Jul
24
2013

Paleontology lab work brings fossil fish remains to light

Green River Formation slab: The head end of Lepisosteus has been partially prepared.The rest of the fossil is hidden beneath a layer of rock matrix.
Green River Formation slab: The head end of Lepisosteus has been partially prepared.The rest of the fossil is hidden beneath a layer of rock matrix.Courtesy Mark Ryan
I've had the great fortune of being able to volunteer in the paleontology lab at the Science Museum of Minnesota. I'm in my fourth month there and it's been a real blast. My first project was preparing (cleaning) the skull of a small oreodont collected from the White River Formation in Wyoming. This is the same formation exposed in the fossil-rich South Dakota Badlands. By cleaning, I mean removing all the rock (matrix) in which the skull is encased. I've also helped patch up the casts of a couple of lambeosaurus skulls, and spent a few days puzzling over a crocodile skull reduced to about 1000 pieces.

Removing matrix: Science Museum of Minnesota fossil preparator Becky Huset uses an air scribe to methodically remove the thin rock veneer covering the remains of Lepisosteus, a fossil gar.
Removing matrix: Science Museum of Minnesota fossil preparator Becky Huset uses an air scribe to methodically remove the thin rock veneer covering the remains of Lepisosteus, a fossil gar.Courtesy Mark Ryan
At the moment, preparators been working on the remains of a 52 million year-old gar collected from the Green River Formation in southwestern Wyoming. Most of the work is being done by the more experienced volunteers in the lab but I've been able to help a little, taking my turn with the air scribe to reveal some caudal scales in their rocky grave. This particular specimen, an ancient member of Lepisosteus, was collected in Lincoln County, Wyoming. It's fascinating work uncovering something that last saw sunlight more than 50 million years ago. Now, at least, its remains can bask in the glare of the paleo lab's artificial lights.

Lepisosteus tail comes to light: SMM paleo lab technician, Kay Blair, works at revealing the 52 million year-old gar's tail rays and posterior scales.
Lepisosteus tail comes to light: SMM paleo lab technician, Kay Blair, works at revealing the 52 million year-old gar's tail rays and posterior scales.Courtesy Mark Ryan
Fifty some million years ago, the gar lived in a large body of water known as Fossil Lake, one of three intermountain lakes that existed at different times in a sub-tropical environment in that part of Wyoming. The intermountain basin in and around the lake teemed with both floral and faunal life that over about 4000 years lived and died and were fossilized forming one of the great Lagerstätten in the world. The surrounding mountains were composed mainly of limestone, and the rivers and streams eroding those mountains carried high levels of calcite (CaC3) into the lake, resulting in a high sedimentation rate that added to the ideal fossilization environment.

(More about the Green River lakes and fossils in previous Buzz story).

Most of the fossils coming out of the Fossil Lake strata have been fossilized by a process called permineralization, where mineral-rich water permeates all the spaces and pores in the skeleton and the minerals (in this case calcite) crystallize out of the water replacing bone material down to the cellular level. Some carbonization is also involved. This process depletes the remains of volatiles and is caused by the heat and pressure of sediment compression, which also crushes and flattens the fossils, and tends to color them either brown or black.

Scattered bones: The fossil gar's head was blown apart by gases that built up during its decomposition.
Scattered bones: The fossil gar's head was blown apart by gases that built up during its decomposition.Courtesy Mark Ryan
That's very apparent with our gar. Although only portions of the fish's remains have been exhumed (its head and tail) the fossil is already providing some information about what followed the gar's death (taphonomy). Lepisosteus favored the shallow, swampy edges of Fossil Lake and when it died it probably floated on the surface for a while giving bacteria time to enter its mouth and gills and begin their decomposition work before the corpse was buried beneath sediments.

We can deduce this scenario by the manner the remains are preserved. The bones of the gar's skull and jaws are scattered and jumbled in a mish-mash of bones and scales. The head appears to have been blown apart, and that's probably what happened. As the microbes feasted on the fish's head, they released gases inside the corpse which built up, and bloated the gar to a point where it burst from the internal pressure. The mandibles, the cranium, and other bones broke apart before settling to the bottom and are disarticulated. The very end of the tail, however, shows no such disruption. The rays of the caudal fins looking almost as fresh as they did when the gar died half a million centuries ago.

Scales as tough as nails: In life, an enamel-like tissue called ganoin made the diamond-shaped scales of Lepisosteus tough and predator-resistant.
Scales as tough as nails: In life, an enamel-like tissue called ganoin made the diamond-shaped scales of Lepisosteus tough and predator-resistant.Courtesy Mark Ryan
The scales of its mid-section are beginning to come to light. These diamond-shaped structures were covered with ganoin, an enamel-like tissue containing less than five percent organic material. The mineralized tissue gave Lepisosteus a very tough, predator-resistant exterior when it was alive but not so resistant to the bacteria that attacked the gar from the inside after it died. Preliminary work of the mid-section is showing signs of decomposition there but further work required.

One of the major experts on the fossils found in the Green River Formation is Lance Grande, a graduate of the University of Minnesota (and elsewhere) who has been working at Chicago's Field Museum for the past few decades. In the early '80s, Dr. Grande wrote a hefty bulletin titled Paleontology of the Green River Formation for the Wyoming Geological Survey, and has now come out with a new book titled The Lost World of Fossil Lake: Snapshots from Deep Time. In a recent television interview, Dr. Grande talked about his book and about the fossils found in the Green River Formation.

Hundreds of thousands of finely preserved fossils from Fossil Lake deposits can be found in museum displays and on rock shop shelves world-wide. The best fossils were buried quickly and preserved in near pristine condition. Many of these come from what used to be the deep center of the lake where conditions were probably anoxic and burial fairly swift. At times during Fossil Lake's history events like seasonal algal blooms or rapid turnovers of the water column occurred and caused massive die-offs of fishes. Other fish, like our gar, probably just died a regular death.

Progress so far: After several weeks of preparation, the remains of Lepisosteus are becoming more defined.
Progress so far: After several weeks of preparation, the remains of Lepisosteus are becoming more defined.Courtesy Mark Ryan
Every fossil tells a story, and our gar is no exception. Back in the Eocene epoch it lived for a short time in the then subtropic environment of southwest Wyoming, doing what gars do before it finally died along the shores of Fossil Lake. After it was buried, it was fossilized, dug up, and transferred to the collections vault of the Science Museum of Minnesota. A few months ago, it was retrieved from the vault and brought into the paleo lab where it's been worked on each week by several people. Whatever the gar was thinking when it was alive back in the late Eocene, you can be sure it was unaware that its post-mortem life would provide hours of detailed work, study and fascination for another curious life-form 52 million years later.

SOURCES AND LINKS

Green River Fossil Adventures
Fossil Butte National Monument
More about Green River Formation at Fossil News
The Green River Formation: A Fossil Fiesta
Green River Info at UCMP Berkeley

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