Permian
280 to 225 million years ago
All the continental masses are now merged in a single supercontinent which has been called Pangea ("all the earth"). Some seas (actually long gulfs) creep into the huge block: one between India and Antarctica-Australia, another between Northeast Asia and Canada and, finally, on the line of the Equator, the large basin called Tethys, from the name of the sea goddess. Sediments accumulate in it which new movements of the plates will bring out as mountains (the Alps and, later, the Himalayas). During the Permian, the Urals mountains stop emerging. A key phenomenon of this period is the great glaciation that continues the coal-bearing one. Evidence of the existence of glaciers indicates their maximum extent: in addition to Antarctica, southern Australia, India and a large part of Africa and South America were covered with ice.
The climate becomes drier but remains warm enough in the equatorial belt or in the adjacent tropical areas. Insects continue to thrive with interesting solutions to the problem of reproduction, linked to the fact that many swamps dry out (the forms that have non-aquatic larvae increase).
The glaciation in the southern part of Pangea certainly took place in several successive waves with a slow improvement in conditions starting from the middle of the period. At the end of it the ice was very small. Living beings could move on land without ever "getting their feet wet". It could therefore be thought that during the Permian, plants and animals spread uniformly everywhere. The fossil remains, on the other hand, indicate that there were different populations at different latitudes. The barriers no longer consisted of sea arms but of differences in temperature and climate. In the southern part, above all, the story of the glaciation gave birth to particular adaptations which, among other things, took place in several phases, testifying to the alternation of advances and retreats of ice. Other populations are found in the northern regions. In practice, Pangea is divided into three ecological regions: the northern one with a climate that is not too cold but dry; the central zone in which hot-humid conditions remain; the southern area, affected by the glaciation. In the South, Pteridosperms and Conifers are widespread. We know that the Hylonomus, found in the layers of the final Carboniferous, is considered a reptile. The judgment, in this regard, is formulated on the basis of the structure of his skeleton. However, the most important clue has not yet been found: the carboniferous layers have not yet provided us with traces of an egg with a shell. The earliest egg fragments are found in Permian layers. At the glaciers of the meridional region of Pangea, reptiles enhanced their ability to maintain constant body temperature with their hair. Hairy reptiles? The skulls of some shapes have series of small hollows similar to those that can be seen in the bones of the "snout" of modern mammals equipped with "whiskers" (like the cats for example). Vibrissae and hairs are formations of the same nature. It is therefore very likely that the "cold" reptiles had a fur, an effective insulating coating. Research on "mammal-like" forms (classified, in the reptile class, as Therapsids) has recently led to very interesting proposals to which we will return later. However, let's say right away that various scentists suggest that these animals are no longer "strange" reptiles, but intead very primitive mammals. A new name has also been proposed: paramammals.
The Permian reptiles present other notable adaptations, in addition to the very important ones we have already talked about, which have brought them closer to homeothermy. Still in the evolutionary "direction" of mammals, for example, there is the development of dentitions made up of different elements: "front" teeth suitable for biting (such as the incisors of mammals), fangs (such as canines), teeth to tear (such as premolars) of today's carnivores, the so-called "feral teeth".
Obviously these teeth are typical of predators: we find them, among the pelicosaurs, in the Dimetrodon, or, among the therapsids, in the Lycaenops and similar forms. In herbivores there are sometimes strange chewing plates while there are only two fangs in the jaw, perhaps more developed in males (as in Lystrosaums, a form widespread in the southern part of Pangea, from Australia, Antarctica, from southern Africa to all India). Needle-like teeth, very numerous and not differentiated, occur in small aquatic reptiles called Mesosaurs. These animals were avid predators of fish and crustaceans in inland waters and shallow waters near coasts. The webbed legs and powerful tail were perfectly adapted to swimming.
