tiiny Jurassic synapsids, like Amphitherium prevosti (von Meyer 1832; Middle Jurassic, 170 mya) and Docodon victor (Marsh 1881; Late Jurassic, 2 cm skull length), are known only from mandibles with teeth (Fig. 1).
We can guess what the skull looks like
because the molars occlude and the rest of the teeth interlock, slide past one another or meet at or near their tips. Plus we have clues from sister taxa that set parameters for possibilities in a method known as phylogenetic bracketing. In such cases some scores are less risky to guess, like the number of molars. Others are more risky, like the presence of caniniform canines.
Figure 1. Amphitherium and Docodon with skulls imagined. The large number of molars nests both these taxa with Monotremata.
Marsh OC 1881. Notice of new Jurassic mammals: American Journal of Science, ser. 3, 21: p. 511-513.
Meyer H von 1832. Palaeologica, zur Geschichte der Erde und ihrer Geschöpfe. Schmerber, Frankfurt a/M, xi, 560 pp.