The evidence is the iridium found in the K-T layer. Apparantly aliens make their ships out of an iridium alloy to withstand the high temperatures encountered at flying at Warp speed. Unfortunately, when arriving at earth their lithium crystals ( which had a fatal flaw ) developed a hairline crack that went unnoticed for some time necessary at least to set up a colony. A chaotic feedback loop finally pushed the core to breach. The resulting explosion aprayed iridium in the upper atmosphere, and the dust finally settled in the K-T layer. The habitable part of the ship they landed since they had no way to get back home. You can find the ship in the same place as Noah's Arc. Being stranded made them mad as hell and they blasted away at every living thing they could find with a scorched earth policy at the god foresaken planet. The dinasaurs fought back with tooth and nail but were no match and alas, finally the planet was lost, hence the mass extinction. Some of these galliant fighters survived living hidden from view in swamps or taking to the air and generations persevered.
They still live among us. We are the aliens.
That's my History channel version.