Therizinosaurs migrated into South America (along with many other clades) during the great faunal interchange when North and South America linked with each other, about 3 million years ago.  In the plains, viriosaurs (basal ornithopods) could not be shaken from their dominance (this clade actually migrated north, and ousted the native small herbivores from their niches).  However, the therizinosaurs did find an open niche in South America's high mountains, the Andes.  One group of North American therizinosaurs, the seculsaurids, had already conquered the mountains of their home continent and the leap from the Rockies to the Andes was, evolutionarily speaking, a short one.
(Text by Daniel Bensen)

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