Inland Canada Sheds New Light on Old Crab

Submitted by Deborah Cramer

Inland Canada Sheds New Light on Old Crab
Paleontologists digging in this dusty rock near Williams Lake, Canada found fossils of the sea’s oldest horseshoe crabs.
© Dave Rudkin, Royal Ontario Museum

Horseshoe crabs, with their armored shells, spike-like tails, and antediluvian appearance, look as if they’re relics from the past. “Living fossils,” they roamed the sea’s shallow coast long before dinosaurs arrived on land. The oldest known horseshoe crab fossils were thought to date from 350 million years ago, until paleontologists began studying two new, and unexpectedly rich, fossil sites. An artist discovered one site: prospecting for rock flat enough for his painting, he accidentally turned up a fossil eurypterid, or sea scorpion.

Earth’s fossil record is scant, accounting for only a tiny percentage of the hundreds of millions of species of plants and animals that have lived here. Mudslides, or ash from erupting volcanoes, or in this case, highly saline water devoid of oxygen, can create spectacular fossil sites. These sites preserve soft tissue of animals that would otherwise be eaten by scavengers or decomposed by bacteria. Lagerstätten, as the sites are known, offer an extraordinary window into the distant past.

Two Lagerstätten in Manitoba contain fossils of earth’s oldest horseshoe crab. One site is in a cove on Hudson Bay outside Churchill, and the other among jack pines near William Lake, a five hour drive north of Winnipeg. Its founders named the animal Lunataspis aurora, “crescent moon shield of dawn,” for the crescent shape of its large, shield-like body and for its appearance much closer to the dawn of animal life than scientists previously understood.

Luna lived in lagoons and tidal flats of a tropical sea that once covered much of North America, a sea full of coral and sea lilies, fierce, coiled cephalopods, sea scorpions, and giant trilobites. Many animals from these ancient times are now extinct, but horseshoe crabs endure: the 445 millionyearold Luna is strikingly similar to its modern descendant Limulus.

Spawning horseshoe crabs, Delaware Bay
Spawning horseshoe crabs, Delaware Bay
© Jay Fleming

p.s. Stay tuned! Even older horseshoe crabs have been reported from the rocky desert of Morocco, in rocks that once belonged to the sea. Ancient seafloor is still revealing the secrets of earth’s history.

References and more information

Young, G.A. et al. 2007. Exceptionally preserved Late Ordovician biotas from Manitoba, Canada. Geology 35:883-886.

Rudkin, David M., Graham A. Young, and Godfrey S. Nowlan. 2008. The oldest horseshoe crab: a new xiphosurid from Late Ordovician Konservat-Lagerstätten deposits, Manitoba, Canada. Palaeontology 51:1-9.

Rudkin, D. M. and G. A. Young. 2009. Horseshoe crabs – an ancient ancestry revealed. In The Biology and Conservation of Horseshoe Crabs, John.T. Tanacredi, Mark L. Botton and David R. Smith, eds. 25-44. New York: Springer.

Paleontologist Graham Young of the Manitoba Museum’s blog on the discovery of Luna
http://ancientshore.com/2009/02/06/catching-a-crab/

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