Fossils of Futalognkosaurus were found in the Neuquén province of Argentina in 2000, and were scientifically described in 2007. The genus name is derived from the localindigenous languageMapudungun and is pronounced foo-ta-logn-koh-sohr-us: "futa" means "giant" and "lognko" means "chief". It is based on three fossil specimens, yielding an estimated 70% of the skeleton in total. The fossil team described the find as "the most complete giant dinosaur known so far".
Description
The holotype of the type species, Futalognkosaurus dukei, was originally estimated at in length. In 2008 this was down-sized to. In 2012 Holtz estimated it at long and with an approximate weight of 43.5 to 51 tonnes. An estimate by Gregory S. Paul in 2016 was that Futalognkosaurus had a maximum length of and a weight of 50+ tonnes . In 2016, using equations that estimate body mass based on the circumference of the humerus and femur of quadrupedal animals, it was given an estimated weight of 38.1 tonnes. In 2019, Paul estimated the weight of the holotype specimen, at 29 tonnes, and later, in 2020, Molina-Pérez and Larramendi estimated its length at, and its weight at 30 tonnes. Its long neck contained 14 vertebrae, and was over due to its extremely tall neural spines which had a distinctive . The hips were also extremely large and bulky, reaching a width of nearly.
Classification
In their phylogenetic analysis, Calvo and colleagues found Futalognkosaurus to be a member of the Titanosauridae, and most closely related to Mendozasaurus. They defined a new clade for the group containing both Futalognkosaurus and Mendozasaurus, their common ancestor, and all descendants, which they named the Lognkosauria. The authors found Malawisaurus to be the sister group of this new clade. Another, much later member of Lognkosauria is the colossal Puertasaurus, which may be the biggest dinosaur so far known. Besides Futalognkosaurus, other fauna was discovered in the Futalognko site, including two further undescribed sauropod taxa, specimens of Megaraptor, Unenlagia and some pleurodiran turtles. The following cladogram shows the results of an analysis by Calvo et al. in 2007, where they placed Futalognkosaurus within the group Titanosauria, more precisely the clade Lognkosauria:
