Featured Fossil: Eocene Egg

By Sean Moran

During July and August of this summer, a group of vertebrate paleontologists from the Florida Museum of Natural History spent four weeks combing the badlands of Nebraska looking for ~34 million-year-old (Ma) fossils. The northwestern corner of Nebraska, in addition to containing some of the state’s most beautiful scenery, also preserves some amazing geological and paleontological history. In fact, all five biostratigraphic North American Land Mammal Ages, or NALMAs for short, from 38-20.6 Ma take their names from towns in this region of Nebraska. The earliest of these is the Chadronian, named after the Nebraska college town of Chadron. The Chadronian is the last NALMA of the Eocene Epoch which dates from about 38-33.9 Ma and is preserved in this part of North America by the Chadron Formation. Overlying the Chadron Fm is a package of rocks known as the Brule Formation deposited during the Oligocene Epoch. The Orella Member of this formation overlies the Chadron Fm and dates from 33.9-33.3 Ma, or the Orellan NALMA. The namesake of this formation and land mammal age is a tiny ghost town located in the Oglala National Grasslands. More interestingly, the contact between this two units of rock records the largest magnitude extinction since the extinction of the dinosaurs. The Eocene-Oligocene Transition (EOT) as it is known is marked by the formation of permanent Antarctic ice caps and a global decrease in temperature of ~7°C, though there is still much debate as to the cause.

Annotated figure of the formations in the field.

The goal for our crew was to collect fossils across this boundary in Nebraska to better understand how mammals, in particular, responded to this major climate event. Luckily for us, we have a US Forest Service permit to collect fossils for the Florida Museum collections in the Oglala National Grasslands. This permit gives us legal access to some highly fossiliferous outcrops that can help to address these questions.

Towards the very end of the month-long excursion, one of our museum volunteers, Bob Tarnuzzer, was prospecting an outcrop near Toadstool Geologic Park. We weren’t sure where we were in the rock layers at the time, but that became more clear in the last few days of the field season. It turns out we were just below the PWL, which is an ash bed that marks the boundary between the Chadronian and Orellan. This means we were prospecting in the latest Eocene right before the major EOT climate event. As Bob was prospecting this one section of outcrop, he picked up what he thought was just a fragment of fossil bone. Instead, upon inspecting it closer, he realized that the fossil had a remarkable texture unlike the other bones we had been collecting. That texture, as well as the shape, confirmed that Bob had found a complete 34 Ma bird egg. Bob’s discovery is only the third bird egg in the Florida Museum’s half century of Nebraska collection and by far the best-preserved specimen. Further research into the pattern of the eggshell should help us to identify which species of Eocene bird the egg may have come from. It was certainly one of the best finds of the trip!

Fossilized egg found by Bob!


To learn more:
Check out this museum video about fossil collecting in the Badlands.