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HOME > PAST ISSUE > May-June 2004 > Article Detail

MARGINALIA

A Tour of Geological Spacetime

Philip Morrison

Life History

North of Perth in Western Australia, the coastal Hamelin Pool hosts a layered shoal of colorful bacterial mats. Forming in brackish seawater, layer after layer of mats have left behind clusters of small, sedimentary columns. The dry, fossilized equivalent (with matching microbial content) lies close by on the same shore, marooned billions of years ago by a changing coastline.

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In Lyme Regis, a popular beach resort in England, erosion of the sea cliffs feeds the beach shingle with fossilized, tropical shells. These ancient, aquatic relics are common for miles along the maritime bluffs. People come to pick over the beach wrack for the coiled, weather-revealed shells. The site also yields the occasional fossilized sea dragon, creatures that are both large and ancient. The bones of such an ichthyosaur, recovered here by young Mary Anning sometime around 1810, constitute the first giant skeleton recognized in Britain. In more recent times, the remains of juveniles from related species have been discovered in the area.

The biggest dinosaur skeleton ever assembled is 74 feet long. Such huge fossils of herbivores and their grim-jawed predators are found around the globe. The Royal Tyrrell Museum in Alberta, Canada, has one of the biggest collections, owing to a nearby source: A few miles from the museum, the Red Deer River has incised a winding valley through green rolling farmlands, and it has revealed in the colorful canyon walls impressively large skeletons now on display at Tyrrell. Paleontologists believe that many more fossils await discovery in the area. A new find is always under excavation.

The remains from other Mesozoic sites, while not so large, are especially showy. In the Bavarian town of Solnhofen, a dozen miles from the Danube, residents have used the white, amazingly flat limestone slabs of their quarries for floors, walls and roofs since pre-Roman times. The demand for local stone increased dramatically when at the end of the 18th century, Alois Senefelder showed how to use the finely grained slabs for lithography, a technique for printing ink on paper using a flat, greased stone.

As a result of the demand for lithographic plates, much of the area was excavated, revealing a large number of very well preserved fossils. Our own book shelves hold one such heavy limestone block from Solnhofen that bears the impression of a large dragonfly, naturally cast at the plate surface. Such stones were useless to the early quarriers, although they would often take the specimens home as curiosities.

The Solnhofen site is rife with spectacular finds owing to its situation near the edge of the ancient Tethys Sea where salty, oxygen-starved lagoons often formed. (The fineness of the chalk powder at this site is so remarkable that it can have settled only in a ponded body of water long undisturbed even by tiny waves.) One hundred fifty million years ago, these pools were only kept wet by infrequent surges from shallow seas nearby. Evidence of many animals has been preserved in the even layers of sediment, including two complete specimens of Archeopteryx lithographica, the first bird, which sported reptilian teeth and a feathered body.




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