FUNCTION AND IMPLICATIONS OF THE LATERAL CORONOID FORAMEN IN EXTANT AND EXTINCT SLOTHS (MAMMALIA: XENARTHRA: FOLIVORA)

Among all modern and extinct mammals, only sloths of the Order Pilosa bear a laterally opening foramen just anterior to the root of the coronoid process on the mandible, or Lateral Coronoid Foramen (LCF). This passageway is a branch off the mandibular canal, carrying sensory nerves through the dentary for the face and dentition. While somewhat similar to a passage seen in other basal Afrotherians, those foramens open posterior to the ultimate tooth and are not directed laterally as in the sloths but are anteriorly oriented along the long axis of the dental series. Recent dissection of extant sloths by the authors revealed for the first time the contents of the LCF to be entirely nervous tissue, with no blood vessels evident in either of the modern sloth genera. The nerve is a branch of the mandibular division of the trigeminal nerve (CN V₃) given its relation to the mandibular canal; however, its specific identification and homology to that of other CN V₃ branches is currently unknown. As the LCF occurs in all sloth taxa, the morphology and placement of the opening has implications for its identification and function. Examination of representatives from the four major sloth clades reveals the LCF location to be influenced in part by the masticatory muscles, as it is ventrally displaced by the temporalis attachment on the coronoid process. This removes the possibility of the nerve being a unique innervation path for those muscles. The exiting path of the LCF runs anteriorly just below the level of the buccal line, which additionally rules out the nerve as a motor branch of the facial nerve (CN VII), but leaves open sensory innervation for lesser salivary glands at that location and/or the skin and lips. Placement of the LCF is at the level of the last molariform (m4) in all sloths, except for the extant genus Bradypus (three-fingered sloths) where the foramen in positioned more anteriorly at the penultimate (m3) molariform. This last feature is indicative of a renewed movement that considers this taxonomically difficult genus to be the result of paedomorphic speciation.