| Named By: | Ismar de Souza Carvalho, Leonardo dos Santos Avilla & Leonardo Salgado in 2003 |
| Time Period: | 125-100 Ma Early Cretaceous |
| Location: | Brazil, Legal Amazon - Itapecuru Formation |
| Size: | Estimated up to 12 meters long, but lack of remains make it impossible to be certain |
| Diet: | Herbivore |
| Fossil(s): | Partial post cranial remains including dorsal (back) and caudal (tail) vertebrae, ribs and a fragmentary pelvis |
| Classification: | | Chordata | Reptilia | Dinosauria | Saurischia | Sauropodomorpha | Sauropoda | Diplodocoidea | |
Amazonsaurus ( AM-a-zan-SAWR-as, "Amazon lizard") is a genus of diplodocoid sauropod dinosaur from the Early Cretaceous Period of what is now South America. It would have been a large-bodied quadrupedal herbivore with a long neck and whiplash tail. Although more derived diplodocoids were some of the longest animals ever to exist, Amazonsaurus was probably not more than 12 meters (40 ft) long. Gregory S. Paul estimated in 2010 its weight at 5000 kg.
Despite the fact that other dinosaurs have been found in Brazil, this is the first named genus from territory in the Amazon Basin. The generic name is derived from the Brazilian Legal Amazon region and the Greek word sauros ("lizard"). There is one named species, (A. maranhensis), which is named after the Brazilian state of Maranhao. Both genus and species were named in 2003 by Brazilian paleontologists Ismar de Souza Carvalho and Leonardo dos Santos Avilla, and their Argentine colleague, Leonardo Salgado.
Fossils of Amazonsaurus, including some back and tail vertebrae, ribs, and fragments of the pelvis, are the only dinosaur remains identifiable at the generic level from the Itapecuru Formation of Maranhao. This geologic formation dates back to the Aptian through Albian epochs of the Early Cretaceous Period, or about 125 to 100 million years ago. Amazonsaurus was recovered in sediments which are interpreted by geologists as floodplain deposits near a river delta.
The tall neural spines on the tail vertebrae identify Amazonsaurus as a diplodocoid sauropod, but the fragmentary nature of the only known specimen makes it difficult to place A. maranhensis more specifically within the superfamily Diplodocoidea. However, some features of these vertebrae suggest it may be a late-surviving member of a line of basal diplodocoids. At least one published cladistic analysis shows Amazonsaurus to be more derived than rebbachisaurids, but basal to dicraeosaurids and diplodocids within Diplodocoidea (Salgado et al., 2004).