Evolution Diary

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.)

A newfound fossil species of horseshoe crab (left) dates back at least 455 million years—roughly a hundred million years earlier than horseshoe crabs were previously thought to exist.

The ancient animal is remarkably similar to modern horseshoe crabs (right), according to the team that found the fossils in Manitoba, Canada.

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.

Tiny Fossils

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.

It is not known if Lunataspis ever came up on land to mate and bury its eggs, as horseshoe crabs do today.

But the setting in which the fossils were discovered suggests that the ancient creatures’ environment and way of life were similar to those of its modern relatives, Rudkin said.

“The rocks [at the fossil locations] show evidence of being formed from sediments deposited in shallow water along the shorelines of extensive inland seas,” he said.

Lunataspis shared those waters with sea scorpions, trilobites, and other long-extinct marine organisms.

The 1.5-inch-long (3.8-centimeter-long) fossils are much smaller than modern horseshoe crabs, but scientists don’t know if this is because the ancient species was diminutive or if the remains are those of young individuals.

A paper describing the new fossils appears in the current edition of the journal Palaeontology.

At first, Rudkin said, the fossil hunters were unsure what they had discovered.

“We didn’t seriously consider the possibility of a true horseshoe crab affinity until we discovered more-or-less complete specimens at two separate locations,” he said.

Horseshoe crab fossils are uncommon, Rudkin noted, because the animals’ flexible shells are made of an organic compound called chitin that usually degrades before fossils can be formed.

Derek Briggs of Yale University in New Haven, Connecticut, called the fossils “a remarkable discovery.”

“The new horseshoe crab [comes from] a setting where evidence of tissues that normally decay was preserved,” he said.

“Examples of such exceptional preservation from rocks of this age are rare. They provide important windows on the life of the past.”

Surviving Body

The reasons why horseshoe crabs have remained largely unchanged for so long are far from clear, Rudkin said.

The answer may be that the creatures are well adapted to their environment, and their environment has persisted.

Even as the continents and oceans have shifted and ice ages have come and gone, shallow coastal marine habitats have always been present.

Or horseshoe crabs may be subject to genetic and developmental constraints that tend to lock certain physical characteristics into place.

“It’s a fortuitous blend of evolutionary and ecological factors that permits long-term survivorship of certain body plans,” Rudkin said.

“I hasten to point out [though] that similarity in external appearance doesn’t equate to an absence of evolution,” he noted.

“[The horseshoe crab of today] is most certainly not the same thing as Lunataspis.”

News from National Geographic