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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.
Primate-like Mammals. The first primate-like mammals, or proto-primates , evolved in the early Paleocene Epoch (65.5-55.8 million years ago) at the beginning of the Cenozoic Era. They were roughly similar to squirrels and tree shrews in size and appearance.