Andesaurus ( ; "Andes lizard") is a genus of basal 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
In 1991,
Jorge Orlando Calvo and José Fernando Bonaparte named
Andesaurus, which refers to the
Andes and also includes the
Ancient Greek word
sauros ("lizard"), because of the proximity of this animal's remains to the Andes.
Andesaurus 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 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.
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.] 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.
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 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
Rebbachisauridae Limaysaurus.
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.
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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.
Further reading
-
Calvo, J.O. & Bonaparte, J.F. 1991. "Andesaurus delgadoi gen. et sp. nov. (Saurischia-Sauropoda), dinosaurio Titanosauridae de la Formacion Rio Limay (Albiano-Cenomaniano), Neuquén, Argentina" Andesaurus Ameghiniana''. 28: 303-310. In
-
Wilson, J.A. & Upchurch, P. 2003. A revision of Titanosaurus Lydekker (Dinosauria – Sauropoda), the first dinosaur genus with a ‘Gondwanan’ distribution. Journal of Systematic Palaeontology 1(3): 125–160.
