Антарктидадағы қалқанды динозавр: Antarctopelta оliveroi

Extinct genus of dinosaurs

Antarctopelta (annTARKtohPELtə; meaning 'Antarctic shield') is a genus of ankylosaurian dinosaur, a group of large, quadrupedal herbivores, that lived during the Campanian stage of the Late Cretaceous period on what is now James Ross Island, Antarctica. Antarctopelta is the only known ankylosaur from Antarctica and a member of Parankylosauria. The only described specimen was found in 1986, the first dinosaur to be found on the continent, by Argentine geologists Eduardo Olivero and Robert Scasso. The fossils were later described in 2006 by paleontologists Leonardo Salgado and Zulma Gasparini, who named the type species A. oliveroi after Olivero. It was a medium sized ankylosaur, reaching in length or more, and showed characteristics of two different families, making more precise classification difficult for many years. This was until 2021, when a nearly complete skeleton of the similar Chilean genus Stegouros was described. This led to the recognition of Parankylosauria, containing Antarctopelta, Stegouros, and Kunbarrasaurus. The head was small, with proportionally large teeth compared to other ankylosaurs' and spikes above the orbits. The neck and back vertebrae were short and circular in cross section, whereas the tail vertebrae were elongated and flattened. Its tail likely terminated in an arrangement of spiked osteoderms known as a macuahuitl, which resembled an Aztec weapon of the same name. Osteoderms were present on other parts of the body and came in six different shapes, with some being large and flat while others were tall and keeled. It was discovered in rocks of the Gamma Member of the Snow Hill Island Formation, which bears a variety of other fossils, many of them unique as they evolved in the isolation of Antarctica after the breakup of Gondwana. Antarctopelta coexisted with the ornithopod dinosaur Trinisaura in addition to a menagerie of mosasaurs, plesiosaurs, and sharks.

During an expedition to James Ross Island off the coast of Antarctica, an incomplete skeleton of an ankylosaur was discovered by Argentine geologists Eduardo Olivero and Robert Scasso in January 1986. However, excavations would not be finished for a decade due to ground frost and harsh weather conditions. Olivero and Scasso had found the specimen in strata from the Gamma Member of the Snow Hill Island Formation, which dates to the upper Campanian age of the Late Cretaceous period. The material all came from a single individual, but was spread over a area and collected over several field seasons. The bones were heavily worn due to freeze thaw weathering, causing many to become fragmented and broken. It was theorized that one of the phalanges came from a different individual, though this has been disproven. At this site, the specimen was unearthed with a tooth of the shark Notidanodon, likely due to scavenging, as well as bivalves. This implies that the ankylosaur died and floated out to sea, a phenomenon observed in other ankylosaurs. The holotype (specimen used as the basis for the taxon) MLP 86 X 28 1 is the only known examplar 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 with another tooth in situ, some other skull fragments, vertebrae of the neck, back, hips and tail, some shoulder and hip bones (scapula, ilium) a thigh bone (femur), foot and hand bones (five metapodials and two phalanges), and numerous pieces of armor, representing approximately 15% of the skeleton. The genus name refers to its location on the continent of Antarctica and its armored nature. Antarctica is derived from the Greek words αντ/ant ('opposite of') and αρκτος/arktos ('bear' referring to the constellation Ursa Major, which points north). The Greek πελτη/pelte ('shield') is commonly used to name genera of ankylosaurs (Cedarpelta and Sauropelta, 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. The head was small, with jaws lined with small, leaf shaped teeth. The limbs were short and about equal in length, the forefeet having five toes while the hindfeet had six. Very little of the skull is known, but all of the preserved skull fragments were heavily ossified for protection. One bone in particular, identified as a supraorbital (brow ridge bone), bore a short spike which would have projected outwards over the eye. Other fossils likely from the quadratojugals (cheek bones), supraorbitals, and right parietal (cranium bone) were identified in 2006, though their poor condition gives little information. The leaf shaped teeth are asymmetrical, with the majority of the denticles on the edge closest to the tip of the snout and large furrows on the cingula. Seven to eight mesial denticles are found on each Antarctopelta tooth, the highest number known from Parankylosauria, a trait distinguishing it from the related Stegouros. This compares to the much larger North American Euoplocephalus, in body length, which had teeth averaging only across. A fragment from the left dentary (lower jaw bone) preserving these teeth was recovered, which has a curved tooth row like other ankylosaurs'. In 2021, a new group of ankylosaurs, Parankylosauria, was dubbed after the discovery of Stegouros. Parankylosauria is a group of small ankylosaurs from the Southern Hemisphere that, instead of a full tail club, had a macuahuitl. The clade has only three known members, the largest of which is Antarctopelta. However, a 2019 histological analysis by Argentine paleontologist Ignacio Cerda and colleagues found that the holotype individual was sexually mature. Samples from three osteoderms, a bone shaft, a metapodial, several undetermined fragments, ossified tendons, and dorsal ribs were used in the study. Based on the spacing and organization of the Outer Circumferential Layer of the bone cortex, Cerda et al concluded that the specimen had reached sexual maturity and was close to adult maturity. During their study, the authors found that one section of bone had an abnormal bone tissue that may have been caused by a tumor or other periosteal reaction. The growth patterns of the bones were not very dissimilar to those of ankylosaurs from lower latitudes. This suggests that the growth rates remained the same, despite different climatic or environmental conditions.

The holotype skeleton was collected about from the base of the Gamma Member of the Snow Hill Island Formation. It is one of only two major dinosaur bearing rock formations found on Antarctica, bearing all but two of the continent's named dinosaurs. The floral composition, habitat and climate are similar to modern volcanic arches. During the time in which Antarctopelta lived, Earth's climate was much warmer and more humid than it is today and as a result Antarctica was without ice. The environment was mainly dominated by large dense conifer forests, cycads, and ginkgos. The animals inhabiting Antarctica at this time would still have had to endure long periods of darkness during the winter, much like in modern day Antarctica. 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. The Antarctic Peninsula, including James Ross Island, was connected to South America throughout this time period, allowing interchange of fauna between both continents. In fact, the recent discovery of the Chilean parankylosaurian Stegouros shows that these dinosaurs inhabited also South America. Some of the environment may have been wet and similar to peat bogs, as evidenced by the presence of Sphagnaceae (peat mosses) and several other groups including the clubmoss Selaginella, the firmoss group Lycopodiaceae, and the clade Ericaceae. The Gamma Member of the formation has yielded several other vertebrate remains, such as the ornithopod Trinisaura, a vertebral centrum of a lithostrothian sauropod, an aquatic elasmosaurid, and the carnivorous tylosaurine mosasaurs Taniwhasaurus and Hainosaurus. As for fishes and chondrichthyes, the bony fish Enchodus and Apateodus, an indeterminate ichthyodectiformes, and several sharks such as Sphenodus, Cretalamna, and Notidanodon have been collected. Ammonites, a kind of aquatic, shelled cephalopod, are also found in the layers of the Gamma Member.