The one therory that is most likely to have happened is the earth was hit by a large meteor thus kicking up a large sum of dust that the sun was unable to penetrate, thus all plant life would die and in the process the dinosaurs in turn die.

The one I believe is probably the theory where they all get sick by some fever.

As I said in another reply; drought with accompanying loss of water would make a corridor for teeth disease and its othe rlife giving properties. Sea water would be affected by asteroidal impacts as well. Dubious any creature would survive even if it is a slow kill idea from drought.

There are several theories on what caused the extinction at the end of the Cretaceous where the dinosaurs (and many other forms of life)became extinct. It could be a combination:

1. Disease because land bridges opened and allowed microbes that were minor in one locality to spread to other localities where they were a disease. One example among humans was when Europeans explored the rest of the earth. Many microbes -- smallpox, whooping cough, measles, chicken pox -- were minor diseases in Europe, but were fatal to the American Indians and Pacific Islanders who had never been exposed before. So, if you have dinos from Asia mixing with those of North America, then they trade diseases, causing massive epidemics among both.

2. Massive volcanic eruptions over several thousands of years in the Deccan plateau in India. So much dust and gas that it cooled the earth and caused massive ecological changes.

3. A giant meteor impact at what is today Chixulub Mexico that caused immediate massive global weather changes that resulted in extinction within 1,000 years.