Embryo of Neomeris, showing dorsal plates [now Neophocaena spp - the Black Finless Porpoise]
These “plates” develop into dermal ossicles (small bony skin concretions) as the embryo develops. These ossicles are similar to the skin of reptiles, and are rare in mammals, as fur cannot grow from them, and fur is the primary dermal protection in most mammals. The giant ground sloths had them on their stomachs, though, and this would have prevented the pointy sticks and thorns of plants from injuring them, as they ate the fruits and nuts from high in trees.
Cetaceans tend to have dermal ossicles on their easily-injured extremities, such as their dorsal and pectoral fins. In the Black Finless Porpoise, the dorsal fin is absent, but the dermal ossicles still remain.
A Book of Whales. F. E. Beddard, 1900.