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. Besides Andesaurus, the formation also contains fossils of the theropods Buitreraptor and Giganotosaurus, as well as other sauropods including the 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)