It has been proposed that the progressive increase in body sizes among nothrotheriids over their evolution is related to their increasingly terrestrial habits and lifestyles. During the
Late Miocene and
Pliocene, the sloth genus
Thalassocnus of the west coast of South America became adapted to a shallow-water marine lifestyle. However, the family placement of
Thalassocnus has been disputed; while long considered a nothrotheriid, one 2017 analysis moves it to
Megatheriidae, while another retains it in a
basal position within Nothrotheriidae. The only known nothrotheriid in North America was
Nothrotheriops, which appeared at the beginning of the
Pleistocene, about 2.6 Ma ago.
Nothrotherium reached Mexico (
Nuevo Leon) by the late Pleistocene. ized
Nothrotheriops shastensis dung in Rampart Cave,
Arizona (
NPS, 1938) The last ground sloths in North America belonging to Nothrotheriidae, the Shasta ground sloth (
Nothrotheriops shastensis), died so recently that their dried
subfossilized dung has remained undisturbed in some cavessuch as the Rampart Cave, located on the
Arizona side of the
Lake Mead National Recreation Area as if it were just recently deposited. A Shasta ground sloth skeleton, found in a
lava tube at
Aden Crater in
New Mexico, still had skin and hair preserved, and is now at the Yale
Peabody Museum. The largest samples of
Nothrotheriops dung can be found in the collections of the
National Museum of Natural History. ==Phylogeny ==