Achillesaurus
Encyclopedia
Achillesaurus is a genus
of alvarezsaurid theropod dinosaur
from the Santonian
-age Upper Cretaceous Bajo de la Carpa Formation
of Rio Negro
, Argentina
. It was a relatively large, basal
alvarezsaurid, and a contemporary of Alvarezsaurus
. Achillesaurus is based on
MACN-PV-RN 1116, a partial skeleton
including a sacral
vertebra, four tail vertebrae, part of the left thighbone
, shin
and foot, and the left ilium
. Agustín Martinelli and Ezequiel Vera, who described the specimen, performed a phylogenetic analysis
and found their new genus to be an alvarezsaurid with an unresolved relationship to Alvarezsaurus and more derived
alvarezsaurids.
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 alvarezsaurid theropod 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...
from the Santonian
Santonian
The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series. It spans the time between 85.8 ± 0.7 mya and 83.5 ± 0.7 mya...
-age Upper Cretaceous Bajo de la Carpa Formation
Bajo de la Carpa Formation
The Bajo de la Carpa Formation is a geologic formation that outcrops in Patagonia, in the provinces of Río Negro and Neuquén, Argentina. It is the first of two formations belonging to the Río Colorado Subgroup within the Neuquén Group...
of Rio Negro
Río Negro Province
Río Negro is a province of Argentina, located at the northern edge of Patagonia. Neighboring provinces are from the south clockwise Chubut, Neuquén, Mendoza, La Pampa and Buenos Aires. To the east lies the Atlantic Ocean.Its capital is Viedma...
, Argentina
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
. It was a relatively large, basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
alvarezsaurid, and a contemporary of Alvarezsaurus
Alvarezsaurus
Alvarezsaurus is a genus of small alvarezsaurid dinosaur from the Late Cretaceous period of Argentina, approximately 86 - 83 million years ago. Estimates suggest that it measured about 2 meters in length and weighed approximately 20 kg...
. Achillesaurus is based on
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
MACN-PV-RN 1116, a partial skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
including a sacral
Sacrum
In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
vertebra, four tail vertebrae, part of the left thighbone
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...
, shin
Tibia
The tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
and foot, and the left ilium
Ilium (bone)
The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium.The name comes from the Latin ,...
. Agustín Martinelli and Ezequiel Vera, who described the specimen, performed a phylogenetic analysis
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
and found their new genus to be an alvarezsaurid with an unresolved relationship to Alvarezsaurus and more 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...
alvarezsaurids.