Having read the book that Sunken Civ has posted on this thread, I am inclined to think it was something more solid that a comet, or else a very large one, perhaps a broken up asteroid. For example, there is a picture of the bottom of Lake Michigan which appears to have been hit by either 2 or 3 large objects in a row. There is also evidence of some things striking Siberia or Scandinavia.
The early Australians had many tens of thousands of years to kill the mega fauna. They also may have killed off a pre sapiens homonid about 12,000 years ago called Kow swamp people. In the wall of human evolution in the Smithsonian one of the Kow skulls is in the far right lower corner. It is called homo sapien, but after comparing it to Neanderthal and earlier Heidelbergensis skulls, I am of the opinion it had more in common with the Heidelberg skulls. Really, the only basis for calling it sapiens is the 12,000 age. when the Europeans arrived in Australia the Tasmanian Devil and Tiger were still alive, and I believe other large animals that are now extinct had survived the aboriginies. They did not survive firearms. There are still significant killings and species depletion occurring in Madagascar as well.
No doubt some of it at the hands of Man but when accompanied by evidence of catastrophe??? ONLY if one ignores the catastrophe I suppose. Anyhow, here in East Texas we have something similar occurring, after a fashion. Fire ants! Yep, fire ants have been credited with creating havoc amongst critters that nest on, near or under the ground. Larger young critters are even at risk if mama drops her newborn on or near a fire ant hill. Ground nesting birds like quail are nothing but a memory. I miss the night calls of Bobwhites and Whippoorwills.