Carboniferous
345 to 280 million years ago
The continental blocs have come very close, in particular Africa is beginning to "push" North America (and the Appalachians arise), while Europe and Asia are about to join up (birth of the Urals). The North Pole touches the far-eastern part of Asia, while the South Pole is in the middle of Antarctica.
A good part of the emerged lands is therefore found in the belt between the tropics: in this area mild and humid climatic conditions predominate. Antarctica and the contiguous areas are affected by an ice age that had phases of varying intensity. Throughout the Carboniferous, lowering and raising of coastal areas or low lands in general follow one another: lagoons and estuaries are formed which later dry up.
There are therefore important deposits derived from the accumulation of sand and mud (sedimentary rocks, sandstone). In the phases of raising that lead to a withdrawal of the water, the plants immediately invade the lands and form forests; these are then submerged, for a new lowering, and covered with mud. Taken from the air, the plants (and in particular the trunks: many plants have large stems), do not undergo decomposition, do not rot and, also due to the compression exerted by the sedimentary layers that continue to accumulate, they carbonize. The process involves the "hard" part (wood) of terrestrial plants, formed by molecules similar to sugar (cellulose) or others (lignin), all made up of carbon, hydrogen, oxygen. The wood, following various slow chemical reactions, it loses oxygen and practically all hydrogen, becoming hard coal. It should be noted that the large coal deposits are found in areas that, in the Carboniferous, were included in the equatorial belt and were therefore covered by large forests.
These are areas of Europe and North America: this fact has had significant consequences. Amphibians are numerous in the coal forests. Different evolutionary branches are distinguished. Forms quite similar to the current salamanders and newts arise, but also forms that suggest crocodiles. All these amphibians have teeth in which the dentin and the enamel are variously folded section resembles the design of a labyrinth, therefore the animals are called Labyrinthodons. These teeth already existed in various steep slopes: it is therefore not an "invention" of amphibians. Very often amphibians appear "old and new" at the same time: in the Diplovertebron, of the late Carboniferous, an animal more than 1 m long, many skeletal elements in the areas corresponding to the "wrists" and "ankles" are cartilaginous and the dorsal chord is persistent in adults. The connection with water for the affairs of reproduction (eggs laid in water, without shell, aquatic tadpoles) could be disadvantageous: it was necessary to lay a lot of eggs to cope with the drying up of the swamps and the hunger of the predators. Again we must not think of a "pioneering spirit" of animals, in this case of amphibians who intend to colonize new environments, but to the rigid need to survive and to guarantee at least the survival of some young (tadpole larva) in the most critical phases of periodic desiccation. Some amphibians soon managed to lay eggs that could resist, without drying up, out of the water: they were eggs with shells. These amphibians well suited to cope with unfavorable conditions were already Reptiles. The remains of the Hylonomus are dated 280 million years ago: one of the first representatives of the new class. The tendency to exploit and improve the use of the legs contrasts with another, evident in various amphibians unrelated to the group of labyrinthdons. These animals favored the serpentine movement and in many cases underwent a total reduction of the limbs (Aistopodia, genus Phlegetontia). A nod to the animal forms living in the seas. Brachiopods and ammonites abound. The first Belemnites develop among the cephalopod molluscs. Bony fish and cartilaginous fish dominate the food chains.
