Furthermore, the wide, straight and relatively flat claws of these sloths resemble those of living mammals that dig. Forelimb bone strength in mylodontids was also high and shows that the forelimbs were resistant to impact with the ground (Bargo et al. Recent studies have shown that the length of the olecranon process relative to the rest of the ulna is a good indicator of digging ability in mammals as the olecranon provides the attachment area for the triceps, the main muscle used in digging. Mylodontids had a particularly prominent olecranon process on the ulna. The forelimbs of most sloths are about subequal in length to the hindlimbs, the most prominent exceptions being the long-armed tree sloths of the genus Bradypus. Some fossil sloths had squarish or subrectangular teeth and, in these forms, transverse ridges between the valleys are particularly prominent. As the teeth occlude against those in the opposite jaw, valleys and cusp-like structures are formed as the two kinds of dentine erode differentially (Naples 1989, 1995). When sloth teeth erupt they are devoid of the cusps and basins seen normally in mammalian teeth and are simple and cylindrical in form.
Sloth teeth lack enamel and are composed instead of two different kinds of dentine plus an outer layer of cementum, the softer dentine forming the innermost region of the tooth. In the Pleistocene megalonychid Megalocnus from Cuba, and in certain other genera, the two most anterior upper jaw teeth have been described as ‘pseudorodentiform’ and are more incisiform than caniniform. The upper caniniforms of these sloths are ahead of the lower caniniforms and, while some evidence suggests that the upper caniniform in Choloepus is a true canine, this probably isn’t the case for the lower caniniform. The living tree sloth Choloepus, as well as some mylodontids, megalonychids and nothrotheriids, possess caniniform teeth separated from the other teeth by a diastema. Incisors are absent, and it is not really possible to distinguish between the similar premolars and molars. 2006) that grow continuously throughout life, and the lack of a replacement dentition has made it difficult to homologise sloth teeth with those of other mammals. They do not possess deciduous teeth but have a single set of high-crowned, open-rooted teeth (Bargo et al. In sloths with particularly long-rooted teeth there is a distinct bulge on the ventral margin of the lower jaw. The tip of the sloth mandible is usually spout-shaped and there is a foramen, representing an external opening of the mandibular canal, on the side of the lower jaw. The sloth palate is rugose and covered in pits and grooves and there are distinctive deep laminae that descend ventrally from the pterygoid bones (Gaudin 2004). Some megalonychids had a domed cranium resulting from marked enlargement of the sinuses within the frontal bones. Sloth skulls are diverse in form and range from the deep and broad, snub-faced morphology seen in Bradypus and some megalonychids to the elongate almost horse-like skulls of megatheriids and others (Gaudin 2004). This article – an excerpt from Naish (2005) (though with citations added that were absent in the published article) – briefly reviews the anatomy of fossil sloths, though there are references to the living forms where appropriate.Ī typical fossil sloth can be imagined as a rather bear-shaped, shaggy-furred mammal with particularly powerful forelimbs, a barrel-shaped ribcage, a stout tail, prominent curved hand and foot claws and a markedly broad, robust pelvis. Everybody loves sloths, and whenever we talk about sloths we have to remember that the two living kinds ( Bradypus – the four species of three-toed sloth – and Choloepus – the two species of two-toed sloth) are but the tip of the iceberg when it comes to sloth diversity.
