A Rare Fossil
The thin exoskeleton of the horseshoe crab does not usually survive in the fossil record, but a German stone quarry has yielded some astonishing finds.

	The supercontinent of Pangaea began to break up during Jurassic times, and the landmasses that would ultimately become North America and Eurasia began to appear. The North Atlantic Ocean began to open up, as the result of rifting between North Africa and the eastern coast of North America.

During the Jurassic Period, a shallow sea covered part of present-day Germany. Many marine animals flourished along the margin of the sea, including mollusks, sponges, fish and arthropods such as the horseshoe crab.

Deeper parts of this sea basin may have been low in oxygen and inhospitable to life. Occasionally a storm transported an animal like this Mesolimulus walchi to the deeper basin, where it survived at least briefly—as we can see from its trail—but then died. Soft carbonate mud then buried it in place, protecting it from scavengers and currents. When the exoskeleton of this animal became fossilized, it was preserved in pristine condition for 150 million years and ultimately excavated from a limestone quarry in Solhnofen, Germany.

Life in the Jurassic Oceans
The Jurassic Period is often called the "Age of Dinosaurs," but other animals also flourished—in the oceans as well as on land.

Today's horseshoe crab (Limulus polyphemus) bears an obvious resemblance to its extinct ancient relative Mesolimulus walchi. Horseshoe crabs date back to more than 400 million years ago and have changed relatively little in the last 300 million years.

As the global climate warmed, sea levels rose, drowning the coastal areas of continents and flooding inland regions. The shallow seas that developed were home to increasingly diverse and complex marine life.

Of the smaller marine animals that thrived in Jurassic times, many are still familiar today. Mollusks burrowed into the ocean sediment. Starfishes, sea urchins and arthropods found food along the sea bottom, as did the ancient relatives of the horseshoe crab. Sponges and sea fans grew, while bony fishes, sharks and rays roamed above. Now unfamiliar are the larger marine reptiles, ranging from the dolphinlike ichthyosaurs to the pliosaur, a carnivorous swimming reptile that grew up to 40 feet long.