Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

Paleozoic Era: (543-248 mya)

Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian

Carboniferous Period (354-290 mya)

Hot, humid conditions pervade across the great northern landmass, Laurussia (Euramerica), which lies near the equator. Colder weather dominates southern lands, including glacier-ridden Gondwana, which lies over the South Pole. In the plant kingdom, early spore-bearing club mosses, tree ferns, and horsetails grow to heights of 50-100 feet in swamps and estuaries. All of these are quite different from modern trees. Seed-producing plants, including seed ferns, also flourish.

Six-foot millipedes, dragonflies with wingspans of up to two feet, and other insects thrive on the organic matter that litters the lush forest floor. Among the vertebrates, amphibians thrive. A key evolutionary moment occurs toward the middle of the period, when reptiles evolve as the first strictly land-dwelling animals.

In the Carboniferous seas, sharks and bony fishes take over from the jawless agnathans and armored placoderms, victims of the late Devonian extinction. Recovering reefs host starfish, gastropods, and sea urchins, among other marine invertebrates.

350 mya: Carbon sinks form in "coal forests"

Lush Carboniferous forests flourish in wetlands at the continental margins, where they are prone to episodic flooding. The underwater plant material cannot fully decompose, and it sinks and becomes buried in silt and mud. Heat and pressure eventually transform this carbon-rich organic matter into coal.

350 mya: Reptiles

Early reptiles share many skeletal similarities with amphibians of the time. Some differences in the skull and vertebrae, however, establish their relationship to later distinctive reptiles, such as turtles and dinosaurs.

	Read more From water to land: Counting their eggs (350 mya) The earliest reptiles are distinguishable from amphibians only by size -- early reptiles are small, while some amphibians are quite large -- and by slight skeletal differences. It is the reptiles' reproductive cycle that truly sets them apart from their ancestors. We know from living groups that reptiles have penetrative sex, delivering sperm directly to the egg while it is still protected within the female's body. Except, perhaps, for the earliest reptiles, which may have laid smaller, softer, amphibian-like eggs, reptile embryos develop within a fluid-filled membrane that is divided into separate compartments, called an amnion. The amnion itself is surrounded by a shell. Scientists call all animal groups that produce this type of egg -- including reptiles, birds, and mammals -- amniotes. Amniote eggs provide protection, nourishment, separate storage for waste products, and a supply of air to the developing embryo. Because reptile eggs are larger (on a relative basis) and can sustain embryos longer than most amphibian eggs, reptile embryos develop into juveniles without having to pass through a larval stage. When it's time to hatch, reptiles are exposed immediately to air. With this significant evolutionary development, reptiles expand the territory of four-legged animals ("tetrapods") to drier regions of the land. The evolution of life from water to land is complete.

300 mya: Conifers

Tall-growing conifers evolve and soon dominate the forests. Conifers are a class of gymnosperms, plants that produce "naked" seeds that develop on the outside of the plant. The reproductive organs of conifers are contained in cones. Male pollen cones, which are smaller, scatter their pollen in dry conditions. Female seed cones contain receptor organs, called ovules, on their scales. Fertilized eggs produce seeds, which, when mature, are dispersed. Thus, reproduction occurs without the need for water.

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Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era