The Clarkia Flora: 16-million-year-old plants offer a window into the past.

Near the small town of Clarkia in Shoshone County, Idaho in the USA, exists a rich and unique fossil deposit. The Clarkia fossils, or Clarkia Flora, as the deposit is mostly called due to the abundance of fossil plants, is so well preserved that the assemblage is referred to as a “lagerstätte”, a scientific term reserved for the world’s very finest fossil deposits. The Clarkia fossils are found in sediments that are now known to be about 16 million years old and belong to a period in Earth history called the Miocene. By this time, the (non-avian) dinosaurs were long extinct (the last of these dinosaurs disappeared about 66 million years ago), the Earth’s continents were more or less in the same position as today, and many of the animals and plants would have started looking familiar to modern humans (who emerged much later, about 200,000 years ago).

Fig. 1. The entrance to the “Fossil Bowl”
motocross racetrack and fossil locality near
Clarkia, Idaho.

Among the Clarkia fossils can be found various insects, fish and occasionally the remains of small mammals. However, most striking is the wealth of plant fossils in the form of exceptionally well-preserved leaves, nuts, seeds and wood. Impressively, one can find leaves of oak, laurel, pine and birch that look virtually identical to those we find today. If you look quickly when a new fossil is newly exposed from within the host sediments, you may occasionally even see the original green or autumn red leaf colour. This, and many other original features of the leaves, were locked in to the rock record due to special conditions that have continued to protect the leaves through the millennia.

Fig 2. A well-preserved fossil bony fish, a little larger than a typical modern-day anchovy.
Fig. 3. Miocene fossil leaves, clearly belonging to the Oak group: a familiar large and widely
distributed plant group today. Sometimes, acorns are also found at the Clarkia fossil localities.

Although initially discovered 45 years ago, scientists have still not fully capitalised on these ancient floral treasures and much more research needs to be done. The authors of this article, who are from the Swedish Museum of Natural History and Stockholm University in Sweden, recently visited Idaho to study the Clarkia Flora. Returning with new fossil material, they hope to kick-start a new wave of research into life and climate 16 million years ago.

Massive volcanoes

The sediments containing the Clarkia fossils were deposited in a lake that formed when massive outpourings of lava, known as the Columbia River Basalts, blocked the mouth of a large valley, thereby damming several rivers to create a lake. The Miocene Lake Clarkia was a long, narrow body of water, surrounded by forested hills. The lake filled quickly with sediments – or quickly in geological terms that is – in about 10,000 to 100,000 years. The sediments were mostly fine-grained silts and clays. With the water and suspended sediments came plant and animal remains, which, together with the seasonal fall of autumn leaves, created a supply of natural treasures descending to the lake bottom. The rather deep (about 100 to 150m) Lake Clarkia did not get fully stirred by wind, and anaerobic (devoid of oxygen) and toxic conditions developed at the bottom. These conditions were perfect for preserving fossils – sinking plant and animal remains lay undisturbed, because the scavengers and bacteria that usually decompose organic remains could not survive there. The fast sedimentation rates and close vicinity of the forest to the lake contributed to the unique preservation, by minimising transport-related damage to incoming plant or animal remains, and quickly buried potential fossils in a muddy, lake floor made up of layer-cake of sediments.

Fig. 4. An approximately two-metre-high layer-cake section of mud and siltstone at the Clarkia
Fossil Bowl locality.

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