Researchers have found a new type of dinosaur egg from the Lower Cretaceous, or Early Cretaceous, in northwest China. The strata encompasses fossils dated between 100 million and 145 million years old.

Dinosaur eggs from the Lower Cretaceous are hard to find. Eggs from the Upper Cretaceous -- a bit younger at 66 million to 100 million years old, and typically closer to the surface -- are much more common.

Researchers use a parallel but separate system of taxonomy called "parataxonomy" to classify fossilized eggs. The newly discovered egg required scientists to establish a new "oogenus," "oospecies" and "oofamily."

In a new paper on the egg, researchers listed the features that set it apart from other oospecies: "branched eggshell units lacking a compact layer near the outer surface; interlocking or isolated multi-angular eggshell units, as viewed in tangential sections; and irregular pore canals."

The scientists gave it the oospecies name Polyclonoolithus yangjiagouensis, placing it within the new family, Polyclonoolithidae.

"Dinosaur eggs from China largely come from the Late Cretaceous deposits, with occasional reports from the Early Cretaceous in Liaoning Province, northeastern China," study co-author Zhang Shukang, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology, said in a news release. "The new discovery expands the geological and geographical distribution of the fossil record of dinosaur eggs in China and may reveal the origin of eggshell microstructures of spheroolithid eggs."

Early Cretaceous outcroppings in Gansu Province's Lanzhou-Minhe Basin have revealed many dinosaur fossils, but never an egg. Researchers are hopeful that their discovery will lead to the excavation of more fossilized eggs in the future.