Until the discovery of 90-million-year-old Nothronychus in New Mexico, therizinosaurs were known only from fossils found in Asia. Giant ground sloths evolved in South America and reached North America around nine million years ago. Another similarity is that both giant ground sloths and therizinosaurs represent migrations into North America from other places. Nothronychus was named in 2001 by my colleagues Jim Kirkland (Utah Geological Survey) and Doug Wolfe (Zuni Dinosaur Institute for Geosciences) - the name means "sloth claw". Both groups evolved into large-bodied bipedal plant-eaters. Despite their very different evolutionary origins, giant grounds sloths and therizinosaurs share some remarkable similarities: long arms ending in huge claws, wide bodies to accommodate a long digestive tract for processing plant matter, short robust legs, and short tails. However, therizinosaurs evolved into big feathery herbivores. Therizinosaurs, like this Nothronychus mounted at the Natural History Museum of Utah in Salt Lake City, are part of Maniraptora, a group of theropod dinosaurs that also includes predators like Velociraptor and our living dinosaurs, the birds. As a group, sloths are related to armadillos and anteaters. Like the little slow-moving, tree-dwelling sloths of today, giant ground sloths, like this Paramylodon mounted at Western Science Center, were herbivorous. One of the most bizarre is that of giant ground sloths and therizinosaurs. And yet, there are some amazing cases of convergent evolution among them. In many ways, dinosaurs and mammals are very different types of animal.
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