An illustration of a Diplodocus skeleton and the caudal vertebrae (A and B). Diplodocus is a genus of diplodocid sauropod dinosaur whose fossils were first discovered in 1877 by S. W. Williston. It lived in what is now western North America at the end of the Jurassic Period. Diplodocus is one of the more common dinosaur fossils found in the Upper Morrison Formation, a sequence of shallow marine and alluvial sediments deposited about 150 to 147 million years ago, in what is now termed the Kimmeridgian and Tithonian stages.
Encyclopaedia Britannica (11th ed., vol. 8) (New York, NY: The Encyclopaedia Britannica Company, 1910)