Many believe that a meteor caused the death of dinosaurs, and a multitude of other species of animals, during the Cretaceous–Tertiary extinction event, but new technology has uncovered the truth. A virus now believed to have first appeared in the species Coronasaurus is believed to be the cause.
Coronasaurus were 39 foot long predators of the Cretaceous, towards the later half of the Cretaceous they contracted a highly contagious virus. Within a matter of days after contracting the virus the host's soft innards would be consumed and the creature would bleed to death. However, they did not stay dead, the virus would essentially restart the dinosaurs body so that it could continue to synonymously feed and transmit the virus.
After several weeks the dinosaur's rotting body would collapse in a festering pile unable to hunt for the virus any more. Many scavenging species were still able to contract the virus from infected carcasses. Relatively quickly the virus had spread around the world causing the mass extinction.
Bellow is an artist's rendering of what an infected Coronasaurus might have looked like after being reanimated.