For 225 years, confusion and controversy have surrounded the spelling and/or authorship of Megalonyx, a giant ground sloth from North America. In a review of the earliest paleontology research, Prof. Loren Babcock shows that Megalonyx was validly named according to the rules of zoological nomenclature by Thomas Jefferson in 1799, even though its type species, M. jeffersonii, was not described until nearly a quarter-century later. Megalonyx was the first-named genus of fossil vertebrate animal from the United States. Publication of Jefferson's study on this giant ground sloth marked the beginning of the science of paleontology in America. This interesting story was published in ZooKeys, a leading journal in the field of systematic and evolutionary biology.