''Atacamatitan'' (meaning "

Atacama Desert The Atacama Desert () is a desert plateau located on the Pacific Ocean, Pacific coast of South America, in the north of Chile. Stretching over a strip of land west of the Andes Mountains, it covers an area of , which increases to if the barre ...

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 (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

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 b ...

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

from the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent 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 ''cre ...

Tolar Formation of

Chile Chile, officially the Republic of Chile, is a country in western South America. It is the southernmost country in the world and the closest to Antarctica, stretching along a narrow strip of land between the Andes, Andes Mountains and the Paci ...

Discovery and naming

In February 2000, with the purpose of looking for

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

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

National Museum of Brazil National may refer to: Common uses * Nation or country ** Nationality – a ''national'' is a person who is subject to a nation, regardless of whether the person has full rights as a citizen Places in the United States * National, Maryland, ...

heading to

Antofagasta Region The Antofagasta Region (, ) is one of Chile's Administrative divisions of Chile, sixteen first-order administrative divisions. Being the second-largest region of Chile in area, it comprises three provinces, Antofagasta Province, Antofagasta, El ...

. 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. In July 2001, a second expedition was organized and with this, the excavation of the formation. The results ended on the discovery of the

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

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 (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...

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

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

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 thr ...

s 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, ''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 (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

, the proximal end of a

humerus The humerus (; : humeri) 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 (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

, two dorsal vertebrae, posterior

caudal vertebrae Caudal vertebrae are the vertebrae of the tail in many vertebrates. In birds, the last few caudal vertebrae fuse into the pygostyle, and in apes, including humans, the caudal vertebrae are fused into the coccyx. In many reptiles, some of the caud ...

, dorsal

ribs The rib cage or thoracic cage is an endoskeletal enclosure in the thorax of most vertebrates that comprises the ribs, vertebral column and sternum, which protect the vital organs of the thoracic cavity, such as the heart, lungs and great vessels ...

and a possibly fragmentary element of the

sternum The sternum (: sternums or sterna) 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 bl ...

, 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 Casting, casts of organisms. Carried by water, these minerals fill the spaces within organic tissue. Because of the nature of the casts, p ...

. 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. 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'' and '' Uberabatitan'', 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 elongated section at the rear end of a bilaterian animal's body; in general, the term refers to a distinct, flexible appendage extending backwards from the midline of the torso. In vertebrate animals that evolution, evolved to los ...

. 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 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 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 Pneumatics (from Greek 'wind, breath') is the use of gas or pressurized air in mechanical systems. Pneumatic systems used in Industrial sector, industry are commonly powered by compressed air or compressed inert gases. A centrally located a ...

foramina In anatomy and osteology, a foramen (; : foramina, or foramens ; ) is an opening or enclosed gap within the dense connective tissue (bones and deep fasciae) of extant and extinct amniote animals, typically to allow passage of nerves, arter ...

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 MUSCULAR (DS-200B), located in the United Kingdom, is the name of a surveillance program jointly operated by Britain's Government Communications Headquarters (GCHQ) and the U.S. National Security Agency (NSA) that was revealed by documents release ...

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 Synapomorphy, 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 (cladistics), outgroup taxa, not ...

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. 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 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'' an ...

References