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 vertebrates to climb out of the sea and colonize dry (or at least swampy) land, a key evolutionary transition that occurred somewhere between 400 and 350 million years ago. Crucially, the first tetrapods descended from lobe-finned, rather than ray-finned, fish, which possessed the characteristic skeletal structure that morphed into the fingers, claws and paws of later vertebrates. (Oddly enough, some of the first tetrapods had seven or eight toes on their hands and feet, and thus counted as evolutionary "dead ends.")
During the Carboniferous period--from about 360 to 300 million years ago--terrestrial life on earth was dominated by prehistoric amphibians. Often considered a mere way station between earlier tetrapods and later reptiles, amphibians were crucially important in their own right, since they were the first vertebrates to figure out a way to colonize dry land (however, these creatures still needed to lay their eggs in water, which severely limited their mobility). Today, amphibians are represented by frogs, toads and salamanders, and their population is rapidly dwindling under environmental stress.
4. Terrestrial Reptiles
About 320 million years ago--give or take a few million years--the first true reptiles evolved from amphibians (with their scaly skin and semi-permeable eggs, reptiles were free to leave bodies of water behind and venture deep into dry land). The earth's land masses were quickly populated by pelycosaurs, archosaurs (including prehistoric crocodiles), anapsids (including prehistoric turtles), prehistoric snakes, and therapsids (the "mammal-like reptiles" that later evolved into the first mammals); during the late Triassic period, two-legged archosaurs spawned the first dinosaurs, the descendants of which ruled the planet until the end of the Mesozoic Era 175 million years later.
5. Marine Reptiles
At least some of the first reptiles led partly (or mostly) aquatic lifestyles, but the true age of marine reptiles didn't begin until the appearance of the ichthyosaurs ("fish lizards") during the early to middle Triassic period. The ichthyosaurs overlapped with, and were then succeeded by, long-necked plesiosaurs and muscular pliosaurs, which themselves overlapped with, and were then succeeded by, the exceptionally sleek, vicious mosasaurs of the late Cretaceous period. All of these marine reptiles went extinct 65 million years ago along with their terrestrial dinosaur cousins.
6. Avian Reptiles
Often mistakenly referred to as dinosaurs, pterosaurs ("winged lizards") were actually a distinct family of reptiles that evolved from archosaurs during the early Triassic period. The pterosaurs of the early Mesozoic Era were fairly small, but some truly gigantic breeds (such as the 200-pound Quetzalcoatlus) dominated the late Cretaceous skies. Like their dinosaur and marine reptile cousins, the pterosaurs went extinct 65 million years ago; contrary to popular belief, they didn't evolve into birds, an honor that belonged to the small, feathered theropod dinosaurs of the Jurassic and Cretaceous periods.
It's difficult to pin down the exact moment when the first true prehistoric birds evolved from their dinosaur forebears; most paleontologists point to the late Jurassic period, about 150 million years ago, on the evidence of distinctly bird-like dinosaurs like Archaeopteryx. However, it's possible that birds evolved multiple times during the Mesozoic Era, most recently from the small, feathered theropods (sometimes called "dino-birds") of the late Cretaceous period. By the way, following the classification system known as "cladistics," it's perfectly legitimate to refer to modern birds as dinosaurs!
8. Mesozoic Mammals
As with most such evolutionary transitions, there wasn't a bright line separating the most advanced therapsids ("mammal-like reptiles") of the late Triassic period from the first true mammals. All we know for sure is that small, furry, warm-blooded, mammal-like creatures skittered across the high branches of trees about 230 million years ago, and coexisted on unequal terms with dinosaurs right up to the K/T Extinction. Because they were so small and fragile, most Mesozoic mammals are represented in the fossil record only by their teeth, though some species left surprisingly complete skeletons.
9. Megafauna Mammals
After the dinosaurs and marine reptiles vanished off the face of the earth 65 million years ago, the next big theme in vertebrate evolution was the rapid progression of mammals from small, timid, mouse-sized creatures to the giant megafauna of the middle to late Cenozoic Era, including oversized wombats, rhinoceroses, camels and beavers. Among the mammals that ruled the planet in the absence of dinosaurs and mosasaurs were prehistoric cats, prehistoric dogs, prehistoric elephants, prehistoric horses and prehistoric whales, most species of which went extinct by the end of the Pleistocene epoch (often at the hands of early humans).
Technically, there's no good reason to separate prehistoric primates from the other mammalian megafauna that succeeded the dinosaurs, but it's natural to want to distinguish our human ancestors from the mainstream of vertebrate evolution. The first primates appear in the fossil record as far back as the late Cretaceous period, and diversified in the course of the Cenozoic Era into a bewildering array of lemurs, monkeys, apes and anthropoids (the direct ancestors of modern humans). Paleontologists are still trying to sort out the evolutionary relationships of these fossil primates, as new "missing link" species are constantly being discovered.