Cretaceous
135 to 65 million years ago
The Cretaceous period lasts 70 million years and ends the Mesozoic. During this phase of the history of the Earth, the definitive fragmentation of the supercontinents takes place. The clods that form the present continents separate. Only Antarctica and Australia still remain, at least in part, connected with southern Amenca. India approaches the equator and crosses it at the end of the period. Madagascar also detaches itself from Africa. The Atlantic is now quite wide and continues to widen. The elements of the Italian peninsula and the Balkans are approaching the rest of Europe from the south.
Of great interest is the presence in all continents of vast seas of shallow water: they separate the eastern areas of the two Americas from the western ones: they still divide Europe from Asia and the western part of this continent from Siberia and the Far East. Orient; they also divide Africa into a western part and a larger portion comprising the present eastern, central and southern regions of the continent.
The displacements of the continental masses lead to the "birth" of many mountains (gold genesis) and to a strong volcanic activity: in areas where previously "it was hot" snow falls; in others the Sun is obscured by the ashes of an eruption. Situations arise in which it is difficult to find food.
There are two essential components that act on living forms in the Cretaceous period: the isolation of the various emerged lands and the instability, even temporary or local, of environmental situations. Intense solicitations therefore, which resulted in a truly spectacular wealth of forms, especially among dinosaurs.
This great activity, however, is covered by a sinister reflection when we consider that the Cretaceous also saw the disappearance of the legendary "rulers".
A lush season, like a beautiful autumn, full of many fruits and food for everyone. The discontinuity of the land is obviously matched by the continuity of the seas. The movements of the continents, however, often isolated the internal seas or the areas close to the coasts and local evolutions or survivals took place. The seas of the Cretaceous are frequented by the last plesiosaurs (Elasmosaurus, up to 15 m long, of which 7 pertaining to the neck alone), by many pliosaurs (gigantic forms: the Kronosaurus, 13 m long, had a 3 m long skull) and by a wide range of bony fish. Ichthyosaurs, in decline, have very specialized forms: the Ophthalmosaurus, for example, had a toothless mouth (evidently it swallowed its prey whole, especially ammonites). The eyes of this animal were protected by a ring of bony plates: a typical adaptation to pressure variations due to diving to depth. There were large turtles (Archelonischyros, over 4 m long) quite similar to the current forms. In the Cretaceous the Mosasaurs develop and immediately triumph. Interpretable as ancestors of today's monitor lizards, they descended from lizards adapted to aquatic life. The largest forms reach 15 m. Huge mouth with large conical teeth, back decorated with crests or series of triangular plates, long and tapered tail, the mosasaurs must have been very similar to the "sea snakes" of the legends. Their eyes were also equipped with rings of bony elements. Among the best known forms are those of the genera Tylosaurus and Mosasaurus. Georges Cuvier is credited with having recognized the Mosasasurus fossil unearthed in 1780 near Maastricht, in Federal Germany, near Aachen, on the Meuse as a marine lizard. We do not know if the mosasaurs were viviparous. The marine environments were also frequented by pterosaurs (Pteranodon ingens, toothless, with a pocket mouth similar to the beak of the pelican and with a wingspan of almost 8 m); by birds with atrophied wings (Hesperornis regalis), specialized in swimming and fishing near the coasts; by flying birds and fishermen (Ichthyornis victor). All these predators did not have to be in direct competition for food: there was no shortage of fish and shellfish. However, it is probable that mosasaurs, pterosaurs and birds frequented the same environments: their bones are often found in the same layers. One of these findings is at the origin of a notable misunderstanding about the Ichthyornis. The skeleton was found without the head, but with a small mosasaur head among the other bones. Cone teeth are typical. For years, reconstructions of the bird have been seen with a beak-covered mouth with teeth. In fact, it is likely that Ichthyornis had a toothless beak. Ammonites are abundant in the Jurassic and Cretaceous marine ecosystems. The shells of these Moli); We know the earliest stages of development of ceratopsis because some Protoceratops nests have been found in Mongolia.
The eggs were laid in groups in holes in the sand, as is the case today for crocodiles. Some of the eggs discovered contained embryos apparently dead before completing development. Many of the ceratopsis later come from North American sites. It seems that these dinosaurs had two main areas of diffusion: one in central-eastern Asia and another in North America. The genera described are up to now 20. The best known species is Triceratops prorsus, described by Marsh in 1889.
Probably the triceratops, up to 11 m long and even 8 t heavy, behaved like the current rhinos: they systematically grazed the palms, but if they were disturbed or feared an attack by some predator, they hurled themselves with all their mass and they certainly put the enemy to flight or treated him very badly. The frontal horns of the triceratops were also long m. Perhaps the real enemies that the ceratopsis had to guard against were not the large predators, but the small "egg-stealing" dinosaurs (such as the oviraptor) or the mammals with the same habits. In the relatively arid contexts to which they were well adapted (palm trees also grow in areas on the edge of deserts: they are the plants of oases) there was not much food and the small predators had specialized in raiding nests. These are obviously hypotheses, which however appear quite reasonable, given the environmental situation. We mention 1 'Ornithomimus, 1' Ovraptor, the Velociraptor.
In Deinonychus ("terrible nail") the hind legs have four toes with claws. The first toe is very small; the second has an enormous very sharp claw: the animal kept it "bent up" while walking (on the third and fourth toes), to prevent it from being consumed. Cassowaries, the large Australian running birds, behave in a similar way. Confronting its victim, the Deinonychus stood up on one leg and struck a deadly blow with its sickle-claw. The complex balance game was facilitated by the long tail-counterweight stiffened by particular bony structures associated with the vertebrae that slipped into each other like cases. The forelimbs had three strong claws. The tyrant and the others Found at the beginning of the century in Montana (USA) in two specimens and described by Osborn in 1905, the Tyrannosaurus rex is undoubtedly one of the best known and "popular" dinosaurs. To the great charm of the "giant" it associates the sinister charm of the "villain". And the ogre of fairy tales transformed into an animal: it is scary, but distant in time (what somehow calms the viewer, like us, at the skeleton in a museum). It is therefore understandable why the tyrannosaurus has a firm place in the world of comics, in that of science fiction stories and films. His "big mouth" is in front of our eyes (casts of the skull can be found in the main natural history museums): 60 teeth with serrated edges, many of them a span long. The skull is as long as a calf. Today this "bad" and less alone, after various discoveries have accompanied other very similar forms: the Albertosaurus of Canada (this genus can also include animals previously ascribed to the genus Gorgosaurus) and the Tarbosaurus haraar of Mongolia (the lying of this region have provided also new skeletons of the same tyrannosaurus). How did the "tyrannosaurus lizard" live (in Greek tyrannos is the absolute master of a city)? He ate meat, of course, but what meat? And how much? The average length of the tyrannosaurus is 15 m , the estimated weight about 10 t. A weight of meat equivalent to that of the animal could be provided by half a 20 t sauropod or three 3 t hadrosaurs. How often the tyrannosaurus had to take this quantity of food? Comparing its size with that of a "cold-blooded" animal such as a crocodile or a Komodo monitor, we could conclude that such a booty was enough for the tyrannosaurus for months. Mammals are present from the Triassic to the Cretaceous with small or very small forms. Towards the end of the Cretaceous, however, a certain specialization of various groups is already noted: animals similar to Opossum (Marsupials), insectivores and primates (two groups of Placentates) are documented. The small animals that, for millions of years, have operated in the evening, at night, or underground, begin to come forward, to explore the rapidly changing environments. The forests, with the dense undergrowth of shrubs, present themselves as a very suitable "place". These animals are small and very agile: they can slip anywhere, hide. They have a fur covering: therefore they are not afraid of the thorns of plants and pointed twigs, on the contrary they find a sure protection there. The hair makes homeothermics extremely efficient: these animals do not need to receive much heat from the environment, even the light-shadow conditions of the forests and undergrowth appear excellent. Their advance finds no obstacles. Mammals eat all kinds of food: insects, worms, leftover meat, leaves, bark, fruit, hard seeds. Their young are fed with mother's milk which causes them to develop very quickly. When later, the "rulers", the dinosaurs, encounter great difficulties in hostile environments, the mammals will calmly replace them. The Mesozoic closes. The new era opens, the Cenozoic ("of recent life"), or Tertiary. One should not think of these "closings" and "openings" as if they were doors or passages from one room to another. The "passage" in geological terms is given by a certain change in the layers of the soil, a change that is usually detected because the fossil evidence changes. Here is the reality "written in the rocks": in the layers that follow those of the Cretaceous there is no longer any trace of the following groups of vertebrates: saurisks, ornithischians, pterosaurs, mosasaurs, pliosaurs, ichthyosaurs. Among the invertebrates, the disappearance of the Ammonites and Belemnites is important. And the great "mystery" of the fall of the giants, of the disappearance of the rulers. It is important to immediately note that the "great collapse" did not only affect dinosaurs, but also other living forms: flying forms (pterosaurs) and marine forms (including invertebrates of the mollusc type).
Will it be possible to identify a single "cause" capable of explaining the extinction of living forms in three different environments? Prudence and logic prompt us to examine the possibility that there have been multiple causes. First of all we note that it is not possible, dealing with events so distant in time, to ascertain what was the "speed" of the phenomenon. In short, we cannot say if a certain day, or year, or century the dinosaurs were still there and if the day, or the year, or the next century were no longer there. Overall, it can be said that the last chapter in the history of the "giants" lasted at least ten million years. We have no difficulty in admitting that "anything" could have happened in such a long time, that several and twenty have occurred, some of which are exceptional. It is nevertheless necessary to put a limit on the "exceptionality" of such events.
