Recently scientists discovered a clutch of 150-million-year-old fossil eggs that may be an important “missing link” in evolutionary history. The eggs are dated to be from the Late Jurassic period and contain actual fossilized embryos. This find allows paleontologists a rare insight into the early lives of a group of dinosaurs known as theropods, which include the iconic Tyrannosaurus rex and modern birds. The discovery also addresses questions of about which shells came from which particular species.

“Most of the time, you find eggs without the embryos, or the embryos without the eggs,” said said Ricardo Araújo to National Geographic. Araújo is a vertebrate paleontologist at Southern Methodist University in Dallas, Texas, and lead author of the new study.

The majority of fossilized eggs found up until now have been from the Cretaceous period—about 80 million years ago. The gap between the Cretaceous period and the Early Jurassic period (190 million years ago) has confounded scientists, leaving them without the necessary information to make concrete conclusions about the “bird like” qualities seen in the later dinosaur species. These qualities are believed to have occurred during the gap between the Late Jurassic period and Cretaceous period. The newly discovered clutch of theropod eggs and embryos are exactly what paleontologists needed to solve the evolutionary mystery.

The theropod eggs belong to a group called Trovosaurus, which are extremely porous, leading scientists to believe the eggs were at one time buried. The pores allowed gas exchange between the inside and outside of the egg while underground. The embryos found inside the theropod eggs were formed enough to have significant skeletal structure. This means that the eggs are from a very late stage of incubation, possibly the last week of development before they would have hatched—another impressive aspect of this particular scientific discovery.