Megalonyx prehistoric mammal fossil. 19th-century illustration of a fossil of the lower jaw of the prehistoric ground sloth Megalonyx jeffersonii, first discovered in 1796. It lived from 10 million to 11, 000 years ago in North America. This illustration is Plate 29 from the 1840 volume on fossil mammals ('Fossil Mammalia'), part of the multi-volume work 'The Zoology of the Voyage of HMS Beagle'. This work described the animals and fossil specimens collected and observed by the British naturalist Charles Darwin during the survey voyage of HMS Beagle during the years 1832 to 1836. This expedition established Darwin's reputation as a naturalist. Edited by Darwin, the work was published between 1838 and 1843. The volume on fossil mammals was written by British naturalist and palaeontologist Richard Owen.