750 million years ago, Los Angeles would have been an east coast city

That's according to Dinosaur Database, a terrific site with an interactive ancient earth feature that lets viewers input a location and a time period to see what things looked like at the time.

LA would probably also have been under a glacier at the time:

Cryogenian Period. Glaciers may have covered the entire planet during the greatest ice age known on Earth. New types of life such as red and green algae appear during this period.

As the North American Plate moved into its current position, LA and many other cities were submerged at various times. The whole site is a lot of fun and very informative!