Millipedes were among the first animals to walk on Earth. They took their first steps at least 428 million years ago, venturing onto land to eat the few simple, mosslike land plants that existe back then. By 350 million years ago, the plants had evolved into trees and the millipedes had become giants, too. Biggest of all was Arthropleura.
As big as a crocodile, it was the largest invertebrate ever to live on land.
■ When: 350 million years ago (Early Carboniferous)
■ Fossil location: Scotland
■ Habitat: Forests
■ Length: Up to 8.1/2 ft (2 m)
■ Diet: Unknown
Arthropleura lived on the dark, damp floor of tropical jungles during the Carboniferous Period. Fossils of its mouth have not been found, making its diet a mystery, but traces of ferns in its gut suggest it was a plant-eater. Although able to breathe out of water, it probably stayed in damp places and may have had to return to water to shed its skin as it grew. Some scientists think it could also swim under water.
Despite all the legs, they are slow walkers, their tiny feet swinging forward in waves. They feed on rotting plant matter, burrowing into soil to find it.
Centipedes, in contrast, are fast-moving hunters that kill with venomous claws.