by Todor Nikolov and Viara Minkovska


    Dinosaurs are the most exiting group reptiles among the varied animal world of the past geologic epochs. The history of dinosaurs marks a culmination in the vertebrates' evolution, although they are often seen as a brilliant failure of evolution because of their mass extinction at the end of the Mesozoic Era.
    Dinosaurs are part of the Archosauria subclass (archosaurs) of the Reptilia class (reptiles). Besides the dinosaurs, the archosaurs class includes also the thecodonts, the crocodiles and the pterosaurs (the flying reptiles). Archosaurs are the most developed and widely distributed Mesozoic reptiles group.
    The fast distribution of reptiles on land was enabled by one of the greatest discoveries in the evolution of land vertebrates: the shell-egg called the amniote egg. In this way the big Amniota group was set up including the reptiles, the birds and the mammals. The children's question: "Which came first, the chicken or the egg?" was answered more than 310 million years ago (310 Ma B.P.) in the late Carboniferous Epoch, i.e. the birds received the egg as a present from the reptiles.
    A significant phenomenon in the dynamics of the planet during the Mesozoic was the break of the united supercontinent Pangea producing the present continents. The oldest known finds of dinosaurs date back to the end of Middle Triasic - the beginning of Late Triasic (230 Ma B.P.). According to the fossils there are some 380 genera and more than 800 species of dinosaurs. It is assumed that during their whole history, from the Middle Triasic till the end of Cretaceous, the dinosaurs were represented by some 50,000 genera and more than 500,000 species. In spite of the incompleteness of the geological record it seems that there have been at least some dozens of thousands genera of dinosaurs during the Mesozoic including 150-200 genera represented by giant forms.
    Generally the dinosaurs show an evolutionary trend toward giantism, a trend common to many rapidly evolving groups. Some dinosaurs were truly monsters. There were also a few small dinosaurs.
    The great divergence of the phylogenetic lineages beginning in the Late Triasic was most fully realised in the Jurassic after the changes in the structure of the pelvic girdle. The clear difference between the two kinds of movement (the two-legged and the four-legged, bipedal and quadrupedal) manifested itself after the changes in the structure of the pelvis of the Jurassic representatives.
    The choice of dinosaurs to be strong and quick, and not flexible and clever, was of fateful significance. In the initial stages of dinosaurs' evolution in the remote Late Triassic the dilemma "quick or clever" was solved in favour of the quick. The Eoraptors pushed aside the terapsids reptiles. The clever lost to the quick, but this was just the beginning of a long run through the hills of time. In the next stage the clever (the mamals) showed that they had more perspectives for the future.
    Dinosaurs were not primary and typic warm-blooded animals. They were inherently exothermic animals with inertial homeothermy, whose higher temperature depended on their activity and behaviour to look for warmer places, where they obtained warmth for their huge bodies.
    What was the intelligence coefficient of dinosaurs? The saying goes that "fools are not dinosaurs and will never disappear". Does it mean that dinosaurs were clever?
    Probably, dinosaurs were no fools, although the comparative studies showed that in the average the volume of their brains was 10 times smaller than that of the mammals with similar proportions. Big or small most of dinosaurs had disproportionately tiny brains for their size.
    The highest IQ close to that of birds and mammals had the species of the Stenonychusaurus genus belonging to the group of ostrich-like dinosaurs, called also celurosaurs. They were probably the ancestors of the birds.
    Remarkably enough, the largest carnivorous dinosaurs ( the carnosaurs) and the ostrich-like dinosaurs (the celurosaurs) with a higher IQ, compared to the other dinosaur groups, were much more active and flexible forms.
    There were proofs showing that the brain (the cerebrum) of some dinosaur groups was very small. For example, the well-known Diplodocus, Brontosaurus, Brachiosaurus, Stegosaurus, etc. had a brain with a volume of a kitten's brain. That was why these animals developed secondary cerebral centres in the pectoral and pelvic girdles. Furthermore, the volume of the cerebral cavity in the pelvic area was several times bigger than the cranial-cerebral cavity (f. e. genus Diplodocus).
    It is hard to say how effective this decentralisation of the anyway small amount of brain substance should have been, which has explicitly manifested itself in some dinosaurs, but its has most likely facilitated the control of the separate parts of the huge body reaching up to 30-50 tons for some forms.
    The tragic end of those dinosaurs showed that "thinking with the head" was still more productive than "thinking" with the ischial body parts.
    All dinosaurs propagated through eggs. Eggs including embryos of dinosaurs were known from more than 200 deposits in different parts of the world. Many fossilised bones on egg nests have been found, which is assumed as a proof of the nesting behaviour of the dinosaurs. In many cases bones have been found around the nests. During the Cretaceous Period the eggs shells of many dinosaurs show a progressive decrease in thickness in younger and younger rocks beds.
    Developing giant proportions, powerful jaws with sharp teeth, forming spikes, protective plates and shells was the result of searching protection against carnivora. This was the longest arms race demonstrated in the evolution of dinosaurs. Nevertheless, nothing of these arms could save the giant reptiles and they left the living environment defeated by the weaknesses of their own morphophysiological organisation, which was destroyed by external factors.
  The mystery of dinosaurs is fascinating: they appeared on the planet some 230 million years ago, conquered all ecologic niches of the continents, inhabiting the air and water media by their relatives, becoming giants, establishing a full and unquestionable dominance over the whole Earth, becoming rulers over all living beings on the planet in the course of more than 165 million years, and collapsed in the end of the Mesozoic Era at the boundary of the Cenozoic Era (Cretaceous-Tertiary boundary = K-T boundary). This was also the end of the biggest empire in the animal world.
    The mystery becomes even more complicated because of the fact that the mass extinction of dinosaurs in the end of the Mesozoic was accompanied by the ruin of many other systematic groups.
    Because of this, the end of the Mesozoic, the Cretaceous-Tertiary boundary (65 million years ..), marked one of the greatest crises in the history of life, when nearly 650 families died out, mostly representatives of the animal kingdom.
    When empires are being destroyed, they always carry away in their ruin not only what has been closely related to them, but also some peripheral groups, usually not directly connected with the empire itself. It is characteristic of the global mass extinctions in the geologic past that whole systematic groups have disappered, also not related ecologic organisms.
    The dinosaur development was influenced by strong interacting factors of biological, geological and astronomic nature.
    Many main characteristics in the morphology, physiology and the behaviour of dinosaurs gradually turn into their weaknesses forcing them into a struggle for survival. The explicit tendency to giantism and shell forming seemed nonproductive from a strategic point of view, because it led to sluggishness, i.e. a lack of flexibility not only in their behaviour, but also in the possibility to adapt themselves to the changing environmental conditions.
    Among the geological factors an important role played the globals eustatic sea level fluctuations, the sharp changes of climate, the volcanism, the inner Earth dynamics, and the drift of the litospheric plates.
    A great influence on the tragic end of dinosaurs had the astronomic forcing phenomena (the orbital factors and the related gravitate and other phenomena, especially the impact phenomena). During its own history the Earth has been constantly hit by meteorites of different sizes.
    Many researchers think that the fall of huge comets, meteorites or asteroids caused the catastrophic mass extinctions in the history of the Earth assuming that the dinosaurs were killed by a giant asteroid.
    An undisputable hit among the hypothesis of the impact of the external dynamic factors was the idea of Luis and Walter Alvarez, Frank Asaro and Helen Michel published in 1980.
    The studies of the team of Alvarez, like the studies of many other researchers from different countries, showed that all over the - boundary, in all deep-sea segments, there was a thin clay bed (with thickness of some centimeters), where the content of iridium (Ir) exceeded 20 to 160 times the usual quantity of this element in the neighbouring lower and upper beds. The abnormal high quantity of Iridium in this bed was accompanied by the so-called shock-quartz.
    The sudden increase of iridium (Ir) in the K-T boundary beds might be due to an explosion of SuperNova near the Solar system, when elements heavier than iron including iridium could have been emitted. The increased quantity of Ir could be explained by a crash with a giant meteorite, asteroid or comet, or it could be of a volcanic origin.
    This crash produced a large quantity of dust darkening the Earth for a long time. This led to slowing down the speed of photosynthesis on land and of the oceanic phytoplankton. The amount of atmospheric oxygen decreased and the relations in the trofic tracts of the ecosystems were disrupted. As a result, almost no animal over the weight of 25 kg survived this cosmic impact. Those results are comparable to a nuclear holocaust.
    The end of dinosaurs was a natural finish of their long evolution, in which they became rulers of the Earth establishing the biggest empire in the history of our planet. In the last stage of their development, which lasted some 30 million years (during the Late Cretaceous Epoch), dinosaurs fought an incredible survival struggle; but a survival struggle in evolution has always decresed the possibilities of improvement. In the end of the Mesozoic they seemed to be quite an old-fashioned general morphophisiological organisation, heavy and inflexible giants, trying to protect themselves mainly through development of power and shells, but without having possibilities of improving their brains. Thus, the influence of the external factors forced them to face the catastrophe. The asteroid impact was like a mass brain stroke provoking not just a coma, but their final extinction from the Earth leaving their bones in the rock record for the other species' generations to draw a lesson from.
    The crises and the extinctions of the organismic groups are a normal natural phenomenon in the history of evolution and are part of the survival struggle. They correlate with the general geologic cycles.
    Mankind can learn from the dinosaur history. The main conclusion is that the thread for our planet does not come from natural catastrophic events and phenomena related to the planet's dynamics and the influence of astronomic forcing phenomena, but rather from its attitude towards its own cradle. Unfortunately, this attitude is very often on the borderline of irresponsibility, arrogance and stupidity. These three qualities were characteristic of the dinosaur behaviour. This behaviour, figuratively speaking, was one of the most important reasons for the decay and the collapse of the dinosaur empire.
Compared to dinosaurs, the intellectual capabilities of man are practically unlimited, but his brain does not keep up with his development. Everything attainable with the intellectual capabilities of Homo sapiens regarding the wellbeing of mankind and the life on our planet has to be controlled by the mind and the morals. Otherwise, mankind will have to face the dinosaur fate.

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Last modified: 27 September 2005

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