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Beneath Tropical Seas


Fri, Mar 2nd, 2001
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Monday, February 5, 2001

Waves roll in and then recede, as the ocean endlessly caresses this lonely tropical shore. Shallow, crystal clear and deliciously warm are the waters of the all-encompassing sea. The white sand beach is deserted, polished smooth, and except for the gentle sounds of the surf is eerily silent, peaceful and serene. Not even a single insects persistent whining buzz splinters the mood of pristine tranquility.

Although reading like one of those tourist agency advertisements for some South Pacific Island paradise, the location described in the opening paragraph above is far away from any place so exotic. Could it be somewhere down in the Caribbean then, out in the Indian Ocean, or perhaps maybe one of those isolated rocks that dot the Sea of Cortez?

The answers, however, to each of these questions remains no, as our quiet, wave lapped shoreline is to be found much more closer to home. Somewhere right here in southeastern Minnesota in fact, since it is time and not distance that separates us from this seemingly mythological destination.

To reach our beach we must first travel back into the heart of the early Paleozoic, the "Era of Ancient Life" somewhere in the neighborhood of 400 million years ago. More specifically, we are in the Ordovician, a unit of geologic time lasting some 65 million years and named for a "Stone Age" Celtic Tribe that once inhabited Wales where rocks of this period were first identified.

From outer space the Earth, while certainly recognizable, probably appeared far different than it does today. For starters, all of the present day continents were most likely fused together into a single gigantic mass of land. Known as Pangaea, this island "super-continent" straddled the equator placing what was to eventually become todays Minnesota well within the worlds more tropical zones.

As was the case in the immediately preceding Cambrian Period, shallow saltwater seas cover much of future North America. The name of this earlier period 500 million years before the present has Celtic connections as well, being derived from "Cambria," which is what the ancient Romans called the Celts homeland of Wales.

The Ordovician shoreline, on dry land at least, is equally alien and unfamiliar. There are no trees, shrubs, flowers, grasses, nor are there any ferns or mosses. Away from the waters edge, the landscape is barren and desolate, almost as lifeless as the moon. Without vegetation there is nothing for land-dwelling animals to eat and it will be at least another 100 million years before even the insects arrival.

While less bizarre overall, the environment of the underwater world of the Ordovician is likewise strangely different. With the possible exception of a few rare and extremely primitive "jawless" forms related to the lampreys, fishes are conspicuous by their absence. Instead, an assortment of oddly flattened, segmented arthropods called Trilobites swim or scuttle across the bottom as they forage for meals and mates. Believed to be related to the still living Horseshoe Crabs, the Trilobites are now long since extinct despite dominating Paleozoic seas.

Other "spineless" invertebrate animals abound in the shallow marine waters as well. Poorly understood forms leaving no known living relatives include the presumably colonial Graptolites with jagged-edged, lightening bolt shaped body parts and the oddly "tooth-like" Conodonts. Exactly how these animals lived and what they actually looked like, as well as their respective places in the animal kingdom still remain highly controversial today.

More familiar invertebrate life forms, at least in terms of general appearance, are also abundant and widespread in the Ordovician seas. Most prominent among these are the Cephalopods, tentacled squid-like animals encased within conical shells. Capable of reaching lengths in excess of ten feet, and close relatives of todays Chambered Nautilus, the Cephalopods are perhaps the top ranking predators of the age.

Sponges, primitive corals, and the multi-branched bodies of the so-called "Moss Animals" or Bryozoans encrust underwater rocks or litter the oceans floor. Long stalked, "flower-headed" Crinoids or Sea Lilies, starfish and sea urchin relatives despite their plant-like form, sway and bend to the rhythm of the waves. Snails slowly creep about and the currents are filled with tiny single celled animals known as Foraminiferans.

Perhaps most conspicuous of all are the Brachiopods, a diverse assortment of "bivalve" or "two-shelled" species present in almost dizzying profusion. As is the case with oysters, Brachiopods often congregate in dense, widely spread underwater fields that may literally blanket the floor of the sea like a bed. Despite this rather oyster-like behavior, however, and their overall clam-like appearance, the Brachiopods are not very closely related to clams and oysters.

Brachiopods along with Snails, Bryozoans, Crinoids, Corals, Sponges and single cell Protozoa still exist in the worlds oceans today. Although currently buried under an ice-encrusted shroud of snow, evidence that these animals and other extinct Ordovician organisms once inhabited Minnesota surrounds us everyday as well. The limestone, shale and sandstone making up our steep sided Bluffs are often impregnated with the fossilized remains of those marine life forms, which died beneath a long vanished tropical sea.

By John Levell

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