What is the basic reason for the death of dino period?

Dinosaurs reigned supreme throughout the Secondary era, that is to say for 160 million years. Of course, during this period they have evolved. But they remained the dominant species on Earth. At the end of their reign, mammals already existed, but they were just small little bigger beasts than current chat. Actually, dinosaurs, both herbivores than carnivores, occupied space. The remaining space for mammals (niche) was very small, and they could not really develop.

Multiple theories have been scaffolded to explain this disaster, but few are resisting serious analysis. Among them, there are biological theories, and geological or astronomical theories.

Les premières envisagent des attaques virales ou bactériennes, qui auraient tué tous les dinosaures. Mais ce genre d’explication paraît douteux, car il existait une grande diversité de dinosaures : très grands ou tout petits, tropicaux ou polaires, carnivores ou herbivores… Il est difficile d’admettre qu’un même agent pathogène puisse détruire ces différentes espèces. A fortiori on ne comprend pas qu’il s’attaque aussi à des animaux marins.

The first of these theories presented by Vincent Courtillot, is based on the observation of a geological phenomenon that occurred just when the dinosaurs disappeared; it is an episode cataclismique volcanic eruptions, hot point type in the Deccan (province of India). In a short time (in geological terms), several kilometers thick lava has spread on the surface of the Earth. During eruptions, it is possible that a significant amount of material from the mantle of the Earth, so rich in iridium, were projected into the stratosphere, and then are later impact by covering the entire surface of the planet. Such an explanation is closer to the phenomena observed in 1908 during the eruption of Cracatoa, whose ashes have obscured the Earth for two years, causing a decrease in temperature, or more recently the Pinatubo eruption, less violent, but sensitive consequences. If many such eruptions occur within a short time, the accumulation of dust in the stratosphere can reduce solar radiation to the ground, lower the temperature, and limit photosynthesis. Such conditions could be very damaging to many animals, regardless of species.

Still the number of theories are diverse to explain all of them, but the one that I believe on is the meteorites one cause it's so close to be real.