Pachycorioolithus
Pachycorioolithus is an oogenus of small, thin-shelled fossil egg from the early Cretaceous in China. It probably belongs to a bird, though there is a possibility the parent was a non-avian theropod. It was named in 2016, based on a single specimen found in Zhejiang. Description Pachycorioolithus is represented in the fossil record by a single, 80% complete fossil egg. It is elongated, measuring long by across, and smooth-surfaced. Thin pores (20 to 30 µm in diameter) cut through the eggshell. The eggshell is very thin, measuring only 166 µm thick, and is composed of three structural layers: the mammillary, continuous, and external layers. Pachycorioolithus is unique for having an external layer thicker than its continuous layer, with an external to continuous layer ratio of 1.6:1. This sets it apart from nearly all other known eggs. An unnamed Brazilian fossil egg also has an external layer thicker than continuous layer, but only barely thicker. Also, a few modern bird eggs have a similar, extremely thick external layer. These examples appear to result from convergent evolution, as they represent isolated cases in different clades. Based on its unique characteristics, Lawver et al. (2016) assigned Pachycorioolithus to its own monotypic oofamily, Pachycorioolithidae.