Pachycorioolithusis anoogenusof small, thin-shelledfossil eggfrom the earlyCretaceousinChina.It probably belongs to abird,though there is a possibility the parent was a non-aviantheropod.It was named in 2016, based on a single specimen found inZhejiang.[1]
| Pachycorioolithus Temporal range:
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Egg fossil classification
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| Oofamily: | †Pachycorioolithidae Lawveret al.,2016 |
| Oogenus: | †Pachycorioolithus Lawveret al.,2016 |
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Description
editPachycorioolithusis 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 166μmthick, and is composed of three structural layers: the mammillary, continuous, and external layers.Pachycorioolithusis 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 unnamedBrazilianfossil 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 fromconvergent evolution,as they represent isolated cases in different clades.[1]
Based on its unique characteristics, Lawveret al.(2016) assignedPachycorioolithusto its ownmonotypicoofamily, Pachycorioolithidae.[1]
Parentage
editThough no embryo was found in the egg,Pachycorioolithuscan be referred to Theropoda by the characteristics of its eggshell. However, it is uncertain whether the parent was abirdor not.[1]Most non-avian dinosaurs have a two- or one-layered eggshell, whereas birds typically have a three-layered eggshell (likePachycorioolithus).[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]Lawveret al.(2016) believed it likely thatP. jinyunensispertains 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
editThe only knownP. jinyunensisspecimen was discovered in a quarry nearShuhong,a town inJinyun CountyinZhejiang.This site is part of theLiangtoutang Formation,dating to theAlbian.[1]
Discovery and Naming
editPachycorioolithuswas first described in2016by a team of paleontologists consisting ofDaniel R. LawverandFrankie D. Jacksonof theMontana State University,Jin Xingshengof theZhejiang Museum of Natural History,andWang Qiongyingof theJinyun Museum.It was based on a single egg specimen housed in the Jinyun Museum. They gave it the namePachycorioolithus 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]Thespecific epithetmeans "from Jinyun", the county where it was found.[1]
References
edit- ^abcdefghijLawver, Daniel R.; Jin, Xingsheng; Jackson, Frankie D.; Wang, Qiongying (2016)."An Avian Egg from the Lower Cretaceous (Albian) Liangtoutang Formation of Zhejiang Province, China".Journal of Vertebrate Paleontology.36(3): e1100631.doi:10.1080/02724634.2016.1100631.
- ^Laura E. Wilson, Karen Chin, Frankie D. Jackson, and Emily S. Bray.II. Eggshell morphology and structure.UCMP Online Exhibits: Fossil Eggshell
- ^Jackson, Frankie; Varricchio, David (2010)."Fossil eggs and eggshell from the lowermost Two Medicine Formation of western Montana, Sevenmile Hill locality".Journal of Vertebrate Paleontology.30(4): 1142–1156.doi:10.1111/j.1475-4983.2011.01114.x.