The Formation and Preservation of the Solnhofen Fossils

Our new Archaeopteryx exhibition has stunning complete fossils of fish, turtles, crocodiles, shrimp, sharks and much more, all from Solnhofen, Germany. In this blog, Dr. Bakker explains why Solnhofen produced and preserved so many spectacular, intact specimens.

The Mystery of Tropical Germany

From the first diggings in the late 1700’s,  Solnhofen presented a profound puzzle: Why was  Germany tropical in the Jurassic?

The fossil evidence was perplexing:

Fish
Amiopsis Lepidota

Big, long-lived reefs grow only in the tropics – how could northern Europe have supported the Solnhofen reef?

Large crocodiles thrive only in the warmest climate – how could giant sea-crocodiles flourish at Solnhofen?

Huge tree ferns today are emphatically warmth-loving plants – how could tropical ferns grow luxuriously at Solnhofen?

The mystery was world-wide. In the Jurassic, big crocodilians, tree ferns and reefs had spread all over Europe, Asia and North America. The tropical belt must have extended into Alaska and far south into Argentina.

Solnhofen was part of the proof that the Jurassic was one of the warmest periods in the history of life. Since the end of the Jurassic, on average Europe and North America suffered a gradual decrease in winter warmth.

Solnhofen – A Real Jurassic Park

Big-Budget movies have made the Jurassic Period  the most famous sector of geological time in our modern world. But in fact, the Jurassic was already world-renowned by the 1830’s. The first carnivorous dinosaurs known from good skeletons came from the Jurassic of Oxford. The first dinosaur tracks discovered in abundance were from the Jurassic of Massachusetts. The first complete skeletons of giant sea-reptiles were excavated from the Jurassic of southern England.

But no locale has gave finer fossils from the Jurassic than Solnhofen, Germany. Beginning in the mid 1700’s, Solnhofen has provided a never-ending stream of petrified animals and plants.

Fish
Liodesmus Sprattiformis

The exquisite skeletons lie in lithographic limestone, a rock that records not only bones but  impressions of skin and other soft tissue. Vertebrate bodies are preserved in exceptional detail. The pterodactyls at Solnhofen often have fossilized wing membranes. Crustaceans and mollusks are often fossilized as complete bodies. Even the most delicate  parts of squid – tentacles, eyes, and ink sacs – are recorded as high-resolution impressions.

Solnhofen lithographic stone has captured a more complete picture of Jurassic life than any other kind of sediment. Fossils are not common – hundreds of rocks slabs must be split to expose a single animal. Fortunately, the discovery of fossils is encouraged by commercial interests. Beginning in 1798, the lithographic stone has been quarried to make stone plates used to print high-resolution images of paintings, etchings and, later, photographs.

Many scientific publications about Solnhofen fossils have been illustrated by drawings of specimens reproduced via lithographic limestone plates.

Why are Solnhofen fossils so magnificent? The environment  around a tropical reef  was perfect for preservation. Reef-building organisms – sponges, microbes, corals – built up an arc of hard calcium carbonate that shielded a quiet lagoon. All manner of salt-water fish and invertebrates hunted for food in the upper warm waters. Land-living animals came to the beach to search for washed-up carcasses. In the air flew ‘dactyls and, on occasion, a  bird.

When an animal died and sank to the bottom of the lagoon, the water chemistry offered protection from  the forces of decay and dismemberment. The hot tropical climate concentrated the salts in the quietest part of the lagoon, so that most decomposers – organisms that would destroy the carcass – were kept away. Salt-loving microbes spread a thin film over the bottom, and this film functioned like a death-shroud, further protecting the body of dead animals. Perfect fossils were formed when the microbial mat excluded every crab, snail and  bottom-living shark that would otherwise destroy the body.

Extinct Sea Turtle
Eurysternum Wagleri

Solnhofen brings to us a picture of half-way evolution. The rich fish fauna was being modernized by natural selection. Old-fashioned armored fish were going extinct. New styles of jaws and fins were being developed among what would become the dominant fish families in the modern world. Many Solnhofen fish were living-fossils in their own day, representing evolutionary designs that had first appeared two hundred million years earlier. Other Solnhofen fish were the first successful members of clans that dominate today.

Pterodactyls and sea-reptiles too were about half-way in their Darwinian trajectory. Sea-turtles had not yet evolved their specialized flipper. Sea-crocodiles were about to suffer extinction and replacement by the new ocean-going species of the Cretaceous Period. Crustaceans were starting the wave of evolution that would continue as modern crabs and shrimp and lobsters.

There collection displayed here in our exhibit is one of the finest samplings of the entire Solnhofen biota. The Archaeopteryx at the center of the exhibit is the only Archaeopteryx in the New World.

Gems and Minerals

The Houston Museum of Natural Science is currently hosting a special exhibition, The Nature of Diamonds, so this month our booklist features Gems and minerals.

According to www.rocksforkids.com a mineral is the same all the way through, and  there are about 3000 known minerals on earth.  A rock, on the other hand, is made from two or more minerals. 

For young children, Let’s Go Rock Collecting by Roma Gans is a great introduction to rocks.  The book begins by saying that people collect many things, and that the oldest thing you can collect is rocks.  In simple terms, with wonderful illustrations and photographs, Ms. Gans explains the three types of rocks—igneous rocks, sedimentary rocks and metamorphic rocks.

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Creative Commons License photo credit: Walter Rodriguez

You will learn how magma becomes lava which becomes igneous rocks. You will also see photographs of granite, quartz and basalt –  all igneous rocks.  Sandstone and limestone are examples of sedimentary rocks.  The Egyptian pyramids were made from limestone. In modernt times, limestone is mixed to make cement. Metamorphic rock means changed.  Slate is a metamorphic rock that used to be shale before being exposed to intense heat and pressure.

Children are encouraged to collect rocks, and examples of simple rock collections are pictured.  Rocks are everywhere, so collecting rocks is an inexpensive introduction to science.  And, who knows?  You might grow up to be president of the Houston Museum of Natural Science!

Byrd Baylor’s Everybody Needs a Rock begins “Everybody needs a rock.  I’m sorry for kids who don’t have a rock for a friend.”  Baylor gives the reader rules for finding a special rock, something you might keep forever.  The rules say you can find a rock anywhere, but make your choice when things are quiet.  You need to look the rock in the eye to make sure it is the perfect size, color, shape and smell.  Do not let anyone help you make the choice — the decision is yours alone. 
Baylor’s words paired with Peter Parnall’s simple black and bronze drawings work together to create quite a book that will make children anxious to begin the search for their own rock.

Gemstones by Ann O. Squire is a nonfiction introduction to gems.  You learn that deep within the earth, high temperatures and pressure transform minerals into crystals which can be cut, polished and sold for thousands of dollars.  A crystal must pass 3 tests to be considered a gemstone:  it must be rare; it must be beautiful; and it must be hard enough to resist scratching or breaking.

bariteSquire says that gemstones began forming millions of years ago up to 100 miles beneath the earth’s surface.  The intense heat caused the rocks to become magma which contains tiny mineral crystals.  Pressure caused the magma to erupt from the earth as a volcano or flowing between layers of rock.

Diamonds, rubies, sapphires and emeralds are inorganic minerals, meaning they have never been alive.  A pearl, however, comes from a living source — an oyster.  Amber comes from the sap of trees that lived long ago and coral is made from the skeletons of tiny sea creature.
Squire briefly explains some of the superstitions involving gemstones and tells how the idea of birthstones began.

Don’t miss the Cullen Hall of Gems and Minerals including the Lester and Sue Smith Gem Vault on the second floor of the Houston Museum of Natural Science.  You will see the most incredible collection of gems and minerals in the world.