Wilderness Underfoot: Fossil shrimp cocktail

StoryImage( ‘/Images/Story//Auto-img-116966719921781.jpg’, ”, ‘A "living fossil" Anaspides tasmaniae from Australia.‘);
StoryImage( ‘/Images/Story//Auto-img-116967071521774.jpg’, ”, ‘Palaeocaris typus from Australia.‘);
StoryImage( ‘/Images/Story//Auto-img-11696670322220.jpg’, ”, ‘A jumbo shrimp — Belotelson magister, as it was in life, and as a fossil.‘);

Our state has fossils of at least 15 different species of shrimp.

Having spent much of its prehistoric history under water, Illinois is home to a treasure trove of marine and freshwater fossils, such as squids, fishes, worms, crustaceans, clams, snails and corals.

In the lower two-thirds of Illinois is a region of Carboniferous Period fossils, about 300 million years old, that might appeal to any time-traveler with an appetite for shrimp. During the Coal Age, inland seas swept in and out of the region, burying layer upon layer of tropical forests. Marine and freshwater shrimp were abundant here in a variety of shapes and sizes.

Today, Coal Age shrimp fossils from Illinois are most commonly found in Will, Grundy and Kankakee counties. One of the easiest shrimp fossils to find in that region is a chunky creature known as Belotelson magister. This shrimp can be readily identified by its large carapace (the shell that covers its head) and long rostrum (the beak that extends out of its carapace). Usually, Belotelson is fossilized lying on its side, and the fossil concretion containing the shrimp shows an inside view of its body. You’ll often find that the fossil reveals the shrimp’s long digestive tract filled with its last meal!

Another of the more common fossil shrimps found here in Illinois is tiny Palaeocaris typus. Easily overlooked, this shrimp belongs to the order of Syncarids, which had long been considered extinct—that is, until a living species was found in the mountains of Australia. The mountain shrimp Anaspides tasmaniae could be considered a “living fossil” of sorts, hardly distinguishable from its ancestors when compared side by side.

Most of these ancient shrimp species fed on detritus, or tiny organisms collected on the sea floor or river bottom, but some shrimp were carnivores. One group of carnivorous shrimps, known as Tyrannophontids, filled a niche that is occupied today by the aggressive mantis shrimp.

Shrimps, lobsters and crabs belong to an order of crustaceans known as Decapods (meaning “10-footed” in reference to their five pairs of legs). The earliest known fossil of a Decapod comes from the Late Devonian Period, 360 million years ago. That ancient creature, which scientists named Palaeopalaemon newberryi, appeared to have characteristics common to both shrimps and lobsters. Even earlier fossils from this group of crustaceans may yet be found.

From the Jan. 24-30, 2007, issue

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