Deciduous dentition, tooth replacement, and mandibular growth in the Late Jurassic docodontan Haldanodon exspectatus (Mammaliaformes)

Abstract

The Late Jurassic docodontan Haldanodon exspectatus from the Guimarota coal mine in Portugal provides important information on the evolution of the dental replacement pattern in mammaliaforms. Haldanodon shows diphyodont replacement of antemolars and non-replacement of molars. Lower incisors are replaced in alternating order with early replacement of i2 and i4, and late replacement of i1 and i3. Upper and lower premolars were replaced sequentially from front to back. In the maxilla and mandible, four deciduous premolar positions are present (dP1-4, dp1-4), but only three permanent premolars (P1/p1, P3/p3, P4/p4) erupt with loss of the dP2/dp2 position. The anterior deciduous premolars (dP1-2/dp1-2) are small and peg-like, dP3-4/dp3-4 are much larger and molariform. The lower canine and dp4 are the last lower teeth to be replaced. The permanent lower premolars are premolariform and consist mainly of large main cusp a. The ultimate permanent lower premolar (p4) erupts at the same time as m4. After replacement of the antemolars, one or two more molars (m5-6) are added at the posterior end of the tooth row of the mandible. Growth of the ramus occurs at the anterior and posterior ends as evident from the much larger permanent canine and addition of m5-6. In the maxilla, all permanent premolars differ morphologically clearly from the molars. The replacing P1 is small and consists mainly of labial cusps A, B, and C. The P3 and P4 are increasingly larger and show a lingual extension with cusps X and Y which is shorter than in the molars. The adult dental formula of Haldanodon is 6I/4i, 1C/1c, 3P/3p, 5 M/5-6m, the deciduous dental formula is ?dI/4di, 1dC/1dc, 4dP/4dp. The tooth eruption sequence in the mandible is m1 → p1 + m2 → i2 + i4 → ?i1 + ?i3 → p3 + m3 → c + p4 + m4 → m5 → m6. Our results corroborate the earlier suggestions that “Peraiocynodon inexpectatus” is based on deciduous lower premolars (dp1-4) of Docodon victor, and that “Peraiocynodon major“ and Cyrtlatherium canei are based on deciduous lower premolars.