| Probactrosaurus Temporal range: Early Cretaceous,
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| Complete skeleton mounted at the Giga Dinosaur Exhibition 2017 | |
Scientific classification 
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| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Ornithopoda |
| Superfamily: | †Hadrosauroidea |
| Genus: | †Probactrosaurus Rozhdestvensky, 1966 |
| Type species | |
| †Probactrosaurus gobiensis Rozhdestvensky, 1966
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| Synonyms | |
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Probactrosaurus (meaning "before Bactrosaurus") is an early herbivorous hadrosauroid iguanodont dinosaur. It lived in China during the Early Cretaceous period.
In 1959 and 1960 a Soviet-Chinese expedition uncovered the remains of a euornithopod in Inner Mongolia near Maortu. The type species is Probactrosaurus gobiensis, described and named by A. K. Rozhdestvensky in 1966.[2] The generic name refers to Rozhdestvensky's hypothesis that Probactrosaurus would be the direct ancestor of Bactrosaurus, a notion now discarded.
The specific name refers to the Gobi Desert. The holotype specimen, PIN 2232/1, a partial skeleton with skull, was found in layers of the Miaogou Formation (Maortu locality; originally interpreted as the nearby Dashuigou Formation).[1] Another partial skeleton, PIN 2232-10, was found along with numerous other fragments.[2]
In 1966 Rozhdestvensky also named a second species, Probactrosaurus alashanicus, based on fragmentary material. Its specific name refers to the Alxa League. In 2002 David B. Norman published a revision of the genus, in which he reported the holotype specimen of P. alashanicus, the back of a skull, had been lost after being dispatched from Moscow to Beijing. He concluded that the species was a synonym of P. gobiensis.[3]
In 1997 Lü Junchang named a third species, Probactrosaurus mazongshanensis, based on holotype IVPP V.11333 found in 1992. The specific name refers to the Mazong Shan region.[4] Today, this form is seen as more closely related to Equijubus and Altirhinus rather than to P. gobiensis and is therefore commonly referred to as "Probactrosaurus" mazongshanensis.[3] It was moved to the new genus Gongpoquansaurus in 2014.[5]
Probactrosaurus was a herbivorous dinosaur. In 2010 Gregory S. Paul estimated its length at 5.5 metres (18 feet) and its weight at one tonne.[6] Probactrosauurus was lightly built, with relatively long and slender arms and hands and only a small thumb spike. It had a narrow snout, an elongated lower jaw and tooth batteries, each consisting of a superimposed double row of flattened cheek teeth; a third row of replacement teeth was incipient. Probably predominantly quadrupedal, it shared some common features with the later duck-billed dinosaurs.[7]
Probactrosaurus was originally assigned by Rozhdestvenky to the Iguanodontidae.[2] Today it is seen as a basal member of the Hadrosauroidea, relatively closely related to the Hadrosauromorpha.[8]
