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."
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."
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