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| Ramoprismatoolithus okurai Uematsu, Tanaka, Kozu, Isaji & Shimojima, 2022 |
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.
KEYWORDS: Eggshells, Testudoolithidae, Prismatoolithidae, Early Cretaceous, Okurodani Formation, Japan
Oofamily Prismatoolithidae, Hirsch 1994a emend. Moreno-Azanza et al. 2014
Ramoprismatoolithus oogen. nov.
Etymology. ‘Ramo’ from Latin rāmus meaning branch, referring to the reticulate ridges on the eggshell outer surface.
Ramoprismatoolithus okurai oosp. nov.
Diagnosis. Combination of the following characters distinguishes Ramoprismatoolithus from all other ootaxa: low reticulate ridges on the outer surface; eggshell thickness 0.3–0.6 mm including ornamentation; eggshell consists of two distinct layers: the inner mammillary layer and the outer prismatic layer delimited by a gradual boundary; mammillae composed of acicular crystals; the mammillary layer to prismatic layer thickness ratio of 1:4–1:5; columnar extinction patterns under PLM.
Holotype. GPM-Fo-1925, eggshell fragments up to 3 × 4 mm in size mounted on a SEM stub.
Referred specimens. Five isolated eggshell fragments and four outer surface impressions that are the external moulds of the eggshell fragments (<12 × 17 mm; GPM-Fo-1924, 1926, 1927 and 1928).
Locality and horizon. All referred specimens were discovered in black mudstone layers from the upper part of the Hauterivian to Barremian Okurodani Formation at the Kobudani Valley in the Shokawa area, Takayama City, Gifu Prefecture, Japan.
Etymology. The specific name honours the late Masatoshi Okura who pioneered the discovery of fossil eggshells and other vertebrate remains in Shokawa.
Conclusion:
The current study describes fossil eggshells collected from the Hauterivian to Barremian Okurodani Formation in Shokawa, northwestern Gifu Prefecture, which represents so far the oldest fossil eggshells in Japan. The Shokawa eggshells are ascribed to a turtle and non-avian theropod species, suggesting the faunal composition that is yet to be recognised in skeletal remains from the deposits. Notably, a new oospecies Ramoprismatoolithus with peculiar ornamentation sheds light on the presence of unreported maniraptorans during the early Early Cretaceous. This finding reveals the distribution of small-bodied non-avian theropods in the eastern margin of Asia at that time.
Rina Uematsu, Kohei Tanaka, Shohei Kozu, Shinji Isaji and Shizuo Shimojima. 2022. Fossil Eggshells from the Early Cretaceous Okurodani Formation, northern central Japan. Historical Biology: An International Journal of Paleobiology. DOI: 10.1080/08912963.2022.2142910
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