Important Green River Formation fossils come to New York

Stuart Wilensky and Douglas Miller (USA)

In the early Eocene Epoch, drainage from the newly uplifted Rocky Mountains filled an inter-mountain basin to form what geologists call Fossil Lake. The climate of Fossil Lake was subtropical, similar to the climate of Florida today. The lake persisted for about two million years, and was home to palm trees, turtles, birds and an abundance of fish. On numerous occasions, unique conditions came together to result in some of the best-preserved fossils ever discovered. The sediments of Fossil Lake were first discovered in the 1860s, near the town of Green River Wyoming, and the area was named the “Green River Formation,” which is well-known in the scientific community and by amateur collectors.

Palaeontologists have long theorised that the lake was deep enough to be anoxic (devoid of oxygen) at the bottom. This prevented scavengers from disturbing the plants and animals, and inhibited decomposition. Algae, and other plant and animal life, would die and fall to the bottom as in lakes and ponds today. Storms brought runoff from the mountains, covering the flora and fauna with mineral-rich material that would ensure their preservation.

Recently, scientists have asserted that a kind of “red tide” may have been responsible for the many perfectly preserved fossils found. (“Red tide” is a common name for algal blooms, which are large concentrations of aquatic microorganisms, such as protozoans and unicellular algae. These can cause a severe decrease oxygen levels in the water column, leading to mass mortality events.) We may never know exactly how and why the plants and animals of Fossil Lake were so well fossilised, but we can all agree that they are to be treasured.

“Ancient Treasures: Fossils from The Eocene” is a ground-breaking exhibition running from 9 May to 3 August 2019 at the Wilensky Gallery in New York, featuring historically important fossil murals from the Green River Formation, unearthed and preserved by the Green River Stone Company (see https://www.iloveny.com/event/ancient-treasures%3A-fossils-from-the-eocene/50850/ and http://www.greenriverstone.com/). Here, we share some historically important highlights from the upcoming show.

One of the outstanding fossils included in the exhibit is this softshell turtle. This large mural contains a giant fossil softshell turtle, Axestemys byssinus, and is one of the finest turtles ever found. The fossil is extremely dark against a light matrix stone adding to its beautiful graphic nature. The exquisite composition features all the limbs and a beautifully articulated shell, and this turtle is 95% complete. Fossil turtles from the Green River Formation are extremely rare. We estimate less than 25 complete A. byssinus have ever been found. They are amongst the most sought after of all fossils due to their size, beauty and rarity. Discovered in the upper layers of the Green River formation of south-western Wyoming, this turtle dates to the early Eocene (50 to 52 million years old).

Gigantism in turtles was likely caused by a warm, subtropical environment, which allowed for “cold-blooded” animals to grow quite large. Softshell turtles belong to a specialised family of turtles called the Trionychidae – a unique group of turtles, which lack the hard-keratinised shell common to other kinds of turtles. The upper shell (carapace) and lower shell (plastron) of soft shells are covered in soft, leathery skin, which is flexible around the edges.

Quarried from Fossil Lake stone, this mural contains a rare Masilliosteus Janeae. (Up until 2012, the only example of the M. janei was housed at the Chicago Field Museum and was thought to be a one-of-a-kind holotype.) To our knowledge, less than ten such gar have ever been found. This is a short nose gar, which had flat grinding teeth, unlike the sharp pointed teeth found on most long nose gar. The fossil measures 47″ head to tail. Presented in a heavily textured light limestone matrix, the mural measures 36” tall by 80” wide by 21 1/2” deep (framed).

Another spectacular piece on view is this gallery-sized mural, quarried from the Steel Blue Lake stone layer. This mural contains a rare palm frond, Sabalites powelli. It is exceptional in both beauty and fossil detail. With this palm, one can see detail down to the small veins in the leaf. The palm has less than 10% restoration. Prized by collectors throughout the world, Green River palms are both rare and beautiful works of natural art. Their size and graceful graphic nature make for a most striking presentation. The present specimen, S. powelli, measures about 6’ tall and is dramatic in composition and colour. The fossil dates to the early Eocene (50 to 52 million years ago). The palm was discovered in the Green River formation of Lincoln Co, Wyoming.

Typically, one or two high quality palms are uncovered by the Green River Stone Company a year. The fossilisation of plants and leaves is quite thin and delicate. Therefore, the discovery process involves splitting sedimentary stone, layer by layer, looking for fossils. The fossil plants are only revealed when the stone splits directly on the plant or a small portion is exposed.

The exhibit brings another rare turtle fossil to the public. Most turtles found in the Green River Formation belong to the Trionychidae family (or softshell turtles). This one is a Baenidae, a hard-shell turtle, and much rarer. To our knowledge, only eight complete adults have ever been found. A land and water turtle, it would have used its long tail to move along the bottom of the water in search of food. Most Baenidae that are discovered are very small juvenile turtles or partial remains and, as mentioned, the adults are quite rare. Of the two types of Baenidae found in the Green River Formation, Baena arenosa is the rarest.

This extinct genus of turtles, Baena, was first described during the famous Hayden Geological Survey of 1870. It is known from only a few complete specimens discovered over the 145 years since Hayden. At a length of 36″, this is amongst the most beautiful fossils the Green River Stone Company has found. The fossil is presented in a dark limestone matrix measuring 44” tall by 28” wide by 23/4” deep (framed).

Perhaps the most spectacular piece in the show is the crocodile Borealosuchus wilsoni, which measures 13’ 5” feet long from head to toe. The skull portion measures 26” in length and 10 1/2” in width. The framed stone measures 57 1/2” tall by 141 1/2” wide by 3 1/4” deep. The specimen is a wall mounted presentation. The fossil is supplied attached to a wood structural armature designed to support its 1,000lb weight. It has a two-part alder wood, French cleat for easy hanging, and is framed in a 3/4” thick, handmade, wenge wood frame. The armature structure has a removable, metal hanging bar, so it can be transported and installed using an overhead lift or forklift truck.

This rare fossil crocodile contains details that are unique. For example, within the front and sides of the skull, there are multiple sets of large and small teeth. Along the entire body, one can see multiple perfectly arranged dorsal scutes, which are protective bony plates, located just under the skin. In addition, the spine can be seen protruding outward from the body, starting at the back of the skull and down through the body, and then turning sideways through its tail. The crocodile has multiple ribs that are partially exposed along the lower belly line. This is truly an amazing fossil, exceptional in its level of completeness and fossil preservation.

Fossils have been collected and studied in Wyoming’s Green River fossil formation for over 150 years. The vast majority of fossils found in the ancient lake bed sediments are fish. Plants such as palm fronds, and reptiles such as turtles, are considered rare and valuable finds. Crocodiles are far scarcer; to our knowledge, less than ten complete, or near complete, specimens have ever been found. This particular specimen is among the best of its kind. At 13’ 5”, it is larger than the famous crocodile housed at the Chicago Field Museum.

“Ancient Treasures: Fossils from The Eocene” offers a rare glimpse into the prehistoric past. A connoisseur’s delight, it celebrates 50 million years of history and life with its compilation of almost 25 rare, never before seen fossils.