''Andesaurus'' ( ; "Andes lizard") is a genus of Basal (phylogenetics), 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.

History of discovery

Andesaurus-Scale-Diagram-SVG-Steveoc86-001

In 1991, paleontologists Jorge Orlando Calvo and José Fernando Bonaparte named ''Andesaurus'', which refers to the Andes and also includes the Ancient Greek, 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 vertebral body, centra. Elements of the pelvis were also discovered, including two ischium, ischia and a pubis (bone), pubis bone, along with rib fragments and an incomplete humerus and femur.

Description

''Andesaurus'' was a medium-sized sauropod. Total length estimates place the known remains at Paul, G.S., 2010, ''The Princeton Field Guide to Dinosaurs'', Princeton University Press p. 205 to long,Holtz, Thomas R. Jr. (2012) ''Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages,'
Winter 2011 Appendix.
/ref> and around in body mass. 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 Somphospondyli, 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 Synapomorphy, 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 geologic formation, 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. Besides ''Andesaurus'', the formation also contains fossils of the theropods ''Buitreraptor'' and ''Giganotosaurus'', as well as other sauropods including the Rebbachisauridae, rebbachisaurid ''Limaysaurus''.

References

Bibliography

* Novas, F.E., Salgado, L., Calvo, J.O., & Agnolin, F. 2005. Giant titanosaur (Dinosauria, Sauropoda) from the Late Cretaceous of Patagonia. ''Revista del Museo Argentino de Ciencias Naturales 7(1): 37-41. * Salgado, L., Coria, R.A., & Calvo, J.O. 1997. Evolution of titanosaurid Sauropods. I: Phylogenetic analysis based on the postcranial evidence. ''Ameghiniana''. 34: 3-32. * Tidwell, V., Carpenter, K. & Meyer, S. 2001. New Titanosauriform (Sauropoda) from the Poison Strip Member of the Cedar Mountain Formation (Lower Cretaceous), Utah. In: Mesozoic Vertebrate Life. D. H. Tanke & K. Carpenter (eds.). Indiana University Press, Eds. D.H. Tanke & K. Carpenter. Indiana University Press. 139-165. * Upchurch, P., Barrett, P.M, & Dodson, P. 2004. Sauropoda. In: Weishampel, D.B., Dodson, P., & Osmolska, H. (Eds.). ''The Dinosauria'' (2nd Edition). Berkeley: University of California Press. Pp. 259–322.