An articulated skeleton of Thalassocnus natans (Xenarthra: Nothrotheriidae) and a review of some other fossil sloths provide new information on sesamoid bones located at the knee joint. A sesamoid bone and an ossified meniscus have been identified at this joint. The cyamo-fabella (posterior sesamoid of the tibio-femoral articulation) of T. natans may act like a pulley through which the tendon of the m. gastrocnemius would have passed. The ossified meniscus, which is also present in Megatheriidae and Mylodontidae, could be related to the pedolateral stance and a large rotation of the knee joint during locomotion. © 2005 Elsevier SAS. All rights reserved.