Amurosaurus

***Amurosaurus*** (/əˌmʊərəˈsɔːrəs/; "Amur lizard") is a genus of lambeosaurine hadrosaurid dinosaur found in the latest Cretaceous period (66 million years ago) of eastern Asia. Like most lambeosaurs, it would have been a primarily bipedal herbivore with a "duckbill" shaped snout and a hollow crest on top of its head, although such a crest has not been found. Fossil bones of adults are rare, but an adult would most likely have been at least 6 metres (20 ft) long. According to Gregory S. Paul, it was about 8 metres (26 ft) long and weighed about 3,000 kilograms (6,600 lb).

Russian paleontologists Yuri Bolotsky and Sergei Kurzanov first described and named this dinosaur in 1991. The generic name is derived from the Amur River and the Greek word *sauros* ("lizard"). The Amur (called Heilongjiang or "Black Dragon River" in Chinese) forms the border of Russia and China, and is near where this dinosaur's remains were found. There is one known species (*A. riabinini*), named in honor of the late Russian paleontologist Anatoly Riabinin, who conducted the first Russian expeditions to recover dinosaur remains in the Amur region in 1916 and 1917.

All fossils of *Amurosaurus* have been recovered from a single bonebed locality, discovered in 1984 within the city limits of Blagoveschensk in the Amur Oblast of far eastern Russia. This bonebed is found in the Udurchukan Formation, the oldest geologic formation in the Tsagayan Group of far eastern Russia and northeastern China. This formation is thought to belong to the Maastrichtian stage of the Late Cretaceous Period, and was deposited about 68 million years ago, or just prior to the Lancian faunal stage of North America. The sediments were laid down in the floodplain of a river, which transported the fossils, but only a short distance, judging by the randomly assorted, disarticulated, but well-preserved bones within the bonebed, including fragile skull elements. Only a small section of the bonebed has been uncovered, but 90% of the remains found so far belong to lambeosaurines like *Amurosaurus*, mostly juveniles, with the rest belonging to other taxa, such as the hadrosaurine *Kerberosaurus*. Theropod teeth are also abundant, and there are many toothmarks on the bones, made by predators or scavengers.

The holotype, or original specimen, consists of only a maxilla (upper jaw bone), and a dentary (lower jaw bone), both from the left side of the same individual. However, most of the other bones of the skull and skeleton have also been preserved in the bonebed, albeit of many different individuals. This other material was described more recently, making *Amurosaurus* the most abundant and completely known Russian dinosaur.

## Description

*Amurosaurus* is characterized by many autapomorphies, or unique features, of the skull, as well as the sigmoidal shape of the ulna (a lower arm bone) when viewed from the front or side. Most other known lambeosaurines have hollow crests on the top of their skulls, and although the bones that would make up such a crest are unknown in this dinosaur, the bones of the roof of the skull are modified to support one, so it can be assumed that *Amurosaurus* was crested as well.