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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.
Most mammals today are placentals. Mammalian diversity was not so simple 125 million years ago, said Anne Weil, a research associate in the department of biological anthropology and anatomy at Duke University in Durham, North Carolina. First of all, Eomaia is not a placental mammal.t is an early, primitive representative of the lineage that eventually led to placental mammals,  said Weil. Properly, it is called a eutherian mammala mammal that is more closely related to living placentals than to living marsupials.. Fossil evidence dates the earliest mammals to 220 million years ago.