Therizinosaur: The Mystery of the Sickle Clawed Dinosaur

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The Therizinosaur Nothronychus, plesiosaur and shark. Artwork by Victor Leshyk.


The Therizinosaur Nothronychus, plesiosaur and shark. Artwork by Victor Leshyk.


Much of North America was under water 93 million years ago. The Cretaceous Interior Seaway separated today’s North American land mass. Dominant predators in these ancient seas included gigantic plesiosaurs, fishes and sharks.


Mosasaurs were giant marine lizards, distantly related to living monitor lizards, who roamed the seas in the Cretaceous Period.


Several kinds of pliosaurs swam the seas of what is now southern Utah and northern Arizona in the time of the sickle clawed dinosaur Nothronychus about 93 million years ago. Pliosaurs were marine reptiles, not dinosaurs.

The sea turtle Desmatochelys has been excavated from marine beds in northern Arizona.Desmatochelys belongs to an extinct group of sea turtles thought to have fed on ammonites, Mesozoic Era relatives of the modern Nautilus.


Reconstruction of the bone bed where the Therizinosaur fossil was found, in marine sediments at the bottom of the Cretaceous Interior Seaway. How did the sickle clawed dinosaur Nothronychus, an animal who lived on land, wind up in deposits 60 miles out to sea? Visit the museum to explore this mystery.


, the sickle clawed dinosaur. This is the most complete Therizinosaur specimen known.

Zuniceratops, a member of the great ceratopsian lineage that includes Triceratops, was discovered by an Arizona Museum of Natural History expedition to the Arizona-New Mexico border. Zuniceratops lived in the same region and at the same time as Nothronychus.


The Arizona Museum of Natural History discovered the first Nothronychus known from North America. Casts of the fossil bones placed on the illustration show which bones of the Therizinosaur were recovered. You may see real fossil bones from the animal nearby, and put together a Therizinosaur puzzle.


Possibly the Therizinosaur used its long claws to stir up the bottom mud in swamps to feed on small life forms such as crayfish and amphibians. Artwork by Victor Leshyk.



Perhaps Nothronychus used mangrove swamps to hide from predators, to eat mangrove leaves, or to eat crabs. The claws here are raking leaves from branches towards the mouth. Artwork by Victor Leshyk.