Land crabs unique to Hawaii, as big as a human fist and able to travel huge distances inland, were wiped out by the first human colonists around 1,000 years ago, scientists have deduced.
Fossils have been found at altitudes of 1,000m (3,000ft) - unusual for a crab.
Writing in the journal PLoS One, researchers say they identified the species by comparing it with living relatives on other Pacific islands.
Early settlers brought animals such as pigs and rats - wiping the crabs out.
The researchers describe this as the first documented extinction of a crab in the human era.
"They'd already vanished from the islands by the time the Europeans got there - nobody's ever seen one alive," said Gustav Paulay, curator of the Florida Museum of Natural History in Gainesville.
"There are stratigraphic sequences (layers of rock) that have been studied mostly for bird extinctions, and we can date when Polynesian settlers came in, partly by occurrence of Polynesian rats: and when that happens, you don't see any more crabs," he told BBC News.
Land crabs appear particularly vulnerable to contact with animals associated with human spread, and there are documented cases on islands elsewhere where they have fared very badly when rats have been introduced.
Settlement of the Hawaiian islands brought huge changes to the ecology, with many native species including birds and amphibians swiftly becoming extinct.
The same fate appears to have befallen this once mighty land crab, which the researchers have dubbed Geograpsus severnsi.
"When you look at the islands of the Pacific, things don't get there easily, so land animals are scarce - and the biggest things on some of them are crabs," said Professor Paulay.
"They control the ecosystems on atolls to an incredible extent.
"This particular species would have been important as a predator and as an omnivore - it would probably have had a big impact on the insect population, on land snails, and also maybe on nesting birds."
The fossils have been known for many years from various sites around the Hawaiian islands, including caves at altitudes highly unusual for crabs, several kilometres from the coast.
Many crabs cannot live in these conditions as they need regular immersion in salt water.
But G severnsi appears to have evolved the capacity to roam further afield than most, including closely related species found on other Pacific islands.
Even land-dwelling crabs return to the sea to produce larvae - and once in the sea, can be carried from island to island, even across the thousands of kilometres that lie between the Hawaiian archipelago and neighbours such as Kiribati.