Megatherium, Thalassocnus and Hapalops

Megatherium americanum (Cuvier 1796; Pliocene; 6m length) is the largest ground sloth. The medial pedal digits are absent or vestiges. There is little space for an auditory bulla and none was preserved. Metacarpal 4 and metatarsal 4 are the longest.

Hapalops ruetimeyeri (Ameghino 1887; Miocene; 1 m length) is a smaller ground sloth, derived from Megatherium and basal to tree sloths in the LRT. 26 species has been named.

Note the reduced medial and lateral fingers and toes. This informs the digits remaining in the three-toed sloth, Bradypus.

Thalassocnus natans (de Muizon and McDonald 1995, Late Miocene to Late Pliocene, 7–3mya) is a long-bodied, aquatic sloth related to Hapalops. Fossils were found in lagoonal to near-shore environment.

Ameghino F 1887. Enumeración sistemática de las especies de mamíferos fósiles coleccionados por Carlos Ameghino en los terrenos eocenos de la Patagonia. Boletín del Museo de La Plata 1: 1–26.

Cuvier G 1796. Notice sur le squelette d’une très-grande espèce de quadrupède unconnue jusqu’a présent, trouvé a Paraguay, et déposé au cabinet d’histoire
naturelle de Madrid. — Magasin Enciclopedique: ou Journal des Sciences, des Lettres et des Arts 1: 303–310, 2: 227–228; Paris.

de Muizon C and McDonald HG 1995. An aquatic sloth from the Pliocene of Peru. Nature. 375 (6528): 224–227.

wiki/Hapalops
wiki/Megatherium
wiki/Thalassocnus