Pachycorioolithus

Pachycorioolithus Temporal range: Albian PreꞒ Ꞓ O S D C P T J K Pg N
Egg fossil classification
Oofamily:	†Pachycorioolithidae Lawver et al., 2016
Oogenus:	†Pachycorioolithus Lawver et al., 2016
Oospecies
†P. jinyunensis Lawver et al., 2016 (type)

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.^[1]

Description[]

Pachycorioolithus is represented in the fossil record by a single, 80% complete fossil egg. It is elongated, measuring 50 mm (2.0 in) long by 32 mm (1.3 in) across, and smooth-surfaced. Thin pores (20 to 30 µm in diameter) cut through the eggshell.^[1]

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.^[1]

Based on its unique characteristics, Lawver et al. (2016) assigned Pachycorioolithus to its own monotypic oofamily, Pachycorioolithidae.^[1]

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.^[1] Most non-avian dinosaurs have a two- or one-layered eggshell, whereas birds typically have a three-layered eggshell (like Pachycorioolithus).^[2] However, most Mesozoic bird eggs have only two layers^[1] and some non-avian dinosaur eggs have three layers, so this trait alone cannot be used to identify bird eggs.^[3] 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.^[1]

Distribution[]

The only known P. jinyunensis specimen was discovered in a quarry near , a town in Jinyun County in Zhejiang. This site is part of the Liangtoutang Formation, dating to the Albian.^[1]

Discovery and Naming[]

Pachycorioolithus was first described in 2016 by a team of paleontologists consisting of and of the Montana State University, of the Zhejiang Museum of Natural History, and of the . 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".^[1] The specific epithet means "from Jinyun", the county where it was found.^[1]

References[]