Antarctosaurus (; meaning "southern lizard") is a genus of titanosaurian sauropod dinosaur from the Late Cretaceous Period of what is now South America. The type species, Antarctosaurus wichmannianus, and a second species, Antarctosaurus giganteus, were described by prolific German paleontologist Friedrich von Huene in 1929. Three additional species of Antarctosaurus have been named since then but later studies have considered them dubious or unlikely to pertain to the genus.
The type species, A. wichmannianus, is controversial because there is uncertainty as to whether all the described remains belong to the same individual or even genus. The second species, A. giganteus, is considered dubious, but the fragmentary remains represent one of the largest dinosaurs known.
Remains of this dinosaur were first mentioned in print in 1916,[1] although they were not fully described and named until a 1929 monograph written by paleontologist Friedrich von Huene.[2] Antarctosaurus does not refer to the continent of Antarctica since it was first found in Argentina, although it does have the same derivation, from the Greek words αντι-, anti- meaning 'opposite of', αρκτός, arktos meaning 'north' and Greek, Ancient (to 1453);: σαυρος, sauros meaning 'lizard'. The generic name refers to the animal's reptilian nature and its geographical location on a southern continent.
Antarctosaurus wichmannianus is the type species of the genus, named in 1929 after the discoverer of its remains in 1912, geologist Ricardo Wichmann. Von Huene used the name A. wichmannianus to describe a large assemblage of bones, which are considered to come from the Anacleto Formation in Río Negro Province of Argentina, which is probably early Campanian in age.[3] Two additional limb bones, found in the Chubut Province in 1924, were also referred to A. wichmannianus by von Huene in 1929. Later studies, however, have doubted their referral to the species.
Von Huene also named a fragmentary second species of Antarctosaurus in the same 1929 monograph, which he tentatively called cf. Antarctosaurus giganteus because of its enormous size. These fossils were recovered in Neuquén Province of Argentina, from the Plottier Formation,[4] which dates to the Coniacian-Santonian stages of the Late Cretaceous Period. The Plottier, like the younger Anacleto, is a member of the Neuquén Group.
Very few remains are known of this species and it is regarded as a nomen dubium by some.[5] Other researchers regard A. giganteus as a likely valid species but probably belonging to a new genus.[6] In 1969, Leigh Van Valen considered A. wichmannianus and A. giganteus to be growth stages of the same species and favored the name A. giganteus.[7] This idea is problematic because A. wichmannianus was named earlier in the same paper and it is known from more material, it should, therefore, get priority over A. giganteus. The two species are also not from the same geological formation which suggests they did not belong to the same time period.[8]
In 1933, Von Huene and Charles Matley described another species, Antarctosaurus septentrionalis, meaning "northern". The remains were found in the Lameta Formation of Madhya Pradesh State in India.[9] [10] This species does preserve important anatomical information but has since been assigned to its own genus in 1994; Jainosaurus.[11]
Antarctosaurus jaxarticus from Kazakhstan is known from a single femur.[12] It was named by Soviet paleontologist Anatoly Riabinin in 1938, and was the first sauropod species from Kazakhstan. It was reported from a certain locality in the Kyzylkum Desert, but the exact location is unknown. It may have come from the Syuksyuk Formation (originally described as Dabrazinskaya Svita) which dates to the Santonian stage of the Late Cretaceous. Other researchers have considered it as either, Titanosauridae incertae sedis, as a nomen dubium, or as a nomen nudum.[13] [14]
In 1970, two fragmentary limb bones and a partial vertebra were found in the Adamantina Formation (originally described as Bauru Formation; has also been reported as the São José do Rio Preto Formation[15]) of the northern Paraná Basin in Brazil. The remains were described by their discoverers Fahad Moysés Arid and Luiz Dino Vizotto in 1971 as A. brasiliensis.[16] Other researchers have considered this species as either, a nomen dubium,[17] or an indeterminate titanosaur.[18]
Describing Antarctosaurus is problematic because the type species consists of elements that are of questionable association and none of the species described are known from complete remains, which has caused a confused taxonomy of the genus. Of the four additional species that have been assigned to Antarctosaurus over the years, three have been considered dubious and "Antarctosaurus" septentrionalis, was given its own genus, Jainosaurus.
The remains that have been described belong to sauropods, most probably titanosaurs,[19] a group of large-bodied, quadrupedal herbivores, usually possessing a long neck and tail, with a small head.[20]
The assemblage of fossil remains that became known as A. wichmannianus were given the specimen number MACN 6904. The known material includes several skull fragments, including a braincase and an incomplete mandible (lower jaw), a cervical (neck) vertebra, a caudal (tail) vertebra, rib fragments, and numerous limb bones including a femur which measures 1.39m (04.56feet) tall. None of the individual fossils were designated the holotype specimen so MACN 6904 is considered to be an assemblage of syntypes.[21] The total length of A. wichmannianus has been estimated at around 17m (56feet).[22] An additional femur and tibia were also referred by von Huene to A. wichmannianus; the femur, FMNH P13019, is over 1.85m (06.07feet) tall. In one study the dimensions of this femur were used in a regression analysis to estimate the mass of A. wichmannianus at about 34MT.[23] The referral of the additional femur and tibia has been questioned by later researchers. In 2003 Jaime Eduardo Powell tentatively referred them to cf. Argyrosaurus and in 2012 Philip Mannion and Alejandro Otero considered it an indeterminate titanosaur.[24]
The incomplete mandible attributed to A. wichmannianus is squared-off at the front with each dentary bone being ''L'' shaped.[25] The teeth were restricted to the front of the lower jaw and were small and slender. The squared-off jaws suggest specialised feeding habits, such as feeding near a surface plane like low vegetation on the ground or floating plants in water.