for National Geographic News
A new fossil species of horseshoe crab shows that the primitive marine creatures have existed for at least a hundred million years longer than previously believed, researchers say.
The well-preserved fossils, found in Manitoba, Canada, suggest that the animals scuttled through shallow tropical seas nearly half a billion years ago.
The ancient animals were remarkably similar to modern horseshoe crabs, the discovery team noted. (See a photo of a modern horseshoe crab.)
Horseshoe crabs have long been known as "living fossils" because they have survived since ancient times with little change in physical form, and they have no close modern relatives.
From the time the newfound species lived to the present, the animals have weathered five major mass extinctions that eliminated a large percentage of Earth's species, said team leader David Rudkin, of the Royal Ontario Museum in Toronto.
"They made it through all of these events, not necessarily unscathed, but in a continuously recognizable form," Rudkin noted.
The new fossils are from the Late Ordovician period and are at least 455 million years old, Rudkin said.
"And the record must go back deeper still," he added.
"We might well be able to trace the genealogical roots of horseshoe crabs into the Cambrian period," more than 490 million years ago.
Horseshoe crabs are not true crabs, but a unique group of marine invertebrates distantly related to spiders and scorpions.
The new fossil species, dubbed Lunataspis aurora, lived at a time when plant and animal life on land was just getting established.
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