There were two mass extinction throught out the Earth's history. The first, took place about 250 milion years ago(the transition period of Paleozoic-Mesozoic) and the second one was approximated to be about 65 million years ago(the transition of Mesozoic-cenozoic). For both of the incidence there have been some compelling evidences which point towards meteorite showers as the main reason of the extinction.
This is of course derived from the findings of the layer of sediments with an exceeding amount of iridium concentration, one in which can only be found due to cosmic activities(and therefore frequently associated with celestial bodies such as meteoroids).The second evidence leading to this breakthrough, is the location of impact itself.
The crater that was found in Mexico, was formed due to an meteorite impact 65 million years back and the crater found in the coast of Australia corresponds to the time in which the preceding mass extinction(250 million yrs) took place.
So, can we say that it was only due to meteor showers?No.
The meorite showers may have been catalyst or a synergist for more catastrophic events such as tsunami(and therefore caused flood), spontaneous volcanic eruptions acroos the globe(a common layer of volcanc ash of a certain depth was found out as well, and radioisotope dating indicated the years, which again, corresponds), and due to the combination of this myriad of events, the Earth's climate was modified.(Right after the fall of meteorites, the Earth must have been exceedingly hot but the thick volcanic ash contered off the effect, leading into a cooling process)
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