''Pachycorioolithus'' is an

oogenus 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 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 ...

from the early

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

. It probably belongs to a

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

, though there is a possibility the parent was a non-avian

theropod Theropoda (; from ancient Greek , (''therion'') "wild beast"; , (''pous, podos'') "foot"">wiktionary:ποδός"> (''pous, podos'') "foot" is one of the three major groups (clades) of dinosaurs, alongside Ornithischia and Sauropodom ...

. It was named in 2016, based on a single specimen found in

Zhejiang ) , translit_lang1_type2 = , translit_lang1_info2 = ( Hangzhounese) ( Ningbonese) (Wenzhounese) , image_skyline = 玉甑峰全貌 - panoramio.jpg , image_caption = View of the Yandang Mountains , image_map = Zhejiang i ...

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 (Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a unit of length in the International System ...

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, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

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 comm ...

, 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 "unisp ...

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 Lishui (; Lishuinese: ''li⁶ syu³'' ) is a prefecture-level city in the southwest of Zhejiang province, People's Republic o ...

. This site is part of the Liangtoutang Formation, dating to the

Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...

Discovery and Naming

''Pachycorioolithus'' was first described in

2016 2016 was designated as: * International Year of Pulses by the sixty-eighth session of the United Nations General Assembly. * International Year of Global Understanding (IYGU) by the International Council for Science (ICSU), the Internationa ...

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 land-grant research university in Bozeman, Montana, United States. It enrolls more students than any other college or university in the state. MSU offers baccalaureate degrees in 60 fields, master's d ...

, Jin Xingsheng of the

Zhejiang Museum of Natural History The Zhejiang Museum of Natural History () is the provincial natural history museum of the Zhejiang province in China. The Hangzhou branch is located in the West Lake Cultural Square in Hangzhou, which also houses the Wulin building of Zhejiang ...

, 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 In Taxonomy (biology), taxonomy, binomial nomenclature ("two-term naming system"), also called binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin gramm ...

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

References

{{Taxonbar, from=Q24909522 Fossil parataxa described in 2016 Dinosaur reproduction Egg fossils