Explore the world of 345-290 million years ago – when giant millipedes were one of the largest terrestrial species on Earth!

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De Agostini Picture Library via Getty Images In the oxygen-rich air of the Carboniferous period, between roughly 300 million and 360 million years ago, some animals swelled to monstrous sizes.

The team analyzed heads that belonged to two juvenile creatures who were only around 3 cm (1 inch) long. But oxygen levels in the Carboniferous were higher than today, allowing arthropods like ...