''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 ...

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 ...

. 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 (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

, 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). Avian clutch size, Clutch size may vary latitudinally within a species. Some birds lay eggs even when the eggs have not been ...

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 is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

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 an 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 ...

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 c ...

s and 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 ...

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, an ...

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 ( ''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 fossils in Jap ...

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 125.77 Ma (million years ago) and 121.4 ± 1.0 Ma (Historically, this stage was placed at 129.4 million to approximately 125 million years ago) It is a ...

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 radiometr ...

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 Paraphyly, paraphyletic group of freshwater green algae, called charophytes (), sometimes treated as a Division (taxonomy), division, yet also as a Division (taxonomy), superdivision. The terrestrial plants, the Embryophyta em ...

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 (geology), epoch of the Jurassic Period, and it spans the geologic time scale, geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic stratum, strata.Owen ...

, fossil eggs from before the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent 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 ''cre ...

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") is a genus of 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 ...

'', '' Fukuisaurus'', ''

Fukuititan ''Fukuititan'' (meaning "giant from Fukui prefecture") is a genus of Sauropoda, sauropod dinosaur that lived during the Early Cretaceous (either Barremian or Aptian age) in the Kitadani Formation in what is now Japan. The genus contains a single s ...

'', ''

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 J ...

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

amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

s, dinosaurs, birds, and

pterosaurs Pterosaurs are an extinct clade of flying reptiles in the Order (biology), order Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 million to 66 million years ago). Pterosau ...

are known from the Kitadani Formation.

References

{{Taxonbar, from=Q24661062 Egg fossils Fossil parataxa described in 2015 Extinct animals of Japan Fossils of Japan