The peninsula Avalon in Newfoundland is separated by 4000 km of ocean from legendary Avalon in Wales. Trilobites show that the two Avalons were conjoined half a billion years ago. Fortuitous congruence of names? Or the influence of the Arthurian magician Merlin?

Paradoxides davidis from Manuels River, Newfoundland. The trilobite is 40 cm long. The yellow colour is limonite. (Photo by Riccardo Levi-Setti (c).)

Fossils have been used to reconstruct ancient continental position ever since the idea of continental drift was first proposed. The German meteorologist Alfred Wegener who published the first scientific exposition of continental drift in 1915 and the South African geologist Alexander du Toit showed that the distribution of the Glossopteris flora and the fresh-water reptile Mesosaurus demonstrated that Africa, South America, India, Antarctica and Australia were conjoined during the late Paleozoic. But it's one thing to use terrestrial fossils to reconstruct continents. What about marine fossils, at a time when there were no larges terrestrial organisms?

The answer came in the unfolding of a remarkable scientific revolution that convulsed the earth sciences during the decade of the 1960s. Incorporating elements of continental drift and sea-floor spreading with data about the dynamics of mountain building and the structure of the deep interior of the earth, the plate tectonic revolution is a genuine example of that overused term, paradigm shift. Most of the evidence for mobilist models of the Earth came from "hard" earth science fields such as paleomagnetics, geochemistry, deep crustal geophysics and seismology, but in a landmark paper published in 1966, the Canadian geophysicist J. Tuzo Wilson drew largely on evidence provided by the distribution of Cambrian trilobites in the North Atlantic region, particularly those of the Avalon Peninsula of eastern Newfoundland.

Middle Cambrian trilobites of the Avalon Peninsula are virtually identical to those of New Brunswick, Massachusetts and, farther afield, North Wales and England. This trilobite province is called "Atlantic". The emblematic "Atlantic" trilobite is the giant Paradoxides davidis which takes its name from Britain's smallest city, St. David, on the Welsh coast. Middle Cambrian trilobites are entirely different in the rest of North America, including western Newfoundland, Scotland, Norway and Greenland. This province is (confusingly) called "Pacific". Why are "Atlantic" trilobites so different from "Pacific" trilobites of the same age? A land barrier between the two might explain it, but geologic evidence of such an "Isthmus of Panama" could not be found. The curious distribution of the "Atlantic" and "Pacific" provinces on both sides of the Atlantic suggests that the movement of continents was responsible.

Tuzo Wilson took a fresh look and concluded that the Cambrian trilobites are different because they lived on different continents during the Cambrian -- the "Pacific" province on Laurentia was separated by a deep Iapetus Ocean from the "Atlantic" province on the European continent. This ocean narrowed during the Ordovician and closed in the Silurian, juxtaposing the "Pacific" and "Atlantic" provinces and abutting Africa against eastern Laurentia. These continents remained fused as part of Pangaea until this supercontinent began to fragment in the Jurassic. The Atlantic Ocean began to open along a path close to the sutured Iapetus Ocean -- but not exactly. Fragments of Laurentia and "Pacific" Province were left on the "wrong" east side of the Atlantic; and similarly, fragments of Europe and "Atlantic" Province remained on the "wrong" west side of the Atlantic.

Tuzo Wilson's scenario has been greatly modified since 1966, but his insight about Middle Cambrian trilobites of the Avalon Peninsula provided the first evidence for recognizing a vanished ocean.

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