When a sparrow-sized bird died about 110 million years ago, she had an egg inside her body. Over time, pressure crushed and flattened that egg. Now scientists report it’s the first unlaid bird egg found inside a fossil.
Scientists unearthed the fossil 11 years ago in northwestern China. Last year, paleontologists led by Alida Bailleul took a closer look. Bailleul works in China at the Key Laboratory of Vertebrate Evolution and Human Origins in Beijing.
At once the researchers noticed something odd: The bird had a strange sheet of tissue between her pubic bones. Bailleul examined a piece of the tissue under a microscope. She found that it had come from an egg. The researchers shared their discovery March 20 in Nature Communications.
The egg was a first. The bird was too — a new species. The researchers named it Avimaia schweitzerae in honor of Mary Schweitzer. She’s a paleontologist who works on fossilized soft tissues.
Further analyses turned up more surprises. The mother bird’s skeleton holds traces of medullary bone. This is a calcium-rich tissue that helps to form eggshells. Modern birds make this tissue while they’re producing eggs. The new finding is the strongest evidence yet that ancient birds did the same thing.
The scientists also found tiny mineral spheres in the shell’s outermost layer, or cuticle. There are similar spheres in the egg cuticles of modern water birds, such as quails and ducks. The spheres may protect embryos from microbial infections. Until now, no one had ever seen them in a fossilized egg.
This bird and her embryo had some problems, though. The eggshell has two layers instead of the usual one. That suggests that the egg had stayed in the bird’s body for too long. And the egg’s layers are extremely thin — thinner than a sheet of paper.
In today’s birds, these symptoms can point to a deadly condition called egg-binding. That’s especially true in small birds that are under extreme stress. In egg-binding, a bird is unable to lay the egg. The researchers think this ancient, unlaid egg may have been what killed its mother.
birds Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.
calcium A chemical element which is common in minerals of the Earth’s crust and in sea salt. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.
cuticle Term for a tough but bendable protective outer shell or cover of some organism, or of parts of an organism.
egg The unfertilized reproductive cell made by females.
embryo The early stages of a developing organism, or animal with a backbone, consisting only one or a few cells. As an adjective, the term would be embryonic — and could be used to refer to the early stages or life of a system or technology.
evolution (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed.
fossil Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.
infection A disease that can spread from one organism to another. It’s usually caused by some type of germ.
medullary bone A type of porous, spongy tissue that develops within the core of bones in birds that are preparing to lay eggs (and therefore female). The same tissue recently showed up in the fossil remains of a Tyrannosaurus rex.
microscope An instrument used to view objects, like bacteria, or the single cells of plants or animals, that are too small to be visible to the unaided eye.
mineral Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.
paleontologist A scientist who specializes in studying fossils, the remains of ancient organisms.
pressure Force applied uniformly over a surface, measured as force per unit of area.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
stress (in biology) A factor — such as unusual temperatures, movements, moisture or pollution — that affects the health of a species or ecosystem. (in psychology) A mental, physical, emotional or behavioral reaction to an event or circumstance (stressor) that disturbs a person or animal’s usual state of being or places increased demands on a person or animal; psychological stress can be either positive or negative.
symptom A physical or mental indicator generally regarded to be characteristic of a disease. Sometimes a single symptom — especially a general one, such as fever or pain — can be a sign of any of many different types of injury or disease.
tissue Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.
vertebrate The group of animals with a brain, two eyes, and a stiff nerve cord or backbone running down the back. This group includes amphibians, reptiles, birds, mammals and most fish.
Journal: A.M. Bailleul et al. An Early Cretaceous enantiornithine (Aves) preserving an unlaid egg and probable medullary bone. Nature Communications. Published online March 20, 2019. doi: 10.1038/s41467-019-09259-x.