Focal Point An egg that crystallized into a geode over fifty million years

What to know: This fossilized egg was discovered in the 1930s by a girl on her family ranch in Wyoming — but it was laid by a bird approximately fifty million years earlier. Soon afterward, the egg cracked and its embryo died. Ancient grass flies then laid their eggs on the embryo, their hungry larvae reducing it to a pile of bones; small, dark spots on the shell are the fly puparia. Over time, the egg was buried in sediment that slowly turned to rock, and the shell, puparia, and bird bones fossilized. Mineral-rich groundwater seeped into the shell, crystallizing white calcite on its walls. The resulting egg geode weighs just two pounds: a compact, eon-spanning record of the cycle of life. 

From an expert: “We don’t know for sure what kind of bird laid the egg,” says Daniel Brinkman, museum assistant in vertebrate paleontology at the Yale Peabody Museum. “However, some researchers think that the bird most likely inhabited near-shore aquatic habitats. Others think it might be the egg of the large, fully terrestrial flightless bird Diatryma, which until recently was considered to be part of the genus Gastornis. Diatryma bones are the most common fossil bird remains found in the approximately 50-million-year-old Willwood Formation of Wyoming, from which this specimen came.”

Exchanging hands: Lillian Jones Hopkin was only twelve years old when she found this egg. She donated it to the Princeton University Museum in 1965; twenty years later, Princeton presented the geode to the Peabody, along with more than 20,000 other vertebrate fossils — a gesture of inter-university cooperation.