Sloths arose in South America during its long period of isolation and eventually spread to a number of the Caribbean islands as well as North America. It is thought that swimming led to oceanic dispersal of pilosans to the Greater Antilles by the Oligocene, and that the megalonychid "Pliometanastes" and the mylodontid "Thinobadistes" were able to colonise North America about 9 million years ago, well before the formation of the Isthmus of Panama. The latter development, about 3 million years ago, allowed megatheriids and nothrotheriids to also invade North America as part of the Great American Interchange. Additionally, the nothrotheriid "Thalassocnus" of the west coast of South America became adapted to a semiaquatic and, eventually, perhaps fully aquatic marine lifestyle. In Peru and Chile, "Thalassocnus" entered the coastal habitat beginning in the late Miocene. Initially they just stood in the water, but over a span of 4 million years they eventually evolved into swimming creatures, becoming specialist bottom feeders of seagrasses, similar to extant marine sirenians.