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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.he first true primates evolved by 55 million years ago or a bit earlier, near the beginning of the Eocene Epoch. Their fossils have been found in North America, Europe, and Asia. They looked different from the primates today.