we looked at phylogenetic miniaturization, the process whereby evolution produces new clades by phylogenetically shrinking the adults.
Among the earliest mammals
was tiny Megazostrodon. But even smaller was tiny Early Jurassic Hadrocodium wui (Fig. 1, Luo, Crompton and Sun 2001), known only from a skull. Hadrocodium was originally considered a juvenile basal mammal, but later was considered an adult.
has a relatively larger brain size and more advanced ear structure than Megazostrodon. It was thus more mammal-like. Even so, with a limited synapsid/mammal taxon list, these two nest together in the large reptile tree.
Distinct from Megazostrodon,
Hadrocodium had relatively larger teeth, a diastema posterior to the canines, and a bulbous cranium extending posterior to the parietals, a trait common to burrowing reptiles. The post-crania remains unknown.
Luo Z-X, Crompton AW and Sun A-L 2001. New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics. Science 292 (5521): 1535–1540. Bibcode:2001Sci…292.1535L. doi:10.1126/science.1058476. PMID 11375489.