Antarctopelta

Antarctopelta

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Antarctopelta was a genus
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 ankylosauria
Ankylosauria
Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are first known to have appeared in the early Jurassic Period of...

n 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...

 with one known species
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. oliveroi, which lived in Antarctica during the Late Cretaceous Period. It was a medium-sized ankylosaur, reaching no more than 4 meters (13 ft) in length, and showed characteristics of two different families, making more precise classification difficult. The single known fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 specimen was discovered on James Ross Island
James Ross Island
James Ross Island is a large island off the southeast side and near the northeastern extremity of Antarctic Peninsula, from which it is separated by Prince Gustav Channel. Rising to , it is irregularly shaped and extends in a north-south direction. It was charted in October 1903 by the Swedish...

 in 1986, constituting the first dinosaur remains ever discovered on Antarctica, although it is the second dinosaur from the continent to be formally named.

Description and classification


Like other ankylosaurs, Antarctopelta oliveroi was a stocky, herbivorous
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...

 quadruped
Quadruped
Quadrupedalism is a form of land animal locomotion using four limbs or legs. An animal or machine that usually moves in a quadrupedal manner is known as a quadruped, meaning "four feet"...

 protected by armor plates
Osteoderm
Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the skin. Osteoderms are found in many groups of extant and extinct reptiles, including lizards, various groups of dinosaurs , crocodilians, phytosaurs, aetosaurs, placodonts, and hupehsuchians...

 embedded in the skin. Although a complete 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...

 has not been found, the species is estimated to have reached a maximum length of 4 meters (13 ft) from snout to tail tip. Very little of the 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...

 is known, but all of the known skull fragments were heavily ossified
Ossification
Ossification is the process of laying down new bone material by cells called osteoblasts. It is synonymous with bone tissue formation...

 for protection. One bone in particular, identified as a supraorbital, included a short spike which would have projected outwards over the eye. The leaf
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....

-shaped teeth are asymmetrical
Asymmetry
Asymmetry is the absence of, or a violation of, symmetry.-In organisms:Due to how cells divide in organisms, asymmetry in organisms is fairly usual in at least one dimension, with biological symmetry also being common in at least one dimension....

, with the majority of the denticles on the edge closest to the tip of the snout. These teeth are also proportionately large compared to those of other ankylosaurs, with the largest measuring 10 millimeters (0.4 in) across. This compares to the much larger 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...

n Euoplocephalus
Euoplocephalus
Euoplocephalus was one of the largest genera of ankylosaurian dinosaurs, at about the size of a small elephant. It is also the ankylosaurian with the best fossil record, so its extensive spiked armor, low-slung body and great club-like tail are well documented.-Description:Among the...

, 6–7 m (20–23 ft) in body length, which had teeth averaging only 7.5 mm (0.3 in) across.

Vertebrae from other sections of the tail were found. Although the tip of the tail did not fossilize, some of the smaller vertebrae recovered would have been situated near the end of the tail in life, and these were associated with ossified tendons on the upper and lower sides. In ankylosaurids, these tendons help to stiffen the end of the tail in support of a large, bony tail club. If such a club existed in Antarctopelta, it has yet to be discovered. Six different types of osteoderms were found along with the skeletal remains of Antarctopelta, but very few were articulated with the skeleton, so their placement on the body is largely speculative. They included the base of what would have been a large spike. Flat oblong plates resembled the ones that guarded the neck of the nodosaurid Edmontonia rugosidens
Edmontonia
Edmontonia was an armoured dinosaur, a part of the nodosaur family from the Late Cretaceous Period. It is named after the Edmonton Formation , the unit of rock it was found in.-Description:...

. Large circular plates were found associated with smaller, polygon
Polygon
In geometry a polygon is a flat shape consisting of straight lines that are joined to form a closed chain orcircuit.A polygon is traditionally a plane figure that is bounded by a closed path, composed of a finite sequence of straight line segments...

al nodules, perhaps forming a shield over the hips as seen in Sauropelta
Sauropelta
Sauropelta is a genus of nodosaurid dinosaur that existed in the Early Cretaceous Period of North America. One species has been named although others may have existed. Anatomically, Sauropelta is one of the most well-understood nodosaurids, with fossilized remains recovered in the U.S. states of...

. Another type of osteoderm was oval-shaped with a keel running down the middle. A few examples of this fifth type were found ossified to the rib
Rib
In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...

s, suggesting that they ran in rows along the flanks of the animal, a very typical pattern among ankylosaurs. The final group consisted mainly of small bony nodules which are often called ossicles, and were probably scattered throughout the body. Several ribs were also found with these ossicles attached.

Antarctopelta shares several features with the nodosaurids, mainly in the teeth and armor, while the possibly-clubbed tail is far more similar to those of ankylosaurids. This mosaic of characters makes assignment to a specific family difficult. It has been designated as Ankylosauria
Ankylosauria
Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are first known to have appeared in the early Jurassic Period of...

 incertae sedis
Incertae sedis
, is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any...

, but has never been subjected to a phylogenetic analysis
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

. A new phylogenetic analysis performed by Thompson et al., 2011 suggests that Antarctopelta is the basalmost
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:...

 known nodosaurid.

Discovery and naming


The holotype
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...

, or original specimen, is the only known example of this genus and species, and was the first dinosaur ever found in Antarctica. It consists of three isolated teeth, part of the lower jaw
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...

 with another tooth in situ, some other skull fragments, vertebrae (neck, back, hips and tail), some partial limb bones (scapula
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....

, 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 ,...

 and femur
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...

), toe bones (five metapodial
Metatarsus
The metatarsus or metatarsal bones are a group of five long bones in the foot located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the medial side : the first, second, third, fourth, and fifth...

s and two phalanges), and numerous pieces of armor. This specimen was initially located in January 1986 on James Ross Island, off the Antarctic Peninsula
Antarctic Peninsula
The Antarctic Peninsula is the northernmost part of the mainland of Antarctica. It extends from a line between Cape Adams and a point on the mainland south of Eklund Islands....

. It was discovered by Argentine
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...

 geologist
Geologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...

s Eduardo Olivero and Roberto Scasso, but excavation was not completed for almost a decade due to the frozen ground and harsh weather conditions. The material was collected from an area of 6 square meters (64.5 sq ft) over several field seasons but is assumed to have belonged to a single individual. Much of the skeleton is in poor condition, as many of the bones nearest to the surface were subjected to years of fragmentation by freeze-thaw weathering.

Although the material had been known for decades and written about in three separate publications, Antarctopelta oliveroi was not named until 2006, by Argentine paleontologists Leonardo Salgado and Zulma Gasparini. It was therefore the second named genus of dinosaur from Antarctica after Cryolophosaurus
Cryolophosaurus
Cryolophosaurus was a large theropod dinosaur, with a crest on its head that looked like a Spanish comb. Due to the resemblance of this feature to Elvis Presley's pompadour haircut from the 1950s, this dinosaur was at one point informally known as "Elvisaurus".Cryolophosaurus was excavated from...

in 1993, despite being discovered first. The genus name refers to its location on the continent of Antarctica and its armored
Armour (zoology)
Armour in animals is external or superficial protection against attack by predators, formed as part of the body , usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species...

 nature. Antarctica is derived from the Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 words αντ/ant- ('opposite of') and αρκτος/arktos ('bear' referring to the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

 Ursa Major
Ursa Major
Ursa Major , also known as the Great Bear, is a constellation visible throughout the year in most of the northern hemisphere. It can best be seen in April...

, which points north). The Greek πελτη/pelte ('shield') is commonly used to name genera of ankylosaurs (Cedarpelta
Cedarpelta
Cedarpelta is an extinct genus of basal ankylosaurid ankylosaur, based on material recovered from the Lower Cretaceous of North America. The skull lacks extensive cranial ornamentation, a trait which has been interpreted as plesiomorphic for ankylosaurs....

and Sauropelta
Sauropelta
Sauropelta is a genus of nodosaurid dinosaur that existed in the Early Cretaceous Period of North America. One species has been named although others may have existed. Anatomically, Sauropelta is one of the most well-understood nodosaurids, with fossilized remains recovered in the U.S. states of...

, for example). The single known species, A. oliveroi, is named after Eduardo Olivero, who discovered the holotype, first mentioned it in print, and has worked in Antarctica for decades.

Earlier work suggested that the James Ross Island ankylosaur was a juvenile. More recent research indicates that the different parts of the vertebrae are completely fused together, while a juvenile would be expected to have visible sutures between the neural arch
Vertebral arch
The vertebral arch is the posterior part of a vertebra.It consists of a pair of pedicles and a pair of laminae, and supports seven processes:* four articular processes* two transverse processes* one spinous process...

 and body
Body of vertebra
The body is the largest part of a vertebra, and is more or less cylindrical in shape. For vertebrates other than humans, this structure is usually called a centrum....

 (centrum) of the vertebrae. A preliminary histological
Histology
Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...

 analysis of several bones also indicates a level of remodeling that would not be seen in newly-formed bone.

Environment


The holotype
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...

 skeleton was collected about 90 m (300 ft) from the base of the Gamma Member of the Santa Marta Formation
Santa Marta Formation
The Santa Marta Formation is a geologic formation in Antarctica. It, along with the Mount Kirkpatrick Formation and the Snow Hill Island Formation, are the only formations yet known on the continent where dinosaur fossils have been found. The formation outcrops on James Ross Island off the coast of...

 in Antarctica. This member
Geologic unit
A geological unit is a volume of rock or ice of identifiable origin and age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features that characterize it....

 was deposited in a shallow marine
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 environment
Natural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....

 and also preserves marine fossils such as shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

 teeth, remains of the mosasaur
Mosasaur
Mosasaurs are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...

  Lakumasaurus antarcticus, ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...

s, bivalves, and gastropods. Index fossil
Index fossil
Index fossils are fossils used to define and identify geologic periods . They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil...

s like ammonites suggest the rocks were deposited in the late 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
Faunal stage
In chronostratigraphy, a stage is a succession of rock strata laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.Rock...

 of 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, or about 74 to 70 Ma (million years ago). Despite being found in marine sediment, Antarctopelta, like all ankylosaurs, lived on land. Other ankylosaurs have also been found in marine sediments, likely as a result of carcasses washing out to sea.

Although Antarctica in the Cretaceous was in the southern polar region
Polar region
Earth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...

, the Earth had a much warmer climate during this time period, and the continent would have been ice-free. Animals like Antarctopelta oliveroi would have lived in forests of conifers and even deciduous trees. Despite the higher temperatures, darkness would still have descended for the winter, just as it does today at high latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

s. The Antarctic Peninsula, including James Ross Island, was connected to South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

 throughout this time period, allowing interchange
Biological dispersal
Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...

of fauna between both continents. However, no evidence has yet been found to support a common ankylosaur fauna between Antarctica and South America.