The total amount of iridium in the worldwide iridium layer is about what you would expect to find in a 10 kilometer wide asteroid. Relatively few dinosaurs were killed by being hit directly by the asteroid. Slightly more were killed by the blast which formed the Chicxulub crater. Fine dust stays in the upper atmosphere for months prior to settling down to earth and forming the iridium layer. Dust in the atmosphere blocks out sunlight, making the Earth much darker and colder than normal.
Plants die from lack of sunlight.
This estimate is supported by the fossil record, which shows roughly one mass extinction per 100 million years. More information is available from the Asteroid and Comet Impact Hazard page , provided by the NASA Ames Space Science Division.