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.