Puzzling distribution of the fossil and living genus Hiatella (Bivalvia)
Abstract The bivalve Hiatella has been present for the last 150 Ma and is consistently linked to cool-temperate and polar regions. Its origin can be traced back to at least the Jurassic, with earliest known records being temperate forms that occurred on the northern side of the Tethys. The subsequent fossil record suggests two possible alternative migration routes. The more plausible one is that Hiatella invaded the southern seas from the Atlantic by the Eocene. From there, it migrated northward to eastern Asia, with first valid North Pacific records (in the Far East region of Russia, Japan and Alaska) from the Oligocene to Miocene. A second explanation is that during the Mesozoic Hiatella penetrated into the circumpolar zone and reached the Boreal–Pacific province, before the Arctic became isolated in the Late Cretaceous (60 Ma) due to the formation of Beringia. As its subsequent oldest records in the North Pacific are from the Oligocene to Miocene, Hiatella probably evolved during the Paleogene in the North Pacific and migrated to southern latitudes (Antarctica, southern South America, New Zealand and Australia), as connecting seaways formed in the trans-Pacific route. In any case, the North Pacific lineage has been separated from the circum-Atlantic lineage of Hiatella since at least the opening of the Bering Strait (3.0–3.5 Ma) during the Late Pliocene, when it probably participated in the trans-Arctic migration. During the Late Pliocene–Pleistocene Hiatella evolved, adopting its present widespread cold-temperate distribution in both hemispheres, although best developed in the polar regions of the Northern Hemisphere. Hiatella probably achieved extinction resistance by virtue of broad geographic distributions associated with dispersal facilities and great plasticity in its habitat and mode of life.