Theory
Evidence
They’d been around too long and just ran out of ways to evolve.
The dinosaurs were a group of organisms that lasted around 180 million years, evidence suggests that these organisms ran out of ways to evolve and as a result could not cope with changing conditions.
They were poisoned by the newly evolving plant life which produced chemicals to protect themselves from predation.
This theory is plausible as it ties in with the first one, as the dinosaurs were not prepared for this change in the plant life. Evidence shows that extinction is caused by many things, these include the loss of food; due to the loss of plants as a food source the herbivores were dying out, this meant that the carnivores also lost their food source (the herbivores).
It got too hot.
Some life on earth is specially adapted to a certain climate and if this climate changes rapidly then it could lead to extinction as some of these species would not be able to adapt to the new climate quick enough to survive. This causes a break in the food chain which has a domino effect on the rest of the organisms in the food chain, and as we know food chains are very interlinked meaning that if the wrong species dyed out it could cause mass extinction.
It got too cold.
This is the same concept as the one above.
The sea level changed.
Evidence shows that the sea level may have changed around the time of the extinction of the dinosaurs. This may have caused a loss of habitat for sea life such as ammonites.
A massive meteorite hit the earth.
Scientists have found a crater that could have been produced by this asteroid. If a meteorite big enough hit the earth then in theory the dinosaurs close enough would be killed instantly. The dinosaurs that survived would die out as a result of the thick cloud of ash a dust thrown up by the meteorite blocking the sun causing a destruction of plant life and therefore a food source (this theory is like the nuclear winter