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Maintained by pioweb@ucsc.edu. The common ancestor of placental mammals probably looked like Eomaia scansoria, the earliest known placental mammal, shown here in an artist's reconstruction based on a 125-million-year-old fossil skeleton found in China in 2002.aintained by pioweb@ucsc.edu. The common ancestor of placental mammals probably looked like Eomaia scansoria, the earliest known placental mammal, shown here in an artist's reconstruction based on a 125-million-year-old fossil skeleton found in China in 2002.
The truth, though, is very different: in fact, the first mammals evolved from therapsids (mammal-like reptiles) at the end of the Triassic period, and coexisted with dinosaurs throughout the Mesozoic Era.idelphodon A Cretaceous ancestor of the modern opossum. Docodon Perhaps not a true mammal, but a mammal-like reptile.. Eomaia This early mammal is known from a single, spectacular fossil. Eozostrodon This Triassic critter may (or may not) have been the first true mammal.