Avimaia

Between the pubic bones of the fossil, a flattened egg is present, running from the last back vertebra to the front of the pygostyle, the fused outer tail vertebrae. It covers a surface of 5.2 cm². It is the first time a fossil bird egg has been discovered within the abdomen of the female. The egg is not merely an outline of organic remains: the egg shells themselves have been preserved, which is again unique. The combined shells have a thickness of 0.2 to 0.4 millimetres.[1]

A section showed the presence of four to six separate shells stacked on top of each other, each about 0.05 millimetres thick. Each shell consists of three layers, as with modern birds and generally in derived dinosaur eggs. There is an inner mammillary layer, an intermediate prismatic layer and an outer crystalline layer. The three layers combined are rather thin: earlier reported shells of Enantiornithes are thicker. Another anomaly consists in the fact that the prismatic layer and outer layer combined are thinner than the mammillary layer, whereas normally the opposite is true. On the surface of the egg a brown organic layer is visible, which was identified as the cuticle. In the cuticle nanospheres of calcium phosphate were discovered, which are also present in the cuticle of some extant birds. Additionally, remains were identified of the egg membrane, the membrana testacea. The fibres of the membrane are individually visible and have a length of 1.5 to 4 micrometres. They possibly contain part of the original proteins. It is likely that during fossilisation, the egg shells shifted in relation to each other and were otherwise distorted.[1]

The presence of four to six shells on top of each other is difficult to explain. The authors considered the possibility that the shells represented two eggs, but this hypothesis was rejected. In case of two eggs, the two top shells should have mirrored each other, as they then should have been the opposite shells of a top egg. In reality, they mirror the lower shells. Furthermore, it is generally assumed that the Eumaniraptora only have one functional oviduct. It was concluded that the stacking was caused by a pathology, egg binding, which is not uncommon among some modern bird species, such as the chicken, and can be caused by stress. Retention of the egg in the oviduct would have led to the deposition of multiple shells on a single egg. Such a condition is potentially fatal and it was suggested that it led to the demise of the holotype individual.[1]

In the thighbone, section showed that an inner layer of medullary bone was present around the medullary cavity. Such a medullary bone is used by modern bird females to extract chalk from during the laying of eggs. Its presence thus confirms that the holotype is a female and the structure in its abdomen is indeed an egg.[1]

What little of the plumage has been preserved, seems to indicate that long tail feathers were absent. It has been suggested that in the Enantiornithes such long display feathers were limited to the males only, as an instance of sexual dimorphism. Their absence thus is a further corroboration of the hypothesis that the type specimen is female.[1]