Dinosaurs - Their history
The dinosaurs, those terrible lizards
No living thing in the past has aroused as much interest as i Dinosaurs, from the Greek "terrible lizards", whose first remains were discovered in 1600. These monstrous reptiles, perhaps due to the large size reached by many of its specimens, for the appearance of some species and for their sudden disappearance with the end of the Mesozoic, from the beginning of the studies have sparked the imagination of paleontologists, as even the names that were given to the various genera, such as: Lizard shaking the earth(Seismosaurus), Heavy lizard (Barosaurus),Double beam (Diplodocus), Monstrous lizard (Pelorosaurus),Thunder Lizard (Brontosaurus), Whale lizard (Cetiosaurus),Big-toothed snake (Macrodontophion), etc., almost similar to the names of the legendary leaders of the Redskin, a privilege that has never been reserved for other groups of animals.
It should be noted that other Reptilesmonstrous in appearance and size, in addition to Dinosaurs, they lived not only on land, but also in the seas (Ichthyosaurs) and in the skies (Pterosaurs) for the entire Mesozoic Era. Very often these exceptional specimens, with superficial simplification, are "smuggled"like Dinosaurs in more or less sci-fi reconstructions for magazines and films, misleading readers and spectators.
Dinosaurs were exclusively terrestrial animals having failed in their evolution to adapt to living in the waters, despite being able to float and simulate swimming by moving their legs, as other terrestrial animals still do.
Likewise, they did not come to hover in the air, although the rare findings of the remains of a very small reptile with a head and other skeletal parts similar to those of Dinosaurs, and with feathers on the upper limbs (Archaeropteryx), have made some speculate that this genus could prove the ancestry of Birds from Dinosaurs.
In the absence of new discoveries of unequivocal remains, in order to be able to reconstruct the entire evolutionary process from Dinosaurs to Birds, the case remains a fascinating hypothesis for study. It cannot be excluded that some convergence between the bone structure of some Dinosaurs and the Birds, in particular the presence of cavities inside them, can only represent an unsuccessful attempt to conquer the skies, or perhaps an expedient to lighten the weight. and thus acquire greater agility, to chase prey or escape predators.
On the other hand, the whole study of Paleontology is studded with similar situations, due to the lack of links in the evolutionary chain, not only between large groups, as can be the one between Reptiles and Birds, but also within the more restricted of numerous orders.
Just think of the ongoing diatribes in the field of Paleoanthropology, to establish the relationships between the hominids and, in particular, to establish from which hominid modern man descended. Perhaps one day when the studies will not be based only on morphological elements, but will have the help of in-depth studies of molecular paleontology (DNA), it will be possible to better understand that fascinating and largely unknown phenomenon of Evolution. The path of molecular paleontology will be long and difficult, but already in paleoanthropology it has begun to give good results.
In reality from whom did these Dinosaurs originate and who were they? What facilitated their enormous growth? What environments saw their domain?
APPEARANCE AND ENVIRONMENT OF DINOSAURS
They appeared at the end of the Trias, on that single continent called Pangea (see on Elicriso the Chapter on the Drift of Continents), in a scenario characterized by a predominantly arid climate in the equatorial areas, while towards the poles, where given the high temperature they were the glaciers completely disappeared, a relatively humid climate was established, which facilitated the development of large flora such as Araucarias, Sequoias, Pines, Cypresses, Ferns, and the appearance of the first flowering plants, which spread widely in the Cretaceous and, subsequently, in the Tertiary.
The crushing into plates of the Pangea and the relative Drift of the Continents in all directions, has had a basic function in the evolution of fauna and flora in all times.
With the formation of the first arms of the seas, which were penetrating the emerged land and expanding to form the embryos of the current oceans, there were changes in the climate, decisive for the development of the whole complex of animals and plants, with imaginable consequences, given their interdependence
Remains of dinosaurs have been found on all continents, particularly in North America, perhaps because research in this area has been more intense. As far as Italy is concerned, for a long time it was believed that the Dinosaurs did not inhabit our regions, as, during the Mesozoic, Italy was mainly submerged by waters. But for some decades it has begun with the discovery of the footprints, subsequently of the fossil remains and nests of eggs and, then, almost complete skeletons.
HOW WERE THEY MADE? WERE ALL REALLY BAD?
A big distinction must be made between Saurischiis Ornithischians: the former are carnivores and herbivores, while the latter are exclusively herbivores. The subdivision of the two groups is mainly based on the structure of the pubic bone and pelvis: the Saurischi had them similar to the current Crocodiles, while the Ornithischians are more reminiscent of the Birds.
The first Dinosaurs, belonging to the Saurischi, originated from small carnivorous reptiles, which soon reached large proportions compared to their ancestors and spread over vast areas. Among the most representative examples of the first Triassic Dinosaurs we mention the Coelophysis,carnivorous biped of limited size. The Ornithischians appeared later, in the Jura
Dinosaurs were oviparous, like crocodiles and birds, closely related to them, so much so that many consider them intermediate forms between the two groups.
There is no evidence to suppose that there have been viviparous species. This is evidenced by the numerous 50-60 cm sized egg nests found in relatively small spaces, suggests that the dinosaurs lived in groups, as if to constitute families, as many animals still do, to facilitate the defense by predators. species to protect the offspring.
It cannot be excluded that carnivorous specimens lived and moved isolated, with the disadvantage, however, of being more easily attacked.
It is generally believed that herbivores far outnumbered carnivores, as is also the case today, for example. in Africa, where herds of herbivores in the wild are more numerous than carnivorous predators
Dinosaurs were originally bipedal, but later, due to the enormous weight reached by some genera, both herbivores and carnivores, they turned into quadrupeds. The possibilities of growth in size found in some genera, particularly among herbivores, were linked to the luxuriant vegetation of the period, which represented an exceptional availability of food.
At the same time, the development in height of the plants almost pushed for some species to elongate the neck, counterbalanced by that of the tail, in order to allow the animal to reach the highest tops of the trees.
A bit like what happened among living mammals, in more recent times, such as Giraffes and Elephants, with the development of the neck or trunk, but in very different proportions.
As the herbivores had to "grind huge quantities of food" their dental apparatus was provided by hundreds of teeth, with a considerable number of teeth underneath, ready to function as the ones above were consumed.
As evidence of the ferocity that herbivores had to face, is the development on the body of horns, beaks and plates, equipped with quills, of great thickness, to give the specimen an increasingly monstrous appearance, as is clearly visible in the genus Triceratops.
In carnivorous predators obviously there was a development of the fangs and claws to increase their aggressive power, and at the same time of the means of defense. Obviously, the ferocity was exclusive to carnivores exercised not only towards herbivores, but also towards their peers, as some genera exercised "cannibalism".
WERE THE DINOSAURS COLD OR HOT BLOOD?
Until a few decades ago, the majority of paleontologists believed that Dinosaurs must have had a cold blood, like the current Reptiles, but some scholars have pointed out that animals of that size, with a complex metabolism and with agility, must have been much more similar to mammals and birds than to reptiles. On the other hand, it has never been shown that Dinosaurs had hair or feathers to protect them from the cold, which allowed them to keep their body temperature within constant limits, in the event of severe drops in temperature.
Furthermore, some genera were equipped with vertical plates, placed along the back, but given their position it is doubtful that they had only a defensive purpose, while, due to the presence of vascularizations within them, it is hypothesized that they had a thermoregulatory function some blood.
For the supporters of an endothermic system there are substantially valid arguments regarding carnivores, which, being bipedal, endowed with a lot of agility and possessing a more developed brain, compared to herbivores, which allowed them to develop more advanced hunting techniques, needed warm-blooded body circulation, typical of endothermic animals.
While for the herbivorous quadrupeds, slow in their movements for a body in general of large size, which allows a poor dispersion of the body temperature, and not needing large reserves of energy for their metabolism, they were probably cold-blooded.
Therefore the question has not yet found an explanation. For now, many scholars are inclined to believe that Dinosaurs were endowed with an intermediate circulatory system between that typical of warm-blooded animals and that typical of warm-blooded ones, in order to ensure them the most favorable conditions. Just to increase the mysteries surrounding this group of reptiles.
ECATOMBE OF DINOSAURS AND NOT ONLY
With the Cretaceous period, that is, with the period that closes the Mesozoic Era, there was the disappearance of many groups of animals, especially those that had reached considerable dimensions. Almost exclusively small mammals survived, part of the fish fauna, birds and a few reptiles such as crocodiles, snakes and turtles, which were the protagonists of the Tertiary Era together with plants with flowers.
This sudden disappearance of many groups of animals was not exclusive to the end of the Mesozoic, but had occurred several times in the history of the Earth, such as the one at the end of the Paleozoic, when 90% of all the fauna disappeared. It is estimated that since the appearance of life on planet Earth, only 99% of the surviving species have survived to this day.
This is not surprising, because it is nothing other than one of the aspects of the continuous evolution of Nature: just as the single individuals of living beings have their own life cycle, from birth to death, so the great groups of fauna and flora have their own duration. , during which the evolutionary cycle has always had a process, sometimes complex, which ended with the extinction of its specimens. Always keep in mind that these extinctions take place over several million years, as in geology the conception of time has nothing to do with that of man in evaluating events in his life.
What should surprise however is the still presence of forms that appeared hundreds of millions ago, which can be considered an anomaly in the history of all living beings. The borderline case is undoubtedly represented by some forms of bacteria, in part equal to the living ones, which testified to the beginnings of life in the Universe 3-4 billion ago, found in rocks and meteorites.
So much hype around the Dinosaurs is disproportionate, for their sudden disappearance, since they were not the only ones to face this event. Just think of the vast group of Ammonites, the same age as the Dinosaurs, who after having dominated the seas of the time, both for their diffusion and for their size, suddenly disappeared with the end of the Mesozoic, despite the fact that there were more than 5,000 species at the time. The only surviving genus from the Paleozoic to today is the Nautilus, represented by a few species limited to restricted areas of the Far Eastern seas.
WHAT ARE THE CAUSES OF THIS QUICK DISAPPEARANCE?
The mechanism that causes the large and small contemporary disappearances of so many forms of life, to represent a true decimation of the fauna and flora, is not known precisely. The most probable thing is the concomitance of several biological and environmental causes, on which many hypotheses have been formulated.
All animals have undergone and undergo a slow but constant evolution, which has led to the creation of new species, connected to each other, such as the branches of a tree (filum), with the consequent proliferation of numerous genera, families, etc. due to the lack of connecting links in the evolutionary line, it is not always easy to trace the original species.
The engine that sets this mechanism in motion is complex and not always clear, but at the base there is a 'interaction of biological and environmental factors. The biological ones are represented by the molecular mutations of the cell, which can occur spontaneously or triggered by external interventions, sometimes environmental or chemical-physical factors, such as d. particular chemical compounds and the presence of cosmic rays, X rays, ultraviolet rays etc. These mutagens have the main function of accelerating the process of spontaneous mutations and, according to some scholars, they would have played a fundamental role in the emergence of life in the Universe.
This ability to resort to the mutation of its own DNA, allows the species to adapt to new environmental conditions, so much so as to make changes to the anatomical structure to meet new physiological needs.
But this cannot happen if the species has reached a certain "biological fatigue", that is, it no longer has the possibility of generating spontaneous mutations of its biological system and therefore, unable to react to new solicitations from mutagenic agents, it is destined to extinction, in a more or less short time.
The evolutionary cycle, illustrated schematically in this way, has involved, without exception, all animals and plants. Perhaps the Dinosaurs have undergone flashy evolutionary processes, given an exceptional biological predisposition in conjunction with environmental events of considerable magnitude, such as continental drift and other factors also of an astronomical nature.
The consequence was the differentiation into about 450 species, initially carnivorous, to subsequently originate herbivorous species, which through progressive mutations have reached high specializations (e.g. considerable size and therefore the need for large quantities of food), which prevented them from support pronounced environmental mutations with adaptation.
Just think, for the mammoth herbivores of the most recent Dinosaurs, what could have represented a warming of the climate such as to create an environment of drought, causing the reduction of vast areas of luxuriant vegetation. Some forms have managed to emigrate to areas with a less ungrateful climate, but the large specimens have obviously been forced to succumb.
For carnivores, depending on their nourishment on the spread of herbivores, they obviously suffered the same fate, as it was increasingly difficult for them to procure prey. At the same time they too did not have the evolutionary capacities that their ancestral forms had had, therefore they did not they could evolve towards species adapted to the new conditions of life.
It is the end of the Mesozoic Era, characterized by a massacre of many species of animals and plants, which left alive the forms most suited to the environmental conditions of the time, and, in particular, to those forms that had the potential to evolve towards other species, which will explode in the Tertiary sector and which will be protagonists for over 60 million years, culminating with theappearance and development of hominids, an event that concerns us most closely.
Dr. Pio Petrocchi