Pure science fiction. Leg could have been ripped off for any number of reasons, and at any time hundreds (thousands?) of years prior to the comet. It’s not like they were all standing around holding hands and singing songs with Barney.
“This looks like an animal whose leg has simply been ripped off really quickly. There’s no evidence on the leg of disease, there are no obvious pathologies, there’s no trace of the leg being scavenged, such as bite marks or bits of it that are missing.” Of course they were also able to
pull apart the chemistry and identify the composition of that material.
So it is possible the impact only exhumed deaths that had occurred years earlier and there may be other species at the site that are not as circumstantial (the fish) but it should be an interesting presentation of the evidence regardless.