Fossil eggshells from the Early Cretaceous Okurodani Formation, northern central Japan

ABSTRACT

Seven isolated eggshell fragments and six eggshell impressions were collected from the Okurodani Formation (Hauterivian to Barremian) in Shokawa, Takayama City, Gifu Prefecture, Japan. To date, these specimens represent the oldest fossil eggshells in the country. Microscopic observations classified the eggshells into Testudoolithidae indet., Ramoprismatoolithus okurai oogen. et oosp. nov. and indeterminate type. For Testudoolithidae indet., the eggshell microstructure and estimated egg size suggest that they could be laid by any of the cryptodiran turtles reported by skeletal remains from the formation: Trionychoidea, Xinjiangchelyidae and Sinemydidae. Ramoprismatoolithus okurai bears prismatic microstructure and ramifying ridges on the outer surface, the combination of which is unusual for prismatoolithid eggshells. Based on morphological observations and phylogenetic analyses, Ramoprismatoolithus was ascribed to troodontid or closely related non-avian maniraptorans. The remaining specimens consist of eggshell surface impressions without original fragments, for which oospecies and taxonomic affinity are indeterminate. These findings demonstrate that eggshell fragments shed light on small-bodied taxa that are poorly represented by skeletal remains in the region. While the fossil record of small maniraptoran species from the early Early Cretaceous is relatively scarce worldwide, Ramoprismatoolithus adds to this record and extends the confirmed geographic range of this clade.http://www.zoobank.org/urn:lsid:zoobank.org:act:E023F0B5-59F3-47DD-B804-345A92750BEF

KEYWORDS:

• Eggshells
• Testudoolithidae
• Prismatoolithidae
• Early Cretaceous
• Okurodani Formation
• Japan

Acknowledgments

The testudoolithid eggshells and a thin section analysed in this study were collected and made by the late Masatoshi Okura. He was a keen fossil collector exploring many strata and had an inestimable contribution to paleontological research in Japan. This study would not have been possible without his genuine interest in fossils. We are also grateful to Mitsuhiro Nagata (Tono Geoscience Center, Japan Atomic Energy Agency) for his constructive suggestions on the dating of the Tetori Group in Shokawa, and Ai Matsushita for her technical input and the two thin sections of Ramoprismatoolithus made by her.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.