Vertebrate animals have come a long way since their tiny,
translucent ancestors swam the world's seas over 500 million
years ago. Here's a roughly chronological list of the major
vertebrate animal groups, ranging from fish to amphibians to
mammals, with some notable extinct reptile lineages (including
archosaurs, dinosaurs and pterosaurs) in between. 1. Fish and
Sharks Between 500 and 400 million years ago, life on earth was
dominated by prehistoric fish. With their bilaterally symmetric
body plans, V-shaped muscles and protected nerve chords running
down the lengths of their bodies, these ocean dwellers
established the template for later vertebrate evolution (it also
didn't hurt that the heads of these fish were distinct from
their tails, another surprisingly basic innovation of the
Cambrian period). The first prehistoric sharks evolved from
their fish forebears about 420 million years ago, and quickly
swam to the apex of the undersea food chain. The proverbial
"fish out of water," tetrapods were the first v