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How far back does the horseshoe crab lineage extend in the fossil record?
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horseshoe crab
Horseshoe crab (Limulus polyphemus).
The horseshoe crab lineage is an extremely ancient group, with fossil relatives recognized as far back as the Ordovician Period, which occurred approximately 485.4 million to 443.8 million years ago. While these ancient ancestors represent the deep history of the group, forms that closely resemble modern-day horseshoe crabs date back to the Jurassic Period, roughly 201.3 million to 145 million years ago.1