| Antarctosaurus | |
|---|---|
 | |
| HypotheticalAntarctosaurus wichmannianus skull diagram showing the bones illustrated by von Huene in 1929. It's not certain that the mandible and braincase belong to the same individual or even same genus. | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropoda |
| Clade: | †Macronaria |
| Clade: | †Titanosauria |
| Clade: | †Colossosauria |
| Genus: | †Antarctosaurus von Huene, 1929 |
| Species | |
| Synonyms | |
| |
Antarctosaurus (/ænˌtɑːrktoʊˈsɔːrəs/; meaning "southern lizard") is agenus oftitanosauriansauropoddinosaur from the LateCretaceousPeriod of what is nowSouth America. Thetype species,Antarctosaurus wichmannianus, and a second species,Antarctosaurus giganteus, were described by prolificGermanpaleontologistFriedrich von Huene in 1929. Three additional species ofAntarctosaurus 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 evengenus. 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 1929monograph written bypaleontologistFriedrich von Huene.[2]Antarctosaurus does not refer to the continent ofAntarctica since it was first found inArgentina, although it does have the same derivation, from theGreek words αντι-,anti- meaning 'opposite of', αρκτός,arktos meaning 'north' andσαυρος,sauros meaning 'lizard'. The generic name refers to the animal'sreptilian nature and its geographical location on a southern continent.
Antarctosaurus wichmannianus is thetype species of the genus, named in 1929 after the discoverer of its remains in 1912, geologistRicardo Wichmann.[2] Von Huene used the nameA. wichmannianus to describe a large assemblage of bones, which are considered to come from theAnacleto Formation inRío Negro Province ofArgentina,[3] which is probably earlyCampanian in age.[4] Two additional limb bones, found in theChubut Province in 1924, were also referred toA. wichmannianus by von Huene in 1929.[2] Later studies, however, have doubted their referral to the species.[5]
Von Huene also named a fragmentary second species ofAntarctosaurus in the same 1929 monograph, which he tentatively calledcf.Antarctosaurusgiganteus because of its enormous size.[3] These fossils were recovered inNeuquén Province of Argentina, from thePlottier Formation,[3][6] which dates to theConiacian-Santonian stages of theLate Cretaceous Period.[4] The Plottier, like the younger Anacleto, is a member of theNeuquén Group.[4]
Very few remains are known of this species and it is regarded as anomen dubium by some.[7] Other researchers regardA. giganteus as a likely valid species but probably belonging to a new genus.[8] In 1969,Leigh Van Valen consideredA. wichmannianus andA. giganteus to be growth stages of the same species and favored the nameA. giganteus.[9] This idea is problematic becauseA. wichmannianus was named earlier in the same paper and it is known from more material, it should, therefore, getpriority overA. giganteus. The two species are also not from the samegeological formation which suggests they did not belong to the same time period.[10]
In 1933, Von Huene and Charles Matley described another species,Antarctosaurus septentrionalis, meaning "northern". The remains were found in theLameta Formation ofMadhya Pradesh State inIndia.[11][12] This species does preserve important anatomical information but has since been assigned to its own genus in 1994;Jainosaurus.[13]
Antarctosaurus jaxarticus fromKazakhstan is known from a singlefemur.[14][15] It was named bySoviet paleontologistAnatoly Riabinin in 1938,[14] and was the first sauropod species from Kazakhstan.[15] It was reported from a certain locality in theKyzylkum Desert, but the exact location is unknown. It may have come from theSyuksyuk Formation (originally described asDabrazinskaya Svita) which dates to the Santonian stage of the Late Cretaceous.[15][16] Other researchers have considered it as either, Titanosauridaeincertae sedis, as anomen dubium, or as anomen nudum.[7][16][17][18]
In 1970, two fragmentary limb bones and a partial vertebra were found in theAdamantina Formation (originally described asBauru Formation;[19] has also been reported as the São José do Rio Preto Formation[20]) of the northernParaná Basin inBrazil. The remains were described by their discoverers Fahad Moysés Arid and Luiz Dino Vizotto in 1971 asA. brasiliensis.[19][21] Other researchers have considered this species as either, anomen dubium,[7][8][21] or an indeterminate titanosaur.[22]
DescribingAntarctosaurus is problematic because thetype species consists of elements that are of questionable association and none of the species described are known from complete remains, which has caused a confusedtaxonomy of the genus.[7] Of the four additionalspecies that have been assigned toAntarctosaurus over the years, three have been considered dubious[3][7][17][21] and"Antarctosaurus" septentrionalis, was given its own genus,Jainosaurus.[13]
The remains that have been described belong tosauropods, most probably titanosaurs,[3] a group of large-bodied,quadrupedalherbivores, usually possessing a long neck and tail, with a small head.[23]
The assemblage of fossil remains that became known asA. wichmannianus were given the specimen number MACN 6904.[2] The known material includes severalskull fragments, including a braincase and an incompletemandible (lower jaw), a cervical (neck)vertebra, a caudal (tail) vertebra,rib fragments, and numerous limb bones including a femur which measures 1.39 meters (4.6 ft) tall. None of the individual fossils were designated theholotype specimen so MACN 6904 is considered to be an assemblage ofsyntypes.[17] The total length ofA. wichmannianus has been estimated at around 17 meters (56 ft).[24]
An additional femur andtibia were also referred by von Huene toA. wichmannianus; the femur, FMNH P13019, is over 1.85 meters (6.1 ft) tall. In one study the dimensions of this femur were used in aregression analysis to estimate the mass ofA. wichmannianus at about 34 metric tons (37 short tons).[25] The referral of the additional femur and tibia has been questioned by later researchers. In 2003Jaime Eduardo Powell tentatively referred them tocf. Argyrosaurus and in 2012 Philip Mannion and Alejandro Otero considered it an indeterminate titanosaur.[5]
The incompletemandible attributed toA. wichmannianus is squared-off at the front with each dentary bone being ''L'' shaped.[26] The teeth were restricted to the front of the lower jaw and were small and slender.[3] 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.[8]
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 arebbachisaurid sauropod similar toNigersaurus.[27][28][29] However, the jaw ofBonitasaura, described in 2004, is similar in overall shape and is clearly associated with titanosaur skeletal remains, indicating that the lower jaw may belong toA. wichmannianus after all.[30] In 2013 and 2018 respectively,Brasilotitan andBaalsaurus were described which also possessed squared-off jaws.[26][31] It was noted thatBrazilotitan, Bonitasaura, Antarctosaurus, and other titanosaurs show up three teeth peralveolus (tooth socket) whereas the rebbachisauridNigersaurus shows up to seven teeth.[31]Brazilotitan andBaalsaurus were described as a titanosaurs, closely relatedto A. wichmannianus.[26][31]
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.[7][27][32] In 2005,Jeffrey Wilson considered the braincase as being referable toNemegtosauridae but noted that other skull remains require further study.[17] A study, published in 2012 by Ariana Paulina Carabajal,CT scanned theA. wichmannianus braincase which revealed the complete brainendocast and theinner ear structures. The brain endocast and inner ear share several features with other titanosaurids such as shortolfactory tracts andolfactory bulbs that are horizontally projected.[33]
Powell compared the width of the cranium to the length of the limb bones of bothA. wichmannianus andSaltasaurus; this led him to conclude that the skull was proportionally small inA. wichmannianus, this might imply that the skull and limb elements could belong to different individuals or a differenttaxa. He noted, however, that the comparison was potentially misleading because the overall anatomy ofSaltasaurus is shorter and stouter which might facilitate a bigger skull.[8]
Von Huene assigned twotarsal (ankle) bones toA. wichmannianus, which he described as anastragalus and acalcanium. 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.[8]
Von Huene described acaudal vertebra which was found close to the skull material. This vertebra was the first caudal, belonging to the base of the tail just after thesacrum (vertebrae attached to the hip). The vertebra features abiconvex centrum, a feature shared with other titanosaurs.[3] Von Huene noted that the first caudal could possibly belong toLaplatasaurus.[3][27]
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.[27] There is a lack of field documentation to aid in the referral of all the material to one individual.[17][34] Powell thought it was probable that von Huene correctly assigned the material toA. 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.[8]
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The type specimen ofA. 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.[2] The two gigantic femora measure 2.35 meters (7.7 ft) in length, which are among the largest of any known sauropod.[25] Even though the femurs are large, they are also somewhat gracile in construction.[3]
A reconstruction ofA. giganteus, published in 1956 by Carlos Rusconi, was given a length around 30 meters (98 ft). In 1969, van Valen considered it as similar in size toGiraffatitan brancai (then calledBrachiosaurus brancai). Based on an earlier mass estimate ofG. brancai byEdwin Harris Colbert in 1962, van Valen gaveA. giganteus an estimated mass of about 80 metric tons (88 short tons).[9] In 1994,Gregory S. Paul estimated the weight of bothA. giganteus andArgentinosaurus at between 80 and 100 metric tons (88 and 110 short tons) and lengths of 30 to 35 meters (98 to 115 ft) long.[35] Extrapolating from the femur's parameters, a 2004 study by Gerardo Mazzetta and colleagues estimated the mass ofA. giganteus at approximately 69 metric tons (76 short tons); slightly smaller thanArgentinosaurus which in the same study was estimated at nearly 73 metric tons (80 short tons). This would makeA. giganteus among the heaviest known land animals.[25] In 2006,Kenneth Carpenter used the relatively short-neckedSaltasaurus as a guide and estimated a length of 23 meters (75 ft) long.[36] In an encyclopaedia supplementary, Thomas Holtz gave a length of 33 meters and an estimated weight equivalent to that of nine elephants, or around 33 to 65 metric tons (36 to 72 short tons), assuming 3.6 to 7.3 metric tons (4 to 8 short tons) per elephant.[37][38]
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 39.5 metric tons (43.5 short tons) in weight.[39] In 2019,Gregory S. Paul estimated the mass ofA. giganteus in the 45 to 55 metric tons (50 to 61 short tons) range, based on newer titanosaur reconstructions.[40] In 2020 Molina-Pérez and Larramendi estimated its length at 30.5 meters (100 ft) and its weight similar to Paul's estimation at 45 metric tons (50 short tons).[41] Due to the incompleteness of the remains, any size estimates are subject to a large amount of error.[9]
"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 toJainosaurus (then called"Antarctosaurus" septentrionalis).[14][16] PaleontologistTeresa Maryańska noted that, whilstA. jaxarticus was named, it was not properly described or diagnosed.[15] The femur possibly belongs to the titanosaur cladeLithostrotia.[16]
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 righthumerus GP-RD-3, and an incompletedorsal vertebra (backbone) GP-RD-4. The femur is 1.15 meters (3.8 ft) preserved and was estimated at 1.55 meters (5.1 ft) if it were completed. The humerus is 65 centimeters (2.13 ft) preserved and estimated at 95 centimeters (3.12 ft) completed. The dorsal vertebracentrum is 17 centimeters (6.7 in) long.[19]
Antarctosaurus has been included in few phylogenetic analyses, only being added to the matrix of Philip Mannionet al. in2019. It was coded along withVahiny,Jainosaurus,Normanniasaurus and additional non-titanosaurs.Antarctosaurus placed as sister taxon to a clade ofVahiny andJainosaurus, consistently close to taxa of the cladeLognkosauria.[42]
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