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. Parentage. Though no embryo was found in the egg, "Pachycorioolithus" can be referred to Theropoda by the characteristics of its eggshell. However, it is uncertain whether the parent was a bird or not. Most non-avian dinosaurs have a two- or one-layered eggshell, whereas birds typically have a three-layered eggshell (like "Pachycorioolithus"). However, most Mesozoic bird eggs have only two layers and some non-avian dinosaur eggs have three layers, so this trait alone cannot be used to identify bird eggs. Lawver "et al." (2016) believed it likely that "P. jinyunensis" pertains to  a bird because of its small size and extremely thin shell, but they could not exclude the possibility that it was laid by a small, non-avian, theropod. Distribution. The only known "P. jinyunensis" specimen was discovered in a quarry near Shuhong, a town in Jinyun County in Zhejiang. This site is part of the Liangtoutang Formation, dating to the Albian. Discovery and Naming. "Pachycorioolithus" was first described in 2016 by a team of paleontologists consisting of Daniel R. Lawver and Frankie D. Jackson of the Montana State University, Jin Xingsheng of the Zhejiang Museum of Natural History, and Wang  Qiongying of the Jinyun Museum. It was based on a single egg specimen housed in the Jinyun Museum. They gave it the name "Pachycorioolithus jinyunensis", from the Greek "pachys" (thick) and "corys" (helmet), a reference to its thick outer layer, combined with "oolithus", the standard suffix for oogenus names, meaning "stone egg". The specific epithet means "from Jinyun", the county where it was found.