Period· The columns of time

Jurassic

201 Ma → 143 Ma

In the Jurassic, from 201 to 143 million years ago, the supercontinent broke up, the largest land animals in history reached their full size and the first birds appeared.

Period of the break-up of the supercontinent, in which the largest land animals in history reach their maximum size and the first birds appear.

What the evidence shows

Pangaea fractures along successive rifts, each marked by a basalt outpouring, and the coastlines and shallow seas that break-up opens alter climate as much as geography. Sauropods reach ten times the mass of the largest land mammal, a result no single character produces: a very long neck reaches food beyond the reach of other herbivores; a small head makes that neck possible, and it stays small because these animals do not chew; numerous eggs mean that no gestation limits body size; and air sacs extending into hollow bones lighten the skeleton and perhaps remove heat from an animal too large to lose it through its surface. Feathers, the furcula and hollow bones had appeared in small carnivorous dinosaurs well before flight; what the Late Jurassic adds, around 150 million years, is diversity: Archaeopteryx, with its long bony tail, and several related Chinese forms.

Bounds and conventions

A golden spike opens it. Its top, at 143.1 million years, is among the few Phanerozoic limits that still lacks one, and its estimated age has varied by several million years. Which form lies on the lineage leading to birds remains open, Archaeopteryx having been removed from it and reinstated more than once.