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.