Period· The columns of time
Jurassic
201 Ma → 145 Ma
The supercontinent comes apart, the largest animals that ever walked reach their full size, and small feathered dinosaurs take to the air.
What the evidence shows
Pangaea splits along one rift after another, each marked by a flood of basalt, and the new coastlines and shallow seas change the climate as much as the geography. Sauropods reach ten times the mass of the largest land mammal, and no single feature explains it: a very long neck reaches food no other herbivore can; a small head is what lets the neck be so long, and the head can stay small because nothing is ever chewed; eggs laid by the dozen mean that no pregnancy sets a ceiling on body size; and air sacs running through hollow bones lighten the skeleton and may also carry off the heat of an animal too big to lose it through its skin. Feathers, wishbones and hollow bones had appeared among small meat-eating dinosaurs long before anything flew; what the Late Jurassic adds, around 150 million years ago, is variety: Archaeopteryx with its long reptilian tail, alongside several Chinese relatives.
Bounds and conventions
A golden spike opens it. Its top, at 145 million years, is one of the few Phanerozoic boundaries still without one, and its estimated age has moved by several million years. Which of the feathered Late Jurassic animals sits on the line leading to birds is unsettled, and Archaeopteryx itself has been moved on and off that line more than once.
Sources
- Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
- Sander et al. (2010), Biological Reviews · Biology of the sauropod dinosaurs: the evolution of gigantism
- Xu et al. (2011), Nature · An Archaeopteryx-like theropod from China and the origin of Avialae
- Frizon de Lamotte et al. (2015), Tectonics · Style of rifting and the stages of Pangea breakup