| Atacamatitan | |
|---|---|
 | |
| Holotype SGO-PV-961 | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropoda |
| Clade: | †Macronaria |
| Clade: | †Titanosauria |
| Clade: | †Lithostrotia |
| Genus: | †Atacamatitan Kellneret al., 2011 |
| Type species | |
| †Atacamatitan chilensis Kellneret al., 2011 | |
Atacamatitan (meaning "Atacama Deserttitan") is agenus oftitanosauriansauropoddinosaurs from theLate CretaceousTolar Formation ofChile.
In February 2000, with the purpose of looking forMesozoic fossils in Chile, Chilean and Brazilian researchers mounted a preliminary expedition that was organized by theChilean National Museum of Natural History and theNational Museum of Brazil heading toAntofagasta Region. During the expedition, they discovered the deposits of theTolar Formation, located about 150 km north ofCalama town and 50 km east fromEl Abra copper mine.[1]
The deposits of this formation consists of well-stratified, red succession of breccias,conglomerates andsandstones.[2] In July 2001, a second expedition was organized and with this, the excavation of theformation. The results ended on the discovery of theholotype ofAtacamatitan:SGO-PV-961. The fossil remains were found in an outcrop surface of 2 m2 near Conchi Viejo town, Atacama Desert in Antofagasta Region, and they were associated to a single individual.[1] The geological age of this formation is uncertain, ranging from theLate Cretaceous to thePaleocene. However, the discovery ofAtacamatitan seems to support aCretaceous age.Atacamatitan was later named and described by Alexanderet al. 2011, becoming one of the most completetitanosaurs known from Chile. Thegeneric name,Atacamatitan, is in reference to the Atacama desert, were the remains were found, and the Greek wordΤιτάν, meaningtitan. Thespecific name,chilensis, refers to Chile. The holotype is currently housed at the Chilean National Museum of Natural History.[3]
The holotype, SGO-PV-961, consists of a rightfemur, the proximal end of ahumerus, twodorsal vertebrae, posteriorcaudal vertebrae, dorsalribs and a possibly fragmentary element of thesternum, other fragmented caudal vertebrae and indeterminatebones. Most elements are slightly distorted, have a reddish coloration and are quite ponderous, due to extensivepermineralization.[3] Although the body estimates are quite uncertain,Thomas Holtz estimated its possible weight between 8 and 16 t (8,000.0 and 16,000.0 kg).[4]
According to Kellner et al. 2011,Atacamatitan can be recognised in having dorsal centra with pleurocoels that are curved and do not point posteriorly, dorsal vertebrae with concave ventral surface, posterior caudal vertebrae with laterally compressed neural spine having a blade-like anterior margin, femur with the proximal end getting gradually reduced until two thirds of its total length.[3]
The two preserved dorsal vertebrae are fragmented, with partialneural arches. Both vertebrae are opisthocoelic. Dorsal vertebra SGO-PV-961a features an elongatedcentrum, withpleurocoels in the lateral sides that are rounded and less elongated compared to other titanosaurs. The neural arch is missing, and due to the preserverd dimensions, it is likely that it was wider than high. The second dorsal vertebra SGO-PV-961b is antero-posteriorly compressed with the neural arch partially preserved. These dorsal vertebrae differ from other titanosaurs such asGondwanatitan,Saltasaurus,Trigonosaurus andUberabatitan, due to the strongly concave ventral margin of the centrum. By taking into account all the characteristics of the dorsal vertebrae,Atacamatitan differs from other titanosaurs by having pleurocoels rounded and less elongated not pointing posteriorly.[3]
Although some caudal vertebrae were discovered, only two are well preserved. Both vertebrae came from the middle and posterior end of thetail. Caudal vertebra SGO-PV-961c was preserved only the centrum, it is dorsoventrally compressed and lacking pleurocoels. The centrum preserve two processes to attachchevrons, and the ventral and lateral surfaces are curved. Second caudal vertebra SGO-PV-961h is nearly complete, only lacking rightprezygapophysis. The centrum is similar to SGO-PV-961c, only being smaller and posteriorly oriented. The prezygapophyses are elongated and reach nearly 30% of the anterior centrum. Postzygapophyses are reduced and positioned at the ventral margin of theneural spine. The neural arch is preserved in the anterior half of the centrum.[3]
Ribs were unearthed too, the most complete is SGO-PV-961d, though it is unknown if the ribs ofAtacamatitan hadpneumaticforamina as evidenced in other titanosaurs. It is anteroposteriorly compressed with the dorsoventral diameter longer than the anteroposterior one.Capitulum andtuberculum are fragmented. The fragmentary sternal plate is thin and has a smooth border; due to its fragmentary nature, it is unclear the form. The right humerus is very fragmented, only preserving the proximal end and although the distal expansion is missing, it has a notable reduced distal end. Anteriorly, it preserves a developed depression formuscular attachment.[3]
The preserved right femur is more gracile compared to other elements, indicating that the animal had stronger forelimbs. This is seen as a possible and potentialautapomorphy for this species. It measures 1.10 m (110 cm), the femoral head is missing and posteriorly, it preserves a well developed fourthtrochanter, extending to the proximal quarter.[3]
In the original description,Atacamatitan was placed in theTitanosauridae.[3] However given the fragmentary nature of the holotype, most cladistic analysis excludeAtacamatitan. Nevertheless, in 2012, Rubilar-Rogers and Gutstein conducted a preliminary cladistic analysis which ended in a polytomy withinNemegtosaurus,Rapetosaurus,Titanosaurus and theSaltasauridae.Atacamatitan was placed within theLithostrotia.[5]
