Jurassic
190 to 135 million years ago
The Jurassic period, in the middle of the Mesozoic, is characterized by a certain stability in the climatic situation. Of course, given the length of the period (about 60 million years), such an indication has only an approximate value. The fragmentation of the supercontinent Pangea causes interesting changes in the shape of the land. However, the majority of these remain in a wide belt that excludes the South Pole and barely touches the North Pole. Therefore in the Jurassic there were no intense colds and probably the precipitations were abundant and frequent.
It was therefore an ideal situation for the plant, which in fact prospered, and of course for herbivores: in this period the largest "plant eaters" ever lived on planet Earth developed. In the Jurassic the plate of North America detaches from Gondwana: in the space between this continental mass and Africa the North Atlantic begins to open. The rift between North America and Eurasia also occurs in this period. Vast seas of shallow water form, from North to South, in North America and, in Eurasia, between Europe and Asia.
Antarctica, Australia and South America break away from Africa; towards the end of the period a "sketch" of the South Atlantic already exists. The movements of the plates give rise to the Andes and the Rocky Mountains. A rotation of the polygon that will become the Iberian Peninsula causes the Bay of Biscay to open and the Pyrenees to rise. In the later stages, India is already detached from East Africa and begins its "journey" towards the North, while still remaining south of the Equator. There are many areas covered by swamps and marshes on all continents. Seas that crept between the various continental masses created extensive Seas. The jurassic was an ideal period for marine animals. Te molluscs abounded and in addition to the creeping forms on the seabed, there were numerous cephalopods (belemnites and above all ammonites, but also forms similar to squid and cuttlefish). Many were the cartilaginous fishes (sharks and rays). Bone fishes (also common in inland waters) spread and assert themselves, with a continuous "growing". All these animals constituted an interesting source of food and among the reptiles the most suitable forms to prey on them multiplied.
There are three main groups of reptiles adapted to marine life: Plesiosaurs, Ichthyosaurs and Crocodiles. The latter undergo a notable development towards the end of the period (Geosaurus, length up to 4 m; Sreneosaurus, length up to 9 m). The limbs of the Jurassic marine crocodiles are very similar to flippers but, in the skeleton, they retain the structure of the distinct fingers and with a normal number of phalanges. In the plesiosaurs, on the other hand, and above all in the ichthyosaurs, the phalanges become very numerous and the fingers come closer together forming a sort of mosaic of ossicles that supports the flippers; these, due to the structure of the upper part of the limbs, are less mobile in ichthyosaurs than they are in plesiosaurs.
In the second half of the Jurassic, several short-skulled plesiosaurs with large and long skulls evolved: they are the Plesiosaurs. The fins of these animals could also move upwards: therefore, in addition to pushing the body, they allowed its immersion. The plesiosaurs know the maximum diffusion at the beginning of the following period (Cretaceous), with even gigantic forms. Why have such huge forms developed? Let's give some of the more logical answers. Because the large mass was an efficient means of helping to keep the animal's temperature fairly constant. Because the bulk discouraged predators. Because the availability of vegetable food was considerable and these forms, herbivores, took advantage of it. Because the vegetables, often leathery, required a long digestion process, largely entrusted to fermentation.
Such a process had to be carried out on large accumulations of chewed substances in very large stomachs: the sauropods were perhaps only enormous transport systems for an enormous digestive system, immense stomachs equipped with four legs.
How did they move? In what environment did they live? The limb bones were practically solid (i.e. not hollow as in most vertebrates): they had to operate as true columns to support the great weight of the body.
The postenor paws had (in the Diplodocus) five toes with claws on the three inside. The two fingers had more. External paws were included in a thick, hard pad, similar to that which exists in the legs of elephants.
In the forelegs the first toe had a rather large claw. We observe the footprints in some cases very well preserved in the muds that have become stone. Sometimes there are series of footprints-holes (as big as the wheels of a bus) from which we deduce that the animals moved in groups. Other times the footprints (more and more individuals) are curious: from time to time only the signs relating to the "front legs" are noticed. Excluding that the beasts jumped or gave themselves to balancing exercises, we must admit that at least in some cases they lived in shallow water and pushed themselves with rare paw strokes on the bottom, just like hippos do today. The structure of the vertebrae gives us another clue: they are massive in the region of the tail; they become much lighter and hollow in the trunk area and become long but hollow and almost fragile in the neck. The reconstruction of the sauropod with the back emerging from the water, the floating neck and the tail dangling, almost like an anchor, becomes quite acceptable. On the other hand, in some forms (Brachiosaurus for example) the long neck and the front legs longer than the hind ones, suggest a reconstruction of the "elephant-giraffe" type. Columnar legs for walking even on dry soil and a very long neck to eat sprouts on plants. The teeth of some sauropods (Diplodocus) were relatively few and peg-shaped: a set of teeth suitable for chewing on soft aquatic plants or algae. In other species (Camarasaurus and Brachiosaurus).
Insects, spiders, millipedes abounded on the emerged lands, which were preyed upon by the very first lizards, by many forms of the group today represented only by the tuatara (still a famous "living fossil") and other small vertebrates. These in turn were hunted by various reptiles of medium size: Celurosaurs descending from the forms lived in the Triassic.
Coelurosaurs were agile and slender animals; bipeds, they moved very rapidly. We all know that to catch a fleeing lizard you need the dexterity and speed of action of the cat: the celurosaurs had to be just as fast. It has therefore been proposed by various scholars to attribute these capacities to a considerable availability of "own" energy, therefore to a high metabolism. In short, it is possible that the agile celurosaurs were homeotherms. The Ornitholesres was 2m long, but tall, "standing", less than a child; Compsognathus was smaller than a chicken. Descendants from Ornithosuchus and similar forms are the large bipedal predators called Carnosaurs.
At the beginning of the Jurassic the 6 m long Dilophosaurus is documented; towards the end of the period we have the Ceratosaurus, the Allosaurus, the Megalosaurus. In some genera the skull is decorated with crests and bony protuberances. The megalosaurus is the first dinosaur whose remains have been found and described. Insects had long been "masters" of the air. Vertebrates also tried, in the late Triassic and then in the Jurassic, to establish themselves in this environment. Why the air? The answer comes, once again, from food considerations.
In the air there are insects, therefore many prey, even if usually very small: moreover, the air constitutes a new "means" to get closer to other prey. Raining down from the sky you can better surprise an animal on the ground or you can even fish, completing the "dive" with a short dip. A small late Triassic reptile, Podopteryx mirabilis (about twenty centimeters long), had skin membranes both between the hind limbs and the tail, and between the short forelimbs, the hips and the "thighs" of the hind limbs. Little more than a small living kite with the largest part of the "veil" on the side of the tail. The Podopteryx launched itself from the trees to glide perhaps over its prey (certainly tiny) or to escape its enemies. All Pterosaurs descend from animals similar to Podopteryx. The structure of the pterosaurs remained roughly the same for 130 million years. The species are many and are usually distinguished by the shape of the skull and above all by the teeth. The organ of flight is always a membrane of skin stretched between the sides of the chest and the fourth finger of the forelimb enormously long (in proportion we should have a ring finger at least 4 m long). The skulls are very fragile, but in some cases the internal petrified "cast" of the space occupied by the brain has been preserved. It was thus possible to ascertain that the brains of these animals were quite large and that in particular the part relating to vision was well developed. The eye sockets are large: the pterosaurs must therefore have seen us quite well. The structures of the brain related to the coordination of movements are also well developed. It is probable that the pterosaurs knew how to make glides, but also pull-ups, vaulting and other "maneuvers" with skill and speed.
In Dimorphodon (early Jurassic, wingspan of 70 cm), the skull is large and the teeth are peg-shaped at the end of the mouth and smaller and more solid inward. Perhaps there was a "pocket" under the jaw, as in pelicans. In Crenochasma the jaw is bristling with very thin points similar to the baleen of whales. In Rhamphorhynchus the teeth are sparse and similar to daggers: a fishing tool similar to the harpoons of underwater hunters. It is probable that the majority of these animals ate fish or other marine animals; some perhaps caught insects. The discovery in 1971 in Kazakhstan (USSR) of a very well preserved specimen of a pterosaur made it possible to definitively shed light on a problem that had been debated for years. Referable to the Upper Jurassic, the animal of Kazakhstan has been called Sordes piloO5U5 (roughly "hairy crap"): it is a pterosaur as big as a pigeon, has a toothed mouth and a body thickly covered with hair. Some suspicion about the hairiness of pterosaurs had already been had from traces associated with other skeletons. So these animals were hairy. Why would they have developed such a characteristic if they had not needed to "conserve" their own heat, a heat that is evidently produced by themselves? Pterosaurs were homeotherms: this also explains their ability to lead a very active life. The excellent adaptation to the needs of flight is also testified by the fact that it led to the development of real feathers and therefore of birds. Five more or less complete skeletons of an animal with teeth, long tail, well developed front limbs (with three fingers), runner hind limbs (with four fingers) have so far been found in the fine limestone of the Jurassic period of Bavaria.
The skeleton is similar, also for the size, to that of a celurosaurus (for example of a Compsognathiis). Here, however, is the remarkable detail: the bones are associated, very recognizable, with the imprints of a whole series of feathers and feathers, corresponding to the antenon limbs and the tail. A feathered reptile? Or a bird? The animal was named Archeopteryx lithographica and is considered the first bird. The basin is birdlike, but a relationship with ornithischian dinosaurs does not seem possible.
Perhaps this animal was actually a celurosaur adapted to live in the woods. Perhaps it ran and jumped short leaps supporting itself with its "wings", or it climbed trees (with free fingers and clawed front limbs) and then let itself glide. The technical solution represented by the feathers offered, among the trees, some advantages compared to the membrane of the pterosaurs: this could tear in the impact against a branch, while the set of feathers offered no resistance and spread out, letting a foreign object pass. Homeothermics found a very useful accessory in the lining of feathers and feathers. The "scales-feathers" became established (we note that the scales of the reptiles and the feathers are formed from the same substance and have the same origin, that is, they are homologous). The feathers of Archeopteryx have been counted: they are 10 primary and 14 secondary on each wing, just like in present-day birds.
