A Cretaceous floodplain
During the Cretaceous Period, between 83 and 120 million years ago, the Gyeongsang Basin was a large, non-marine depression filled with alluvial fans, rivers, and vast lakes. The geology of the region is dominated by the Gyeongsang Supergroup, a sequence of sedimentary and volcanic rock up to 9 kilometers thick. This environment, with its soft, muddy floodplains, provided ideal nesting grounds for a diverse population of dinosaurs. Over millennia, repeated nesting events in the same areas led to an extraordinary accumulation of fossilized eggs.
The first dinosaur egg fossil in Korea was discovered here in 1972. Since then, hundreds of eggs and numerous clutches have been unearthed at multiple sites along the southern coast, including Boseong, Goseong, Hadong, and Hwaseong. At the Boseong Bibong-ri site alone, more than 200 individual eggs have been found across at least ten nests. Some nests measure up to 1.5 meters in diameter. The eggs are typically found in reddish or purple sandy mudstone, a type of paleosol suggesting a semi-arid climate with distinct wet and dry seasons.
A colonial nesting ground
The arrangement of eggs shows colonial nesting, a behavior where many animals gather to nest in the same location. Dozens of nests are often found in close proximity, sometimes in multiple layers of sediment, indicating that dinosaurs returned to these same preferred spots over many nesting seasons. The eggs themselves come in various shapes and sizes. At the Boseong site, egg diameters range from 9 to 15 centimeters. In Tongyeong, a clutch of the oospecies Macroelongatoolithus goseongensis contained eggs measuring up to 39 centimeters long, likely belonging to a large oviraptorosaur.
Most eggs discovered belong to herbivorous dinosaurs. However, the discovery of both carnivorous and herbivorous dinosaur egg fossils in close proximity is rare. This cohabitation suggests a complex ecosystem. The sites also contain fossilized footprints from both theropods and sauropods, alongside the skeletal remains of ornithopods like Koreanosaurus boseongensis, a small, burrowing dinosaur. The presence of such varied species records the Cretaceous life that once populated these ancient lakeshores.