Fossils Lend Clues to Alaska's Eurasian Roots

Hayley Rutger
for National Geographic Magazine

November 18, 2008

The tiny, prehistoric seashells swirl, spiral, and twist. Some curl like soft-serve ice cream, others sport crowns of fragile, hollow spines. They evoke tropical reefs, but geologist David Rohr found them lodged in gray Alaskan limestone.

These 18 Paleozoic-era snails—half of them new to science—did live on reefs some 420 million years ago, when jawless fishes spread throughout the seas and the ancestors of spiders and centipedes began creeping about on land.

Many of the snails resemble no other fossils from North America's landmass. Instead, they're linked to creatures whose fossils have been discovered as far away as Eastern Europe and Russia's Ural Mountains. These new finds are adding to a growing body of evidence about Alaska's diverse and far-flung geological roots.

(See more photos and a map of where fossils have been found.)

Fossil Mosaic

Modern Alaska is a geological puzzle, a mosaic of fragments from other parts of the world.

Geologists suspect the state contains only a small triangle of original North America, located along Alaska's east-central boundary with Canada.

Fossils suggest that the rest of Alaska was formed from a patchwork of small land chunks, known as terranes, that collected against North America like flotsam during the Mesozoic and early Cenozoic eras, between 251 million and 60 million years ago.

Paleontologists began noting weirdly similar fossils in Alaska and Eurasia as far back as 1907, and they've been working ever since to trace their links and decipher their origins.

Rohr, chair of Earth and physical sciences at Sul Ross State University, in Alpine, Texas, believes the spreading prehistoric seafloor carried his snails' limestone grave to the southeast panhandle of Alaska on a 100,000-square-kilometer chunk of land called the Alexander terrane.

With support from the National Geographic Society's Committee for Research and Exploration, he and his colleague Robert Blodgett have found striking similarities between the Alaskan fossil shells and shells from Europe and Russia that date back to the same time, the Paleozoic era's Silurian period, between 417 million and 443 million years ago.

"We were impressed because some of these [fossils] seemed to match species in Europe," Rohr says.

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.