In Japan, a patch of rock the size of a tennis court has yielded 90 fragments of fossilized dinosaur eggs, researchers reported June 29 in the journal Cretaceous Research. The find includes five types of eggs and hints at the first dinosaur nesting site discovered in the Japanese Islands.

Dinosaur eggs have turned up at hundreds of fossil sites around the world, but such finds are rare in Japan. Just like modern bird eggs, dinosaur eggs can be easily destroyed, smashed or flattened. On top of that, Japan’s geology and volcanism compresses rock layers, making fossilized eggs hard to distinguish.

“It is difficult to find fossil eggshell fragments in Japan because the rock is so hard and needs to be broken apart manually,” Darla Zelenitsky, a paleontologist at the University of Calgary and co-author on the study, explained in a statement.

In 2006, an amateur fossil hunter led researchers to a riverside site in southern Japan. Over the last few years, the site has yielded the remains of ancient mammals, frogs, lizards and a few dinosaurs. Sifting through samples from the site, Zelenitsky and her colleagues stumbled upon fragments of 110-million-year-old eggshells from different dinosaurs. Under a microscope, the structural patterns of eggs can point to the species that produced them.

Most of these eggs likely came from meat-eaters called theropods (the group that produced T. rex and modern birds), but a few came from an ornithopod, a larger dinosaur that munched on plants. Some of the theropod eggs they discovered were extremely small — researchers estimate their eggs weighed between one and five ounces — making them some of the tiniest theropod eggs ever unearthed.

The presence of so many eggs suggests that the site may have been used as a nesting site for lots of different species. “[These eggshell fragments] can tell us a lot about the evolution, reproduction, and biology of dinosaurs in this region,” Kohei Tanaka, a paleontology grad student in Zelenitsky’s lab who did most of the analysis, noted in a statement.

In the meantime, researchers plant to continue the hunt for more dino eggs and perhaps even fully preserved nests at this rare riverside site in Japan.