Probactrosaurus

*Probactrosaurus gobiensis*

***Probactrosaurus*** (meaning "before club lizard") is an extinct genus of ornithopod hadrosaurid dinosaur that lived in China during the Late Cretaceous period. The type and only species of this genus is *P. gobiensis*.

## Discovery and species

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. 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 Dashuigou Formation. Another partial skeleton, PIN 2232-10, was found along with numerous other fragments.

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. alshanicus*, 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*.

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. 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*. It was moved to the new genus *Gongpoquansaurus* in 2014.

## Description

*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. *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.

## Classification

*Probactrosaurus* was originally assigned by Rozhdestvenky to the Iguanodontidae. Today it is seen as a basal member of the Hadrosauroidea, relatively closely related to the Hadrosauromorpha.

## JPInstitute.com Description

Probactrosaurus was an early member of the family known as the iguanodonts. In fact, it is kind of a cross between those dinosaurs and the duck-billed hadrosaurs. Like duck-billed dinosaurs, it had hundreds of teeth packed closely together to form what is called a dental battery. These batteries were used to grind tough plant fiber into mushy pulp so that it could be digested easier. It could walk on either two or four legs.

Probactrosaurus represents an evolutionary step between Iguanodon and Bactrosaurus. It had a number of features found on both iguanodonts and the later hadrosaurs, and it marks an important step in the evolution of two important families. The type specimen is considered by some to be a juvenile based on the large size of its feet.

## Links

https://web.archive.org/web/20030813131754/http://www.jpinstitute.com/dinopedia/dinocards/dc\_proba.html