Horseshoe Crabs: What Shit They've Seen

If you wanted to talk to someone who had witnessed the upheavals on planet Earth through the last 450 million years or so, you could do worse than a horseshoe crab, assuming that the horseshoe crab you chose to talk to, had the patience to describe 500 million years of Earth's violent history in a fun and an accessible way without scaring you to death.

Horseshoe crabs predate dinosaurs by about 100 million years, which means they have been around - in some form or the other - for 450 million years. That is 50 million years short of half a billion years. Only other creatures that have been around for this long are jellyfish.

Humans, by comparison, have been around for a 200,000 years, though (strictly speaking) our ancient ancestors have been around for about 5 - 6 million years. But that's a blink compared to how long arthropods (horseshoe crabs are arthropods) have crawled on Earth.

The head spins.

How have these creatures survived so long? What have these creatures seen?

First let's imagine for a moment, if we could, the devastation that has occurred in the last 500 million years that the arthropods (horseshoe crab is an arthropod) have witnessed up close. To our knowledge, two major extinction events happened in this time frame -

1. P-T Extinction Event

2. K-T Extinction Event.

The P-T Extinction Event is the greatest extinction on our planet. The Permian-Triassic (P-Tr) Extinction Event, which is the extinction event at the boundary of the Permian and Triassic geological periods, also roughly known as "the Great Dying".

It's a fair statement considering how much of life on Earth (most of it, really) was wiped out.

Up until 20 or so years ago, it was believed there weren't enough rock formations spanning this critical Permian-Triassic boundary to determine what had happened during this period.

But as it happens, this is possible now. Using Uranium-Lead (U-Pb) dating in Meishan, China, scientists have dated the event to a stunning degree of accuracy. This extinction happened 252 million years ago, or if you wanted to be painstakingly precise, 251.9 million years ago.

The P-Tr Extinction Event happened in three ways, each mind-bogglingly severe, and the sequence of catastrophic events triggered a level of devastation hard to imagine as I sit at my table with my laptop while it is a sunny, clear day outside and a perfect 65F.

First -

• Meteorite event: For the longest time this was only a hypothesis, but evidence of a 250 million-years-old crater has been found in northwest Australia - read "A Possible End-Permian Impact Crater Offshore Northwestern Australia” in the NASA write-up here.

Massive Asteroid Entering Earth's Atmosphere

As the write-up says (work is preliminary and ongoing still, but...) it is a clear possibility:

The paper reports on the identification of a large submarine impact structure off western Australia that is dated at 250.7 +/- 4.3 million years (an argon-argon date from a single plagioclase crystal). This geological rise had originally been thought to be volcanic, but a re-examination of drill cores by this team shows clear evidence of impact materials, including abundant shocked mineral grains. The preliminary work suggests that this original crater was nearly as large as the Chicxulub crater, which caused the KT extinction 65 million years ago.

• Volcanic event - In Siberia, Russia, massive volcanic eruptions that created the Siberian Traps happened approximately 250 million years ago. These continued for one million years. These weren't a single, but a series of massive eruptions. They literally "cooked" rocks, threw up billions of tons of gases and toxic metals into the atmosphere (see this) which then came down (as what goes up must...) as sulphuric acid rain.

Subsequently...

• Climate change - ocean acidification due to the rise in temperatures and the increase in carbon dioxide, methane, and sulphur in the atmosphere wiped out most of marine life. Approximately 95% of marine species and 70% of terrestrial life were gone.

THAT is the crap these poor creatures (or rather their ancestors) have been through.

I - for one - would like to show these poor horseshoe crabs some respect.

---

Next post: The K-T extinction.