During the Carboniferous (‘coal bearing’) period, huge and very dense forests developed in the extensive lowland swamps around the rivers and lakes near the seas. The lycopods Lepidodendron and Sigillaria attained heights thirty and forty metres (100 and 130 ft). Calamites, closely related to modern horsetails, reached heights fifteen metres (50 ft) or more. Nearer the ground there were tree-ferns such as Psaronius growing up to three metres (9 ft) high, together with other seed-bearing trees that vaguely resembled conifers.
The lowland swamp and estuarine areas were periodically flooded by the sea. The forest produced thick layers of leaves, while flooding knocked over the trees to produce the organic layers that over millions of years broke down and compressed into coal seams often several metres thick. When the seas retreated the forests grew up again and the cycle repeated itself over tens of millions of years.
Insects developed flight in the Devonian, beginning perhaps by making gliding descents from the trees. Of equal importance was the explosive radiation of winged insects in the Late Carboniferous. Giant dragonflies having wingspans of up to sixty cm (2 ft) preyed on smaller arthropods. Other insects in the Carboniferous forests included centipedes, cock-roaches, spiders and springtails. Giant millipedes reached two metres (6 ft) in length.
The fishes looked very different from those of the Devonian. Acanthodians, armoured agnathans, lungfishes, placoderms and rhipidistians were in decline. Sharks and bony fishes dominated. Early forms of tetrapods were superseded by a wide variety of creatures that ranged between small creatures, generally known as lepospondyls, and larger forms, known as temnospondyls. The temnospondyls adapted to a wide range of habitats: aquatic, amphibious and terrestrial and their remains have been found on every continent.
Lepospondyl microsaurs (‘little lizards’ but they were not lizards) mostly lived on land. Although closer to amphibians than living reptiles they showed that some of the early tetrapods could survive away from water for a considerable time. The closest relatives to the early reptiles appear to be the amphibian anthracosaurs (‘coal lizards’).
The big difference between living amphibians and reptiles is that amphibians lay jelly-covered eggs in water, whereas reptiles lay shelled eggs on land. It was this development that broke the final link between the invertebrates and water and cleared the way for them to exploit the drier upland areas.
All the earliest reptiles are classed as protorothyrids. They are small and lizard-like. The earliest confirmed reptile is Hylonomus (‘forest mouse’) found in the remains of fossilised tree stumps in Joggins, Nova Scotia. When Westlothiana lizziae was first discovered in East Kirkton, Scotland, it was hailed as the earliest known reptile but this is now debated.
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