Neomonachus Slater & Helgen

Figure 5.Lateral views of crania of a Monachus monachus b Neomonachus schauinslandi, and c Neomonachus tropicalis. Arrows indicate the more developed occipital crest and zygomatic arches, and deeper snout of Monachus compared to Neomonachus species.

Figure 6.Ventral views of palates of a Monachus monachus b Neomonachus schauinslandi, and c Neomonachus tropicalis. The tooth row of Monachus is more crowded, likely as a result of the shorter rostrum, and this results in a more obliquely oriented set of post-canine teeth and the lack of a diastema between the upper canine and the first premolar. In Neomonachus, there is a distinct diastema between C1 and P1, and the post-canine teeth are arranged more linearly. The upper incisor arcade of Monachus is slightly parabolic due to the posterior placement of the lateral incisors, and the anterior premaxilla appears slightly curved. In Neomonachus, the incisor arcade is linear and the anterior premaxilla is straight.

Figure 7.Dorsal view of rostra of a Monachus monachus b Neomonachus schauinslandi, and c Neomonachus tropicalis. Monachus exhibits a well-developed antorbital process on the maxilla, immediately inferior to the fronto-maxillary suture. The process is reduced or absent in Neomonachus. The nasals of Monachus are short and triangular, tapering smoothly posteriorly to produce a point at their union. The nasals of Neomonachus are longer and do not taper smoothly.

Figure 8.Ventral views of crania of a Monachus monachus b Neomonachus schauinslandi, and c Neomonachus tropicalis, showing the pterygoid region. Neomonachus exhibits a well-developed, laterally flared pterygoid hamulus that is visible in dorsal view. The hamulus may be spatulate (Neomonachus schauinslandi) or hook-like (Neomonachus tropicalis). The hamular process is absent or medially flared in Monachus, and is not visible in dorsal view.

Figure 9.Posteroventral view of the basicranium and left bulla in a Monachus monachus b Neomonachus schauinslandi, and c Neomonachus tropicalis. The bulla of Monachus is bordered posteriorly by a ventrally expanded posterior portion of the petro-mastoid complex. The petrosal abuts the bulla’s posterior wall and in ventral view forms the entire lateral and anterolateral border of the posterior lacerate foramen. In Neomonachus, the posterior part of the petrosal is visible in the posterior lacerate foramen but remains superior to the bulla. In ventral view, this gives the impression that the anterior border of the posterior lacerate foramen is formed entirely by the bulla. The posterior carotid canal opens posteroventrally in Monachus. This apparently results from a relatively complete “ring-like” opening, formed by the bulla. This form of opening is apparent in subadult and juvenile Monachus, suggesting that it is not dependent on ontogenetic development or the robusticity of the Monachus cranium relative to Neomonachus. In contrast, the posterior carotid canal of Neomonachus opens directly posteriorly, the opening being an incomplete ring and the dorsal border formed by a flattening of the bulla, perhaps resulting from the bulla’s extension over the petrosal.

Figure 10.Plots of mean upper (a) and lower (b) relative post-canine tooth size. Relative tooth size is computed by dividing the mesio-distal length of each tooth by the length of the 3rd premolar (which is typically largest) in the same row.

Figure 11.Medial view of right dentaries of a Monachus monachus b Neomonachus schauinslandi, and c Neomonachus tropicalis. The mandibular foramen is situated inferior to the mandibular notch in Monachus, and opens immediately to the medial surface of the ramus. In Neomonachus, the foramen is anteriorly displaced and is set in a groove or sulcus that extends from inferior to the mandibular notch. Also note the expanded rugose area for insertion of the pterygoid muscles in Monachus. This region is poorly developed in Neomonachus.