Materpiscis
Materpiscis (Latin for "mother fish") is a genus of ptyctodontid placoderm from the late Devonian period, about 380 million years ago. A fossil of this armored placoderm shows an embryo and umbilical cord inside its body. So Materpiscis is the oldest known vertebrate to show viviparity (giving birth to live young). Viviparity of one kind or another is quite common in certain kinds of fish, for example sharks.