Argentinosaurus

The first Argentinosaurus bone, which is now thought to be a fibula (calf bone), was discovered in 1987 by Guillermo Heredia on his farm "Las Overas" about east of Plaza Huincul, in Neuquén Province, Argentina. Heredia, initially believing he had discovered petrified logs, informed the local museum, the Museo Carmen Funes, whose staff members excavated the bone and stored it in the museum's exhibition room. In early 1989, the Argentine palaeontologist José F. Bonaparte initiated a larger excavation of the site involving palaeontologists of the Museo Argentino de Ciencias Naturales, yielding a number of additional elements from the same individual. The individual, which later became the holotype of Argentinosaurus huinculensis, is catalogued under the specimen number MCF-PVPH 1. The additional material recovered included seven dorsal vertebrae (vertebrae of the back), the underside of the sacrum (fused vertebrae between the dorsal and tail vertebrae) including the first to fifth and some sacral ribs, and a part of a dorsal rib (rib from the flank). Bonaparte and Coria described the limb bone discovered in 1987 as an eroded tibia (shin bone), although the Uruguayan palaeontologist Gerardo Mazzetta and colleagues reidentified this bone as a left fibula in 2004. In 1996, Bonaparte referred (assigned) a complete femur (thigh bone) from the same locality to the genus, which was put on exhibit at the Museo Carmen Funes. This bone was deformed by front-to-back crushing during fossilization. In their 2004 study, Mazzetta and colleagues mentioned an additional femur that is housed in the La Plata Museum under the specimen number MLP-DP 46-VIII-21-3. Though not as strongly deformed as the complete femur, it preserves only the shaft and lacks its upper and lower ends. Both specimens belonged to individuals equivalent in size to the holotype individual. As of 2019, however, it was still uncertain whether any of these femora belonged to Argentinosaurus. ==Description==

Size Argentinosaurus is among the largest known land animals, although its exact size is difficult to estimate because of the incompleteness of its remains. A reconstruction of Argentinosaurus created by Gregory Paul in 1994 yielded a length estimate of . Later that year, estimates by Bonaparte and Coria suggesting a hind limb length of , a trunk length (hip to shoulder) of , and an overall body length of were published. In 2006, Kenneth Carpenter reconstructed Argentinosaurus using the more complete Saltasaurus as a guide and estimated a length of . In 2008, Jorge Calvo and colleagues used the proportions of Futalognkosaurus to estimate the length of Argentinosaurus at less than . In 2013, William Sellers and colleagues arrived at a length estimate of and a shoulder height of by measuring the skeletal mount in Museo Carmen Funes. In 2016, Paul estimated the length of Argentinosaurus at , but later estimated a greater length of or longer in 2019, restoring the unknown neck and tail of Argentinosaurus after those of other large South American titanosaurs. Paul estimated a body mass of for Argentinosaurus in 1994. In 2014 and 2018, Roger Benson and colleagues estimated the mass of Argentinosaurus at , but these estimates were questioned due to a very large error range and lack of precision. In 2016, using equations that estimate body mass based on the circumference of the humerus and femur of quadrupedal animals, Bernardo Gonzáles Riga and colleagues estimated a mass of based on an isolated femur; it is uncertain whether this femur actually belongs to Argentinosaurus. In the same year, Paul moderated his earlier estimate from 1994 and listed the body mass of Argentinosaurus at more than . While Argentinosaurus was definitely a massive animal, there is disagreement over whether it was the largest known titanosaur. Puertasaurus, Futalognkosaurus, Dreadnoughtus, Paralititan, "Antarctosaurus" giganteus, and Alamosaurus have all been considered to be comparable in size with Argentinosaurus by some studies, although others have found them to be notably smaller. The mass of the blue whale, however, which can be greater than , still exceeds that of all known sauropods. The (excavations on the sides of the centra) were proportionally small and positioned in the front half of the centrum. The vertebrae were internally lightened by a complex pattern of numerous air-filled chambers. Such camellate bone is, among sauropods, especially pronounced in the largest and longest-necked species. In both the dorsal and sacral vertebrae, very large cavities measuring were present. The dorsal ribs were tubular and cylindrical in shape, in contrast with other titanosaurs. probably had six sacral vertebrae (those in the hip region), although the last one is not preserved. The centra of the second to fifth sacral vertebrae were much reduced in size and considerably smaller than the centrum of the first sacral. The sacral ribs curved downwards. The second sacral rib was larger than the other preserved sacral ribs, though the size of the first is unknown due to its incompleteness. Sebastián Apesteguía, in 2005, argued the structures seen in Argentinosaurus, which he termed hyposphenal bars, are indeed thickened laminae that could have been derived from the original hyposphene and had the same function. Limbs |alt=Photo of the assigned upper thigh bone on exhibit at the Museo de La Plata The complete femur that was assigned to Argentinosaurus is long. The femoral shaft has a circumference of about at its narrowest part. Mazzetta and colleagues used regression equations to estimate its original length at , which is similar to the length of the other femur, and later in 2019 Paul gave a similar estimate of . By comparison, the complete femora preserved in the other giant titanosaurs Antarctosaurus giganteus and Patagotitan mayorum measure and , respectively. While the holotype specimen does not preserve a femur, it preserves a slender fibula (originally interpreted as a tibia) that is in length. When it was identified as a tibia, it was thought to have a comparatively short , a prominent extension at the upper front that anchored muscles for stretching the leg. However, as stated by Mazzetta and colleagues, this bone lacks both the proportions and anatomical details of a tibia, while being similar in shape to other sauropod fibulae. ==Classification==

Relationships within Titanosauria are amongst the least understood of all groups of dinosaurs. In 2002, Davide Pisani and colleagues recovered Argentinosaurus as a member of Titanosauria, and again found it to be in a clade with Opisthocoelicaudia and an unnamed taxon, in addition to Lirainosaurus. A 2003 study by Jeffrey Wilson and Paul Upchurch found both Titanosauridae and Andesauridae to be invalid; the Titanosauridae because it was based on the dubious genus Titanosaurus and the Andesauridae because it was defined on plesiomorphies (primitive features) rather than on synapomorphies (newly evolved features that distinguish the group from related groups). A 2011 study by Philip Mannion and Calvo found Andesauridae to be paraphyletic (excluding some of the group's descendants) and likewise recommended its disuse. In 2004, Upchurch and colleagues introduced a new group called Lithostrotia that included the more derived (evolved) members of Titanosauria. Argentinosaurus was classified outside this group and thus as a more basal ("primitive") titanosaurian. The basal position within Titanosauria was confirmed by a number of subsequent studies. A 2017 study by Carballido and colleagues recovered Argentinosaurus as a member of Lognkosauria and the sister taxon of Patagotitan. In 2018, González Riga and colleagues also found it to belong in Lognkosauria, which in turn was found to belong to Lithostrotia. Another 2018 study by Hesham Sallam and colleagues found two different phylogenetic positions for Argentinosaurus based on two data sets. They did not recover it as a lognkosaurian but as either a basal titanosaur or a sister taxon of the more derived Epachthosaurus. In 2019, Julian Silva Junior and colleagues found Argentinosaurus to belong to Lognkosauria once again; they recovered Lognkosauria and Rinconsauria (another group generally included in Titanosauria) to be outside Titanosauria. Another 2019 study by González Riga and colleagues also found Argentinosaurus to belong to Lognkosauria; they found this group to form a larger clade with Rinconsauria within Titanosauria, which they named Colossosauria. Topology according to Carballido and colleagues, 2017. }} Topology according to González Riga and colleagues, 2019. }} ==Palaeobiology==

|alt=Reconstructed mounted skeleton in side view The giant size of Argentinosaurus and other sauropods was likely made possible by a combination of factors; these include fast and energy-efficient feeding allowed for by the long neck and lack of mastication, fast growth and fast population recovery due to their many small offspring. Advantages of giant sizes would likely have included the ability to keep food inside the digestive tract for lengthy periods to extract a maximum of energy, and increased protection against predators. Sauropods were oviparous (egg-laying). In 2016, Mark Hallett and Matthew Wedel stated that the eggs of Argentinosaurus were probably only in volume, and that a hatched Argentinosaurus was no longer than and not heavier than . The largest sauropods increased their size by five orders of magnitude after hatching, more than in any other amniote animals. In 2013, Sellers and colleagues used a computer model of the skeleton and muscles of Argentinosaurus to study its speed and gait. Before computer simulations, the only way of estimating speeds of dinosaurs was through studying anatomy and trackways. The computer model was based on a laser scan of a mounted skeletal reconstruction on display at the Museo Carmen Funes. Muscles and their properties were based on comparisons with living animals; the final model had a mass of . Using computer simulation and machine learning techniques, which found a combination of movements that minimised energy requirements, the digital Argentinosaurus learned to walk. The optimal gait found by the algorithms was close to a pace (forelimb and hind limb on the same side of the body move simultaneously). The authors concluded with its giant size, Argentinosaurus reached a functional limit. Much larger terrestrial vertebrates might be possible but would require different body shapes and possibly behavioural change to prevent joint collapse. The authors of the study cautioned the model is not fully realistic and too simplistic, and that it could be improved in many areas. For further studies, more data from living animals is needed to improve the soft tissue reconstruction, and the model needs to be confirmed based on more complete sauropod specimens. ==Palaeoenvironment==

, Argentinosaurus in blue| alt=Silhouettes of dinosaurs from the Huincul Formation as size comparison Argentinosaurus was discovered in the Argentine Province of Neuquén. It was originally reported from the Huincul Group of the Río Limay Formation, These deposits were laid down during the Upper Cretaceous, either in the middle Cenomanian to early Turonian stages The deposits represent the drainage system of a braided river. Fossilised pollen indicates a wide variety of plants were present in the Huincul Formation. A study of the El Zampal section of the formation found hornworts, liverworts, ferns, Selaginellales, possible Noeggerathiales, gymnosperms (including gnetophytes and conifers), and angiosperms (flowering plants), in addition to several pollen grains of unknown affinities. The Huincul Formation is among the richest Patagonian vertebrate associations, preserving fish including dipnoans and gar, chelid turtles, squamates, sphenodonts, neosuchian crocodilians, and a wide variety of dinosaurs. Vertebrates are most commonly found in the lower, and therefore older, part of the formation. and several rebbachisaurids including Cathartesaura, Limaysaurus, and some unnamed species. Theropods including carcharodontosaurids such as Mapusaurus, abelisaurids including Skorpiovenator, Ilokelesia, and Tralkasaurus, noasaurids such as Huinculsaurus, paravians such as Overoraptor, and other theropods such as Aoniraptor and Gualicho have also been discovered there. Several iguanodonts are also present in the Huincul Formation. ==References==