Fossils Lend Clues to Alaska's Eurasian Roots

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The Alaska-Eurasia Connection

One of the fossils, a smooth, loosely spiraled shell, exactly matches specimens of Beraunia bohemica collected in the Czech Republic.

And a tightly swirled cone matches Medfracaulus turriformis shells found in the eastern Urals.

Neither had been found before in North America. These finds provide "substantial support for previous conclusions that the Alexander terrane has little to do, in the Paleozoic, with most of North America," says Oregon State University zoologist Arthur Boucot, an expert on Silurian fossils.

These snails lived in a warm, shallow sea near Siberia during a period when coral reefs first formed. This sea flourished with corals, sponges, mollusks, algae, and brachiopods with clamlike shells.

The Spinicharybdis krizi and S. boucoti snails bristle with delicate, hollow tubular spines that Rohr says probably supported them like tripods on hard reefs.

He speculates that they didn't move around much, but when necessary, could propel themselves along the seafloor or reef surfaces by lifting their shells with their muscular foot and then falling forward.

This would have come in handy when their likely predators, squidlike mollusks, loomed nearby.

A "Volcanic Arc"

Rohr believes the Alexander terrane may have originally taken shape as "a volcanic arc, an island chain, probably off the western edge of the Siberian paleo-continent." (Today that ancient Siberian continental core forms the greater part of eastern Russia.)

The snails are among the oldest known markers of where the terrane likely began its long journey from Eurasia during the Permian period, some 290 million years ago.

It drifted south and escaped being absorbed when the continents huddled to form the supercontinent known as Pangea.

Blodgett says it then veered east and eventually north again, moving between plates and coming to rest against North America, where it formed the island-dotted topography of southeast Alaska's slender panhandle.

Rohr and Blodgett excavated the snail fossils in 2004 from a ton of limestone on Prince of Wales Island, a part of the Alexander terrane that's particularly rich in Eurasian links.

In recent years, the island has yielded at least 11 species of snail fossils with some Eurasian connection. Fossilized sponges found nearby during the 1990s provided paleontologists with another key and previously unconfirmed link between the Alexander terrane and the Urals and southwest Siberia.

Some of the sponges and snails also appear in Alaska's Farewell terrane, a chunk of land that probably broke off ancient Siberia's western coast.

Unlike most Silurian fossils, Rohr's snails aged unusually well and retain exquisite detail. Silica from sediments in their watery home slowly seeped in to replace their delicate calcium carbonate shells, and they look as pristine as if they'd just come "up off the beach," Boucot says.

Rohr and Blodgett were able to free the fossils by dissolving their weaker limestone casing in large tubs of hydrochloric acid—much faster and more precise than chipping through rock.

"The lab work," Rohr says, "was like opening an Easter egg."

Using wire nets to sift out the results, they found shells ranging in size from more than an inch long to something that fits on a penny, ridges and grooves intact on even the tiniest specimens.

Rohr's next project will focus on an island near Prince of Wales, where he may turn up evidence to support the theory that the Alexander terrane had volcanic-island origins.

In the meantime, the most minuscule prehistoric creatures he's already found will continue to offer significant clues to Alaska's eclectic roots.

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