''Atacamatitan'' (meaning "

Atacama Desert The Atacama Desert ( es, Desierto de Atacama) is a desert plateau in South America covering a 1,600 km (990 mi) strip of land on the Pacific coast, west of the Andes Mountains. The Atacama Desert is the driest nonpolar desert in the ...

titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...

") is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

of titanosaurian

sauropod Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their ...

dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

that lived in

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the souther ...

during the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

period.

Discovery and naming

In February 2000, with the purpose of looking for

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Creta ...

fossils in Chile, Chilean and Brazilian researchers established a preliminary expedition that was organized by the Chilean National Museum of Natural History and the

National Museum of Brazil The National Museum of Brazil ( pt, Museu Nacional) is the oldest scientific institution of Brazil. It is located in the city of Rio de Janeiro, where it is installed in the Paço de São Cristóvão (Saint Christopher's Palace), which is ins ...

heading to

Antofagasta Region The Antofagasta Region ( es, Región de Antofagasta, ) is one of Chile's sixteen first-order administrative divisions. The second-largest region of Chile in area, it comprises three provinces, Antofagasta, El Loa and Tocopilla. It is border ...

. During the expedition, they discovered the deposits of the Tolar Formation, located about 150 km north of Calama town and 50 km east from El Abra copper mine. The deposits of this formation consists of well-stratified, red succession of breccias, conglomerates and

sandstones Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

. In July 2001, a second expedition was organized and with this, the excavation of the

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondary ...

. The results ended on the discovery of the

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...

of ''Atacamatitan'': 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. The geological age of this formation is uncertain, ranging from the

to the

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...

. However, the discovery of ''Atacamatitan'' seems to support a

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

age. ''Atacamatitan'' was later named and described by Alexander et al. 2011, becoming one of the most complete

titanosaur Titanosaurs (or titanosaurians; members of the group Titanosauria) were a diverse group of sauropod dinosaurs, including genera from all seven continents. The titanosaurs were the last surviving group of long-necked sauropods, with taxa still th ...

known from Chile. The generic name, ''Atacamatitan'', is in reference to the Atacama desert, were the remains were found, and the greek word ''Τιτάν'', meaning

. The

specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...

, ''chilensis'', refers to Chile. The holotype is currently housed at the Chilean National Museum of Natural History.

Description

The holotype, SGO-PV-961, consists of a right

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...

, the proximal end of a

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...

, two dorsal vertebrae, posterior

caudal vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

, dorsal

ribs The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a semi ...

and a possibly fragmentary element of the

sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. ...

, other fragmented caudal vertebrae and indeterminate bones. Most elements are slightly distorted, have a reddish coloration and are quite ponderous, due to extensive

permineralization Permineralization is a process of fossilization of bones and tissues in which mineral deposits form internal casts of organisms. Carried by water, these minerals fill the spaces within organic tissue. Because of the nature of the casts, permineral ...

. Although the body estimates are quite uncertain, Thomas Holtz estimated its possible weight between . 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.

Vertebrae

The two preserved dorsal vertebrae are fragmented, with partial

neural arches The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

. Both vertebrae are opisthocoelic. Dorsal vertebra SGO-PV-961a features an elongated centrum, with pleurocoels 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 as '' Gondwanatitan'', '' Saltasaurus'', ''

Trigonosaurus ''Trigonosaurus'' (meaning "triangle lizard" after Triangulo Mineiro, where it was found), is a genus of saltasaurid dinosaurs from the Maastrichtian Serra da Galga Formation of Brazil. The type species, ''Trigonosaurus pricei'', was first descr ...

'' and ''

Uberabatitan ''Uberabatitan'' (meaning "Uberaba titan", in reference to where it was found) is a genus of titanosaurian sauropod dinosaur from the Late Cretaceous of Brazil. It is known from bones including neck, back, and tail vertebrae, pelvic bones, and l ...

'', 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. Although some caudal vertebrae were discovered, only two are well preserved. Both vertebrae came from the middle and posterior end of the

tail The tail is the section at the rear end of certain kinds of animals’ bodies; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals ...

. Caudal vertebra SGO-PV-961c was preserved only the centrum, it is dorsoventrally compressed and lacking pleurocoels. The centrum preserve two processes to attach chevrons, and the ventral and lateral surfaces are curved. Second caudal vertebra SGO-PV-961h is nearly complete, only lacking right

prezygapophysis The articular processes or zygapophyses ( Greek ζυγον = " yoke" (because it links two vertebrae) + απο = "away" + φυσις = " process") of a vertebra are projections of the vertebra that serve the purpose of fitting with an adjacent ver ...

. 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 the

neural spine The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

. The neural arch is preserved in the anterior half of the centrum.

Forelimb and hindlimb

Ribs were unearthed too, the most complete is SGO-PV-961d, though it is unknown if the ribs of ''Atacamatitan'' had pneumatic

foramina In anatomy and osteology, a foramen (;Entry "foramen"
in

as evidenced in other titanosaurs. It is anteroposteriorly compressed with the dorsoventral diameter longer than the anteroposterior one. Capitulum and tuberculum 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 for muscular attachment. 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 potential

autapomorphy In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to t ...

for this species. It measures , the femoral head is missing and posteriorly, it preserves a well developed fourth trochanter, extending to the proximal quarter.

Classification

In the original description, ''Atacamatitan'' was placed in the

Titanosauridae Lithostrotia is a clade of derived titanosaur sauropods that lived during the Early Cretaceous and Late Cretaceous. The group was defined by Upchurch ''et al.'' in 2004 as the most recent common ancestor of '' Malawisaurus'' and '' Saltasaurus ...

. However given the fragmentary nature of the holotype, most cladistic analysis exclude ''Atacamatitan''. Nevertheless, in 2012, Rubilar-Rogers and Gutstein conducted a preliminary cladistic analysis which ended in a polytomy within '' Nemegtosaurus'', '' Rapetosaurus'', '' Titanosaurus'' and the Saltasauridae. ''Atacamatitan'' was placed within the Lithostrotia.

References