File:Hadrocodium, Morganucodon & Docodon lower jaws.png

English: Comparison of the mandible of Hadrocodium with the mandibular growth stages of Morganucodon and Docodon. A. The mammaliaform Hadrocodium wui Luo, Crompton, and Sun, 2001 (holotype, IVPP 8275) from the Lower Lufeng Formation, Lower Jurassic of Yunnan, China (arrow indicates the retro-molar space and feature of adult). B, C. Mandibular growth stages of the morganucodontan Morganucodon watsoni Kühne, 1949 (= “Eozostrodon parvus”) from the Lower Jurassic fissure fills of Wales (Parrington 1971); adult (B) and the oldest-known adult (C) (respectively specimens D60 and D120 in Parrington 1971: fig. 3). D–F. Mandibular growth stages of the docodontan Docodon victor Schultz, Bhullar, and Luo, 2019, from the Morrison Formation, Upper Jurassic of Wyoming, USA. D. YPM 23748, juvenile. E. YPM 11823, adult. F. YPM 11826, the oldest-known adult. In H. wui, the ultimate molar is positioned in front of the coronoid process base. This is an adult feature well documented in the successively older (or the oldest-known) adult individuals in the growth series of other mammaliaforms. Morganucodon watsoni shows such a growth pattern: a positional shift of the ultimate molar (m4) anteriorly to the base of coronoid process in successively older individuals; the ultimate molar has a single alveolus for its fused or confluent root(s) in most cases (Parrington 1971; Pamela Gill, personal communication 2021). Docodon victor shows a similar growth pattern of a shift of the coronoid process relative to the last molar(s) of the toothrow. The youngest-available individual D. victor (YMP23748) shows the last molar (m5) is medial to the coronoid process. In the adult (YPM11823) the last molar (m7) is shifted more anteriorly. In the oldest-available individual (YPM11826), the ultimate molar (m8) is shifted to the anterior, as the coronoid process is shifted posteriorly relative to the toothrow (Schultz et al. 2019). The placement of the ultimate molar with a retro-molar space anterior to the base of the coronoid in the oldest-available adult specimen of D. victor is similar to that of H. wui. D–F, stylistic illustrations based on CT visualizations by Schultz et al. (2019).