The creature - related to crabs, lobsters and shrimp - is an ostracod, or a type of crustacean sometimes known as seed shrimp.
It has been named 'Pauline avibella', in memory of the late wife of David Siveter from the University of Leicester, who led the research, the 'Discovery News' reported.
The 0.4-inch-long animal was found, not only with its shell, but also with its soft parts - body, limbs, eyes, gills and digestive system. Such well-preserved remains from that ultra prehistoric period are near unheard of in the fossil record.
"The two ostracod specimens discovered represent a genus and species new to science, named Pauline avibella," Siveter said.
"The genus is named in honour of a special person and avibella means 'beautiful bird', so-named because of the fancied resemblance of a prominent feature of the shell to the wing of a bird," he said.
The discovery of the tiny shelled animal was made in Herefordshire, Welsh Borderland. The rocks at the site date to a time when southern Britain was a sea area on a small continent situated in warm, southerly subtropical latitudes.
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The ostracods and associated marine animals living there were covered by a fall of volcanic ash that preserved them frozen in time.
"Ostracods are the most abundant fossil arthropods, occurring ubiquitously as bivalved shells in rocks of the last 490 million years, and are common in most water environments today," Siveter said.
"The find is important because it is one of only a handful preserving the fossilized soft-tissues of ostracods," he said.