''Pachycorioolithus'' is an oogenus of small, thin-shelled

fossil egg Egg fossils are the fossilized remains of eggs laid by ancient animals. As evidence of the physiological processes of an animal, egg fossils are considered a type of trace fossil. Under rare circumstances a fossil egg may preserve the remains of t ...

from the early

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...

. It probably belongs to a

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

, 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 Zhejiang ( or , ; , also romanized as Chekiang) is an eastern, coastal province of the People's Republic of China. Its capital and largest city is Hangzhou, and other notable cities include Ningbo and Wenzhou. Zhejiang is bordered by Jiang ...

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 The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...

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

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

ian 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 Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

, as they represent isolated cases in different clades. Based on its unique characteristics, Lawver ''et al.'' (2016) assigned ''Pachycorioolithus'' to its own

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

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

or not. Most non-avian dinosaurs have a two- or one-layered eggshell, whereas birds typically have a three-layered eggshell (like ''Pachycorioolithus'').Laura E. Wilson, Karen Chin, Frankie D. Jackson, and Emily S. Bray
II. Eggshell morphology and structure
''UCMP Online Exhibits: Fossil Eggshell'' 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 Jinyun County () is a county of south-central Zhejiang province, China. It is under the administration of the Lishui, Zhejiang, Lishui City. Administrative divisions Towns: *Wuyun, Zhejiang, Wuyun (五云镇), Huzhen, Jinyun County, Huzhen (壶� ...

. This site is part of the

Liangtoutang Formation The Liangtoutang Formation, also referred to as the Laijia Formation is a geological formation located in Zhejiang, China. Its strata date back to the Albian to Cenomanian stages of the Cretaceous period.Albian.

Discovery and Naming

''Pachycorioolithus'' was first described in

2016 File:2016 Events Collage.png, From top left, clockwise: Bombed-out buildings in Ankara following the 2016 Turkish coup d'état attempt; the impeachment trial of Brazilian President Dilma Rousseff; Damaged houses during the 2016 Nagorno-Karabakh ...

by a team of paleontologists consisting of Daniel R. Lawver and Frankie D. Jackson of the

Montana State University Montana State University (MSU) is a Public university, public land-grant university, land-grant research university in Bozeman, Montana. It is the state's largest university. MSU offers baccalaureate degrees in 60 fields, master's degrees in 6 ...

Jin Xingsheng Jin is a toneless pinyin romanization of various Chinese names and words. These have also been romanized as Kin and Chin (Wade–Giles). "Jin" also occurs in Japanese and Korean. It may refer to: States Jìn 晉 * Jin (Chinese state) (晉國), ...

of the Zhejiang Museum of Natural History, and

Wang Qiongying Wang may refer to: Names * Wang (surname) (王), a common Chinese surname * Wāng (汪), a less common Chinese surname * Titles in Chinese nobility * A title in Korean nobility * A title in Mongolian nobility Places * Wang River in Thailand ...

of the

Jinyun Museum Jinyun County () is a county of south-central Zhejiang province, China. It is under the administration of the Lishui City. Administrative divisions Towns: * Wuyun (五云镇), Huzhen (壶镇镇), Xinjian (新建镇), Shuhong (舒洪镇), Dayu ...

. 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 In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

means "from Jinyun", the county where it was found.

References

{{Taxonbar, from=Q24909522 Fossil parataxa described in 2016 Dinosaur reproduction Cretaceous birds of Asia