Lamanna and his colleagues describe Suzhousaurus megatherioides , which means "giant sloth-like lizard from Suzhou," in the current issue of the Chinese journal Acta Geologica Sinica.
Adapted for Plants
Therizinosaurs had long necks, small heads, wide bodies, and unusually short tails. They also had large and powerful forelimbs that were tipped with three sharp claws, Lamanna said.
However, unlike most other theropods, which were meat-eaters, therizinosaurs were vegetarians.
"They've adapted to eating plants full time, and as a result their bodies became modified in very, very unusual ways compared to other theropods," Lamanna said.
He added that the claws were most likely used to fend off predators and may also have played a role in pulling tree branches toward their mouths, as in their namesake giant ground sloths.
Within ten million years of therizinosaurs' first appearance, they began to grow large, perhaps for protection or to better digest more plant food.
"We're not sure, but it does tell us that [this group] attained large size very rapidly after they first appeared," Lamanna said.
The Gobi at the time of Suzhousaurus was more lush than it is today, noted study co-author Kenneth Lacovara, a geologist at Drexel University in Philadelphia.
Conifer and fern trees with underbrush of low-lying ferns likely filled a semi-arid landscape dotted with calm lakes that periodically dried up, according to his analysis of the sedimentary deposits.
Despite the periodic dry spells, Lacovara noted the environment contained enough vegetation to support the relatively large Suzhousaurus.
"In order to support a breeding population of animals that weigh as much as a sack of cows, you need an especially productive environment at the level of photosynthesis," he said.
"So it was obviously lush enough to keep these guys in business."
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