Carboniferous: giant insects and coal forests

Oxygen-rich air and giant arthropods

The Carboniferous lasted from 359 to 299 million years ago, about 60 million years. Oxygen reached about 35 percent, compared with 21 percent today, letting diffusion breathing support huge bodies in warm swamps.

Meganeura dragonflies spanned about 70 centimeters, while 2-meter millipedes crawled through leaf litter. Towering lycopsid trees up to 30 meters tall fell into stagnant water, later buried as coal seams meters thick.

Coal swamps and the amniotic egg

Tropical wetlands covered about 5 million square kilometers at their peak. Fallen trunks decayed slowly in acidic water, locking carbon that today supplies about 30 percent of global coal reserves from this interval.

Around 320 million years ago the first reptiles laid shelled amniotic eggs, freeing breeding from water. When oxygen fell below 25 percent by 290 million years ago, giant insects shrank and forests thinned toward drier Permian landscapes.

• Lasted 60 million years, 359 to 299 million years ago.
• Oxygen peaked near 35 percent.
• Meganeura wingspan about 70 centimeters.
• Trees up to 30 meters formed coal.
• First amniotic eggs about 320 million years ago.