BARCELONA, Spain, July 12 (UPI) -- The discovery of dinosaur eggs with a unique shape -- they are oval -- suggests a link between dinosaurs and modern birds, Spanish researchers say.
Unlike most non-avian dinosaur eggs, which are symmetrical, the oval shape of some non-avian theropod eggs from the upper Cretaceous period discovered in Spain suggests birds and the theropods could have a common ancestor.
The oval form is rarely seen in eggs from the period and is similar to modern day eggs, researchers from Complutense University of Madrid and the Universitat Autonoma de Barcelona said.
The oval shape of bird eggs is associated with their physiology: they take on this shape given the existence of only one oviduct, which can form only one egg at a time.
The region in the oviduct creating the eggshell membrane is what gives the egg its asymmetrical oval shape.
The wider end contains a bag of air that allows the bird to breathe in the last stages of its development, an evolutionary step relatively underdeveloped in most dinosaurs, researchers said.
The egg discovered by the Spanish researchers may therefore represent a missing link between dinosaurs and birds, a release from the Barcelona university said Thursday.