A juvenile specimen of Sinoconodon rigneyi with new information on pattern of tooth replacement

Sinoconodon, an early branching mammaliaform from the Early Jurassic, showcases a unique combination of both plesiomorphic (e.g., sustained and indeterminate growth, over two replacements of teeth) and derived features of mammaliaforms (e.g., jaw joint formed by the squamosal glenoid and dentary condyle, a shallow postdentary trough). Ontogenetic features of its dentition captured by different stages in a growth series are unique among early branching mammaliaforms but detailed knowledge remains scarce. Here we report a newly discovered mandible of Sinoconodon rigneyi from the Lower Jurassic Lufeng Formation, Lufeng, Yunnan, China. It has distinct mandibular and dental features of Sinoconodon, e.g., a prominent mandibular condyle leveled to the postcanine alveolar line, a reduced postdentary trough and its medial ridge, and a Meckelian groove parallel to the ventral border of the mandible. This very small mandible also shows two juvenile features of Sinoconodon: two molariform loci medial to the coronoid process and four lower premolariform loci as indicated by alveoli, without a postcanine diastema. The presence of more complete lower premolariform loci is undocumented and new to the known Sinoconodon specimens, augmenting the understanding of tooth development of early ontogenetic stages in Sinoconodon. It reinforces that the development and enlargement of the postcanine diastema is a growth feature indicating maturity of individuals. The aging-indicative pattern of premolariform loss without replacement and with their alveoli plugged is similar to that in other early mammaliaforms, and enhances our understanding of ontogenetic changes of dentition in early branching mammaliaforms.