New Turtle Egg Fossil from the Upper Cretaceous of the Laiyang Basin, Shandong Province, China

A new type of turtle egg fossil was established: Emydoolithus laiyangensis oogen. et oosp. nov.. Based on its elliptical morphological shape, rigid eggshells, and eggshell characteristics, it is different from other types of round chelonian egg fossils. It is the second chelonian egg fossil found in Cretaceous in China. This discovery shows the Laiyang ecosystem in Late Cretaceous is more diversified than previously thought.


INTRODUCTION
Turtle egg fossils are very rare. Even though they are found from the Jurassic to the Tertiary age, turtle eggs are known only from a few Cretaceous deposits, from localities in China (Fang et al. 2003, Jackson et al. 2008, Mongolia (Mikhailov et al. 1994), Japan (Isaji et al. 2006), India (Mohabey 1998), Brazil (Azevedo et al. 2000), and the United States (Hirsch 1996, Bray and Hirsch 1998, Kohring, 1999. A review of previous work on the turtle egg fossils has been done by Jackson et al. (2008).
Testudoolithus jiangi, which was found in Tiantai, Zhejiang Province (Fang et al. 2003, Jackson et al. 2008, is the only valid turtle egg fossil found in China. Although the egg fossils found in Changtu, Liaoning Province, in 1921 were originally thought to be turtle eggs (Yabe and Ozaki 1929), they were later recognized as dinosaur eggs based on their macrostructure characteristics (Chow 1954). Recently, we have restudied the macrostructure and microstructure of these eggs found in Changtu (housed at the Dalian Nature History Museum) and confirmed they are indeed dinosaur eggs (to be reported in another paper). During an excavation in the Laiyang Basin, Shandong Province in 2010 (Fig. 1A), we found a new turtle egg in the first excavation site where the famous Tsintaosaurus was found (Fig. 1B). This is the second valid turtle egg fossil found in China.  Jingangkou Formation, and Changwangpu Formation (Hu et al. 2001), is mainly deposits of alluvial fan, mudflow and braided channel in the lower part, shallow lacustrine deposits and rhythmic fluvial sediments of mudstone, siltstone, sandstone or soils in the middle part, and rhythmic depositions of silty-muddy conglomerate, sandstone and siltstone of mudflow, braided-channel and flooding plain facies (Liu et al. 2011).
There are the famous Late Cretaceous hadrosaurid fauna represented by Tsintaosaurus and Tanius, and the Laiyang dinosaur egg fauna represented by elongatoolithids and ovaloolithids in the Wangshi Group ( Fig. 2) (Wang et al. 2010(Wang et al. , 2012. Up to now, over ten genera and species of dinosaurs, four oofamilies, five oogenera and eleven oospecies of dinosaur eggs, one species of turtle have been reported from the Wangshi Group of the Laiyang Basin. All fossils were found in the Jiangjunding and Jingangkou formations, belonging to the middle part of the Wangshi Group (Wang et al. 2012). In 2010 and 2011 excavations, we found some dinosaurs, dinosaur eggs, turtle, turtle egg, crocodile teeth, gastropods, and plant fragments in two localities (Fig. 1A). Here, we report on the turtle egg found from the Upper Cretaceous Jingangkou Formation in Laiyang in 2010. This is the second turtle egg ever found in China.

MATERIALS AND METHODS
The specimen studied in this paper is a complete egg, found during an excavation in 2010 in the Jingangkou Formation (Upper Cretaceous) in Laiyang Basin (Fig. 2). We used calipers to measure the morphological data of the egg. We took six eggshell samples from the egg to study its eggshell microstructure. Four of them were used to make radial and tangential section with cutting system (EXAKT 300CP and EXAKT 400CS). The other two were used to observe the radial sections with SEM (S-3700N). These two specimens were embedded and polished, and etched 30 seconds in 1% HCl solution, then gilded and observed in operating voltage 10Kv. All the processes were carried out at

DESCRIPTION
IVPP V18544 is a complete symmetrical elongate elliptical egg (Fig. 3A), with a hard calcareous eggshell (Fig. 3B, C). Because an extrusion, eggshell was broken in small pieces (Fig. 3C). The egg was preserved in the red pebbly muddy siltstone, and a gastropod fossil was found nearby (Fig. 3A, D).
The polar axis of the egg is 9.1 cm; the equatorial diameter is 2.2 -2.5 cm. Shape index (the equatorial diameter/polar axis ×100%) is 27.4. Thickness of eggshell is 400 -500 μm. The width of the eggshell units is 100 -200 μm, and the ratio of shell unit height-to-width is 2:1 -5:1. The eggshell is composed of tightly arranged columnar eggshell units (Fig. 4A, B), with a clear boundary between eggshell units (Fig. 4B, C, D). Nuclei center of eggshell units is not in the same horizontal (Fig.  4B). There is actinomorphic acicular aragonite toward inner surface of eggshell in the lower part of eggshell units (Fig. 4B, C).
In tangential section through the lower part of eggshell, irregular shape eggshell units are tightly arranged with a clear boundary between eggshell units (Fig. 4E, F). There are 50 -60 eggshell units per square millimeter. Within the eggshell units, there are one or more crystal particles (Fig. 4E, F), which are also found in the living turtle Podocnemis unifilis (see Schleich and Kästle 1988, Fig. 9).

COMPARISONS AND CONCLUSIONS
Emydoolithus laiyangensis is a turtle egg, because that only chelonian eggs have aragonite eggshell unit characteristics (Hirsch 1983). Hirsch (1996) established two parataxonomic oofamilies of chelonian eggs: Testudoolithidae and Testudoflexoolithidae with the oogenera Testudoolithus and Testudoflexoolithus and three oospecies. Eggshell units are generally wider than high and loosely abutting in soft shell in testudoflexoolithids. Eggshell units are higher than wide, and crystals of adjacent units interlock with each other in rigid eggshell in testudoolithids.
In comparison, Emydoolithus laiyangensis is similar to Testudoolithus jiangi in the thickness of eggshell and the height-to-width ratio of the eggshell unit (Table I). However, elliptical morphological shape of Emydoolithus laiyangensis is different from other round shape turtle eggs (Table I).

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QIANG WANG, XIAOLIN WANG, ZIKUI ZHAO, JIALIANG ZHANG and SHUNXING JIANG  Crystal particles in eggshell microstructure of Emydoolithus laiyangensis is more likely the living turtle Podocnemis unifilisi eggshell microstructure, and many living turtle eggs are symmetrically elliptical like E. laiyangensis. So it is maybe that E. laiyangensis belong to an emydid turtle. Even though, there is one turtle fossil species? Glyptops sp. (Chow 1954) found in the same locality in Laiyang. We also found many turtle carapace and plastron fragments at two sites in 2010 and 2011 excavations. But, we are still not sure the exactly turtle species to which of the fossil egg belongs.

Ootaxon
Age Location  Emydoolithus laiyangensis is the second turtle egg fossil found in China. It differs from other chelonian egg fossils, which are round, while the oospecies here described presents an elliptical shape. Thus, it is represents a new type of turtle oospecies. Also, the discovery of this new turtle oospecies indicates that the Laiyang ecosystem in Late Cretaceous was thriving and more diverse than previously thought.

ACKNOWLEDGMENTS
We thank Cheng Xin, Li Yan, Xiang Long, Pan Rui, Shou Hua-quan, Gao Wei, Zhang Jie, Liu Xinzheng (Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences), Wang Jian-hua (Laiyang) for help in the field work. We thank Wang Hong for specimen preparation, Gao Wei for photos, and Zhang Wen-ding for SEM photos. We also thank De-Sui Miao (Natural History Museum and Biodiversity Research Center, University of Kansas, Lawrence, USA) and two anonymous reviewers for their valuable comments and English correction on the manuscript. This work was supported by the National Science Foundation of China (41172018)