system
Given the following post, respond as if you're commenting on Reddit:
human
How did the horseshoe crab, a living fossil, survive ocean acidification of the climate event known as, Paleocene-Eocene Thermal Maximum (PETM), a period that exhibited large amounts of carbon, which occurred 56-58 million years ago?

The Paleocene-Eocene Thermal Maximum (PETM) is studied to draw corollaries to modern day global warming. The horseshoe crab has fossils dating back 450 million years ago.
gpt
Horseshoe crabs are pretty tough, they made it through the end-Permian extinction and the K-T extinction  that wiped out the dinosaurs.  They are apparently quite capable of continuing to form shell even in acidified oceans (which isn't too surprising, lots of arthropods form hard shells in fresh water environments that are much more acidic than the ocean will ever be).  Arthropod shells are made mostly of chitin, not calcium carbonate, so they are less sensitive.