A holotype of <em>Avimaia schweitzerae</em>.
A holotype of Avimaia schweitzerae. Barbara Marrs

Around 110 million years ago, a sparrow-sized bird died in northwestern China with an entire, imperfectly formed egg in its abdomen. After paleontologists dug up the bird’s preserved remains in the mid-2000s, this unidentified fossil gathered dust for over a decade in storage at China’s Institute of Vertebrate Paleontology and Paleoanthropology. So in 2018, when the researcher Alida Bailleul was combing through specimens in IVPP’s archives in search of soft tissue samples, she didn’t expect to find much, according to a report from National Geographic. After noticing a squashed and leathery lump in the fossil’s abdomen, Bailluel analyzed the membrane-like mass only to discover the lump was an unlaid egg, preserved within the bones of a prehistoric bird.

This remarkable specimen marks the first time scientists have found an egg inside a fossilized bird. It is therefore one of the most notable bird fossils discovered from the Cretaceous Era—especially considering that adult birds only retain fully formed eggs for around 24 hours. Bailleul and her team published their findings on this newly described species, Avimaia schweitzerae, this week in Nature Communications. The team named the specimen in tribute to the paleontologist Mary Higby Schweitzer, who helped establish the field of molecular paleontology and once used soft tissues to identify a Tyrannosaurus rex fossil as a pregnant female.

This egg doesn’t just hold Avimaia’s claim to fame. It might also explain how she died. Most of the shell contained an abnormal two layers (normal bird eggs just have one), with a second layer that lacks a necessary eggshell cuticle. This evidence indicates that the bird held the egg inside its body for far too long. Due to this aberration, the researchers believe the bird suffered from a common condition called “egg-binding,” in which an egg becomes stuck inside an animal. This trauma can lead the bird to add additional shells around the egg, which can suffocate the embryo and even kill the mother. If the researchers’ egg-binding theory is correct, Avimaia would represent the oldest documented case of this reproductive disorder.

It’s not easy to sex bird or dinosaur fossils, making the egg a gift that keeps on giving. When preparing to lay eggs, modern birds fill the empty spaces of their skeleton with a calcium reservoir called medullary bone. Finding this substance in a fossil would ordinarily confirm the specimen was female. But despite its name, medullary bone is a tissue, not a bone, and therefore often doesn’t survive a 110-million-year burial. While Bailleul found layers that resembled medullary bone in the specimen, the physical presence of an egg offers incontrovertible proof that the bird was female, allowing scientists to model sex-specific hypotheses on Avimaia. For example, the fossil lacks elongated tail feathers typical of males of other species of the Avimaia’s bird group enantiornithines, suggesting the species exhibits possible sexual dimorphism. It’s as the saying goes—everything’s better with an egg on it.