Skeletal and fibrous abnormalities in sloths and manatees. A) Fusion of the dorsal spinous processes of the seventh and eighth vertebra in a Choloepus didactylus (RMNH.MAM.7203) (arrow). B) Absence of ossification in the sternum of a Choloepus didactylus (ZMA.335). C) Defective ossification of the sacrum in a Choloepus didactylus (ZMA.335) (arrow). D) Fusion of the second and third vertebra in a Trichechus senegalensis ( ZMA14042) (white arrow). E) large sternal foramen, a midline fusion defect, in a Trichechus manatus (RBINS 1.181). F) Abnormal fibrous band in a Trichechus manatus (U. Nat coll) (white arrow) that connects the rudimentary rib on the left with the sternum. Note the asymmetry of left and right ribs (white arrowheads). The position of the fibrous band suggests that the mesenchymal anlage of the rib was formed, but that there was a cell fate change from chondrification (and later ossification) to the formation of a fibrous band, see text and cf horse with rudimentary first ribs and fibrous band in Figure 4D.