The oldest known avian eggshell, Plagioolithus fukuiensis, from the Lower Cretaceous (upper Barremian) Kitadani Formation, Fukui, Japan

Abstract

Fossil record of Early Cretaceous birds may be geographically biased, and eggs and eggshells predating the Late Cretaceous were unknown. Here, we report the oldest known bird eggshell (FPDM-V-0009175) collected from the upper Barremian Kitadani Formation in Katsuyama City, Fukui, Japan. The Kitadani Formation likely represents fluvial environments. Thin-section and scanning electron microscope analyses revealed diagnostic characters of FPDM-V-0009175, including thin (0.44 mm) shell, smooth external surface, non-branching and narrow pore canals with relatively constant width, three structural layers, oblique crystal orientation from vertical in the external layer, and mammillary to continuous to external layer thickness ratio of 1:1:0.44. These characters allow assignment of FPDM-V-0009175 to a new oogenus and oospecies, Plagioolithus fukuiensis, and suggest it belonging to a bird. The three-layered eggshell structure is seen in extant and extinct birds, Plagioolithus fukuiensis, and non-avian theropods. Therefore, such structure may be plesiomorphic among theropods, appearing in the late Barremian or earlier. As the first bird body fossil from the Mesozoic of Japan, Plagioohlithus fukuiensis suggests extensive distribution and abundance of birds in the Barremian East Asia. Plagioolithus fukuiensis indicates that the late-Barremian birds inhabited and reproduced in the fluvial environments within a basin located along the eastern margin of the Asian continent.http://www.zoobank.org/urn:lsid:zoobank.org:pub:90A8CCE8-57F7-473C-855C-B2CCE2BBD1A2

Keywords::

• fossil egg
• theropod
• dinosaur palaeontology
• Early Cretaceous

Acknowledgements

The authors thank Z. Sano and K. Hirasawa for kindly providing the specimen to Fukui Prefectural Dinosaur Museum. A. Matsushita did a preliminary work on the specimen, which founded this study. The authors acknowledge Fukui Prefectural Dinosaur Museum for preparing the specimen, permitting the authors to study their specimen and granting the authors access to their petrographic microscopes and scanning electron microscope. T. Kubo, K. Miyata, S. Sano, and M. Shibata offered technical assistance for the use of equipment in the museum. F. Jackson greatly contributed to development of the study and organisation of the manuscript. D. Barta, D. Lawver, S. Oser, J. Schmitt, J. Simon and D. Varricchio shared discussion about fossil eggs with authors, which furthered this study.