Elsevier

Cretaceous Research

Volume 114, October 2020, 104519
Cretaceous Research

Exceptionally small theropod eggs from the Lower Cretaceous Ohyamashimo Formation of Tamba, Hyogo Prefecture, Japan

https://doi.org/10.1016/j.cretres.2020.104519Get rights and content

Highlights

  • We describe eggs and eggshells of four ootaxa, all of which likely belonged to small theropods, from new horizons of a fossil egg locality in Hyogo Prefecture, southwestern Japan.

  • Himeoolithus murakamii oogen. et. oosp. nov. and Subtiliolithus hyogoensis oosp. nov. were erected as new ootaxa.

  • The discovery of additional eggshells from this locality indicates that various small theropods were nesting in the Hyogo area in the Early Cretaceous, even though these dinosaurs are poorly represented by skeletal remains.

  • This locality has amongst the highest diversity of eggshells from Lower Cretaceous fossil egg sites known worldwide.

Abstract

The Kamitaki Locality in the Albian Ohyamashimo Formation in the eastern Hyogo Prefecture of southwestern Japan is among the richest Lower Cretaceous fossil egg sites in the world. So far, eggshells of five different ootaxa, one ornithopod and four theropods, have been identified from the Kamitaki Bonebed Quarry. Although previous discoveries of egg remains were limited to isolated eggshell fragments, here we report on the discovery of a nearly complete egg, several partial eggs, as well as numerous eggshell fragments from horizons of the newly excavated Kamitaki Egg Quarry that are 5.5–6.75 m above the Kamitaki Bonebed Quarry. Taphonomical investigations indicate that the new site preserves the remnants of an in-situ nest of Himeoolithus murakamii oogen. et oosp. nov., mixed with scattered and isolated eggshell fragments of other non-avian theropod ootaxa, including Nipponoolithus ramosus, Prismatoolithus sp., and Subtiliolithus hyogoensis oosp. nov. The most abundant ootaxon at the quarry, Himeoolithus, is represented by four eggs and over 1300 scattered eggshell fragments. Himeoolithus is the smallest non-avian theropod egg known to date (9.9 g in estimated mass) and exhibits an unusually elongated shape (length:width ratio of 2.25) for such as small egg. The presence of six dinosaur ootaxa from the two quarries at the Kamitaki Locality reveals a hidden diversity of small dinosaurs, particularly non-avian theropods, in the Hyogo region and indicates the area was utilized for nesting by various small dinosaur species at the end of the Early Cretaceous.

Introduction

Early Cretaceous fossil sites that preserve a diverse assemblage of small theropod dinosaurs are rare in East Asia. One notable exception is the localities of the Jehol Biota in northeastern China that have produced a high diversity of well-preserved skeletons of small avian and non-avian theropods (Zhou et al., 2003, Zhou and Wang, 2010). Small theropod diversity at other Early Cretaceous localities in East Asia is poorly understood, likely due in part to the generally low preservation potential of small skeletons (Brown et al., 2013). When skeletal remains are scarce, fossil eggshells can be useful to understand faunal taxonomic composition and diversity (Tanaka et al., 2016, Zelenitsky et al., 2017a, Zelenitsky et al., 2017b). Fossil egg localities of the Shiwha Basin in South Korea (Lee, 2003) and of the Kamitaki locality in southwestern Japan (Tanaka et al., 2016) both hint at a diversity of small theropods, otherwise poorly known from skeletal remains in these areas during the Early Cretaceous.

The Early Cretaceous (Albian) Kamitaki Locality in the eastern Hyogo Prefecture of Japan has yielded numerous fossil eggshells and bones that indicate a large diversity of dinosaurs in the region (Saegusa and Ikeda, 2014, Tanaka et al., 2016). The diversity of small theropods was shown to be relatively high based on the presence of eggshell remains of at least four distinct ootaxa (i.e., Elongatoolithus, Nipponoolithus, and two species of prismatoolithid: Tanaka et al., 2016), for which skeletal remains are poor or lacking at the site (Saegusa and Ikeda, 2014). Although previous discoveries at the Kamitaki Locality were limited to isolated eggshell fragments, here we report on newly discovered partial to nearly complete eggs and eggshell fragments from new nesting horizons that provide evidence for additional small theropod species in the region.

Section snippets

Geologic setting

The Kamitaki Locality is located in the upper part of the Ohyamashimo Formation (formerly known as the ‘Lower Formation’) of the Sasayama Group (Fig. 1: Hayashi et al., 2017). The upper part of this formation is composed mainly of mudstones, sandstones, and conglomerates, sediments that were deposited in fluvial environments under a semi-arid to subhumid climate (Hayashi et al., 2017). Radiometric dating of tuff beds in the lower part of the Ohyamashimo Formation and in the overlying Sawada

Material and methods

Egg remains at the Kamitaki Egg Quarry were first discovered in 2015 and the quarry was intensively excavated in the Winter of 2019. An area of approximately 24 m2 was mapped (Fig. 2) and aerial photographs were taken with the use of a drone (DJI Mavic Air). Eggs, eggshell fragments, as well as small bones and tooth remains of anurans, lizards, and dinosaurs were collected throughout the mudstone layer. High concentrations of eggs/eggshells were found at two horizons located approximately 30 cm

Systematic paleontology

  • Oofamily. Incertae sedis.

  • Oogenus. Himeoolithus oogen. nov.

  • ZOOBANK ID: urn:lsid:zoobank.org:act:49AE6ABE-5E4D-4992-A5FC-5C13562A1213

  • Type oospecies. Himeoolithus murakamii oosp. nov., by present designation.

  • Diagnosis. As for the type and only oospecies.

  • Etymology. ‘hime’ meaning small, little, or pretty in Japanese, referring to the exceptionally small size of the egg, ‘oo’ meaning egg, and ‘lithos’ meaning stone in Greek. The generic name is masculine in gender.

  • Type locality and horizon. As for

Phylogenetic analyses

In order to determine the phylogenetic position of the ootaxa from the Kamitaki Egg Quarry (i.e., Himeoolithus murakamii, Nipponoolithus ramosus, and Subtiliolithus hyogoensis), three cladistic analyses using the oological data matrices of Zelenitsky, 2004, Zelenitsky and Therrien, 2008b, and Jin et al. (2010) were conducted with the software TNT v.1.5 (Goloboff and Catalano, 2016). These original data matrices were altered in subsequent studies by the addition/modification of ootaxa and/or

Discussion

At least six eggshell morphotypes or ootaxa are recognized from the Kamitaki Locality of the Albian Ohyamashimo Formation of southwestern Japan (Fig. 1), making it one of the most taxonomically diverse Early Cretaceous egg localities known (Fig. 8). The locality has produced both skeletal remains and eggshells from the Kamitaki Bonebed Quarry (Saegusa and Ikeda, 2014, Tanaka et al., 2016), as well as the eggs and eggshells described here from the nearby, but slightly stratigraphically higher,

Conclusions

New egg horizons (Kamitaki Egg Quarry), situated 5.50–6.75 m higher than the previously-known eggshell horizons (Kamitaki Bonebed Quarry), at the Lower Cretaceous Kamitaki Locality in the Hyogo Prefecture provide additional information regarding the diversity of theropod dinosaurs in the region. Four theropod ootaxa were identified from the new egg horizons: Himeoolithus murakamii oogen. et oosp. nov., Subtiliolithus hyogoensis oosp. nov., Nipponoolithus ramosus and Prismatoolithus sp.,

CRediT authorship contribution statement

Kohei Tanaka: Methodology, Data curation, Formal analysis, Writing - original draft, Writing - review & editing. Darla K. Zelenitsky: Writing - original draft, Writing - review & editing. François Therrien: Writing - original draft, Writing - review & editing. Tadahiro Ikeda: Data curation, Formal analysis, Writing - review & editing. Katsuhiro Kubota: Data curation, Formal analysis, Writing - review & editing. Haruo Saegusa: Data curation, Formal analysis, Writing - review & editing. Tomonori

Acknowledgements

We thank the Self-Government Council of Kamikuge, staffs and volunteers at the Museum of Nature and Human Activities, Hyogo, as well as Tamba City and Matsutaka Kogyo Co., Ltd., for their significant effort on the excavation and preparation for the specimens. We also thank Kumiko Handa, Kosuke Nakamura, Hidehiko Nomura, Natsuko Takagi, Chisako Sakata, Makoto Manabe, Mariela Fernández, and Miguel Moreno-Azanza for their support. The manuscript was improved by editor Eduardo Koutsoukos and the

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