multiple paleopathologies, including healed lesions and signs of a developmental abnormality. Dilophosaurus is known from the Kayenta Formation, and lived alongside dinosaurs such as Megapnosaurus and Sarahsaurus.
The Dilophosaurus has been represented on different occasions in popular culture. Its most famous appearance was in the 1993 film Jurassic Park. Although with brilliant special effects, this film depicts Dilophosaurus much smaller than it actually was, In addition, the film credits it with the ability to deploy an extendable collar, like the agamid lizard Chlamydosaurus kingii, and also the ability to project venom onto its prey. The ability to project poison was already present in the description of this species in the novel on which the film is based. These last two characteristics come from the imagination of the authors, since there is no way to verify that this dinosaur had an extendable collar (only for the film) or the ability to spit poison. It was designated as the Connecticut State Dinosaur in 2017.
Description
Dilophosaurus was one of the first large predatory dinosaurs, a medium-sized theropod, though small compared to some of the later theropods. Slender and lightly built, its size was comparable to that of a brown bear. The largest known specimen weighed about 400 kilograms, was about 7 meters long, and its skull was 590 millimeters long. The holotype specimen is smaller, weighing approximately 283 kilograms, 6.03 meters long, with a hip height of approximately 1.36 meters, and its skull was 523 millimeters long. Some researchers have interpreted a resting imprint of a theropod similar to Dilophosaurus and Liliensternus shows feather impressions around the belly and feet, similar to down. Other researchers interpret these impressions as sedimentological artifacts created as they the dinosaur moved, although this interpretation does not rule out that the track maker had feathers.
The most distinctive feature of Dilophosaurus is the pair of rounded crests on its skull, possibly used for display. In 1997, Dodson noted that cranial crests appeared earlier in Dilophosaurus than in other types of theropods. Studies by Robert Gay show that there may have been variation in size between the sexes, but they say nothing about the crest itself. The teeth of Dilophosaurus > are long, but have a fairly small base and broaden basally. Another skull feature was a notch behind the first row of teeth, giving Dilophosaurus an almost crocodile-like appearance, similar to the supposedly piscivorous spinosaurid dinosaurs. This "diastema" it existed due to a weak connection between the premaxillary and maxillary bones of the skull. This conformation led to the early hypothesis that Dilophosaurus was a scavenger, with the front teeth being too weak to bring down and support large prey.
Axial skeleton
The Dilophosaurus had 10 cervical vertebrae in the neck, 14 dorsals in the back, and 45 caudals in the tail. It had a long neck, which was probably flexed almost 90° at the skull and at the shoulder, holding the skull in a horizontal posture. The cervical vertebrae were unusually light; their centers, the "bodies" of the vertebrae, they were hollowed out by pleurocoelos, depressions on the sides and centrocoelos, cavities on the inside. The arches of the cervical vertebrae also had chonoses, conical gaps so large that the bones separating them were sometimes paper-thin. The center was flat-concave, flat to slightly convex at the front and deeply hollowed or concave at the back, similar to Ceratosaurus. This indicates that the neck was flexible, although it had overlapping cervical ribs that were fused to the center. The cervical ribs were thin and may have easily bent. The atlas bone, the first cervical vertebra to attach to the skull, had a small, cuboidal center and had a concavity in front where it formed a cup for the occipital condyle, a bulge that connects to the atlas vertebra, at the back of the skull. skull. The axis bone, the second cervical vertebra, had a heavy spine, and its postzygopophyses, the processes of vertebrae that articulated with subsequent vertebrae, met with long prezygapophyses that curved upward from the third cervical vertebra. The centers and spines of the cervical vertebrae were long and low, and the spines had caps that gave the appearance of a Maltese cross, cruciform, when viewed from above, a distinctive feature of this dinosaur. The neural spines of the dorsal vertebrae were also low and expanded back and forth, forming large attachment sites for ligaments. The sacral vertebrae that ran the length of the ilium that did not appear to be fused. The center of the caudal vertebrae was very consistent in length, but their diameter became smaller towards the back, and they changed from elliptical to circular in cross section.
Members
The scapulae were of moderate length and concave on their inner sides to follow the curvature of the body. The shoulder blades were wide, particularly the upper part, which was rectangular or square, a unique feature. The coracoids were elliptical and did not fuse with the scapulae. The arms were powerful and had deep pits and strong processes for the attachment of muscles and ligaments. The humerus, the arm bone, was large and thin, with strong epipodials, and the ulna, the forearm bone, was thick and straight, with a strong olecranon. The hands had four fingers, the first one was shorter but stronger than the next two fingers, with a large claw, and the next two fingers were longer and thinner, with smaller claws. The third finger was reduced, and the fourth was vestigial, retained, but without function. The crest of the ilium was higher above the iliac peduncle, the downward process of the ilium, and its outer side was concave. The foot of the pubic bone was slightly expanded, while the lower end was much more expanded in the ischium, which also had a very thin shaft. The hind legs were large, with a femur, thigh bone, longer than the tibia, bone of the lower part of the leg, the opposite of, for example, Coelophysis. The femur was massive, its shaft was sigmoid in shape, curved like an S and its greater trochanter was centered on the shaft. The tibia had a developed tuberosity and was expanded at the lower end. The talus bone, the ankle bone, was separated from the tibia and calcaneus, and formed half of the socket for the fibula. It had long, stout feet with three well-developed toes that had large claws. The third toe was the strongest, and the smallest first toe, the hallux, was kept off the ground.
Skull
The Dilophosaurus skull was large in proportion to the overall skeleton, but delicate. The muzzle was narrow when viewed from the front, narrowing towards the rounded top. The premaxilla, frontal bone of the upper jaw, was long and low when viewed from the side, and its external surface became less convex from the snout to the nostril, nostril. The nostrils were placed further back than in most other theropods. The premaxilla was loosely attached to the maxilla, the next bone in the upper jaw, only connected to the middle of the palate, with no connection on the side. Downward, the premaxilla formed a wall for a space between it and the maxilla called the subnarial space, also called the 'fold'. Such a gap is also present in coelophysoids. The subnarial gap resulted in a diastema, a gap in the tooth row, which has also been called a 'notch'. Within the subnarial gap was a deep excavation behind the premaxillary teeth, called the subnarial fossa, which was walled off by a keel down the premaxilla. The external surface of the premaxilla was covered with foramina of different sizes. The upper part of the two rearwardly extending premaxillary processes was long and low and formed most of the upper edge of the elongated nostrils. It tucked inward, making the area concave in its profile. The lower part of the premaxilla that contains the dental alveoli was oval. The maxilla was shallow, and was depressed around the antorbital fenestra, a large opening in front of the eye, forming a rounded hollow towards the front and smoother than the rest of the maxilla. A foramen called the preantorbital fenestra opened into this gap in the forward curve. Large foramina ran down the side of the maxilla, above the alveoli. A deep nutrient groove ran backward from the subnarial fossa along the base of the interdental plates of the maxilla.
The Dilophosaurus had a pair of tall, thin, and arcuate or plate-shaped ridges longitudinally on the roof of the skull. The ridges were formed significantly by the lacrimal bones and partially by the nasal bones. The crests extended towards the roof of the skull and gave the appearance of a twin-crested cassowary. As a distinctive feature, each crest also had a finger-like backward projection. The upper surface of the nasal bone between the ridges was concave, with the nasal part of the ridge overlapping the lacrimal part. As only one specimen preserves the shape of the crests, it is unknown if they differ in other individuals. The lacrimal bone had a single thickened upper border, where it formed the upper border at the posterior part of the anterorbital fenestra. The prefrontal bone formed the roof of the eye socket and had an L shaped bar that made part of the upper surface of the socket concave. The orbit was oval, and narrow towards the bottom. The jugal bone had two upwardly pointing processes, the first of which formed part of the lower margin of the anterorbital fenestra and part of the lower margin of the orbit. A projection of the quadrate bone into the lateral temporal fenestra, the opening behind the eye, gave the eye a kidney-shaped, kidney-shaped outline. The foramen magnum, the large opening at the back of the braincase, was about half the width of the occipital condyle, which was cordate, heart-shaped, and had a short neck and a groove on the side.
Jaw and teeth
The mandible was thin and delicate in front, but the articular region, where it connected to the skull, was strong, and the mandible was deep around the mandibular fenestra, an opening in its side. The mandibular fenestra was small in Dilophosaurus, compared to that of coelophysoids. The retroarticular process of the mandible, a backward projection, was long, and the surangular platform was strongly horizontal. The dentary bone, the front part of the jaw, where most of the teeth met, had a curved rather than pointed chin. The chin had a large foramen at the tip, and a row of small holes ran roughly parallel to the upper edge of the dentary. On the inner side, the mandibular symphysis, where the two halves of the lower jaw connected, was flat and smooth, showing no sign of being fused with its opposite half. A Meckelian foramen ran along the outer side of the dentary.
The Dilophosaurus had four teeth in each premaxilla, twelve in each maxilla, and seventeen in each dentary. The teeth were generally long, slender, and recurved, with relatively small bases. They were compressed on the side, oval in cross section at the base, lenticular, lens-shaped, above, and slightly concave on their outer and inner sides. The largest tooth in the maxilla was either in or near the fourth alveolus, and the height of the dental crowns decreased in the posterior areas. The first tooth of the maxilla pointed slightly forward from its socket because the lower border of the prexamillary process, which projected into the maxilla, turned upward. The teeth of the dentary were much smaller than those of the maxilla. The third or fourth tooth in the Dilophosaurus dentary and some coelophysoids were the largest and appear to have fitted into the subnarial gap of the upper jaw. Most of the teeth had serrations on the anterior and posterior edges, which were offset by vertical grooves, and were smaller at the front. There were 31-41 denticles on the front edges, and 29-33 on the back. At least the second and third teeth of the premaxilla had denticles, but the fourth tooth did not. The teeth were covered in a thin layer of enamel, 0.1 to 0.15 mm thick, which extended beyond their bases. The alveoli were elliptical to nearly circular, and all were larger than the bases of the teeth they contained, so they could have been freely held in the jaws. Although the number of alveoli in the dentary seems to indicate that the teeth were very close together, they were quite far apart, due to the larger size of their alveoli. The jaws contained replacement teeth at various stages of eruption. The interdental plaques between the teeth were very low.
Discovery and research
The first specimens of Dilophosaurus wetherilli were described in 1954, but it was not until a decade later that the genus received its current name. It is known from the Kayenta Formation, and lived alongside dinosaurs such as Megapnosaurus and Sarahsaurus among others during the early Jurassic, approximately 193 million years ago.
First findings
In the summer of 1942, American paleontologist Charles L. Camp led a University of California Museum of Paleontology (UCMP) field expedition searching for fossil vertebrates in Navajo County in northern Arizona. Word of this spread among the Navajo Native Americans there, and Jesse Williams led three members of the expedition to some fossil bones he had discovered in 1940. The area was part of the Kayenta formation, about 20 miles north of Cameron near the town of Tuba on the Navajo Indian reservation. Three dinosaur skeletons were found in a purplish shale, arranged in a triangle, about 9.1 meters long per side. The first was almost complete, missing only the front of the skull, parts of the pelvis, and some vertebrae. The second was heavily eroded, including the front of the skull, the lower jaws, some vertebrae, limb bones, and a jointed hand. The third was so eroded that it only consisted of fragments of vertebrae. The first good skeleton was encased in a block of plaster after ten days of work and loaded onto a truck, the second skeleton was easily picked up as it was almost entirely weathered out of the ground, but the third skeleton was almost lost.
The first nearly complete specimen was cleaned and mounted at the UCMP under the supervision of American paleontologist Wann Langston Jr., a process that took three men two years to complete. The skeleton was mounted on the wall in bas-relief, with the tail curved upwards, the neck straightened, and the left leg turned up for visibility, but the remainder of the skeleton remained in its burial position. Since the skull was crushed, it was reconstructed based on the back of the skull of the first specimen and the front of the second. The pelvis was reconstructed like that of Allosaurus, and the feet were also reconstructed. At the time, it was one of the best-preserved skeletons of a theropod dinosaur, albeit incomplete. In 1954, American paleontologist Samuel P. Welles, who was part of the group that excavated the skeletons, preliminarily described and named this dinosaur as a new species in the existing genus Megalosaurus, M. wetherilli. The nearly complete specimen, cataloged as UCMP 37302, became the holotype, and the second specimen, UCMP 37303, was included in the hypodigmen, the sample of specimens that defines a taxon, of the species. The specific name honored John Wetherill, a Navajo alderman whom Welles described as an "explorer, friend of scientists, and trusted trader." It was Wetherill's nephew Milton who first informed the expedition of the fossils. Welles placed the new species in Megalosaurus due to the similar proportions of the members with M. bucklandii , and because he found no major differences between them. At that time, Megalosaurus was used as a "mixed-box taxon," into which many theropod species were placed, regardless of age or locality.
1964 specimen
Welles returned to the city of Tuba in 1964 to determine the age of the Kayenta Formation, it had been suggested to be Late Triassic in age, while Welles thought it to be Early or Middle Jurassic, and discovered another skeleton at 402, 3 kilometers south of where the 1942 specimens were found. The nearly complete specimen, cataloged as UCMP 77270 was collected with the help of William Breed of the Northern Arizona Museum and others. During preparation of this specimen, it became clear that it was a larger individual than M. wetherilli and would have had two crests on top of its skull. Being a thin plate of bone, a crest was originally believed to have formed part of the missing left side of the skull that had been knocked out of position by a scavenger. When it became apparent that it was a crest, it was also discovered that there would be a corresponding crest on the left side, since the right crest was to the right of the midline, and was concave along its median length. This discovery led to re-examination of the holotype specimen, which was found to have bases of two thin, upwardly extending bones that broke together. These also represented the ridges, but had previously been assumed to be part of a misplaced cheekbone. It was also concluded that the two 1942 specimens were juveniles, while the 1964 specimen was an adult, about a third larger than the others.
Subsequently, Welles and an assistant corrected the wall mount of the holotype based on the new skeleton, restoring the crests, remaking the pelvis, lengthening the neck ribs, and bringing them closer together. After studying the skeletons of North American and European theropods, Welles realized that the dinosaur did not belong to Megalosaurus and needed a new genus name. At this time, no other theropods with large longitudinal crests on their heads were known, and the dinosaur had gained the interest of paleontologists. A cast of the holotype was made and fiberglass casts distributed at various exhibits. To make it easier to label these casts, Welles decided to name the new genus in a short note, rather than wait until the publication of a detailed description. In 1970 Welles coined the new genus name Dilophosaurus, from the Greek words δι', di, meaning "two& #34;, λόφος, lophos meaning "ridge" and σαυρος sauros meaning "lizard" , "two-crested lizard". Welles published a detailed osteological description of Dilophosaurus in 1984, but the 1964 specimen has yet to be adequately described. Dilophosaurus was the first known theropod from the Early Jurassic and remains one of the best preserved examples of that age.
New Findings
In 2001, the American paleontologist Robert J. Gay identified the remains of at least three new Dilophosaurus specimens, this number is based on the presence of three pubic bone fragments and two large femurs. different in the collections of the Museum of Northern Arizona. The specimens were found in 1978 in the Rock Head quadrangle, 190 kilometers from where the original specimens were found, and had been labeled a 'large theropod'. Although most of the material is damaged, it is significant by including items not preserved in earlier samples, including part of the pelvis and several ribs. Some items in the collection belonged to an infant specimen MNA P1.3181, the youngest known example of this genus, and one of the oldest known infant theropods from North America, preceded only by a few specimens. of Coelophysis. The juvenile sample includes a partial humerus, a partial fibula, and a tooth fragment. In 2005, American paleontologist Ronald S. Tykoski assigned a specimen, TMM 43646-140, from Gold Spring, Arizona, to Dilophosaurus, but in 2012 American paleontologist Matthew T. Carrano and colleagues found that it differed in some details.
Related iconotaxa
Several ichnotaxa taxa based on fossil tracks have been attributed to Dilophosaurus or similar theropods. In 1971 Welles reported dinosaur tracks from the Kayenta Formation of northern Arizona, in two levels 14 meters and 112 meters below where the original Dilophosaurus specimens were found. The lowest tracks were tridactyl, with three fingers, and could have been made by Dilophosaurus. Welles created the new ichnogenus and ichnospecies, Dilophosauripus williamsi based on them, naming them after Williams, the discoverer of the first Dilophosaurus skeletons. The type specimen is a plaster cast of a large impression cataloged as UCMP 79690-4, with casts of three other impressions included on it. In 1984, Welles admitted that there was no way to prove or refute that the tracks belonged to Dilophosaurus. theropod. They noted that it could have been made by a very large Dilophosaurus individual, but found this unlikely, as they estimated that the animal that would have made them would have been between 2.83 and 2.99 meters tall at hips, compared to 1.50 to 1.75 meters for Dilophosaurus.
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Polish paleontologist Gerard Gierliński examined tridactyl footprints from the Holy Cross Mountains in Poland and concluded in 1991 that they belonged to a theropod similar to Dilophosaurus. He named the new ichnospecies Grallator (Eubrontes) soltykovensis based on them, with a footprint cast MGIW 1560.11.12 as the holotype. Tracks of an unidentified species of Dilophosaurus have been found in Vallåkra, Sweden. Some of the tracks were taken to museums, but most of these disappeared in natural flooding. In 1994, Gierlinski and Ahlberg assigned these tracks to the Formation Hoganas of Sweden to Dilophosaurus. In 1994, Gierliński also assigned the tracks of the Höganäs Formation in Sweden discovered in 1974 to G. (E.) soltykovensis. In 1996, Gierliński attributed track AC 1/7 to the Turners Falls Formation of Massachusetts, a quiescent trail that he believed showed impressions of feathers, a theropod similar to Dilophosaurus or Liliensternus, and assigned to the ichnotaxon Grallator minisculus. Specimen AC 1/7 represents a "impression of a sitting dinosaur', which was made when a dinosaur was resting by resting its body on the ground, leaving an impression of its belly between a pair of footprints. Interestingly, traces of a fur-like covering can be seen, which Gierlinski interpreted as feather markings, suggesting that Dilophosaurus was a feathered dinosaur. Czech paleontologist Martin Kundrát agreed. that the footprint showed feather impressions in 2004, but this interpretation was disputed by American paleontologists Martin Lockley and colleagues in 2003 and Anthony J. Martin and colleagues in 2004, who considered them to be sedimentological artifacts. Martin and his colleagues also reassigned the track to the ichnotaxon Fulicopus lyellii .
The American paleontologist Robert E. Weems proposed in 2003 that the footprints of Eubrontes were not produced by a theropod but by a sauropodomorph similar to Plateosaurus, excluding Dilophosaurus as a possible track maker. Instead, Weems proposed Kayentapus hopii, another ichnotaxon named by Welles in 1971, as the best match for Dilophosaurus. Attribution to Dilophosaurus relied primarily on the large angle between the impressions of digits three and four shown by these clues, and the observation that the foot of the holotype shows a similarly extended fourth digit. Also in 2003, the American paleontologist Emma Rainforth argued that the separation in the foot holotype was simply the result of distortion, and that Eubrontes would be a good match for Dilophosaurus..
In 2006, Weems defended his 2003 assessment of Eubrontes, proposing an animal such as Dilophosaurus as a possible maker of numerous tracks of Kayentapus from the Culpeper Quarry in Virginia. Weems suggested that the rounded impressions associated with some of these traces represent hand prints lacking digit traces, which he interpreted as a trace of quadrupedal movement. In 2009, American paleontologists Andrew R. C. Milner and colleagues used the new combination Kayentapus soltykovensis, and suggested that Dilophosauripus may not be different from Eubrontes and Kayentapus. They suggested that the long claw marks that were used to distinguish Dilophosauripus may be a dragging artifact. They discovered that the tracks of Gigandipus and Anchisauripus may also represent variations of Eubrontes. They noted that differences between ichnotaxa may reflect how the cueing animal interacted with the substrate rather than taxonomy. They also found Dilophosaurus suitable for a track pathway assigned to Eubrontes of displacement and rest marks cataloged under number SGDS.18.T1 of the St. George dinosaur site in the Moenavede Formation, Utah, although the dinosaur itself is not known from the formation, which is slightly older than the Kayenta Formation.
Previously assigned species
Dilophosaurus "breedorum"
In 1984 Welles suggested that the 1964 specimen, UCMP 77270 did not belong to Dilophosaurus, but to a new genus, based on differences in the skull, vertebrae, and the femur He maintained that both genera had crests, but that the exact shape of the crest was unknown in Dilophosaurus. Welles died in 1997, before he could name this putative new dinosaur, but the idea that the two were separate genera has generally been ignored or forgotten ever since. In 1999, American amateur paleontologist Stephan Pickering privately published the new name Dilophosaurus "breedorum" based on the 1964 specimen, named after Breed, who had helped collect it. This name is considered invalid as it does not meet the requirements and Gay noted in 2005 that there are no significant differences between D. "breedorum" and other specimens of D. wetherilli Carrano and his colleagues found differences between the 1964 specimen and the holotype, but attributed them to variation between individuals rather than species.
Dilophosaurus sinensis
A nearly complete theropod skeleton, KMV 8701, was discovered in the Lufeng Formation, Yunnan Province, China, in 1987. It is similar to Dilophosaurus, with a pair of ridges and a space separating the premaxilla from the maxilla, but differs in some details. Chinese paleontologist Shaojin Hu named it as a new species of Dilophosaurus in 1993, D. sinensis , the specific epithet comes from the Greek Sinai , referring to China.In 1998, American paleontologist Matthew C. Lamanna and his colleagues found that D. sinensis was identical to Sinosaurus triassicus, a theropod from the same formation, named in 1940. This conclusion was confirmed by Chinese paleontologist Lida Xing and colleagues in 2013, and although the paleontologist Chinese Guo-Fu Wang and colleagues agreed that the species belonged to Sinosaurus in 2017, they suggested that it might be a separate species, S. sinensis.
Classification
Dilophosaurus has been examined several times over the years and has been assigned to no fewer than nine theropod groups. Welles in 1954 and most consecutive phylogenetic analyzes during the 1980s and 1990s classified this genus as a large coelophysoid within the taxon Coelophysoidea. However, more recent studies have provided varying degrees of evidence implying that Dilophosaurus and several other "dilophosaurids" they are more closely related to tetanuran theropods than to true coelophysoids.
Welles thought of Dilophosaurus as a megalosaurid in 1954, but revised his opinion in 1970 after discovering that it had crests. In 1974, Welles and American paleontologist Robert A. Long arrived at the concluded that Dilophosaurus was a ceratosaurian. theropods, based on body size, and suggested that this division was inaccurate. He found Dilophosaurus to be closest to the theropods that were generally placed in the family Halticosauridae, particularly Liliensternus .
In 1988, the American paleontologist Gregory S. Paul classified the Halticosaurids as a subfamily of the family Coelophysidae and suggested that Dilophosaurus might have been a direct descendant of Coelophysis. Paul also considered the possibility that spinosaurids were late-surviving dilophosaurids, based on the similarity of Baryonyx's narrow snout, nostril position, and slender teeth. In 1994, American paleontologist Thomas R. Holtz placed Dilophosaurus in the group Coelophysoidea, but separate from Coelophysidae. He placed the Coelophysoidea in the Ceratosauria group.In 2000, the American paleontologist James H. Madsen and Welles divided the Ceratosauria into the families Ceratosauridae and Dilophosauridae, with Dilophosaurus as the only member of the latter.
Lamanna and colleagues noted in 1998 that since Dilophosaurus was found to have crests on its skull and other similarly crested theropods have been discovered, including Sinosaurus, and that this feature is not exclusive to the genus, and of limited use in determining interrelationships within its group. Australian paleontologist Adam M. Yates described the new genus Dracovenator from South Africa in 2005 and found it closely related to Dilophosaurus and Zupaysaurus. His cladistic analysis suggested that they did not belong to the Coelophysoidea, but rather to the Neotheropoda, a more derived or "advanced" group. He proposed that if Dilophosaurus was more derived than the Coelophysoidea, the features it shared with this group may have been inherited from more basal or 'primitive' theropods, indicating that theropods may have passed through a "coelophysoid stage" in its early evolution.
In 2007, American paleontologist Nathan D. Smith and colleagues found the crested theropod Cryolophosaurus to be the sister taxon of Dilophosaurus and grouped them with Dracovenator. and Sinosaurus. This clade was more derived than the Coelophysoidea, but more basal than the Ceratosauria, placing the basal theropods in a ladder-like arrangement. In 2011 Sues and colleagues included Dilophosaurus in an analysis of early theropods where it appears as a sister taxon to the more advanced Jurassic theropods. In 2012 Carrano and colleagues found that the group of crested theropods proposed by Smith and colleagues was based on features that were related to the presence of such crests, but that the features of the rest of the skeleton were less consistent. Instead, they found Dilophosaurus to be a coelophysoid, with Cryolophosaurus and Sinosaurus being more derived, basal members of the Tetanurae group.
Belgian paleontologist Christophe Hendrickx and colleagues defined Dilophosauridae to include Dilophosaurus and Dracovenator in 2015, noting that while there is general uncertainty about the location of this group appears to be slightly more derived than the Coelophysoidea and the sister group of Averostra. Dilophosauridae share features with Coelophysoidea, such as the subnarial gap and forward-pointing maxillary front teeth, while features shared with Averostra include a fenestra on the front of the maxilla and a reduced number of teeth on the maxilla. They suggested that the cranial crests of Cryolophosaurus and Sinosaurus had evolved convergently, or were a feature inherited from a common ancestor.
Phylogeny
Below is a cladogram based on phylogenetic analysis by Sues et al. in 2011, showing the relationships of Dilophosaurus:
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The following cladogram is based on the one published by Hendrickx and colleagues, based on previous studies.
| Neotheropoda |
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Diagnosis
According to Rauhut in 2000, Dilophosaurus can be distinguished based on the following features, Lacrimal bone with a thickened dorso-posterior border, neurocervical spines with a distinctive "cusme" central and a "landing" anterior and posterior, scapulae with a squared distal expansion. Furthermore Carrano et al. in 2012 diagnosed Dilophosurus adding the following feature, paired nasolacrimal ridges extending vertically from the vault skull, each with a finger-like posterior projection.
Paleobiology
Food
The presence and distribution of non-interlocking sutures in the skull of some groups of reptiles, including Dilophosaurus, has been interpreted as evidence of the presence of a leveling system, driven by the jaw muscles., as an aid to predation. Welles in 1984 rejected this hypothesis and interpreted the potential mobility of the skull of Dilophosaurus as a sign of weakness, and stated that the weak connection of the premaxilla precluded the capture and submission of a prey. This led to the idea that Dilophosaurus was a scavenger that ate exclusively carcasses, as its teeth were too brittle to bring down large prey.
Welles discovered that Dilophosaurus did not have a powerful bite, due to weakness caused by the subnarial gap. He thought it used its front premaxillary teeth for pointing and tearing rather than biting, and its rearmost upper teeth for piercing and cutting. He thought that it was probably a scavenger rather than a predator and that if it killed large animals, it would have done so with its hands and feet instead of its jaws. Welles found no evidence of cranial kinesis in the Dilophosaurus skull, a feature that allows the individual bones of the skull to move relative to one another.
In 1986, American paleontologist Robert T. Bakker discovered that Dilophosaurus, with its enormous neck and skull and large upper teeth, had been adapted to kill large prey and strong enough to attack to any herbivore from the Lower Jurassic. In 1988, Paul dismissed the idea that Dilophosaurus was a scavenger and claimed that strictly scavenging land animals are a myth. He claimed that the snout of Dilophosaurus was better armed than previously thought, and that the very large and slender maxillary teeth were more lethal than the claws. Paul suggested that it hunted large animals such as prosauropods, and that it was better able to kill small animals than other theropods of a similar size. Paul also described Dilophosaurus bouncing on its tail as it lunged at an enemy, similar to a kangaroo.
Milner and James I. Kirkland suggested in 2007 that Dilophosaurus had features that indicate it may have eaten fish. They noted that the ends of the jaws expanded to the sides, forming a "rosette" interlocking teeth, similar to those of spinosaurids, known to have eaten fish, and gharials, which eat most fish. The nasal openings were also retracted in the jaws, similar to spinosaurids, which have even more retracted nasal openings, and this may have limited water splashing into the nostrils during fishing. Both groups also had long arms with well-developed claws, which could help catch fish. Dixie Lake, a large lake stretching from Utah to Arizona to Nevada, would have provided abundant fish in the more biologically depleted 'post-catalcismic' world that followed the Triassic-Jurassic extinction event.
Growth
Welles originally interpreted the smaller Dilophosaurus specimens as juveniles, and the larger specimen as an adult, later interpreting them as different species. Paul suggested that the differences between the specimens might be due to due to sexual dimorphism, as was apparently also evident in Coelophysis, which had "robust" and "gracile" of the same size, which might otherwise have been considered as separate species. Following this scheme, the smallest specimen of Dilophosaurus would represent a "gracile" example.
In 2005 Tykoski discovered that most known Dilophosaurus specimens were juveniles, with only the largest adult, based on the level of ossification of the growth plates of the bones. 2005, Gay found no evidence of sexual dimorphism suggested by Paul, but supposedly present in Coelophysis and attributed the observed variation among Dilophosaurus specimens to individual variation and ontogeny, changes during growth. There was no dimorphism in the skeletons, but he did not rule out that there might have been in the crests, more data was needed to determine this. Based on the small nasal crests in a juvenile specimen, Yates had tentatively assigned the related genus Dracovenator, suggested that these would have grown larger when the animal became an adult.
American paleontologist Joe S. Tkach reported a histological study, a microscopic study of internal features, of Dilophosaurus in 1996, performed by taking thin sections of long bones and ribs from the UCMP specimen 37303, the less preserved of the two original skeletons. The bone tissues were well vascularized and had a fibrolamellar structure similar to that found in other theropods and the sauropodomorph Massospondylus. The plexiform structure of the bones suggests rapid growth, and Dilophosaurus may have reached a growth rate of 30 to 35 kilograms per year early in life.
Welles discovered that the replacement teeth of Dilophosaurus and other theropods originated inside bone, decreasing in size the further away they were from the alveolar rim. Usually there were two or three replacement teeth in the sockets, the youngest being a small, hollow crown. The replacement teeth emerged on the outer side of the old teeth. When a tooth approached the gum line, the inner wall between the interdental plates resorbed and formed a notch. When the new tooth emerged, it was moved out to center in the socket, and the notch closed.
Locomotion
Welles envisioned Dilophosaurus as an active, clearly bipedal animal, similar to an enlarged ostrich. He discovered that the forelimbs were powerful weapons, strong and flexible, and not used for locomotion. He noted that the hands were capable of grasping and cutting, of meeting and reaching two-thirds of the way up the neck. He proposed that in a sitting posture, the animal would rest on the large "foot"; of its ischium, as well as its tail and feet. In 1990 the American paleontologists Stephen and Sylvia Czerkas suggested that the weak pelvis of Dilophosaurus could have been an adaptation for an aquatic lifestyle, where the water would help support his weight, and he might have been an efficient swimmer. However, they found it doubtful that it would have been restricted to an aqueous environment due to the strength and proportions of its hind legs, which would have made it agile and agile during bipedal locomotion.
In 2005, American paleontologists Phil Senter and James H. Robins examined the range of motion in the forelimbs of Dilophosaurus and other theropods. They found that Dilophosaurus would have been able to move its humerus backward until it was nearly parallel to the scapula, but could not move forward in any more than vertical orientation. The elbow could approach full extension and flexion at a right angle, but not reach it completely. The fingers do not appear to have been voluntarily hyperextensible, able to extend backwards beyond their normal range, but may have been passively hyperextensible, to resist dislocation during violent movements of captured prey. A 2015 article by Senter and Robins gave recommendations on how to reconstruct forelimb posture in bipedal dinosaurs, based on examination of various taxa, including Dilophosaurus. The shoulder blades were held very horizontally, the resting orientation of the elbow would have been close to a right angle, and the orientation of the hand would not have deviated much from that of the lower arm.
The Dilophosauripus tracks reported by Welles in 1971 were all on the same level, and were described as a "chicken coop" of footprints, with a few forming a path. The footprints had been imprinted in mud, which allowed the feet to sink in at a distance of 5-10 centimeters. The impressions were sloppy, and the variable width of the finger prints indicates that mud had stuck to the feet. The prints varied according to differences in the substrate and the manner in which they were made. Sometimes the foot was planted directly, but often there was a backward or forward slide as the foot came down. The positions and angles of the fingers also varied considerably, indicating that they must have been quite flexible. The Dilophosauripus tracks had a displaced second toe with a thick base and very long straight claws that were in line with the axes of the toe pads. One of the prints was missing the claw of the second toe, perhaps due to injury. In 1984 Welles interpreted the fact that three individuals were found close together, and the presence of crisscrossing tracks nearby, as indications that Dilophosaurus traveled in groups. Gay agreed that they may have traveled in small groups, but noted that there was no direct evidence for this, and that flash floods could have picked up scattered bones from different individuals and having deposited them together.
Milner and colleagues examined the Utah track SGDS.18.T1 in 2009, which consists of typical tail-drag tracks and an unusual rest track, deposited in lake beach sandstone.. The path began with the animal first oriented roughly parallel with the shoreline, and then stopped by a berm with both feet parallel, after which it lowered its body, bringing its metatarsals and the callus around its ischium to the ground. This created "heels" symmetrical and circular impressions of the ischium. The part of the tail closest to the body was kept off the ground, while the far end of the body made contact with the ground. The fact that the animal rested on a slope is what allowed it to bring both hands closer to the ground near the feet. After resting, the dinosaur crawled forward and made new impressions with its feet, metatarsals, and sit bones, but not its hands. The right foot now stepped on the imprint of the right hand, and the second claw of the left foot made a drag mark from the first resting position to the next. After some time, the animal rose to its feet and moved forward, left foot first, and once fully upright, crossed the remainder of the exposed surface, leaving fine drag marks with the end of its tail.
Crouching is a rarely captured behavior of theropods, and SGDS.18.T1 is the only clue with unambiguous theropod hand prints, providing valuable information about how they used their forelimbs. The squatting posture was found to be very similar to that of modern birds, and shows that early theropods held their palms halfway toward each other. As such a posture evolved early in the lineage, it may have characterized all theropods. Theropods are often depicted with their palms facing down, but studies of their functional anatomy have shown that, like birds, they were incapable of pronating or supinating their arms, crossing the radius and ulna of the lower arm to rotate the hand.. The clue showed that the legs were held symmetrically with body weight distributed between the feet and metatarsals, which is also a feature seen in birds such as ratites. Milner and his colleagues also dismissed the idea that the minor track Kayentapus reported by Weems showed a palm print made by a quadrupedal walking theropod. Weems had proposed that the animal would have been able to move in a quadrupedal fashion when walking slowly, while the digits would have been habitually hyperextended so that only the palms touched the ground. Milner and his colleagues found that the inferred posture is unnecessary, and suggested that the track was made in a similar way to SGDS.18.T1 , but without leaving traces of the digits.
Crest function
Welles (1984) proposed that Dilophosaurus traveled in small groups, on the basis that several individuals have been found together. Gay (2001b) noted that there was no direct evidence for this and noted that “flooding suddents could have collected scattered and isolated material from different individuals and deposited them together in the same area.” The skull crests are considered to be ornamentation to be used in sexual displays. This trait makes sense in social or gregarious animals, where other members of the species can observe and interpret messages about sexual status.
Welles conceded that suggestions about the function of Dilophosaurus crests were conjectural, but thought that although the crests did not have grooves to indicate vascularity, they could have been used for thermoregulation. He also suggested that they might have been used for species recognition or ornamentation.The Czerkas noted that the crests could not have been used during battle, as their delicate structure would have been easily damaged. They suggested that they were a visual display for attracting a mate and even thermoregulation. In 1990, American paleontologist Walter P. Coombs stated that the crests may have been color-enhanced for display use.
In 2011, American paleontologists Kevin Padian and John R. Horner proposed that "alien structures" in dinosaurs in general, including crests, frills, horns, and cupules, they were used primarily for species recognition, dismissing other explanations as unsupported by evidence. They noted that too few specimens of theropods with cranial ornamentation, including Dilophosaurus, were known to statistically test their evolutionary role and whether they represented sexual dimorphism or sexual maturity.
In a response to Padian and Horner the same year, American paleontologists Rob J. Knell and Scott D. Sampson argued that species recognition was not unlikely as a secondary function of "alien structures" in dinosaurs, but that sexual selection, used in display or combat to compete for mates, was a more likely explanation, due to the high cost of developing them, and because such structures appear to be highly variable within species. In 2013 British paleontologists David E. Hone and Darren Naish criticized the "species recognition hypothesis" and argued that no extant animal uses such structures primarily for species recognition, and that Padian and Horner had ignored the possibility of mutual sexual selection, where both sexes are ornamented. Studies by Gay in 2005 show no indication that there would be sexual dimorphism in the Dilophosaurus skeleton, but without saying anything about the variation in the crests.
Paleopathology
A Dilophosaurus wetherilli specimen shows potential damage "due to injury or crushing" in a vertebra, and a possible abscess in a humerus. Another specimen of Dilophosaurus wetherilli also had an abnormally small left humerus compared to a very robust right arm, a possible example of "fluctuating asymmetry". Fluctuating asymmetry results from developmental disturbances and is more common in populations under stress and can therefore inform about the quality of a dinosaur's living conditions living under them.
A specimen of Dilophosaurus from the University of California Museum of Paleontology cataloged as 'UCMP 37302 exhibits eight different pathologies in its shoulder girdle and the bones of his arms, including three bone tumors, broken and then healed bones in both arms, misshapen digits and the inability to use one of the forelimbs. Although not known for certain, it is believed that many of the wounds on this specimen were acquired during the course of a fight or accident and that this theropod must have been in severe pain while healing. The animal's pathologies were documented in a study published in the PLOS One journal.
In a 2001 study by Bruce Rothschild and other paleontologists, 60 foot bones referred to as Dilophosaurus were examined for signs of stress fractures, but with negative results.
In 2016, Senter and Sara L. Juengst examined the paleopathologies of the holotype specimen and found that it exhibited the largest and most varied number of such diseases in the pectoral girdle and forelimb of any theropod dinosaur yet described, some of which which are not known in any other dinosaur. Only six other theropods are known with more than one paleopathology in the pectoral girdle and forelimbs. The holotype sample had eight affected bones, whereas no other theropod sample with more than four is known. On his left side he had a fractured scapula and radius and fibrous abscesses on the ulna and the outer phalanx bone of the thumb. On the right side she had a torsion of her humeral shaft, three bone tumors on her radius, a truncated articular surface of her third metacarpal bone, and deformities in the first phalanx bone of her third finger. This finger was permanently deformed and unable to flex. Deformities of the humerus and third finger may be due to osteodysplasia, which has not been previously reported in dinosaurs but is known in birds. This disease, which affects juvenile birds that have experienced malnutrition, can cause pain in one limb, making the birds prefer to use the other limb, which in turn results in torsion.
The number of traumatic events that led to these characteristics is not certain, and it is possible that they were all caused by a single event, such as crashing into a tree or rock during a fight with another animal, which may have caused puncture wounds. with their claws. Since all the wounds had healed, it is certain that the Dilophosaurus survived for a long time after these events, for months, perhaps years. The use of the forelimbs to capture prey must have been compromised during the healing process. Thus, the dinosaur could have endured a long period of fasting or subsisted on prey small enough to dispatch with its mouth and feet, or a forelimb. According to Senter and Juengst, the high degree of pain that the dinosaur may have experienced in multiple locations for long periods of time also shows that it was a resilient animal. They noted that paleopathologies in dinosaurs are underreported, and that although Welles thoroughly described the holotype, he mentioned only one of the pathologies they found.
Paleoecology
The remains of Dilophosaurus were recovered from the Kayenta Formation in northeastern Arizona. The Kayenta Formation is part of the Glen Canyon Group group that includes formations not only from northern Arizona but also from southeastern Utah, western Colorado, and northwestern New Mexico. This is mainly composed of two facies, one dominated by silty depositions and the other dominated by sandstones. The siltstone facies occurs in most of Arizona, while the sandstone facies occurs in areas of Arizona, Utah, Colorado, and New Mexico. The formation was deposited mainly by rivers, with the siltstone facies being the slowest part of the fluvial system. Deposition of the Kayenta Formation was terminated by the encroaching field of dunes that would become the Navajo Sandstone.
Definitive radiometric dating of the formation has not yet been performed, and available stratigraphic correlation has been based on a combination of radiometric dating of vertebrate fossils, magnetostratigraphy, and pollen evidence. The Kayenta Formation has been conjectured to have been deposited during the Sinemurian and Pliensbachian stages of the Lower Jurassic period, that is, between 196 to 183 million years ago, approximately.
The Kayenta Formation has produced a small but growing collection of organisms. Most of the fossils come from the limonite facies, and are generally of vertebrates. Other organisms include microbial or "algal" limestones, petrified wood, plant impressions, freshwater bivalves and snails, ostracods, and invertebrate footprints.
Vertebrates are known from both fossil body parts and footprints. Known vertebrates include hybodont sharks, indeterminate bony fishes, lungfishes, salamanders, the frog Prosalirus, the caecilian Eocaecilia, the turtle Kayentachelys, a sphenodont reptile, lizards, various primitive crocodylomorphs including Calsoyasuchus, Eopneumatosuchus, Kayentasuchus and Protosuchus, the pterosaur Rhamphinion, various theropods such as Dilophosaurus, Kayentavenator, "Syntarsus" kayentakatae, and the so-called "Shake N Bake theropod", the sauropodomorph Sarahsaurus, a heterodontosaurid, the armored dinosaurs Scelidosaurus and Scutellosaurus, the tritylodontid synapsids Dinnebiton, Kayentatherium, and Oligokyphus, morganucodontids, the possible true mammal Dinnetherium, and a mammal haramido. Most of these finds come from the vicinity of Gold Spring, Arizona. The other vertebrate fossils are made up of coprolites and the tracks of therapsids, lizard-like animals, and various types of dinosaurs.
Explorations in the 1970s and 1980s by James M. Clark, Farish Jenkins, and David E. Fastovsky and collection and analysis by William R. Downs have produced several vertebrate specimens. Several mass burial sites have been found in the Kayenta Formation, as well as the remains of three coelophysoid taxa of different sizes, which represent the most diverse fauna of these dinosaurs known.
Taphonomy
Welles described the taphonomy of the original specimens, the changes that occurred during their decay and fossilization. The skeleton of the holotype was found on its right side, and its head and neck were curved backwards, in the " death pose " in which the skeletons of dinosaurs are often found. This posture was thought to be opisthotonous, due to deadly spasms, at the time, but could instead have been the result of how a corpse was embedded in the sediments. The back was straight, and the last dorsal vertebrae were turned on their left sides. The caudal vertebrae extended irregularly from the pelvis, and the legs were jointed, with little displacement. Welles concluded that the specimens were buried at the site of death, without much transport, but that the holotype appears to have been disturbed by scavengers, indicated by the rotated dorsal vertebrae and crushed skull. Gay noted that the specimens found described in 2001 showed evidence of having been carried by a current. As none of the specimens are complete, they may have been transported some distance, or may have lain on the surface and weathered for some time before transport. They may have been carried by a flood, as indicated by the variety of animals found as fragments and broken bones.
In popular culture
In the movie Jurassic Park, Dilophosaurus has a retractable neck membrane similar to that of a chlamydosaurus, and has the ability to spit blinding venom into its eyes. to paralyze their victims. Dennis Nedry and Lewis Dodgson die at the hands of these creatures, after blinding them the Dilophosaurus took the opportunity to bite them to pieces, killing them. There is no evidence that the membrane or the ability to spit existed, which was acknowledged by M. Crichton as literary license. Steven Spielberg's film also reduced the size of Dilophosaurus, leaving it much smaller than it actually was. Derivatives of Jurassic Park included video games featuring the presence of Dilophosaurus, such as the simulation video games Jurassic Park: Operation Genesis and Jurassic World Evolution. The video games based on the sequels, The Lost World: Jurassic Park and Jurassic Park III, also featured Dilophosaurus. The Dilophosaurus of Jurassic Park was recognized as the "only serious departure from scientific veracity" in the film's creation book and as the "most fictionalized" of the movie's dinosaurs in a book about Stan Winston Studios, which created the animatronic. For the novel, Crichton invented the dinosaur's ability to spit venom, explaining how it was able to kill prey, despite its seemingly weak jaws. The art department added another feature, a hood folded against its neck that expanded and vibrated as the animal prepared to attack, similar to that of a chlamydosaurus. To avoid confusion with the Velociraptor shown in the film, Dilophosaurus is rendered as only 1.2 meters tall, rather than its supposed actual height of approximately 3.0 meters. Nicknamed "the spitter," the Dilophosaurus in the film was performed via puppet and required a full body with three interchangeable heads to produce the actions required by the script. The spread legs were also built for a scene where the dinosaur jumps. Unlike most of the other dinosaurs in the film, no CGI was used in showing the Dilophosaurus.
American geologist J. Bret Bennington noted that although Dilophosaurus did not have a frill and could not spit venom as in the film, its bite could have been venomous, as has been claimed for the dragon of Komodo. He found that adding poison to the dinosaur was no less permissible than giving its skin color, which is also unknown. If the dinosaur had a shuttlecock, there would be evidence of this in the bones, in the form of a rigid structure to hold the shuttlecock, or marks where the muscles used to move them were attached. He also added that if he had a flyer, he would not have used it to intimidate his food, but rather a competitor, he might have responded to a character in the film pulling a hood over his head. Welles himself was "delighted" after seeing Dilophosaurus in Jurassic Park and noted the inaccuracies but found minor points and enjoyed the film, and was pleased to find the dinosaur "an internationally known actor". Dennis Nedry and Lewis Dodgson died at the hands of these creatures, after blinding them the Dilophosaurus took the opportunity to bite them to pieces, killing them.
In 2017 Dilophosaurus was designated as the state dinosaur of the US state of Connecticut, to become official with the new state budget in 2019. Dilophosaurus was chosen because they were discovered tracks believed to have been made by a related dinosaur at Rocky Hill in 1966, during excavation for Interstate 91. The six tracks were assigned to the ichnospecies Eubrontes giganteus, which became the fossil State Park of Connecticut in 1991. The area in which they were found had been a Triassic lake and when the importance of the area was confirmed, the highway was rerouted, and the area was made into a state park called Dinosaur State Park. In 1981, a sculpture of Dilophosaurus, the first life-size reconstruction of this dinosaur, was donated to the park. Dilophosaurus was proposed as the state dinosaur of Arizona in 1998, but legislators suggested Sonorasaurus arguing that Dilophosaurus was not unique to Arizona. A compromise was later suggested that would recognize both dinosaurs, but the bill died when it was revealed that Dilophosaurus fossils had been taken without permission from the Navajo Reservation.
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