Antarctosaurus

''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.

Discovery and species

Remains of this dinosaur were first mentioned in print in 1916, although they were not fully described and named until a 1929 monograph written by paleontologist Friedrich von Huene.von Huene, F. 1929. Los saurisquios y ornitisquios del Cretáceo Argentino. ''Anales del Museo de La Plata'' (series 3) 3: 1–196. n Spanish/ref> ''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 , ''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. 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, 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.Upchurch, P., Barrett, P.M, & Dodson, P. 2004. Sauropoda. In: Weishampel, D.B., Dodson, P., & Osmolska, H. (Eds.). ''The Dinosauria'' (2nd Edition). Berkeley: University of California Press. Pp. 259-322. Other researchers regard ''A. giganteus'' as a likely valid species but probably belonging to a new genus. 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.'' 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.

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. This species does preserve important anatomical information but has since been assigned to its own genus in 1994; '' Jainosaurus''.Hunt, A.P., Lockley M., Lucas S. & Meyer C., 1995, "The global sauropod fossil record", In: M.G. Lockley, V.F. dos Santos, C.A. Meyer, and A.P. Hunt, (eds.) ''Aspects of sauropod paleobiology, GAIA'' 10: 261-279

''Antarctosaurus jaxarticus'' from Kazakhstan is known from a single femur. 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''.

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) 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''.Arid, F.M. & Vizotto, L.D. 1971. ''Antarctosaurus brasiliensis'', um novo saurópode do Crétaceo superior do sul do Brasil. In: ''Congresso Brasileiro de Geologia'' 25: 297-305. n Portuguese/ref> Other researchers have considered this species as either, a ''nomen dubium'', or an indeterminate titanosaur.

Description

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, a group of large-bodied, quadrupedal herbivores, usually possessing a long neck and tail, with a small head.

''Antarctosaurus wichmannianus''

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 tall. None of the individual fossils were designated the holotype specimen so MACN 6904 is considered to be an assemblage of syntypes. The total length of ''A. wichmannianus'' has been estimated at around .

An additional femur and tibia were also referred by von Huene to ''A. wichmannianus''; the femur, FMNH P13019, is over tall. In one study the dimensions of this femur were used in a regression analysis to estimate the mass of ''A. wichmannianus'' at about . 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.

The incomplete mandible attributed to ''A. wichmannianus'' is squared-off at the front with each dentary bone being ''L'' shaped. 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.
These bones were, for the most part, not associated with each other but scattered throughout the formation. Consequently, many scientists believe that they may not all belong to the same type of animal. In particular, the very square lower jaw has frequently been suggested to belong to a rebbachisaurid sauropod similar to ''Nigersaurus''. However, the jaw of ''Bonitasaura,'' described in 2004'','' is similar in overall shape and is clearly associated with titanosaur skeletal remains, indicating that the lower jaw may belong to ''A. wichmannianus'' after all. In 2013 and 2018 respectively, ''Brasilotitan'' and '' Baalsaurus'' were described which also possessed squared-off jaws. It was noted that ''Brazilotitan, Bonitasaura, Antarctosaurus,'' and other titanosaurs show up three teeth per alveolus (tooth socket) whereas the rebbachisaurid ''Nigersaurus'' shows up to seven teeth. ''Brazilotitan'' and ''Baalsaurus'' were described as a titanosaurs, closely related ''to A. wichmannianus.''

The back of the skull and the remainder of the skeleton are usually regarded as titanosaurian by researchers, although they do not necessarily belong to the same type of titanosaur. In 2005, Jeffrey Wilson considered the braincase as being referable to Nemegtosauridae but noted that other skull remains require further study. A study, published in 2012 by Ariana Paulina Carabajal, CT scanned the ''A. wichmannianus'' braincase which revealed the complete brain endocast and the inner ear structures. The brain endocast and inner ear share several features with other titanosaurids such as short olfactory tracts and olfactory bulbs that are horizontally projected.

Powell compared the width of the cranium to the length of the limb bones of both ''A. wichmannianus'' and ''Saltasaurus''; this led him to conclude that the skull was proportionally small in ''A. wichmannianus,'' this might imply that the skull and limb elements could belong to different individuals or a different taxa. He noted, however, that the comparison was potentially misleading because the overall anatomy of ''Saltasaurus'' is shorter and stouter which might facilitate a bigger skull.

Von Huene assigned two tarsal (ankle) bones to ''A. wichmannianus'', which he described as an astragalus and a calcanium. Powell suggested it's possible that the calcanium described by von Huene is actually the astragalus of a smaller individual. He also noted that the astragalus seems too small to belong to the same individual as the tibia, being only about half the width.

Von Huene described a caudal vertebra which was found close to the skull material. This vertebra was the first caudal, belonging to the base of the tail just after the sacrum (vertebrae attached to the hip). The vertebra features a biconvex centrum, a feature shared with other titanosaurs. Von Huene noted that the first caudal could possibly belong to '' Laplatasaurus''.

With the exception of an incomplete cervical vertebra and the questionable first caudal, there are no vertebrae that link the skull to the limb material. There is a lack of field documentation to aid in the referral of all the material to one individual. Powell thought it was probable that von Huene correctly assigned the material to ''A. wichmannianus'', arguing that von Huene would have been able to communicate with the discoverers and would have had access to photographs of the discovery site.

''"Antarctosaurus" giganteus''

The type specimen of ''A. giganteus,'' MLP 26-316, includes a left and right femur, a partial left and right pubis, the distal end of a damaged tibia, numerous rib and distal caudal vertebrae fragments, and six large and unidentifiable bones. The two gigantic femora measure in length, which are among the largest of any known sauropod. Even though the femurs are large, they are also somewhat gracile in construction.

A reconstruction of ''A. giganteus'', published in 1956 by Carlos Rusconi, was given a length around . In 1969, van Valen considered it as similar in size to ''Giraffatitan brancai'' (then called ''Brachiosaurus brancai''). Based on an earlier mass estimate of ''G. brancai'' by Edwin Harris Colbert in 1962, van Valen gave ''A. giganteus'' an estimated mass of about . In 1994, Gregory S. Paul estimated the weight of both ''A. giganteus'' and ''Argentinosaurus'' at between and lengths of long. Extrapolating from the femur's parameters, a 2004 study by Gerardo Mazzetta and colleagues estimated the mass of ''A. giganteus'' at approximately ; slightly smaller than ''Argentinosaurus'' which in the same study was estimated at nearly . This would make ''A. giganteus'' among the heaviest known land animals. In 2006, Kenneth Carpenter used the relatively short-necked ''Saltasaurus'' as a guide and estimated a length of long. In an encyclopaedia supplementary, Thomas Holtz gave a length of 33 meters and an estimated weight equivalent to that of nine elephants, or around , assuming per elephant.

In 2016, using equations that estimate body mass based on the circumference of the humerus and femur of quadrupedal animals, it was estimated to be in weight. In 2019, Gregory S. Paul estimated the mass of ''A. giganteus'' in the range, based on newer titanosaur reconstructions. In 2020 Molina-Pérez and Larramendi estimated its length at and its weight similar to Paul's estimation at . Due to the incompleteness of the remains, any size estimates are subject to a large amount of error.

''"Antarctosaurus" jaxarticus and "Antarctosaurus" brasiliensis''

''"Antarctosaurus" jaxarticus'' and ''"Antarctosaurus" brasiliensis'' are both known from very fragmentary remains.

''"Antarctosaurus" jaxarticus'' is known from a single femur which was briefly reported as resembling a femur attributed to ''Jainosaurus'' (then called ''"Antarctosaurus" septentrionalis).'' Paleontologist Teresa Maryańska noted that, whilst ''A. jaxarticus'' was named, it was not properly described or diagnosed. The femur possibly belongs to the titanosaur clade Lithostrotia.

The type specimen of ''"Antarctosaurus" brasiliensis'' is only known from three fragmentary bones that are titanosaurian in nature; a partial left femur GP-RD-2, a partial right humerus GP-RD-3, and an incomplete dorsal vertebra (backbone) GP-RD-4. The femur is preserved and was estimated at if it were completed. The humerus is preserved and estimated at completed. The dorsal vertebra centrum is long.

Classification

''Antarctosaurus'' has been included in few phylogenetic analyses, only being added to the matrix of Philip Mannion ''et al.'' in 2019. It was coded along with '' Vahiny'', '' Jainosaurus'', ''Normanniasaurus'' and additional non-titanosaurs. ''Antarctosaurus'' placed as sister taxon to a clade of ''Vahiny'' and ''Jainosaurus'', consistently close to taxa of the clade Lognkosauria.

References

External links

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Dinosaur Mailing List
detailing the various species of ''Antarctosaurus'' and the remains assigned to them.
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detailing the problematic ''A.wichmannianus.''
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detailing the species that have been referred to ''Antarctosaurus''.
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discussing the material of ''Antarctosaurus wichmannianus'' and showing a reconstruction of the skull

{{Taxonbar, from=Q131738

Lithostrotia
Dinosaur genera
Santonian dinosaurs
Anacleto Formation
Dinosaurs of Argentina
Fossil taxa described in 1929
Taxa named by Friedrich von Huene