Probactrosaurus, which means “before Bactrosaurus”, is an early herbivorous iguanodont dinosaur hadrosauroid. It was found in China during the Late Cretaceous period.

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A Soviet-Chinese expedition discovered the remains of an euornithopod near Maortu in Inner Mongolia between 1959 and 1960. Probactrosaurus bebiensis was the type species, which was first described by A. K. Rozhdestvensky and named in 1966. Rozhdestvensky believed that Probactrosaurus was the direct ancestor to Bactrosaurus. This hypothesis has been discarded.

This specific name refers specifically to the Gobi desert. The Dashuigou Formation layers contained the holotype specimen PIN 2232/1. It was a partial skull and skeleton. PIN 2232-10 was also found, along with many other fragments.

Based on fragmentary material, Rozhdestvensky also referred to a second species as Probactrosaurus. Its exact name refers the Alxa League. David B. Norman published in 2002 a revision to the genus. He reported that the holotype specimen P. alshanicus (the back of a skull) had been lost during transit from Moscow to Beijing. He concluded that the species was synonym for P. gobiensis.

Based on the 1992 holotype IVPP V.11333, Lu Junchang identified a third species as Probactrosaurus mahongshanensis. This specific name refers specifically to the Mazong Shan area. This form is more closely related than ever to P. gobiensis, Equijubus, and Altirhinus. It is commonly referred as “Probactrosaurus” mazongshanensis. It was transferred to the new genus Gongpoquansaurus, in 2014.

Source: Wikipedia