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October 25, 2002
McMenamin
Makes Fossil Discovery
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Photo: Fred LeBlanc
Professor
of Geology Mark McMenamin holds two specimens of the trilobite
Paradoxides. The larger one is part of Mount Holyoke's
collection; the smaller fossil is part of a new discovery
made by the geologist last summer and will eventually end
up in the North Carolina State Museum of Natural Sciences
in Raleigh, North Carolina.
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Working in a weathered
patch of strata in rural South Carolina this past summer, MHC
geology professor Mark McMenamin unearthed fossils of ancient
arthropods called trilobites that support a continental drift
rebound theory known as the Wilson cycle theory. Named for late
Canadian geologist J. Tuzo Wilson, the theory proposes that giant
continents break apart over geologic time, later to reassemble
in a pattern similar to their original configuration.
"The fossils
are significant," says McMenamin, "because they represent
the same species of Avalonian trilobites found near Boston."
Avalonia was a volcanic archipelago that formed in the ocean between
ancient North America and Africa. This ocean was formed when the
supercontinent Rodinia (which was made up of land masses that
now encompass most or all of present-day continents) split apart
700 million years ago, isolating Africa and North America. Over
millions of years, the ocean was destroyed by plate tectonic subduction,
a process that had also formed Avalonia. When Africa and America
were reunited to form part of the supercontinent Pangea, Avalonia
was trapped between them like a fly squished in a door jamb.
Writing in the July
2002 issue of the journal Southeastern Geology, McMenamin
and Patricia Weaver of the North Carolina State Museum of Natural
Sciences argued that there were strong similarities between the
eastern Massachusetts (north Avalonia) and South Carolina (south
Avalonia) sites. The new discoveries, however, are the first to
show that the seafloor trilobites belonged to identical species.
"Avalonia is thus a continuous geological terrane,"
says McMenamin, "a volcanic island chain that had a distinctive
trilobite fauna along its shores. The rocks and their fossils
were crushed and trapped as North America and Africa were rejoined."
This collision led both to the formation of Pangea (which fragmented
250 million years later during the reign of dinosaurs) and to
the uplift of the Appalachian mountain range. Avalonia apparently
first formed closer to Africa than to America, as its trilobites
more closely resemble Old World trilobites than they do typical
American trilobites.
McMenamin is an expert
on the evolution of complex forms of life and the geological events
associated with the emergence of these new life forms. He is the
author of many books and articles, including Hypersea: Life
on Land, which he wrote with his wife, Dianna.
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