Dilophosaurus wetherilli.jpg

''Dilophosaurus'' ( ) is a genus of theropod dinosaurs that lived in what is now North America during the Early Jurassic, about 193 million years ago. Three skeletons were discovered in northern Arizona in 1940, and the two best preserved were collected in 1942. The most complete specimen became the holotype of a new species in the genus '' Megalosaurus'', named ''M. wetherilli'' by Samuel P. Welles in 1954. Welles found a larger skeleton belonging to the same species in 1964. Realizing it bore crests on its skull, he assigned the species to the new genus ''Dilophosaurus'' in 1970, as ''Dilophosaurus wetherilli''. The genus name means "two-crested lizard", and the species name honors John Wetherill, a Navajo councilor. Further specimens have since been found, including an infant. Footprints have also been attributed to the animal, including resting traces. Another species, ''Dilophosaurus sinensis'' from China, was named in 1993, but was later found to belong to the genus '' Sinosaurus''.

At about in length, with a weight of about , ''Dilophosaurus'' was one of the earliest large predatory dinosaurs and the largest known land-animal in North America at the time. It was slender and lightly built, and the skull was proportionally large, but delicate. The snout was narrow, and the upper jaw had a gap or kink below the nostril. It had a pair of longitudinal, arched crests on its skull; their complete shape is unknown, but they were probably enlarged by keratin. The mandible was slender and delicate at the front, but deep at the back. The teeth were long, curved, thin, and compressed sideways. Those in the lower jaw were much smaller than those of the upper jaw. Most of the teeth had serrations at their front and back edges. The neck was long, and its vertebrae were hollow, and very light. The arms were powerful, with a long and slender upper arm bone. The hands had four fingers; the first was short but strong and bore a large claw, the two following fingers were longer and slenderer with smaller claws; the fourth was vestigial. The thigh bone was massive, the feet were stout, and the toes bore large claws.

''Dilophosaurus'' has been considered a member of the family Dilophosauridae along with '' Dracovenator'', a group placed between the Coelophysidae and later theropods, but some researchers have not found support for this grouping. ''Dilophosaurus'' would have been active and bipedal, and may have hunted large animals; it could also have fed on smaller animals and fish. Due to the limited range of movement and shortness of the forelimbs, the mouth may instead have made first contact with prey. The function of the crests is unknown; they were too weak for battle, but may have been used in visual display, such as species recognition and sexual selection. It may have grown rapidly, attaining a growth rate of per year early in life. The holotype specimen had multiple paleopathologies, including healed injuries and signs of a developmental anomaly. ''Dilophosaurus'' is known from the Kayenta Formation, and lived alongside dinosaurs such as '' Scutellosaurus'' and '' Sarahsaurus''. ''Dilophosaurus'' was featured in the novel ''Jurassic Park'' and its movie adaptation, wherein it was given the fictional abilities to spit venom and expand a neck frill, as well as being smaller than the real animal. It was designated as the state dinosaur of Connecticut based on tracks found there.

History of discovery

In the summer of 1942, the paleontologist Charles L. Camp led a field party from the University of California Museum of Paleontology (UCMP) in search of fossil vertebrates in Navajo County in northern Arizona. Word of this was spread among the Native Americans there, and the Navajo Jesse Williams brought three members of the expedition to some fossil bones he had discovered in 1940. The area was part of the Kayenta Formation, about north of Cameron near Tuba City in the Navajo Indian Reservation. Three dinosaur skeletons were found in purplish shale, arranged in a triangle, about long at one side. The first was nearly complete, lacking only the front of the skull, parts of the pelvis, and some vertebrae. The second was very eroded, included the front of the skull, lower jaws, some vertebrae, limb bones, and an articulated hand. The third was so eroded that it consisted only of vertebral fragments. The first good skeleton was encased in a block of plaster after 10 days of work and loaded onto a truck, the second skeleton was easily collected, as it was almost entirely weathered out of the ground, but the third skeleton was almost gone.

The nearly complete first specimen was cleaned and mounted at the UCMP under supervision of the paleontologist Wann Langston, a process that took three men two years. The skeleton was wall-mounted in bas relief, with the tail curved upwards, the neck straightened, and the left leg moved up for visibility, but the rest of the skeleton was kept in its burial position. As 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 after that of '' Allosaurus'', and the feet were also reconstructed. At the time, it was one of the best-preserved skeletons of a theropod dinosaur, though incomplete. In 1954, the paleontologist Samuel P. Welles, who was part of the group who excavated the skeletons, preliminarily described and named this dinosaur as a new species in the existing genus '' Megalosaurus'', ''M. wetherilli''. The nearly complete specimen (catalogued as UCMP 37302) was made the holotype of the species, and the second specimen (UCMP 37303) was made the paratype. The specific name honored John Wetherill, a Navajo councilor whom Welles described as an "explorer, friend of scientists, and trusted trader". Wetherill's nephew, Milton, had first informed the expedition of the fossils. Welles placed the new species in ''Megalosaurus'' due to the similar limb proportions of it and ''M. bucklandii'', and because he did not find great differences between them. At the time, ''Megalosaurus'' was used as a " wastebasket taxon", wherein many species of theropods were placed, regardless of their age or locality.

Welles returned to Tuba City in 1964 to determine the age of the Kayenta Formation (it had been suggested to be Late Triassic in age, whereas Welles thought it was Early to Middle Jurassic), and discovered another skeleton about south of where the 1942 specimens had been found. The nearly complete specimen (catalogued as UCMP 77270) was collected with the help of William J. Breed of the Museum of Northern Arizona and others. During preparation of this specimen, it became clear that it was a larger individual of ''M. wetherilli'', and that it would have had two crests on the top of its skull. Being a thin plate of bone, one crest was originally thought to be part of the missing left side of the skull, which had been pulled out of its position by a scavenger. When it became apparent that it was a crest, it was also realized that a corresponding crest would have been on the left side, since the right crest was right of the midline, and was concave along its middle length. This discovery led to re-examination of the holotype specimen, which was found to have bases of two thin, upwards-extended bones, which were crushed together. These also represented crests, but they had formerly been assumed to be part of a misplaced cheek bone. The two 1942 specimens were also found to be juveniles, while the 1964 specimen was an adult, about one-third larger than the others. Welles later recalled that he thought the crests were as unexpected as finding "wings on a worm".

New genus and subsequent discoveries

Welles and an assistant subsequently corrected the wall mount of the holotype specimen based on the new skeleton, by restoring the crests, redoing the pelvis, making the neck ribs longer, and placing 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 that time, no other theropods with large longitudinal crests on their heads were known, and the dinosaur had therefore gained the interest of paleontologists. A mold of the holotype specimen was made, and fiberglass casts of it were distributed to various exhibits; to make labeling these casts easier, Welles decided to name the new genus in a brief note, rather than wait until the publication of a detailed description. In 1970, Welles coined the new genus name ''Dilophosaurus'', from the Greek words ''di'' (''δι'') meaning "two", ''lophos'' (''λόφος'') meaning "crest", and ''sauros'' () meaning "lizard": "two-crested lizard". Welles published a detailed osteological description of ''Dilophosaurus'' in 1984, but did not include the 1964 specimen, since he thought it belonged to a different genus. ''Dilophosaurus'' was the first well-known theropod from the Early Jurassic, and remains one of the best-preserved examples of that age.

In 2001, the 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 differentially sized femora) in the collections of the Museum of Northern Arizona. The specimens were found in 1978 in the Rock Head Quadrangle, away from where the original specimens were found, and had been labeled as a "large theropod". Though most of the material is damaged, it is significant in including elements not preserved in the earlier specimens, including part of the pelvis and several ribs. Some elements in the collection belonged to an infant specimen (MNA P1.3181), the youngest known example of this genus, and one of the earliest known infant theropods from North America, only preceded by some '' Coelophysis'' specimens. The juvenile specimen includes a partial humerus, a partial fibula, and a tooth fragment. In 2005, paleontologist Ronald S. Tykoski assigned a specimen (TMM 43646-140) from Gold Spring, Arizona, to ''Dilophosaurus'', but in 2012, paleontologist Matthew T. Carrano and colleagues found it to differ in some details.

In 2020, the paleontologists Adam D. Marsh and Timothy B. Rowe comprehensively redescribed ''Dilophosaurus'' based on the by then known specimens, including specimen UCMP 77270 which had remained undescribed since 1964. They also removed some previously assigned specimens, finding them too fragmentary to identify, and relocated the type quarry with the help of a relative of Jesse Williams. In an interview, Marsh called ''Dilophosaurus'' the "best worst-known dinosaur", since the animal was poorly understood despite having been discovered 80 years earlier. A major problem was that previous studies of the specimens did not make clear which parts were original fossils and which were reconstructed in plaster, yet subsequent researchers only had Welles' 1984 monograph to rely on for subsequent studies, muddling understanding of the dinosaur's anatomy. Marsh spent seven years studying the specimens to clarify the issues surrounding the dinosaur, including two specimens found two decades earlier by Rowe, his Ph.D. advisor.

Formerly assigned species

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 femora. He maintained that both genera bore crests, but that the exact shape of these was unknown in ''Dilophosaurus''. Welles died in 1997, before he could name this supposed new dinosaur, and the idea that the two were separate genera has generally been ignored or forgotten since. In 1999, amateur paleontologist Stephan Pickering privately published the new name ''Dilophosaurus'' "breedorum" based on the 1964 specimen, named in honor of Breed, who had assisted in collecting it. This name is considered a'' nomen nudum'', an invalidly published name, and Gay pointed out in 2005 that no significant differences exist between ''D''. "breedorum" and other ''D. wetherilli'' specimens. In 2012, Carrano and colleagues found differences between the 1964 specimen and the holotype specimen, but attributed them to variation between individuals rather than species. Paleontologists Christophe Hendrickx and Octávio Mateus suggested in 2014 that the known specimens might represent two species of ''Dilophosaurus'' based on different skull features and stratigraphic separation, pending thorough description of assigned specimens. Marsh and Rowe concluded in 2020 that there was only one taxon among known ''Dilophosaurus'' specimens, and that differences between them were due to their different degree of maturity and preservation. They did not find considerable stratigraphic separation between the specimens either.

A nearly complete theropod skeleton (KMV 8701) was discovered in the Lufeng Formation, in Yunnan Province, China, in 1987. It is similar to ''Dilophosaurus'', with a pair of crests and a gap separating the premaxilla from the maxilla, but differs in some details. The paleontologist Shaojin Hu named it as a new species of ''Dilophosaurus'' in 1993, ''D. sinensis'' (from Greek ''Sinai'', referring to China). In 1998, the paleontologist Matthew C. Lamanna and colleagues found ''D. sinensis'' to be identical to ''Sinosaurus triassicus'', a theropod from the same formation, named in 1940. This conclusion was confirmed by paleontologist Lida Xing and colleagues in 2013, and though paleontologist Guo-Fu Wang and colleagues agreed the species belonged in ''Sinosaurus'' in 2017, they suggested it may be a separate species, ''S. sinensis''.

Description

''Dilophosaurus'' was one of the earliest large predatory dinosaurs, a medium-sized theropod, though small compared to some of the later theropods. It was also the largest known land-animal of North America during the Early Jurassic. Slender and lightly built, its size was comparable to that of a brown bear. The largest known specimen weighed about , measured about in length, and its skull was long. The smaller holotype specimen weighed about , was long, with a hip height of about , and its skull was long. A resting trace of a theropod similar to ''Dilophosaurus'' and ''Liliensternus'' has been interpreted by some researchers as showing impressions of feathers around the belly and feet, similar to down. Other researchers instead interpret these impressions as sedimentological artifacts created as the dinosaur moved, though this interpretation does not rule out that the track-maker could have borne feathers.

Skull

The skull of ''Dilophosaurus'' was large in proportion to the overall skeleton, yet delicate. The snout was narrow in front view, becoming narrower towards the rounded top. The premaxilla (front bone of the upper jaw) was long and low when seen from the side, bulbous at the front, and its outer surface became less convex from snout to naris (bony nostril). The nostrils were placed further back than in most other theropods. The premaxillae were in close articulation with each other, and while the premaxilla only connected to the maxilla (the following bone of the upper jaw) at the middle of the palate, with no connection at the side, they formed a strong joint through the robust, interlocking articulation between the hindwards and forwards directed processes of these bones. Hindwards and below, the premaxilla formed a wall for a gap between itself and the maxilla called the subnarial gap (also termed a "kink"). Such a gap is also present in coelophysoids, as well as other dinosaurs. 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 toothrow of the premaxilla, called the subnarial pit, which was walled by a downwards keel of the premaxilla.

The outer surface of the premaxilla was covered in foramina (openings) of varying sizes. The upper of the two backward-extending processes of the premaxilla was long and low, and formed most of the upper border of the elongated naris. It had a dip towards the font, which made the area by its base concave in profile. The underside of the premaxilla containing the alveoli (tooth sockets) was oval. The maxilla was shallow, and was depressed around the antorbital fenestra (a large opening in front of the eye), forming a recess that was rounded towards the front, and smoother than the rest of the maxilla. A foramen called the preantorbital fenestra opened into this recess at the front bend. Large foramina ran on the side of the maxilla, above the alveoli. A deep nutrient groove ran backward from the subnarial pit along the base of the interdental plates (or rugosae) of the maxilla.

''Dilophosaurus'' bore a pair of high, thin, and arched (or plate-shaped) crests longitudinally on the skull roof. The crests (termed the nasolacrimal crests) began as low ridges on the premaxillae and were mainly formed by the upwards expanded and . These bones were coossified together (fusion during bone tissue formation), so the sutures between them cannot be determined. The lacrimal bone expanded into a thick, rugose boss, forming an arc at the upper front border of the orbit (eye socket), and supported the bottom of the back of the crest. Uniquely for this genus, the rim above the orbit continued hindwards and ended in a small, almost triangular process behind the orbit, which curved slightly outwards. Since only a short part of the upper surface of this process is unbroken, the rest of the crest may have risen above the skull over a distance of ~. The preserved part of the crest in UCMP 77270 is tallest around the midpoint of the antorbital fenestra's length. UCMP 77270 preserves the concave shelf between the bases of the crests, and when seen from the front, they projected upwards and to the sides at an ~80° angle. Welles found the crests reminiscent of a double-crested cassowary, while Marsh and Rowe stated they were probably covered in keratin or keratinized skin. They pointed out that by comparison with helmeted guineafowl, the keratin on the crests of ''Dilophosaurus'' could have enlarged them much more than what is indicated by the bone. As only one specimen preserves much of the crests, whether they differed between individuals is unknown. CT scans show that air sacs (pockets of air that provide strength for and lighten bones) were present in the bones that surrounded the brain, and were continuous with the sinus cavities in the front of the skull. The antorbital fenestra was continuous with the side of the crests, which indicates the crests also had air sacs (a ridge of bone forms a roof over the antorbital fenestrae in most other theropods).

The orbit was oval, and narrow towards the bottom. The jugal bone had two upwards pointing processes, the first of which formed part of the lower margin of the antorbital fenestra, and part of the lower margin of the orbit. A projection from the quadrate bone into the lateral temporal fenestra (opening behind the eye) gave this a reniform (kidney-shaped) outline. The foramen magnum (the large opening at the back of the braincase) was about half the breadth of the occipital condyle, which was itself cordiform (heart-shaped), and had a short neck and a groove on the side. The mandible was slender and delicate at the front, but the articular region (where it connected with the skull) was massive, and the mandible was deep around the mandibular fenestra (an opening on its side). The mandibular fenestra was small in ''Dilophosaurus'', compared to that of coelophysoids, and reduced from front to back, uniquely for this genus. The dentary bone (the front part of the mandible where most of the teeth there were attached) had an up-curved rather than pointed chin. The chin had a large foramen at the tip, and a row of small foramina ran in rough parallel with 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, and showed no sign of being fused with its opposite half. A Meckelian foramen

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