Live birth is 200 million years older than previously supposed, according to a recent report of a 380 million year old fish (a placoderm) with an embryo, still attached by an umbilical cord:

Until now, scientists thought creatures from these times were only able to develop their young inside eggs.

- "Fossil reveals oldest live birth" by Rebecca Morelle, BBC News (May 28, 2008)

The recently found placoderm dates from the Devonian era, called by some the Age of Fish.

Another fossil unearthed in 1986 was reexamined as a result of this find. It turned out to have three embryos inside that were considered evidence of live birth. In the past, scientists tended to assume that small fish found inside big ones had been eaten, as Carina Dennis explains in "The oldest pregnant mum" (Nature News, 28 May, 2008):

The researchers identified a single embryo in a new Gogo fish genus, and three embryos in a previously described specimen. “When you find a little fish inside a big fish, you tend to think it was dinner,” Long says. But the researchers concluded that the bones were those of embryos, not ingested remains, because they were not crushed or etched by digestive acids. What nailed it, according to Long, was the identification of an umbilical structure and a putative yolk sac.

Finds like this one challenge the widespread belief that live birth is a relatively recent innovation, and that egg-laying is older and perhaps more primitive.