Andesaurus

Andesaurus ( ; "Andes lizard") is a genus of basal titanosaurian sauropod dinosaur which existed during the middle of the Cretaceous Period in South America. Like most sauropods, it would have had a small head on the end of a long neck and an equally long tail. "Andesaurus" was a very large animal, as were many others of its relatives, which included the largest animals ever to walk the Earth.
History of discovery.
In 1991, paleontologists Jorge Orlando Calvo and José Fernando Bonaparte named "Andesaurus", which refers to the Andes and also includes the Greek word "sauros" ("lizard"), because of the proximity of this animal's remains to the Andes. "Andesaurus" fossils were found by Alejandro Delgado, after whom the single known species ("A. delgadoi") is named.
The only known material of "Andesaurus" is a partial skeleton consisting of a series of four vertebrae from the lower back, as well as 27 tail vertebrae, divided up into two series from separate parts of the tail. The vertebrae from the middle part of its tail had elongated centra. Elements of the pelvis were also discovered, including two ischia and a pubis bone, along with rib fragments and an incomplete humerus and femur. 
Description.
Total length estimates place "Andesaurus" at to long. In 1995, Jan Peczkis created mass estimates of dinosaurs and placed them into order of magnitude categories (each category was then subdivided into thirds); using a mass estimation technique that extrapolates from the circumference of the femur, "Andesaurus" was listed in the category. Gregory S. Paul, however, lists "Andesaurus" as in weight. In 2020 Molina-Perez and Larramendi estimated a total length of and .
The dorsal vertebrae of "Andesaurus" bear the typical features of an centrum and large lateral (pneumatic depression). Unlike related "Phuwiangosaurus", there is no ventral keel under any of the vertebrae, and there is a pneumatic feature on the front of the , the latter being found in most neosauropods. Potentially diagnostic for "Andesaurus", there is an additional ridge supporting the lower articulation for the ribs (), although poor preservation in other vertebrae means it can only be identified on one bone. The of "Andesaurus" are smaller than more basal sauropods, but less reduced than in "Argentinosaurus" or "Epachthosaurus", where the surface is only articular ridges, and also less reduced than more derived titanosaurs where the articulation is entirely absent. Pre- and post-spinal are present in the vertebrae of "Andesaurus", like in other somphospondylans.
Tail vertebrae (caudals) of "Andesaurus" are slightly , where the anterior face is concave and the posterior face of convex. This convexity is less prominent than more derived titanosaurs, but is still diagnostic of the clade as a whole. The first caudal has a flat anterior and slightly convex posterior face, different from the subsequent vertebrae similar to in "Epachthosaurus". A prominent depression is present on the bottom surface of some anterior caudals, a feature present in diplodocids and multiple titanosaurs. Anterior caudals are shorter proportionally, the bones becoming almost double the proportional length towards the end of the tail. Lateral pleurocoels are present on some vertebrae, as well as small lateral foramina. These foramina are known in some titanosaurs and non-titanosaurs, but their phylogenetic distribution is poorly understood. Like in all titanosauriformes, the neural arch is on the anterior portion of the centrum, and the neural spines are elongate and rectangular. The variation along the caudal series is similar to "Epachthosaurus" and "Malawisaurus".
Classification.
Several plesiomorphic (primitive) features characterize "Andesaurus" as the most basal known member of Titanosauria. In fact, this clade has been defined to contain "Andesaurus", "Saltasaurus", their most recent common ancestor, and all of its descendants. The most prominent plesiomorphy is the articulations between its tail vertebrae. In most derived titanosaurs, the tail vertebrae articulate with ball-and-socket joints, with the hollowed-out socket end on the front ("procoelous" caudal vertebrae), while in "Andesaurus", both ends of the vertebrae are flat ("amphiplatyan" caudals), as seen in many non-titanosaurian sauropods. "Andesaurus" itself is only characterized by a single feature, the tall neural spines on top of its back vertebrae, and needs further study.
Some other basal titanosaurs from Argentina, including "Argentinosaurus" and "Puertasaurus", were also sauropods of enormous size. The most derived group of titanosaurs, the Saltasauridae, included some of the smallest known sauropods, including "Saltasaurus" itself. Thus it is possible that the largest sizes were attained among the more basal members of the clade.
Palaeoecology.
These fossils were discovered in the Candeleros Formation, the oldest formation within the Neuquén Group of Neuquén Province, Argentina. This formation dates to the Cenomanian stage of the Late Cretaceous period, about 99.6 to 97 million years ago. For the most part, the Candeleros represents an ancient braided river system, and besides "Andesaurus", also contains fossils of theropods like "Buitreraptor" and the enormous "Giganotosaurus", as well as other sauropods such as the rebbachisaurid "Limaysaurus".