McDonald and Carranza-Castañeda—New Miocene ground sloth from Mexico


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Figure 4. CT scan of mandible of Zacatzontli tecolotlanensis n. gen. n. sp. (MPG 1312G): (1) lateral view through sagittal place of alveoli and showing posterior mandibular canal, and (2) anterior view.


entire length. In Zacatzontli n. gen., the length of the spout is 30% of the length of the anterior of the caniniform to the distal margin of the m3. This value is 18–20% in Megalonyx curvidens Matthew, 1924 (FAM 77800 from Pliohippus Draw, Nebraska) and 40% in Pliometanastes protistis Hirschfeld andWebb, 1968 (UCMP 97371 fromSiphonCanal,California).The dorsal surface of the spout is concave and continuouswith the concave surface of the interior surface of the mandibular symphysis. The posterior margin of the symphysis ends at about the midpoint of the diastema and anterior to the first lower molariform. On the anterior midline of the spout there is a well-definedkeelonthe dorsal half; ventral to the keel the surface is flat. There is a shallow concavity on either side of the keel in which is located themental foramen (Fig. 4). The spout of the mandible in Zacatzontli n. gen. is perhaps


one of its most distinctive features when compared to coeval megalonychid sloths in both North and SouthAmerica. Its short triangular shape is reminiscent of Megalonyx, often considered the most derived spout morphology in the family. In the other Hemphillian and older megalonychids, the predental spout of the mandible is elongated and exceeds the length of the diastema (Fig. 5). The diastema is relatively long, ~29% of the length of the


molariform series. It is relatively thick and the lateral side has the concave buccinator fossa, which extends from the posterior margin of the alveolus of the caniniform to the anterior margin of the alveolus of the first molariform. Although the lower molariforms have been lost, the shapes


of the alveoli permit a determination of their outline and size. The lower molariforms are all roughly the same size and only slightly larger than the caniniform. The horizontal ramus at the level of the molariforms is expanded laterally, a feature also seen in the middle Pleistocene genus Meizonyx (Webb and Perrigo, 1985). The ventral margin of the horizontal ramus is not strongly convex and essentially horizontal, so maintains a uniform width from the caniniform to the third molariform and does not become dorsoventrally deep as in Megalonyx and other


members of the family. The outline of the alveolus of the lower first molariform is oval with the long axis perpendicular to the long axis of the tooth row. The outline of the alveolus of the second molariform is triangular with rounded vertices and one


vertex on the lingual side. The outline of the alveolus of the lower third molariform is slightly ovoid with the axis from mesiolingual to distolabial. The anterior margin of the ascending ramus has a posterior


inclination. The base of the ascending ramus is at the midpoint of the third molariform so the distal half of the tooth is not visible when viewed laterally. The masseteric fossa is shallow and restricted to the coronoid process. The fossa is not subdivided into two distinct areas for the insertion of the zygomaticomandibularis and deep masseter muscles, which is present in some other megalonychids. The posteroexternal foramen of the mandibular canal is


positioned on the lateral side of the horizontal ramus opposite the mesial margin of the lower third molariform and the anterior margin and base of the coronoid process (Fig. 4). Although its position is similar to that of Pliometanastes and Megalonyx, it opens anterodorsally and not laterally, as in the other two genera. The posterointernal foramen of the mandibular canal is


below the dorsal margins of the molariform alveoli and below the midpoint of the coronoid process. It opens into a shallow fossa that extends below the notch between the coronoid and condyloid processes. There is a complimentary convexity on the lateral side of the dentary. The long axis of the condyle is from anterolateral to


posteromedial. The articular surface is horizontal and its posterior margin is rounded in dorsal view. The posterior margin of the angular process is broken, but


enough is preserved to indicate its posterior portion curves slightly medially and the ventralmargin has a slight medial curvature. The ventral portion of the lateral surface has a small elongate fossa that extends from the plane at the posterior base of the coronoid process to the posterior edge of the preserved portion of the angular process just above its ventralmargin. In the notch between the angular and condylar process there is a small fossa.


Phylogenetic relationships


McDonald et al. (2013) recovered three major clades within the family Megalonychidae; Caribbean (Megalocnus, Parocnus, Acratocnus,and Neocnus), North American (Pliometanastes and Megalonyx), and South American (Megistonyx and Ahytherium).


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