[waterstraat]

The question is: "What would kill all the 6 foot beavers, and leave all the 2 foot beavers?

Once you have an answer that fits both, then you have the answer to what happened.

[U S Army EOD]

The two foot beavers probably ducked.

[swany]

Only the small ones fit in the first Ark. They had to leave the big ones behind. :)

[e_engineer]

The question is: "What would kill all the 6 foot beavers, and leave all the 2 foot beavers?

Maybe a better question would be: "What would kill all the 6 foot beavers, and not kill all of the 2 foot beavers?"

It's a subtle difference, but the these beds of frozen animals contain many species that are not extinct. In other words, the disaster affected all species but did not make them all extinct.