Abstract
The cervical and thoracic regions including vertebrae, spinal nerves, and brachial plexus were examined by macroscopic and three-dimensional computed tomography observational scans in the two-toed sloth (Choloepus didactylus). This species possesses seven cervical vertebrae unlike closely related sloths, which possess varying number of cervical vertebrae ranging from five to ten. The large axis and the partially vestigial third cervical vertebra are morphologically characteristic of C. didactylus. In addition, the spinal nerve branches of C5, C6, C7, C8, and T1 contributed to the brachial plexus, whereas C4 and T2 did not supply nerves to the plexus. Histological data showed that the branches of C5, C6, C7, and C8 were obviously thicker than that of C4 and T1. The most notable observation was that only the two-toed sloth unlike other sloth species is conservative in these morphological characteristics regarding the cervical vertebrae spinal nerves. By comparing the morphological patterns of the cervical and thoracic spine and the brachial plexus of the two-toed sloth with ancestral morphological patterns and related species, which have derived a repatterned and variable morphology of these structures, it is possible to elucidate the morphological evolution of the cervical and thoracic regions in the evolutionary history of these mammals.
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