But every now and then, scientists' persistence pays off. A recently discovered clutch of 150-million-year-old fossil eggs is being billed as an important missing link in the evolution of dinosaur eggs.

The find from the Late Jurassic period, described in a paper published May 30 in the journal Scientific Reports, gives scientists a picture of early dinosaur eggs and embryos from a group called theropods, which include Tyrannosaurus rex and modern birds.

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

This is the first real picture of what early eggs in theropods might have looked like, said Robert Reisz, a vertebrate paleontologist at the University of of Toronto, who was not involved in the study.

The find will enable researchers to begin to address questions like which aspects of theropod eggshells came from their ancestors, and which ones evolved independently.