A Monitor Lizard from Green River: The Find of a Lifetime

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The fifty-million-year-old shale layers that make up the world-renowned Green River Formation in southwestern Wyoming once lay at the bottom of a large inland lake. Despite the arid environs that currently characterize this rather desolate corner of the American landscape, back in the Eocene Epoch this entire area was part of a subtropical zone rich with an abundance of fauna and flora, including everything from primitive flying bats to swaying palm trees.

It was the ancient lake system itself, however, that provided the area with its most notable inhabitants—a diverse array of fish species, including fresh water perch, gar, and herring whose fossilized remains can today can be found throughout these layers, often in prodigious numbers. Indeed, ever since its discovery as one of the world’s richest paleontological repositories back in the mid-nineteenth century, the Green River Formation has produced fossil fish by the tens of thousands, each specimen sandwiched between layers of easily split shale. Over the years, commercial diggers have literally turned mountains into molehills in their quest for the wonderfully preserved examples that emerge from the formation’s most famed horizon, the “18-Inch Layer.” There, the oil-rich shale has preserved the fossils in a dark chocolate brown, which contrasts dramatically against the mottled tan matrix.

While its beautiful and scientifically significant fish fossils have rightfully garnered this formation international renown, the remains of even more exotic creatures, including stingrays, birds, turtles, and alligators, can also occasionally be found within these rocks—often preserved in exquisite detail with virtually every fin, claw, and vertebra now majestically exhibited in stone. Despite the decades of work that have already gone into the digging, preparation, and study of the diverse fossil material that has emerged from the Green River Formation, however, something sometimes still pops up in these layers that not only surprises … but amazes!

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Specimen of Saniwa ensidens in the Field Museum of Natural History. by Smokeybjb Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)

In 2013, the second complete Eocene-age horse ever found was unearthed in the South Dempsey Quarry (now American Quarry). Astonishingly, though, only a few weeks after that headline-generating discovery was made, a perhaps even more mind-blowing find emerged from these hallowed rocks … a complete monitor lizard! With its rows of fang-like teeth, detailed limbs, and long sinuous tail, this creature stands as one of the most impressive fossils ever to come out of this legendary location.

“This is the largest fossil lizard ever found in the Green River Formation,” says Anthony “Tony” Lindgren, who owns the quarry where the specimen was discovered. His family has been involved in the exploration of the formation’s rich layers for more than three decades, and Lindgren himself has been digging fossils since he was 16.

This amazing fossil represents only the third complete example of the monitor lizard Saniwa ensidens ever uncovered in the Green River Formation. At seventy-four inches in length, it is also the largest. What is more, as Lindgren points out, “it’s one of the largest fossil lizards ever found in North America.”

Lindgren’s extraordinary Saniwa came to light during the summer of 2015—quite by accident. His bulldozer was removing huge sheets of rock in the hopes of uncovering large fish … or perhaps something more special. But after one such run, Lindgren looked down and thought he saw something unusual sticking out of the torn stone. He stopped to investigate. When he first began looking through the rock pieces that covered the lizard his heart both raced … and sank. All he could initially see was the ghostly outline of bone lurking underneath a thin layer of matrix, yet from the size and shape he knew right away that he’d found something unusual. Still, he feared the specimen had been irreparably damaged. Either way, he knew an arduous task lay ahead.

Tom Wiewandt fossil-varanid-lizard_sm_300
Eocene monitor lizard, Saniwa ensidens, from the Green River Formation of Wyoming, USA. This primitive lizard is the largest monitor found in North America; no monitors occur in the Americas today. The specimen measures 29″ from the top of its head to the bottom of its tail curve; its lower jaw is 4.25″ long. Photo ©Tom Wiewandt; used by permission.

Because of the potential importance of the lizard discovery, Lindgren immediately decided to suspend all other digging operations within the quarry. His attentions were then focused solely on the jumble of fossil-containing rock before him and making sure that not even the most delicate piece of skeletal bone was left behind. To enhance his efforts in that regard, Lindgren and two friends constructed a 250-gallon water tank on location, eventually attaching it to a fine mesh screen. The completed contraption was designed to function like an old-time gold sluice, allowing the team to run even the smallest pieces of surrounding matrix through the screen, filtering off debris in the hope of leaving behind dark-colored bone. At times the weeks-long process proved frustrating, often only producing a single small bone as the culmination of a day’s labor. But in the end, this somewhat unconventional process proved to be highly successful, and over 95% of the lizard skeleton was eventually recovered.

Following months of careful sifting and examination, what Lindgren and his team ended up with was a giant fossil jigsaw puzzle … without many end pieces. Dozens of pieces, ranging in size from a few feet to virtually microscopic, were carefully arranged on a table in Lindgren’s lab, located not far from the quarry.

There the meticulous process of preparing, cleaning, and reassembling the find began. It soon became clear that the discovery was going to surpass all expectations. The bone preservation was magnificent, and except for missing a few scattered pieces around the hip joint and tip of the tail, it was all there! Even the skull, with its distinctly pointed snout, was virtually intact, though it had been twisted so that the lower jaw had become somewhat displaced. Still, this specimen represents perhaps the most complete and detailed example of a monitor lizard ever found in the formation.

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Artist Tom Sermon’s scrupulously accurate photo-real illustration of the Lindgren Quarry Saniwa. © Tom Sermon.

As previously mentioned, two other examples of such lizards have been found in the Green River Formation. One, found in 2007, is of an apparent juvenile which features soft-tissue preservation of some scales and cartilage. The other has been, until now, the most “famous” Saniwa ensidens specimen ever found in these layers—a fossil currently housed in Chicago’s Field Museum. That one shows a forty-two-inch lizard seemingly captured in a “swimming” position surrounded by fish, its long tail (twice the length of the body) trailing behind it.

The discovery may also prompt paleontologists to answer a very basic question: How did a monitor lizard end up being buried at the bottom of a lake some 50 million years ago? Did it wander into the water by accident in the pursuit of prey? Was Saniwa ensidens actually semi-aquatic? Even with this amazing new find, scientists may never know the answer to this intriguing natural riddle. Still, a number of institutions have already expressed interest in acquiring the specimen with the intent of unlocking more of its secrets before putting it on public display.

For his part, Lindgren is determined to find the proper home for his Eocene treasure. He knows that for as long as he may continue to mine the rich fossil layers of the Green River Formation, he may never come close to finding anything else as dramatic, beautiful, or valuable as his magnificent monitor lizard.

“Each specimen found adds something to the scientific knowledge of this species,” Lindgren said. “The fossil in the Field Museum is magnificent, and the small one that was found in 2007 is amazing because it does have some of the fossilized soft tissue intact. But this example is special because of the size, the preservation, and the detail. I may be a little prejudiced, but I think it’s the nicest and most important one ever found.”

— Andy Secher


A version of this article appeared in Fossil News, Winter 2017: 49-51. ©Andy Secher.

Andy Secher is a Field Associate in Paleontology with the American Museum of Natural History in New York. His specialty is trilobites, and he currently edits the AMNH’s renowned trilobite website—a site that also features photos drawn from his personal trilobite collection, which houses over 6,000 specimens from locales around the globe. He is currently finishing a book on the subject called Travels with Trilobites which will feature insights on his excursions to many of the world’s leading Paleozoic locations.

Thomas Wiewandt, owner of Wild Horizons Productions (wildhorizons.com), is a professional natural history photographer devoted to art and education. He earned degrees in zoology (M.S., UArizona) and ecology (Ph.D., Cornell) and has been active in ecotourism, publishing, and filmmaking. Tom’s award-winning work has been featured on PBS and in many books, calendars, and magazines worldwide. For twenty-seven years, he has photographed prize fossil specimens, a project destined to become a book.

Tom Sermon, known professionally as PalaeoArt, is an artist who focuses exclusively on producing original illustrations of fossilized remains of prehistoric creatures and critters. Having studied at Oxford University, England, PalaeoArt attempts to bring back the classic style of scientific illustrations but with modern media and techniques. What’s unique is that instead of selling his originals, he trades original art for fossils to add to his collection. Those interested in his art can view some of his work at www.palaeo-art.com and contact him at tomthepom78@gmail.com.