| Plagioolithus Temporal range: | |
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Egg fossil classification  | |
| Oofamily: | incertae sedis |
| Oogenus: | † Plagioolithus Imai and Azuma, 2015 |
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Plagioolithus is an oogenus of fossil egg. It is from the Early Cretaceous of Japan. It was probably laid by a bird, making it the oldest known fossil bird egg. [1]
Plagioolithus is most notable for having a three-layered eggshell. This trait is widespread in modern birds, but is very rare among Mesozoic 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. [1]
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 birds, but some three-layered eggs have been found containing enantiornithines and non-avian theropods, indicating that the presence of a third layer is plesiomorphic among dinosaurs. [1] [2] 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). [1]
Plagioolithus is only known from a single formation, the Kitadani Formation in Fukui, Japan. The date of this formation has generated controversy, [3] but multiple lines of evidence support a Barremian age: zircon fission track dating places a correlated formation (the Bessandani Formation) at 127 million years old. [1] Also, the presence of Nippononaia ryosekiana and the types of charophytes suggest a Barremian age. [1]
Even though birds are known to be as old as the Late Jurassic, fossil eggs from before the Late Cretaceous are very rare. Indeed, Plagioolithus is the oldest known trace of bird eggs in the fossil record. [1]
At the Kitadani Formation, numerous other dinosaurs are known to have coexisted with Plagioolithus, including Fukuiraptor , Fukuisaurus , Fukuititan , Koshisaurus , and an unidentified type of non-avian theropod. [4] Also present were goniopholidid crocodiles, eucryptodire turtles, and amiiform fish. Footprints of various amphibians, dinosaurs, birds, and pterosaurs are known from the Kitadani Formation. [1]
Fukuiraptor was a medium-sized megaraptoran theropod dinosaur of the Early Cretaceous epoch that lived in what is now Japan. Fukuiraptor is known from the Kitadani Formation and possibly also the Sebayashi Formation.
The Fukui Prefectural Dinosaur Museum, located in Katsuyama, Fukui, Japan, is one of the leading dinosaur museums in Asia that is renowned for its exhibits of fossil specimens of dinosaurs and paleontological research. It is sited in the Nagaoyama Park near the Kitadani Dinosaur Quarry that the Lower Cretaceous Kitadani Formation of the Tetori Group is cropped out and a large number of dinosaur remains including Fukuiraptor kitadaniensis and Fukuisaurus tetoriensis are found and excavated.
Dinosaur eggs are the organic vessels in which a dinosaur embryo develops. When the first scientifically documented remains of non-avian dinosaurs were being described in England during the 1820s, it was presumed that dinosaurs had laid eggs because they were reptiles. In 1859, the first scientifically documented dinosaur egg fossils were discovered in France by Jean-Jacques Poech, although they were mistaken for giant bird eggs.
The Kitadani Formation 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 Japan. Dinosaur remains are among the fossils that have been recovered from the formation, but it also preserves a diverse assemblage of plants, invertebrates, and other vertebrates. Most, if not all, of the fossil specimens collected from the Kitadani Formation are reposited at the Fukui Prefectural Dinosaur Museum.
Fukuititan is a genus of titanosauriform sauropod dinosaur that lived in the Early Cretaceous in what is now Japan. It is known from FPDM-V8468, the associated partial skeleton of a single individual, recovered from the Kitadani Dinosaur Quarry of the Tetori Group, at Katsuyama City. The type species, Fukuititan nipponensis, was described in 2010 by Japanese scientists Yoichi Azuma and Masateru Shibata of the Fukui Prefectural Dinosaur Museum. The discovery sheds light on Japanese titanosauriforms, which are poorly known in the region.
Continuoolithus is an oogenus of dinosaur egg found in the late Cretaceous of North America. It is most commonly known from the late Campanian of Alberta and Montana, but specimens have also been found dating to the older Santonian and the younger Maastrichtian. It was laid by an unknown type of theropod. These small eggs are similar to the eggs of oviraptorid dinosaurs, but have a distinctive type of ornamentation.
Dispersituberoolithus is an oogenus of fossil egg, which may have been laid by a bird or non-avian theropod.
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 the once-developing embryo inside, in which case it also contains body fossils. A wide variety of different animal groups laid eggs that are now preserved in the fossil record beginning in the Paleozoic. Examples include invertebrates like ammonoids as well as vertebrates like fishes, possible amphibians, and reptiles. The latter group includes the many dinosaur eggs that have been recovered from Mesozoic strata. Since the organism responsible for laying any given egg fossil is frequently unknown, scientists classify eggs using a parallel system of taxonomy separate from but modeled after the Linnaean system. This "parataxonomy" is called veterovata.
Sankofa is an oogenus of prismatoolithid egg. They are fairly small, smooth-shelled, and asymmetrical. Sankofa may represent the fossilized eggs of a transitional species between non-avian theropods and birds.
Reptile egg fossils are the fossilized remains of eggs laid by reptiles. The fossil record of reptile eggs goes back at least as far as the Early Permian. However, since the earliest reptile eggs probably had soft shells with little preservation potential, reptilian eggs may go back significantly farther than their fossil record. Many ancient reptile groups are known from egg fossils including crocodilians, dinosaurs, and turtles. Some ancient reptiles are known to have given live birth and are therefore not anticipated to have left behind egg fossils.
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" means an old Japanese regional name including Fukui prefecture where fossils of the genus were discovered.
Styloolithus is an oogenus of highly distinctive fossil egg from the Upper Cretaceous Djadokhta Formation and the Barun Goyot Formation in Mongolia.
Dictyoolithidae is an oofamily of dinosaur eggs which have a distinctive reticulate organization of their eggshell units. They are so far known only from Cretaceous formations in China.
Fukuivenator is an extinct genus of therizinosaurian theropod dinosaur from the Early Cretaceous of Japan.
Nipponoolithus is an oogenus of fossil egg native to Japan. It is one of the smallest known dinosaur eggs, and was probably laid by some kind of non-avian maniraptor.
Trigonoolithus is an oogenus of dinosaur egg, representing a basal prismatoolithid. Its eggshell, like avian eggs, is composed of three structural layers, but cladistic analysis suggests that its parent was a non-avian theropod.
Pachycorioolithus is an oogenus of small, thin-shelled fossil egg from the early Cretaceous in China. It probably belongs to a bird, 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.
Symmetrolestes is an extinct genus of small spalacotheriid mammal from the Early Cretaceous period of Japan. The genus contains one species known as S. parvus, the type fossil is from fluvial deposits located in the Dinosaur Quarry in the Kitadani Formation, near the city of Katsuyama which lies alongside valley of the Sugiyamagawa River. It was described by Tsubamoto and Rougier in 2004 keeping the holotype at the National Science Museum, Tokyo, Japan.
Fukuipteryx is an extinct genus of basal avialan dinosaurs found in Early Cretaceous deposits from Japan's Kitadani Formation. It contains one species, Fukuipteryx prima.
Tyrannomimus is an extinct genus of ornithomimosaurian theropod dinosaurs from the Kitadani Formation of Japan. The type species is Tyrannomimus fukuiensis.
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