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Why are horseshoe crabs often called living fossils?
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horseshoe crab
Horseshoe crab (Limulus polyphemus).
Horseshoe crabs are frequently referred to as living fossils because they are an extremely ancient group that has remained essentially unchanged in form for millions of years.12
While their fossil relatives can be traced back to the Ordovician Period (approximately 485.4 to 443.8 million years ago), forms that closely resemble modern-day horseshoe crabs have existed since the Jurassic Period (roughly 201.3 to 145 million years ago). This term highlights their status as organisms that have persisted through vast geologic time with little morphological evolution, often while their close evolutionary relatives have gone extinct.12