''Plagioolithus'' 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 t ...

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 ...

. It is from the Early Cretaceous of Japan. It was probably laid by 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 ...

, making it the oldest known fossil

bird egg Bird eggs are laid by the females and range in quantity from one (as in condors) to up to seventeen (the grey partridge). Clutch size may vary latitudinally within a species. Some birds lay eggs even when the eggs have not been fertilized; it is ...

Description

''Plagioolithus'' is most notable for having a three-layered eggshell. This trait is widespread in modern birds, but is very rare among

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...

fossil eggs. No complete eggs have been found, so the size and shape of ''Plagioolithus'' eggs are unknown. Its eggshell is unornamented and very thin, measuring only 0.44 mm thick. The shell consists of a 0.18 mm mammillary layer (the innermost layer of the shell), a 0.18 mm continuous layer, and a 0.08 mm external layer. The pores are narrow, straight, and constant width throughout the shell.

Parentage

Because no embryos are known, the parent of ''Plagioolithus'' cannot be identified with certainty. It was formerly believed that triple-layered eggshells were unique to

neognath Neognathae (; ) is a infraclass of birds, called neognaths, within the class Aves of the clade Archosauria. Neognathae includes the majority of living birds; the exceptions being the tinamous and the flightless ratites, which belong instead to t ...

birds, but some three-layered eggs have been found containing

enantiornithine The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teeth and cla ...

s and non-avian

theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...

s, indicating that the presence of a third layer is

plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, and ...

among dinosaurs. Therefore, this trait alone cannot be used to assign ''P. fukuiensis'' to birds. However, the combination of the third layer, the thin shell, and the smooth external surface suggests that ''Plagioolithus'' belongs to a bird (though to what type of bird remains unknown).

Distribution

''Plagioolithus'' is only known from a single formation, the

Kitadani Formation The Kitadani Formation ( ja, 北谷層 ''Kitadani-sō'') is a unit of Lower Cretaceous sedimentary rock which crops out near the city of Katsuyama in Fukui Prefecture, Japan, and it is the primary source of Cretaceous-aged non-marine vertebrate f ...

in Fukui, Japan. The date of this formation has generated controversy, but multiple lines of evidence support a

Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 121.4 ± 1.0 Ma). It is a subdivision of the Early Cretaceous Epoch (or Lower Cretaceous Series). It is preceded ...

age: zircon

fission track dating Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium-bearing minerals and glasses. Fission-track dating is a relatively simple method of radiomet ...

places a correlated formation (the Bessandani Formation) at 127 million years old. Also, the presence of '' Nippononaia ryosekiana'' and the types of

charophytes Charophyta () is a group of freshwater green algae, called charophytes (), sometimes treated as a phylum, division, yet also as a superdivision or an unranked clade. The terrestrial plants, the Embryophyta emerged within Charophyta, possibly fro ...

suggest a Barremian age.

Paleobiology

Even though birds are known to be as old as the

Late Jurassic The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 163.5 ± 1.0 to 145.0 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the name ...

, fossil eggs from before the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...

are very rare. Indeed, ''Plagioolithus'' is the oldest known trace of bird eggs in the fossil record. At the Kitadani Formation, numerous other dinosaurs are known to have coexisted with ''Plagioolithus'', including ''

Fukuiraptor ''Fukuiraptor'' ("thief of Fukui") was a medium-sized megaraptoran theropod dinosaur of the Early Cretaceous epoch (either Barremian or Aptian) that lived in what is now Japan. ''Fukuiraptor'' is known from the Kitadani Formation and possib ...

'', ''

Fukuisaurus ''Fukuisaurus'' (meaning "Fukui lizard") is a genus of herbivorous ornithopod dinosaur that lived during the Early Cretaceous in what is now Japan. The type species is ''F. tetoriensis'', which was named and described in 2003. Discovery and nam ...

'', ''

Fukuititan ''Fukuititan'' (meaning "Fukui giant") is a genus of titanosauriform sauropod dinosaur that lived in the Early Cretaceous (either Barremian or Aptian age) in what is now Japan. It is known from FPDM-V8468, the associated partial skeleton of a ...

'', ''

Koshisaurus ''Koshisaurus'' is a monospecific genus of basal hadrosauroid from the Kitadani Formation in Japan. The discovery of the genus suggests that hadrosauroids had higher diversity along the eastern margin of Asia in the Early Cretaceous. "Koshi" mean ...

'', and an unidentified type of non-avian theropod. Also present were

goniopholidid Goniopholididae is an extinct family of moderate-sized semi-aquatic neosuchian crocodyliformes. Their bodyplan and morphology are convergent on living crocodilians. They lived across Laurasia (Asia, Europe and North America) between the Middle Ju ...

crocodiles, eucryptodire turtles, and amiiform fish. Footprints of various

amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

s, dinosaurs, birds, and

pterosaurs Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...

are known from the Kitadani Formation.

References

{{Eggs, state=collapsed Egg fossils Fossil parataxa described in 2015 Extinct animals of Japan Fossils of Japan