I've just been reading (in Wendy Williams' Kraken, which I have cracked now that I'm done with Mieville's--maybe I'll do a joint review?) about ammonites. These famously abundant ancestral cephalopods went extinct along with the dinosaurs at end of the Cretaceous, in what's known as the K-T mass extinction.
Another thing ammonites have in common with a lot of the dinosaurs is that they were already in serious decline before the big meteor strike. Thus, a haiku, complete with seasonal reference:
Declined. Cretaceous autumn.
K-T killed their coils.