From “Fossil Egg Links Dinosaurs to Modern Birds” (Science Daily, July 12, 2012), we learn,
Researchers have discovered a series of dinosaur eggs with a unique characteristic: they are oval in shape. The discovery supports the theory that birds and non-avian theropods, dinosaurs from the Cretaceous Period, could have a common ancestor.
Their shape is a unique characteristic of theropod eggs from the upper Cretaceous period and suggests a connection with bird eggs. Non avian dinosaur eggs are symmetrical and elongated. Asymmetry in bird eggs is associated to the physiology of birds: they take on this shape given the existence of only one oviduct which can form only one egg at a time. In this case the isthmus, the region in the oviduct creating the eggshell membrane, is what gives the egg its asymmetrical shape. Thanks to this shape, the wider end contains a bag of air which allows the bird to breathe in the last stages of its development. This evolutionary step was still relatively underdeveloped in dinosaurs.
Thus, the egg discovered by UCM and UAB researchers may represent the missing link between dinosaurs and birds. Only one other egg, discovered in Argentina and corresponding to a primitive bird from the same period, has similar characteristics.
Other dino to bird stories:
Feathered dinosaur found that is not closely related to birds
Jury’s still out on dino to bird transition?
Therapod dinosaurs’ thermal blankets: highly detailed feathers found in Alberta