Skeletal parts, including jaws and teeth, of the giant jaguar was discovered on the Platte river, of Nebraska, in 1827, by Ferdinand Vandeveer Hayden. He sent it to Joesph Leidy, at the academy of natural sciences in Philidelphia. Who was working on identifying some of the recent pleistocene megafauna discoveries.

Beginning around the 1840s, more findings came forward, from Florida, Maryland to Tennessee.
Even Oregon, California (La Brae), and southwestern Mexico had remains of the giant jaguar.
More pieces became available to scientists, and in the early 1920s, they were able to differentiate and classify the different subspecies, sizes and genetics further.

Growl

Bibliography: Daggett, Pierre M.; Henning, Dale R. (1974). “The Jaguar in North America”. American Antiquity. 39 (3): 465–469. doi:10.2307/279437. ISSN0002-7316. JSTOR279437. S2CID160927286.

McCrady, Edward; et al. (1951). “New finds of Pleistocene jaguar skeletons from Tennessee caves”. Proceedings of the United States National Museum. 101 (3287): 497–512. doi:10.5479/si.00963801.101-3287.497.

Simpson, George Gaylord (1941). “Large Pleistocene felines of North America. American Museum novitates ; no. 1136”. hdl:2246/2262.