The early tetrapods split first into two major groups: the amphibians, which laid their eggs in water, and the amniotes, who laid their cleidoic eggs on land. The amniotes gave rise to two groups both of which became dominant at different times. They were the synapsids, which eventually gave rise to the mammals, and the sauropsids, which gave rise to the dinosaurs and other reptiles. These momentous events took place in the lower part of the Carboniferous, the Mississippian.
The Carboniferous is named after the coal measures, the remains of peat formed by dense tropical wetland forests. New kinds of vascular plants with thick bark grew in the forests. Much of the coal came from this bark. This biota occurred in the upper part of the period, the Pennsylvanian, from 315–300 million years years ago.
These forests were on the equator, and the wetlands, which are always low-lying, stretched across the supercontinent of Laurussia. This included what is now North America in the west, through what is now Europe to China in the east. The river plain which was the heart of the wetland stretched 5000 km from eastern Canada to the Ukraine, and was 700 km wide.p6
This kind of climate and geography has no exact parallel today, but the peat swamp forests in Indonesia and Malaysia, the Amazon Basin, the Mississippi River system and the Okefenokee swamp give some idea. The swamps were dominated by giant clubmosses, including Lepidodendron. They were the earliest trees, later replaced by conifers and flowering plants. Sometimes other plants from the levee got swept down by the river, such as horsetails, ferns and tree-like pteridosperms.
The wetland forests ended when the land level was raised by the pressure of the Gondwana continent against Laurussia. This caused the zone of contact to rise. The end of the coal measures marks the end of the Carboniferous period. China was too far away to be affected. There, the wetland forests continued for another 50 million years, into the Permian.p30
- Clack, Jennifer A. 2002. Gaining ground: the origin and evolution of tetrapods. Indiana University Press, Bloomington IN. ISBN 0-253-34054-3
- Thomas B.A. and Cleal C.J. 1993. The coal measure forests. National Museum of Wales.
- C.J. Cleal & B.A. Thomas 1994. Plants of the British coal measures. The Palaeontological Association.