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Jurassic Times
Apatosaurus was one of the sauropods, a group of long-necked plant-eating dinosaurs that included the largest land animals ever. The sauropods flourished during the Jurassic period, and apatosaurus in particular lived between 147 million and 137 million years ago. Although some sauropod lineages continued into the Cretaceous period, it is almost certain that they became extinct along with most of the other dinosaurs at or before the end of the Cretaceous. No direct descendants of apatosaurus are believed to have survived to the present.
Surviving Dinosaurs
When apatosaurus roamed the earth, there were already living birds, or early ancestors to birds. The famous archaeopteryx, a fossil creature with both feathers and reptile-like teeth, dates back to approximately 150 million years ago. Many believe archaeopteryx to have been the ancestor of modern birds, but it could also have been an evolutionary dead end that retained lizard features long after its cousins had become more birdlike. In any event, the closest relatives to the sauropods, whose descendants are alive today, are the birds, which technically are themselves dinosaurs.
Diapsids: Distant Living Relatives of the Dinosaurs
The diapsids are a group that split off from the other tetrapods (four-limbed vertebrates) some time in the Carboniferous period, some 350 million years ago. They include the dinosaurs (including birds) and all living reptiles except turtles. Snakes and lizards, therefore, are the next closest living relatives of brontosaurus who are still around today. The synapsids, which include mammals, are the next closest after that.
Life on Earth
Brontosaurus, like every other animal, was but a single twig on the vast tree of life. Although all dinosaurs but the birds are extinct, they all grew from the branch of the diapsids, which in turn branched off from the tetrapods, a branch of the limb of vertebrates, leading down in turn to the greater limbs of chordates, animals, eukaryotes and eventually joining up with bacteria and archaea in a single great trunk. In this sense, every living thing on the planet today, from bacteria to dandelions and lobsters to humans, is a modern day relative of the brontosaurus.