Sauropodomorpha

Sauropods are the long-necked, long-tailed giants most people picture first when they think "dinosaur" — and for good reason. The largest land animals in Earth's history belong to this group. By the time the last of them died out at the end of the Cretaceous, sauropods had been walking the planet for roughly 150 million years, having first appeared around 210 million years ago in the Late Triassic.

Their size was made possible by a piece of biological engineering hidden inside their bones. Sauropod neck and back vertebrae were riddled with air-filled sacs, connected to the lungs much like a bird's, which cut the weight of the skeleton by more than half. Without that trick, a neck the length of Brachiosaurus's could never have been lifted, let alone carried.

The sauropods most people know by name — Diplodocus, Apatosaurus, Brachiosaurus — all come from one moment in time and place: the Late Jurassic floodplains of the western United States, preserved today in the rock of the Morrison Formation. Diplodocus has a genuine claim on paleontological history: a specimen unearthed at Como Bluff, Wyoming in 1897 was among the first sauropod skeletons ever scientifically excavated, and within a few years no major natural history museum felt complete without one mounted in its main hall.

By the middle of the Cretaceous, the diplodocids and brachiosaurids of the Jurassic had largely given way to a new sauropod lineage: the titanosaurs. Emerging mainly across the southern continents, titanosaurs like Argentinosaurus pushed sauropod size to its absolute limit — current estimates put it at roughly 35–40 metres long and close to 100 tonnes, likely the heaviest land animal that has ever lived. In one form or another, sauropods held that title for almost the entire age of dinosaurs.