Silurian
440 to 395 million years ago
The two parts of Asia are joining and forming along the "scar" some mountain that remain today (Stanovoj mountains). Europe is approaching the Asian bloc and the areas that will become the Urals begin to rise. The reciprocal movements of Europe and North America will give rise to various reliefs (Caledonids) whose remains are found in Scotland, Scandinavia, Newfoundland, Canada. The bulk of the southern continents (Gondwana) continues to move north. The South Pole is located in parts of South Africa. The Ordovician glaciation has now regressed.
The climate is getting warmer a little at a time and, in various areas, there are conditions of great aridity. A very important phenomenon is taking place in the history of the life. Movements of seas, strong waves: some algae ended up first on the shoreline area and then further on, where there was no more water. The pre-adaptation worked. The algae became "plants", discovering that there was a lot of light on land, more than there was in the water (starting from a certain depth, the light is scarce). Maybe even the "bad weather", during the Ordovician glaciation, had prompted some algae to seek light on the coasts. It is very likely that rather simple plant structures without real vessels (the "tubes" that carry the liquids) have already attempted the big step in the Ordovician: perhaps they were plants similar to mosses. We have no traces of this colonization.
Undoubtedly, however, the earliest terrestrial arthropods "followed" these primordial mosses; and were in turn followed by predatory arthropods. From the Siluriano are the remains of some scorpions, animals very similar to the current forms, even if the appendages are short and stocky. The internal cartilage structures of the first vertebrates were made in the Silurian. The y become more and more complex and could expand to support lateral appendages suitable for swimming. The external rigid coating in several living organisms begins to shrink: it decreases on the tail (which therefore acquires mobility and becomes more efficient for swimming) and is limited to the head region.
Usually the fossil record of these animals concerns only the hard outer covering. How can we be sure of the existence of an internal 'skeleton' which, being made of cartilage, has decomposed?
A clue is provided by the fact that today there are vertebrate animals without jaws (Agnari) and with a cartilaginous skeleton are lampreys and hamsters. The first vertebrates with mobile appendages are called Cephalaspids. How did these animals eat? By filtering the water containing the edible debris, they ate and breathed in a single operation the "used" water came out of a series of small holes and the food particles continued towards the 'intestine'. They were as has been said, "ugly tea strainers".
