Horseshoe Crabs and Velvet Worms

Constance Casey in the New York Times:

ScreenHunter_15 May. 28 11.59Richard Fortey has spent most of his life looking at fossils, the imprints of the skeletons of the very thoroughly dead. Here he sets out — like a more deeply thoughtful David Attenborough, without the cameras — to describe the distinguished groups of organisms that are still recognizable and thriving after millions and millions of years. The horseshoe crabs, velvet worms and other venerable creatures he encounters are Earth’s true conservatives. “We’ve devised a system that works very well for our niche,” they would tell us. “No big changes necessary. Maybe just a tweak at the molecular level.” As Fortey says, “to look at a living horseshoe crab is to see a portrait of a distant ancestor repainted by time, but with many of its features still unchanged.”

Fortey’s dozen or so subjects have survived the many cataclysms the planet has thrown at them over the past 450 million years. As if repeated earthquakes, volcanic eruptions and ice sheets weren’t enough, there were two mass-extinction events. The best known was the disaster 65 million years ago that led to the downfall of the dinosaurs. We’re less familiar with the more devastating earlier extinction — about 251 million years ago — that erased 90 percent of life from the sea and almost as large a percentage of the little things struggling on land. The horseshoe crab made it through; its fossil remains date from 450 million years ago.

Somewhere then, perhaps at the bottom of a poisoned sea, with tsunamis rolling above, some organisms stayed alive, including something we would recognize as the horseshoe crab if it clambered up onto the beach. It’s astonishing to consider that the lucky few — arthropods, snails, clams, jellyfish, worms and a few small four-legged creatures on land — that survived the worst extinction gave rise to everything that followed, including us.

More here.