Achelousaurus
Encyclopedia
Achelousaurus is a genus
of centrosaurine
ceratopsid dinosaur
s from the Late Cretaceous
Period of what is now North America
, dated to 74 million years ago. They were quadrupedal herbivore
s with parrot
-like beaks, rough bosses (raised bony areas) on their snouts and one pair behind their eyes, and a pair of horns on the end of their long bony frills. With body lengths of up to 6 meters (20 feet), Achelousaurus were a medium-sized ceratopsians.
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 skull
s of Achelousaurus have rough bosses in the same places where other ceratopsians have horns, giving them the appearance of having had the horns ripped off. Achelous was also celebrated for his shapeshifting
ability, just as Achelousaurus appear to combine features of other ceratopsia
n dinosaurs.
Early reports suggested that Achelousaurus represented transitional forms 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 genera
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 horneri are 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 specimens are found in the highest levels of the formation, probably closer to the end of that timeframe, 74 mya. Contemporary dinosaur species included Daspletosaurus torosus, Euoplocephalus tutus, Maiasaura peeblesorum, and Einiosaurus procurvicornis.
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.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of centrosaurine
Centrosaurinae
The Centrosaurinae is a subfamily of ceratopsid dinosaurs named by paleontologist Lawrence Lambe, in 1915, with Centrosaurus as the type genus...
ceratopsid dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s from the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
Period of what is now North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, dated to 74 million years ago. They were quadrupedal herbivore
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
s with parrot
Parrot
Parrots, also known as psittacines , are birds of the roughly 372 species in 86 genera that make up the order Psittaciformes, found in most tropical and subtropical regions. The order is subdivided into three families: the Psittacidae , the Cacatuidae and the Strigopidae...
-like beaks, rough bosses (raised bony areas) on their snouts and one pair behind their eyes, and a pair of horns on the end of their long bony frills. With body lengths of up to 6 meters (20 feet), Achelousaurus were a medium-sized ceratopsians.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
(A. horneri) were both named by paleontologist Scott Sampson in 1995. The specific name honors Jack Horner
Jack Horner (paleontologist)
John "Jack" R. Horner is an American paleontologist who discovered and named Maiasaura, providing the first clear evidence that some dinosaurs cared for their young. He is one of the best-known paleontologists in the United States...
, an influential American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
paleontologist famous for his Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
dinosaur discoveries. The generic name Achelousaurus is a complex reference to Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
. Achelous
Achelous
In Greek mythology, Achelous was the patron deity of the "silver-swirling" Achelous River, which is the largest river of Greece, and thus the chief of all river deities, every river having its own river spirit. His name is pre-Greek, its meaning unknown...
, an important Greek
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...
river deity
Deity
A deity is a recognized preternatural or supernatural immortal being, who may be thought of as holy, divine, or sacred, held in high regard, and respected by believers....
, had one of his horns torn off by Hercules
Hercules
Hercules is the Roman name for Greek demigod Heracles, son of Zeus , and the mortal Alcmene...
, in a mythological fight with the legendary hero. All three known skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
s of Achelousaurus have rough bosses in the same places where other ceratopsians have horns, giving them the appearance of having had the horns ripped off. Achelous was also celebrated for his shapeshifting
Shapeshifting
Shapeshifting is a common theme in mythology, folklore, and fairy tales. It is also found in epic poems, science fiction literature, fantasy literature, children's literature, Shakespearean comedy, ballet, film, television, comics, and video games...
ability, just as Achelousaurus appear to combine features of other ceratopsia
Ceratopsia
Ceratopsia or Ceratopia is a group of herbivorous, beaked dinosaurs which thrived in what are now North America, Europe, and Asia, during the Cretaceous Period, although ancestral forms lived earlier, in the Jurassic. The earliest known ceratopsian, Yinlong downsi, lived between 161.2 and 155.7...
n dinosaurs.
Early reports suggested that Achelousaurus represented transitional forms between ceratopsians with modified horns like Einiosaurus
Einiosaurus
Einiosaurus is a medium-sized centrosaurine ceratopsian from the Upper Cretaceous of northwestern Montana. The name means 'buffalo lizard', in a combination of Blackfeet Indian and Latinized Ancient Greek; the specific name Einiosaurus is a medium-sized centrosaurine (“short-frilled”)...
(with which A. horneri shares two horns on the end of the frill), and the derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
, hornless Pachyrhinosaurus
Pachyrhinosaurus
Pachyrhinosaurus is a genus of ceratopsid dinosaurs from the Late Cretaceous period of North America. The first examples were discovered by Charles M. Sternberg in Alberta, Canada, in 1946, and named in 1950. Over a dozen partial skulls and a large assortment of other fossils from various species...
(Horner et al., 1992). While they may or may not form a direct line of descent
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
, all three of these genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
are at least closely related, and are often united in the tribe
Tribe (biology)
In biology, a tribe is a taxonomic rank between family and genus. It is sometimes subdivided into subtribes.Some examples include the tribes: Canini, Acalypheae, Hominini, Bombini, and Antidesmeae.-See also:* Biological classification* Rank...
Pachyrhinosaurini, inside the subfamily Centrosaurinae
Centrosaurinae
The Centrosaurinae is a subfamily of ceratopsid dinosaurs named by paleontologist Lawrence Lambe, in 1915, with Centrosaurus as the type genus...
and the family Ceratopsidae
Ceratopsidae
Ceratopsidae is a speciose group of marginocephalian dinosaurs including Triceratops and Styracosaurus...
(Sampson, 1995; Dodson et al., 2004).
Achelousaurus horneri are known from the U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
state of Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
, in the Two Medicine Formation
Two Medicine Formation
The Two Medicine Formation is a geologic formation, or rock body, that was deposited between 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma , during Campanian time, and is located in northwestern Montana...
, which preserves sediments dated from the Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
stage of the Late Cretaceous Period, between 83 and 74 million years ago. Achelousaurus specimens are found in the highest levels of the formation, probably closer to the end of that timeframe, 74 mya. Contemporary dinosaur species included Daspletosaurus torosus, Euoplocephalus tutus, Maiasaura peeblesorum, and Einiosaurus procurvicornis.
Scientists have so far recovered three skulls and some postcranial material from the Two Medicine, all housed at the Museum of the Rockies
Museum of the Rockies
The Museum of the Rockies, is located in Bozeman, Montana. The museum, originally affiliated with Montana State University in Bozeman, and now, also the Smithsonian Institution, is known for its paleontological collections, although these are not its sole focus...
in Bozeman, Montana. The skull of a full-grown Achelousaurus (including the frill horns) is over 5 feet (1.6 meters) long.