Vaderlimulus horseshoe crab fossil. Photo: New Mexico Museum of Natural History & Science

Before there were birds, mammals and dinosaurs, horseshoe crabs were here. Over the past 450 million years, these relatives of scorpions and spiders (not crabs) passed through five mass extinctions. They stuck to the coasts for the most part so there was little pressure for them to evolve. The horseshoe crabs you see today look similar to their earliest ancestors.

What's new: A recent fossil find in Idaho indicates that in the time in between some ancient horseshoe crabs were more diverse than previously thought. The 245-million-year-old specimen, from when the state sat on the coast of the supercontinent Pangea, is "a missing puzzle piece," Allan Lerner, a research associate in paleontology at the New Mexico Museum of Natural History and Science, tells Axios.

The past: Every now and again, horseshoe crabs ventured out — or rather in — to freshwater environments. When they did, their morphology is thought to have changed. The fossil record during this time is sparse though because horseshoe crabs lack an exoskeleton with minerals that can be preserved except under exceptional conditions.

The discovery: The fossil found in Idaho is so far the only specimen of a new genus called Vaderlimulus with a large — "extravagant would be a good word," Lerner says — helmet and small body that evolved as it expanded into a freshwater environment. Eventually it went extinct though Lerner says they aren't sure why.

The future: This discovery shows horseshoe crabs used to be diverse. But today, only four species remain, and they're in decline due to human actions.

"We shouldn't take for granted that because they have lived as long as they have they will survive," Lerner says.

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