Adeopapposaurus

Adeopapposaurus (meaning "far eating lizard", in reference to its long neck) is a genus of prosauropod dinosaur from the Early Jurassic Cañón del Colorado Formation of San Juan, Argentina. It was similar to Massospondylus. Four partial skeletons with two partial skulls are known. The type specimen, PVSJ568, includes a skull and most of a skeleton to just past the hips. The form of the bones at the tips of the upper and lower jaws suggests it had keratinous beaks. The fossils now named Adeopapposaurus were first thought to represent South American examples of Massospondylus; while this is no longer the case, Adeopapposaurus is classified as a massospondylid. Adeopapposaurus was described in 2009 by Ricardo N. Martínez. The type species is A. mognai, referring to the Mogna locality where it was found.

Achillobator

Achillobator (pronounced /əˌkɪloʊˈbeɪtɔr/; "Achilles' warrior/hero") is a genus of dromaeosaurid theropod dinosaur from the late Cretaceous Period of what is now Mongolia, about 90 million years ago. It was probably an active bipedal predator, hunting with the large sickle-shaped claw on the second toe of each hind foot. It was a large dromaeosaurid, between 15 and 20 feet (4.6 to 6 meters) from snout to tail.

The generic name comes from Achilles, a famous ancient Greek warrior of the Trojan War, and the Mongolian word bator ("warrior" or "hero"). It refers to the large Achilles tendon needed to use the sickle claw on the foot, which was the major combat organ of dromaeosaurids. The one species is named A. giganticus because it is much larger than most other dromaeosaurids.

Discovery and species

The fossil remains of Achillobator were first discovered during a Mongolian and Russian field expedition in 1989, though it was not described and named until ten years later, in 1999,[1] by Mongolian paleontologist Altangerel Perle, and Americans Mark Norell and Jim Clark, although the description was not complete and was actually published without the knowledge of the latter two authors.[2]

Fossil bones of Achillobator were found mostly disarticulated, but associated, including a fragment of the upper jaw with teeth, as well as vertebrae from all sections of the spinal column, ribs, and bones from the shoulder, pelvis, forelimbs and hindlimbs. These remains were found in the Bayan Shireh Formation of Dornogovi Province, Mongolia, which dates to the Late Cretaceous epoch. The exact age is uncertain, with two competing hypotheses; based on comparisons with other formations, the Bayan Shireh fauna seems to correspond best with the Turonian through early Campanian stages of the Late Cretaceous, about 93 to 80 million years ago.[3] However, examination of the magnetostratigraphy of the formation seems to confirm that the entire Bayan Shireh lies within the Cretaceous Long Normal, which lasted only until the end of the Santonian stage, giving a possible Cenomanian through Santonian age, or between 98 and 83 million years ago.[4] Other dinosaurs found in the Bayan Shireh include Alectrosaurus, Segnosaurus, Talarurus, and Bactrosaurus.

Achelousaurus

Achelousaurus (typically pronounced /əˌkiːloʊˈsɔrəs/, though /ˌækɨˌloʊəˈsɔrəs/ might be a better reflection of the butchered etymology;[1] "Achelous's lizard") is a genus of centrosaurine ceratopsid dinosaur from the Late Cretaceous Period of what is now North America. It was a quadrupedal herbivore with a parrot-like beak, a rough boss (raised bony area) on the snout and two more behind the eyes, and two horns on the end of its long bony neck frill. With a total body length of 6 meters (20 feet), Achelousaurus was a medium-sized ceratopsian.

The genus and the one named species (A. horneri) were both named by paleontologist Scott Sampson in 1995. The specific name honors Jack Horner, an influential American paleontologist famous for his Montana dinosaur discoveries. The generic name Achelousaurus is a complex reference to Greek mythology. Achelous, an important Greek river deity, had one of his horns torn off by Hercules, in a mythological fight with the legendary hero. All three known skulls of Achelousaurus have rough bosses in the same places where other ceratopsians had horns, giving it the appearance of having had its horns ripped off. Achelous was also celebrated for his shapeshifting ability, just as Achelousaurus appears to combine features of other ceratopsian dinosaurs.

Early reports suggested that Achelousaurus represented a transitional form between ceratopsians with modified horns like Einiosaurus (with which A. horneri shares two horns on the end of the frill), and the derived, hornless Pachyrhinosaurus (Horner et al., 1992). While they may or may not form a direct line of descent, all three of these species are at least closely related, and are often united in the tribe Pachyrhinosaurini, inside the subfamily Centrosaurinae and the family Ceratopsidae (Sampson, 1995; Dodson et al., 2004).

Achelousaurus is known from the U.S. state of Montana, in the Two Medicine Formation, which preserves sediments dated from the Campanian stage of the Late Cretaceous Period, between 83 and 74 million years ago. Achelousaurus was found in the highest levels of the formation, so it is probably closer to the end of that timeframe. Other dinosaurs found in this formation include Daspletosaurus, Bambiraptor, Euoplocephalus, Maiasaura, and Einiosaurus.

Scientists have so far recovered three skulls and some postcranial material from the Two Medicine, all housed at the Museum of the Rockies in Bozeman, Montana. The skull of a full-grown Achelousaurus (including the frill horns) is over 5 feet (1.6 meters) long.

Abrosaurus

Abrosaurus (pronounced /ˌæbrəˈsɔrəs/; 'delicate lizard' from the Greek αβρος meaning 'delicate' or 'dainty' and σαυρος meaning 'lizard') is a genus of macronarian sauropod dinosaur from the Middle Jurassic Period of what is now Asia, one of many dinosaurs found at the Dashanpu Quarry in the Sichuan Province of China. Like most sauropods, Abrosaurus was a quadrupedal herbivore but it was rather small for a sauropod, not much more than 30 feet (9 m) long. Its head was boxy and topped with a tall bony arch containing the nostrils.

The generic name meaning 'delicate lizard', refers to the nature of the skull, with large openings separated by thin bony struts. The only named species is now known as A. dongpoi, and is named after eleventh century Chinese poet Su Shi, also known as Su Dongpo, who was born in Sichuan.

The naming of Abrosaurus has been a long and convuluted process. Abrosaurus was discovered in 1984 and was first described in the 1986 Ph.D. thesis of Chinese paleontologist Ouyang Hui, with the specific name "A. gigantorhinus". However, this does not meet ICZN standards of publication, so the species "Abrosaurus gigantorhinus" counts only as a nomen nudum, although it has been used incorrectly in at least one paper (Zhang & Chen, 1996). Ouyang formally described this species in 1989 under the specific epithet A. dongpoensis, but in biological nomenclature, the Latin suffix -ensis is correctly used only to honor localities and the name has since been revised to include the more correct -i suffix, which is used to honor male individuals (Peng & Shu, 1999). Abrosaurus dongpoi is now the accepted name for this taxon.

Abrosaurus was originally described as a camarasaurid sauropod, and while it may not turn out to be a member of that particular family, further research has indicated that it is a basal member of Macronaria, much like Camarasaurus itself. However, the remains of Abrosaurus have not been fully described, making its exact placement in the sauropod family tree difficult to determine (Upchurch et al., 2004).

The holotype, or original specimen, of Abrosaurus is a fossil skull which is nearly complete and very well-preserved. A fragmentary skull and a skeleton have also been referred to this taxon but published description is lacking (Zhang & Chen, 1996). All of the material comes from the famous Dashanpu Quarry near Zigong in China, and is housed in the dinosaur museum there. Abrosaurus and at least 4 other species of sauropod are known from the Lower Shaximiao Formation (also called Xiashaximiao) at Dashanpu. These sediments are dated from the Bathonian to Callovian stages of the Middle Jurassic Period, or about 168 to 161 million years ago.

Abrictosaurus

Abrictosaurus (pronounced /əˌbrɪktəˈsɔrəs/; "wakeful lizard") is a genus of heterodontosaurid dinosaur from the Early Jurassic Period of what is now southern Africa. It was a small bipedal herbivore or omnivore, approximately 1.2 meters (4 feet) long, and weighing less than 45 kilograms (100 pounds).

This dinosaur is known from the fossil remains of only two individuals, found in the Upper Elliot Formation of Qacha's Nek District in Lesotho and Cape Province in South Africa, respectively. The Upper Elliot is thought to date from the Hettangian and Sinemurian stages of the Early Jurassic Period, approximately 200 to 190 million years ago. This formation is thought to preserve sand dunes as well as seasonal floodplains, in a semiarid environment with sporadic rainfall. Other dinosaurs found in this formation include the theropod Megapnosaurus, the sauropodomorph Massospondylus, as well as other heterodontosaurids like Heterodontosaurus and Lycorhinus. Remains of terrestrial crocodilians, cynodonts and early mammals are also abundant.

Description

Heterodontosaurids like Abrictosaurus were small, early ornithischians, named for their markedly heterodont dentition. They are best-known for the large, canine-like tusks (often called caniniforms) in both upper and lower jaws. There were no teeth in the front of the jaws, where a hard beak was used to crop vegetation. There were three premaxillary teeth, with the first two small and conical and the third enlarged to form the upper caniniform, counterpart to the even larger lower caniniform, which was the first dentary tooth. In the upper jaw, a large gap (or diastema) accommodated the lower caniniform tooth and separated the premaxillary teeth from the wider chewing teeth of the maxilla. Similar teeth lined the remainder of the lower jaw.[2]

Abrictosaurus is usually considered the most basal member of the family Heterodontosauridae. Lycorhinus and Heterodontosaurus both had high-crowned cheek teeth, which overlapped each other in the jaw, forming a continuous chewing surface analogous to those of Cretaceous hadrosaurids. Abrictosaurus had more widely-separated cheek teeth, with lower crowns, more similar to other early ornithischians. It has been suggested that Abrictosaurus lacked tusks and that this is another primitive feature. However, caniniforms were clearly present on one of the two specimens of Abrictosaurus. The upper caniniform measured 10.5 millimeters (0.4 inches) high, while the lower reached 17 mm (0.67 in). These caniniforms were serrated only on the anterior surface, unlike those of Lycorhinus and Heterodontosaurus, which were serrated on both anterior and posterior edges. Abrictosaurus also had smaller, less powerful forelimbs than Heterodontosaurus and one fewer phalanx bone in both the fourth and fifth digits of the forelimb.

History and naming

Both specimens of Abrictosaurus are housed in the collection of University College London. The holotype specimen was discovered in Lesotho and consists of a partial skull and skeleton (UCL B54). Paleontologist Richard Thulborn, who first described the specimen in 1974, considered it a new species of Lycorhinus and named it L. consors, using the Latin word consors which means 'companion' or 'spouse'. As UCL B54 lacked the caniniforms which had been found in the type species, Lycohinus angustidens, Thulborn believed it to be female.[6] Neither the skull nor the skeletons of Abrictosaurus have been fully described in the literature. A tooth from the latest Triassic of Switzerland has been assigned to Abrictosaurus sp., but this has not been supported, as the specimen does not have unique characteristics of Abrictosaurus, heterodontosaurids, or ornithischians in general.

In 1975, James Hopson redescribed a fragmentary heterodontosaur skull (UCL A100) found in South Africa that Thulborn had previously assigned to Lycorhinus angustidens.[4] After showing that UCL A100 could not belong to L. angustidens but was instead more similar to UCL B54, Hopson erected a new genus to contain both specimens. The generic name Abrictosaurus (from the Greek αβρικτος/abriktos meaning 'wakeful' and σαυρος/sauros meaning 'lizard') refers to Hopson's disagreement with Thulborn's hypothesis that heterodontosaurids underwent periods of aestivation (hibernation during hot and/or dry seasons). The specific name was retained, creating the new binomial Abrictosaurus consors.Despite Hopson's renaming, Thulborn continued to consider Lycorhinus angustidens, Heterodontosaurus tucki, and Abrictosaurus consors to be three species of the genus Lycorhinus. Most paleontologists maintain all three genera separately, although there is no precise definition of a species or genus in paleontology.

Sexual dimorphism

The hypothesis of sexual dimorphism in heterodontosaurids has long centered on Abrictosaurus. Tusks are a sexually dimorphic trait in many modern mammals, including musk deer, walrus, Asian elephants and many pigs, with tusks being present primarily in males. The lack of tusks in UCL B54 led to suggestions that it was female; perhaps even a female of another species. The discovery of caniniforms in UCL A100 showed that A. consors also has this 'male' characteristic, suggesting that it is at least a valid species in its own right. However, UCL B54 may actually be a juvenile, based on its short face and unfused sacral (hip) vertebrae. Therefore, the lack of tusks could be a juvenile trait instead of a secondary sexual characteristic, weakening the case for sexual dimorphism.

Abelisaurus

Abelisaurus (pronounced /əˌbɛlɨˈsɔrəs/; "Abel's lizard") is a genus of abelisaurid theropod dinosaur from the Late Cretaceous Period (Maastrichtian) of what is now South America. It was a bipedal carnivore that probably reached 7 to 9 meters (25 to 30 feet) in length, although it is known from only one partial skull.

The generic name recognizes Roberto Abel as the discoverer of the specimen and former director of the provincial Museum of Cipolletti in Argentina, where the specimen is housed. It also incorporates the Greek σαυρος/sauros, meaning 'lizard'. There is one named species, A. comahuensis, which honors the Comahue region of Argentina, where the fossil was found. Both genus and species were named and described by Argentine paleontologists Jose Bonaparte and Fernando Novas in 1985, who placed it in the newly-created family Abelisauridae.

Classification

Many other abelisaurids have since been discovered, including extremely complete specimens of Aucasaurus, Carnotaurus and Majungasaurus. Some scientists place Abelisaurus as a basal abelisaurid, outside the subfamily Carnotaurinae. Others are less certain of its position.Abelisaurids share some skull features with the unrelated carcharodontosaurids and, since Abelisaurus is known only from a skull, future discoveries may show that this genus was in fact a carcharodontosaurid. However, this is thought unlikely.

Aachenosaurus

The name Aachenosaurus refers to two fossilized fragments of material that were originally thought to be jaw fragments from a duck-billed dinosaur (a hadrosaur). However, the fossils turned out to be petrified wood, to the great embarrassment of the discoverer. The fossil's name means "Aachen lizard", named for the Aachenian deposits of Moresnet (which was a neutral territory between Belgium and Germany), where the fossils were found.

Aachenosaurus was found and named by the scientist (and abbé) Gerard Smets, on October 31, 1888, who named the type species Aachenosaurus multidens. Based on these fragments he determined that the specimen was a hadrosaur reaching an estimated 4 to 5 meters in length which might have had dermal spines. He defended this conclusion, citing that the fossils had been examined visually with the naked eye, magnifying lenses and with the microscope. However, his error was soon demonstrated by Louis Dollo. Smets at first tried to defend his original identification but was again proven wrong by a neutral commission and withdrew from science completely from pure embarrassment.