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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.

Go deeper

Updated 2 hours ago - Politics & Policy

In photos: The Biden and Harris inauguration

President Biden and first lady Jill Biden watch a fireworks show on the National Mall from the Truman Balcony at the White House on Wednesday night. Photo: Chip Somodevilla/Getty Images

President Biden signed his first executive orders into law from the Oval Office on Wednesday evening after walking in a brief inaugural parade to the White House with First Lady Jill Biden and members of their family. He was inaugurated with Vice President Kamala Harris at the U.S. Capitol on Wednesday morning.

Why it matters: Many of Biden's day one actions immediately reverse key Trump administration policies, including rejoining the Paris Agreement and the World Health Organization, launching a racial equity initiative and reversing the Muslim travel ban.

Republicans pledge to set aside differences and work with Biden

President Biden speaks to Sen. Mitch McConnell after being sworn in at the West Front of the U.S. Capitol on Wednesday. Photo: Erin Schaff-Pool/Getty Images

Several Republicans praised President Biden's calls for unity during his inaugural address on Wednesday and pledged to work together for the benefit of the American people.

Why it matters: The Democrats only have a slim majority in the Senate and Biden will likely need to work with the GOP to pass his legislative agenda.

The Biden protection plan

Joe Biden announces his first run for the presidency in June 1987. Photo: Howard L. Sachs/CNP/Getty Images

The Joe Biden who became the 46th president on Wednesday isn't the same blabbermouth who failed in 1988 and 2008.

Why it matters: Biden now heeds guidance about staying on task with speeches and no longer worries a gaffe or two will cost him an election. His staff also limits the places where he speaks freely and off the cuff. This Biden protective bubble will only tighten in the months ahead, aides tell Axios.