Allosaurus
Allosaurus is a genus of theropod dinosaur that lived 155 to 145 million years ago during the Late Jurassic period. The first fossil remains that could definitively be ascribed to this genus were described in 1877 by Othniel C. Marsh. The name "Allosaurus" means "different lizard", alluding to its lightweight, which Marsh believed were unique. The genus has a very complicated taxonomy and includes at least three valid species, the best known of which is A. fragilis. The bulk of Allosaurus remains come from North America's Morrison Formation, with material also known from the Alcobaça, Bombarral, and Lourinhã formations in Portugal. It was known for over half of the 20th century as Antrodemus, but a study of the abundant remains from the Cleveland-Lloyd Dinosaur Quarry returned the name "Allosaurus" to prominence. As one of the first well-known theropod dinosaurs, it has long attracted attention outside of paleontological circles.
Allosaurus was a large bipedal predator for its time. Its skull was light, robust, and equipped with dozens of sharp, serrated teeth. It averaged in length for A. fragilis, with the largest specimens estimated as being long. Relative to the large and powerful legs, its three-fingered hands were small and the body was balanced by a long, muscular tail. It is classified in the family Allosauridae. As the most abundant large predator of the Morrison Formation, Allosaurus was at the top of the food chain and probably preyed on large herbivorous dinosaurs such as ornithopods, stegosaurids, and sauropods. Scientists have debated whether Allosaurus had cooperative social behavior and hunted in packs or was a solitary predator that forms congregations, with evidence supporting either side.
History of discovery
Initial finds and naming of ''Allosaurus''
Allosaurus was discovered during the Bone Wars, a feud between two American paleontologists, Othniel Charles Marsh and Edward Drinker Cope, that led to a surge of fossil discoveries in the Western US. The first described fossil in the taxonomic history of Allosaurus was a bone obtained secondhand by Ferdinand V. Hayden in 1869. It came from Middle Park, near Granby, Colorado, probably from Morrison Formation rocks. The locals had identified such bones as "petrified horse hoofs". Hayden sent his specimen to Joseph Leidy, who identified it as half of a tail vertebra and tentatively assigned it to the European dinosaur genus Poekilopleuron as Poicilopleuron ''valens. He later decided it deserved its own genus, Antrodemus.Allosaurus itself is based on YPM 1930, a small collection of fragmentary bones including parts of three vertebrae, a rib fragment, a tooth, a toe bone, and the shaft of the right . Marsh gave these remains the name Allosaurus fragilis in 1877. Allosaurus comes from the Greek words allos/αλλος, meaning "strange" or "different", and sauros/σαυρος, meaning "lizard" or "reptile". Marsh chose the name 'different lizard' because he believed that the vertebrae were different from those of other dinosaurs due to their lightweight construction. The species epithet fragilis is Latin for "fragile", again referring to the lightening features in the vertebrae. The bones were uncovered by two of Marsh's collectors, Benjamin Mudge and Samuel W. Williston, in the autumn of 1877 at Felch Quarry, in the Garden Park area of Colorado. Marsh and his collectors were unsatisfied with the quality of the collected fossils, so he ordered to close the quarry that same autumn. Yet, Marsh named two new dinosaurs from these remains: Diplodocus and Allosaurus. In 1883, Marsh hired the original discoverer of the quarry, Marshall P. Felch, to continue excavations. Felch's subsequent discoveries made the quarry one of the prime sites of the Morrison, and included the holotype specimens of Ceratosaurus nasicornis, Stegosaurus stenops, and a mostly complete Allosaurus skeleton that would later be selected as the neotype specimen of Allosaurus fragilis.
In 1879, one of Cope's collectors, H. F. Hubbell, found a specimen in the Como Bluff area, but apparently did not mention its completeness and Cope never unpacked it. Upon unpacking it in 1903, it was found to be one of the most complete theropod specimens then known and the skeleton, now cataloged as AMNH 5753, was put on public view in 1908. This is the well-known mount poised over a partial Apatosaurus skeleton as if scavenging it, illustrated as such in a painting by Charles R. Knight. Although notable as the first free-standing mount of a theropod dinosaur and often illustrated and photographed, it has never been scientifically described.
In 1925, Werner Janensch described Allosaurus tendegurensis from the Tendeguru Beds of Tanzania. It was based on a sole tibia. A study from June 2012 by Carrano et. al. dismissed the remains A. tendegurensis'' as undiagnostic, rendering it an indeterminate Tetanuran theropod.
Renaming to ''Antrodemus'' and early discoveries at Dinosaur National Monument
The many names coined by Cope and Marsh complicated later research, with the situation further compounded by the terse descriptions they provided. Even at the time, authors such as Samuel W. Williston suggested that too many names had been coined. For example, Williston pointed out in 1901 that Marsh had never been able to adequately distinguish Allosaurus from Creosaurus. The most influential early attempt to sort out the situation was produced by Charles W. Gilmore in 1920. He came to the conclusion that the tail vertebra named Antrodemus by Leidy was indistinguishable from those of Allosaurus and that Antrodemus should be the preferred name because, as the older name, it had priority. Antrodemus became the accepted name for this familiar genus for over 50 years, until James H. Madsen published on the Cleveland-Lloyd specimens and concluded that Allosaurus should be used because Antrodemus was based on material with poor, if any, diagnostic features and locality information. For example, the geological formation that the single bone of Antrodemus came from is unknown.In 1909, Earl Douglass from the Carnegie Museum discovered what should later become Dinosaur National Monument in Utah. Until 2022, Douglass and his team excavated over of fossils of multiple dinosaur species from a single quarry, including several Allosaurus specimens. Among these finds is CM 11844, which was collected between 1913 and 1915 and comprises much of the skeleton and a fragmentary skull. Since 1938, this skeleton is on display at the Carnegie Museum. During the summer of 1924, the University of Utah uncovered DINO 2560, the best preserved Allosaurus specimen known at that time. The skull of this particularly large individual is on exhibit at the Dinosaur National Monument.
Cleveland-Lloyd discoveries and "Big Al"
Although sporadic work at what became known as Utah's Cleveland-Lloyd Dinosaur Quarry had taken place as early as 1927 and the fossil site itself was described by William L. Stokes in 1945, major operations did not begin there until 1960. Madsen led a cooperative effort between 1960 and 1965 involving nearly 40 institutions, during which thousands of bones were recovered from the site. The quarry is notable for the predominance of Allosaurus remains: the quarry preserves a minimum of 73 individual dinosaurs and at least 46 of those are A. fragilis. The great quantity of well-preserved Allosaurus remains has allowed this genus to be known in great detail, making it among the best-known of all theropods. Skeletal remains from the quarry pertain to individuals of almost all ages and sizes, from less than to long. Because its fossils are common at both this quarry and others in the state, Allosaurus was designated as the state fossil of Utah in 1988.In the early 1990s, a Swiss team led by Kirby Siber set out for commercial fossil excavations at Howe Ranch Quarry, Wyoming. This quarry had originally been worked on in 1934 by Barnum Brown and his crew, who collected more than 30 tons of bones, mostly of sauropods. Because the Swiss team could not locate additional specimens in the quarry, they explored the surrounding area, where they discovered "Big Al" in 1991: a 95% complete, partially articulated Allosaurus specimen. However, because the new site was located on public land, the excavation was taken over by a joint Museum of the Rockies and University of Wyoming Geological Museum team. The specimen, now on exhibit at the Museum of the Rockies, belonged to an individual of about in length. This was below the average size for Allosaurus, as it was a subadult estimated at only 87% grown. The Swiss team later excavated a second Allosaurus, "Big Al II", on private land on Howe Ranch, which is exhibited at the Aathal Dinosaur Museum in Switzerland.
In 1991, Brooks Britt argued that there were at least two species of Allosaurus: A robust species with a short and high skull and pointed lacrimal horns, and a more gracile species with a long and low skull and rounded lacrimal horns. The robust species is geologically younger from localities such as Dry Mesa Quarry and Garden Park, while the gracile species, found at the Cleveland-Lloyd and at Dinosaur National Monument, is older. Already in 1988, Gregory S. Paul made a similar distinction in a popular book, in which he referred to the gracile species as A. fragilis and to the robust species as A. atrox, using a species originally described by Marsh as Creosaurus atrox. However, a series of statistical analyses by David K. Smith between 1996 and 1999 suggested that the differences seen in the Morrison Formation material can be attributed to individual variation.