A new type of dinosaur eggs from Early Cretaceous of Gansu Province, China

XIE Jun-FangZHAnG Shu-KangJIn Xing-ShengLI Da-QingZHOU Ling-Qi

(1Zhejiang Museum of Natural HistoryHangzhou 310014)

(2Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences,Institute of Vertebrate Paleontology and Paleoanthropology,Chinese Academy of SciencesBeijing 100044 * Corresponding author: zhangshukang@ivpp.ac.cn)

(3Gansu Geological MuseumLanzhou 730030)

A new type of dinosaur eggs from Early Cretaceous of Gansu Province, China

XIE Jun-Fang1ZHAnG Shu-Kang2*JIn Xing-Sheng1LI Da-Qing3ZHOU Ling-Qi3

(1Zhejiang Museum of Natural HistoryHangzhou 310014)

(2Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences,Institute of Vertebrate Paleontology and Paleoanthropology,Chinese Academy of SciencesBeijing 100044 * Corresponding author: zhangshukang@ivpp.ac.cn)

(3Gansu Geological MuseumLanzhou 730030)

The Early Cretaceous outcrops in Gansu Province, northern China, have yielded numerous dinosaur skeleton remains and tracks; however, fossil eggs have not been reported in literatures. Here, we describe a new type of dinosaur eggs from the Lower Cretaceous Hekou Group in the Lanzhou-Minhe Basin, representing a new oogenus and a new oospecies, attributed to a new oofamily. The new specimen can be distinguished from other known dinosaur eggs by the combination of the following eggshell micro-features: branched eggshell units lacking a compact layer near the outer surface; interlocking or isolated multi-angular eggshell units, as viewed in tangential sections; and irregular pore canals. Dinosaur eggs from China largely come from the Late Cretaceous deposits, with occasional reports from the Early Cretaceous in Liaoning Province, northeastern China. The new discovery expands the geological and geographical distribution of the fossil record of dinosaur eggs in China and may reveal the origin of eggshell microstructures of spheroolithid eggs.

Lanzhou-Minhe Basin, Gansu; Early Cretaceous; Polyclonoolithidae; dinosaur egg

1 Introduction

Compared to those from Upper Cretaceous deposits, fossil eggs from the Lower Cretaceous worldwide are relatively rare. They have been recovered from China, Mongolia, Thailand, Korea, Japan, Spain, and North and South America, including elongatoolithids, macroelongatoolithids, prismatoolithids, spheroolithids, faveoloolithids, dendroolithids,reptilian eggs belonging to turtle and crocodile, as well as eggs whose parataxonomic positions are still unknown (Nessov and Kaznachkin, 1986; Kurzanov and Mikhailov, 1989; Kohring, 1990; Bray, 1998; Zelenitsky et al., 2000; Azuma, 2003; Buffetaut et al., 2005; Buscalioni et al., 2008; Kim et al., 2009; Canudo et al., 2010; Grellet-Tinner et al., 2012; Moreno-Azanza et al., 2009, 2014a, b). In China, fossil eggs from the Lower Cretaceous were only reported in Liaoning Province, assigned to the oofamily Elongatoolithidae (Zhao and Zhao, 1999). Most of the ootaxa mentioned above were also recovered from Upper Cretaceous deposits.

The Lower Cretaceous Hekou Group in the Lanzhou-Minhe Basin of Gansu Province is very rich in dinosaur bones and tracks (Qi and Yu, 1999; Zhang et al., 2006; You et al., 2005, 2006, 2008), whereas discoveries of dinosaur eggs are rare. In 2009, the Chinese Academy of Geological Sciences discovered some fossil eggshell fragments in Zhongpu, a town in Lanzhou Basin. However, this discovery remains unpublished, possibly because of uncertainty about the locality and formation that yields the fossil material. In November of 2012, fossil eggshells were again discovered in the Lanzhou-Minhe Basin. Here, we provide a detailed description of the new discovery of dinosaur egg and associated eggshell fragments from Gansu Province. Based on the unique microstructure of the eggshell, we established a new oogenus and oospecies, within a new oofamily. The new discovery has important implications for understanding the diversity and the geological and geographical distribution of Early Cretaceous dinosaur eggs in China, as well as the evolution of dinosaur eggshell structure.

2 Material and methods

The incomplete egg (ZMNH M1849) was collected from the Lanzhou-Minhe Basin, by a work team from Gansu Geological Museum (GGM), Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences (IVPP) and Institute of Geology, Chinese Academy of Geological Sciences (IGCAGS) during the field investigation. Laboratory preparation included removal of sediment from the eggshell with small hand tools. The poorly preserved egg exhibits varying degrees of weathering. Therefore, the least weathered fragments were selected for examination, using a Nikon SMZ1000 stereomicroscope to view the outer surfaces. The eggshell fragments were prepared as standard radial and tangential petrographic thin sections (0.03 mm thick) and studied under a Nikon Eclipse polarized light microscopy (Nikon eclipse LV100POL, PLM). Additional eggshells were coated in gold (10 nm), mounted on aluminum stubs, and imaged by a Hitachi S-3700N Scanning Electron Microscopy (SEM) at 20 kV. Structural attributes (eggshell thickness, eggshell unit width) were measured and analyzed with Java image processing software and a digital caliper. The eggshell fragments and the eggshell thin sections are catalogued at the Zhejiang Museum of Natural History (ZMNH).

3 Locality and geological setting

The specimen occurred in outcrops near the border of Yongjing and Lintao counties (Yang et al., 2013: fig. 1), in the central region of the Lower Cretaceous Lanzhou-Minhe Basin. The Lanzhou-Minhe Basin is located on the border of Gansu and Qinghai provinces, and represents a typical Mesozoic-Cenozoic intracontinental rift basin in western China. Within the basin, the Lower Cretaceous Hekou Group is well exposed (Ye, 1980; Song, 1993; Meng, 1994; Ji, 1995; Cai et al., 2001). It unconformably overlies Ordovician beds or contacts with the Middle and Upper Jurassic by faults. The Hekou Group has a thickness of 4000 m and is composed of sandstone, mudstone, and conglomerate (Editorial Committee of Chinese Stratigraphic Standard, 2000). The Hekou Group is formally divided into Lower and Upper formations and eight informal lithostratigraphic units (Li et al., 2002). The rich dinosaur fauna of the Lanzhou-Minhe Basin includes diverse assemblages of dinosaurs, pterosaurs, and bird tracks (Zhang et al., 2006), three basal titanosauriform sauropod dinosaurs (You et al., 2005, 2006; Li et al., 2014), advanced iguanodontidLanzhousaurus magnidens(You et al., 2005) and Polacanthinae (Yang et al., 2013). The fossil egg comes from the fi rst layer of siltstone from the top of the section measured by Yang et al. (2013) of the Hekou Group (Yang et al., 2013: fi g. 2), which indicates a late Early Cretaceous age. This unit records offshore-shallow lacustrine deposits comprised of red brown mudstone with interbedded sage green silty mudstone. The fossil egg was discovered in the same locality with the nodosaurid dinosaur,Taohelong jinchengensis(Yang et al., 2013).

4 Systematic paleontology

Polyclonoolithidae oofam. nov.

Etymology From the type oogenusPolyclonoolithus.

Type oogenusPolyclonoolithusoogen. nov.

Included oogenera The type and only oogenusPolyclonoolithus.

Distribution and age As for the type and only oogenus.

Diagnosis As for the type and only oogenus.

Polyclonoolithusoogen. nov.

EtymologyPolyclon-, in Greek, means “numerous small branches”, in reference to the branched eggshell units;oo, in Greek, from the combining form for ova, meaning egg;lithos, in Greek, means stone.

Type oospeciesPolyclonoolithus yangjiagouensisoosp. nov.

Diagnosis As for the type and only oospecies.

Polyclonoolithus yangjiagouensisoosp. nov.

(Figs. 1, 2)

Etymology Yangjiagou, locality of the fossil egg in Gansu Province.

Holotype An incomplete and highly fragmented egg (ZMNH M1849), housed in ZMNH.

Locality and horizon Yangjiagou Town, border region of Lintao and Yongjing counties, Gansu Province, China; Lower Cretaceous Hekou Group.

Diagnosis 1.9 mm thick on average, branched eggshell units and obvious horizontal accretion lines. Eggshell units typically fused towards the outer surface, but lacking a compact layer. Multi-angular eggshell units are interlocked or isolated in tangential sections. Pore canals irregular in shape with large cavities near the inner surface of the eggshell.

Description The eggshell fragments are surrounded by calcareous sandstone and partly cemented to the matrix, whereas in other areas the eggshells separated easily from sandstone (Fig. 1A). The poor condition provides little information about the egg shape other than it may be less than 10 cm in diameter. Because of extensive weathering, the outer surface of ZMNH M1849 is strongly sculptured, which facilitates identi fi cation of eggshell units and the pore openings in hand sample (Fig. 1B).

Fig. 1 Photograph of the eggshells, ZMNH M1849 A. The largest eggshell fragment, an incomplete egg, scale bar equals 1 cm; B. Eggshell fragment showing highly weathered surface, scale bar equals 1 mm

The eggshell thickness ranges between 1.84 and 2.05 mm, with an average of 1.9 mm (Fig. 2A). The closely arranged cones at the inner surface of the eggshell show radiating structure under PLM and SEM (Fig. 2A-B). The height of the cones is about 1/7 of the eggshell thickness. The highly recrystallized portion of the eggshell above the cones represents about 1/4 of the eggshell thickness. The margins of the eggshell units are rather vague in this area. However, the upper 3/5 of the eggshell is well preserved. Eggshell units branch 2-4 times, displaying a sweeping extinction under cross-polarized light. Each eggshell unit typically has more than five branches. The parallel horizontal accretion linesare distributed evenly throughout the eggshell units. Pore canals between eggshell units or the branches of eggshell units are irregular in shape, gradually narrowing towards the outer eggshell surface. The branches of eggshell units are smaller and more closely spaced near the outer surface of the eggshell (Fig. 2A). Tangential sections of the inner surface show multiangular eggshell units that are tightly packed together (Fig. 2C). There are about 45 cones per square millimeter. In the middle portion of the eggshell, multi-angular eggshell units are separated by large rimiform pore canals, whereas most branches of eggshell units are fused near the outer surface of the eggshell (Fig. 2D-E).

Fig. 2Polyclonoolithus yangjiagouensiseggshell, ZMNH M1849 A. Radial section of eggshell under PLM, showing branched eggshell units and irregular pore canals; the lower white bar marks the boundary between the cones and the upper portion of the eggshell; the upper white bar marks upper 3/5 well preserved eggshell; white arrows point to pore canals; B. Enlargement of cones under SEM; C. Tangential section near the inner surface of eggshell, showing tightly arranged cones; black arrows point to growing centers of cones; D. Tangential section through the middle part of eggshell, showing the isolated eggshell units; white arrows point to pore canals; E. Tangential section through the upper part of eggshell, showing the eggshell units that have fused together; white arrows point to pore canals; scale bars equal 200 μm for A, B, C, and 300 μm for D, E

Comparison With the exception of the oofamilies Dendroolithidae Zhao & Li, 1988, Dictyoolithidae Zhao, 1994 and Similifaveoloolithidae Wang et al., 2011, ZMNH M1849 can be easily distinguished from all other oofamilies by the presence of branched eggshell units.

Compared with eggs of the Dendroolithidae, the eggshell units of ZMNH M1849 are not totally fused together near the outer surface of the eggshell; however, we cannot exclude the possibility that this is a result of weathering. A significant difference between ZMNH M1849 and dendroolithid eggs is that the branches of the eggshell units ofPolyclonoolithus yangjiagouensisare gradually narrowing towards the inner surface of eggshell, probably indicating that these branches are independent eggshell units. Perhaps because of recrystallization, radially arranged calcite crystals are absent in the upper portion of the eggshell under SEM. In contrast to ZMNH M1849, the diameters of branches of the eggshell units are relatively constant in radial sections of dendroolithid eggs. On the other hand, dendroolithid eggs’ eggshell units are round or worm-like in tangential sections (Zhao and Li, 1988; Zhao and Zhao, 1998), differing from the multi-angular eggshell units of ZMNH M1849.

Eggs of the Dictyoolithidae are remarkable for their superimposed branched eggshell units (Zhao, 1994; Wang et al., 2013). But the arrangement of the eggshell units is very irregular, forming a reticulate structure, which is not seen in ZMNH M1849. In tangential sections, the shapes of eggshell units of dictyoolithid eggs are similar to those of dendroolithid eggs, which can be easily distinguished from those of ZMNH M1849.

Eggshell microstructure of similifaveoloolithid eggs in radial sections resembles that of dendroolithid eggs. The diameters of branches of eggshell units are also constant in radial sections. Tangential sections show numerous, evenly distributed pores with irregular shapes. Adjacent eggshell units fuse together to form the walls of pore canals (Wang et al., 2011). Both radial and tangential eggshell microstructures differ from those of ZMNH M1849. According to these comparisons, we erect a new oofamily Polyclonoolithidae based on the features of ZMNH M1849.

5 Discussion

In China, abundant dinosaur eggs from the Upper Cretaceous have been divided intoseveral dinosaur egg faunas, showing a general process of evolution of dinosaur eggs. The dinosaur egg fauna from the Tiantai Basin, Zhejiang Province, mainly consists of faveoloolithid, dictyoolithid and macroelongatoolithid eggs, which represents a group that retains primitive structural attributes. The age of this dinosaur egg fauna is 98-91 Ma, corresponding to the early Late Cretaceous (Cenomanian-Turonian) (Wang et al., 2012). Another primitive dinosaur egg fauna is reported from the Xixia Basin, Henan Province, which primarily includes dendroolithid and macroelongatoolithid eggs. Although the absolute age of these dinosaur eggs is unknown, they are believed to come from Late Cretaceous strata based on the studies of other fossils (Wang et al., 2012).

According to the comparison above, ZMNH M1849 likely shares a close relationship to the oofamilies Dendroolithidae, Dictyoolithidae and Similifaveoloolithidae. Considering the horizon of ZMNH M1849 (Lower Cretaceous), it may represents a more basic type of dinosaur egg than aforementioned oofamilies, which had been extinct in Late Cretaceous. The discovery of this new oofamily possibly indicates there is an unknown dinosaur egg fauna preserved in the Early Cretaceous strata of China.

According to the branched eggshell units and irregular pore canals, ZMNH M1849 should have the same eggshell formation mechanism as dendroolithid, dictyoolithid and faveoloolithid eggs (Zhao, 1993). Interestingly, the multi-angular eggshell units are somewhat similar to the eggshell units in the tangential sections near the inner surface of eggshells of spheroolithid eggs (Spheroolithus spheroides,S. chiangchiungtingensisandS.megadermus) (Zhao and Jiang, 1974; Liu et al., 2013), possibly suggesting that this new type of dinosaur egg has some relationships with spheroolithid eggs. If the hypothesis is true, the eggshell formation mechanism of spheroolithid eggs may be the same as that of ZMNH M1849. The “columnar layer” of spheroolithid eggs which is composed of superimposed eggshell units probably was evolved from the branches of eggshell units of the new oofamily Polyclonoolithidae, otherwise may indicates that the branches of eggshell units are actually superimposed eggshell units.

6 Conclusions

The fossil egg ZMNH M1849 from Gansu, China is referable to a new oofamily Polyclonoolithidae based on following diagnosis: branched eggshell units without a compact layer near the outer surface, interlocked or isolated multi-angular eggshell units in tangential sections and irregular pore canals. The discovery of the new material expands the diversity and distribution of Early Cretaceous dinosaur eggs in China. The new oofamily Polyclonoolithidae should represents a very basic ootaxon among known oofamilies. Furthermore, ZMNH M1849 has the same eggshell formation mechanism as that of dendroolithid, dictyoolithid and faveoloolithid eggs, and shows some relationships with spheroolithid eggs. It may reveal the origin of eggshell microstructures of spheroolithid eggs.

Acknowledgements The authors would like to thank Ji Shu-An from IGCAGS, You Hai-Lu from IVPP and Yang Jing-Tao from China University of Geosciences (Beijing) for fi eld work and explain the stratigraphic information of Lanzhou-Minhe Basin. We thank Frankie D. Jackson from Montana State University for improving the English text.

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甘肅早白堊世恐龍蛋化石新類型

謝俊芳1張蜀康2*金幸生1李大慶3周伶琦3

(1 浙江自然博物館 杭州 310014)
(2 中國科學院脊椎動物演化與人類起源重點實驗室，中國科學院古脊椎動物與古人類研究所北京 100044 * 通訊作者)
(3 甘肅地質博物館 蘭州 730030)

甘肅省早白堊世地層中，出土了大量恐龍骨骼以及恐龍足跡化石，但是至今未有蛋化石的報道。根據(jù)發(fā)現(xiàn)于蘭州-民和盆地下白堊統(tǒng)河口組的蛋殼化石，建立一恐龍蛋新蛋屬、蛋種，并將其歸于一新蛋科：Polyclonoolithidae (多小枝蛋科)。新發(fā)現(xiàn)的蛋化石標本不同于所有已知的恐龍蛋類型，具有獨特的顯微特征組合：分叉的蛋殼單元向外延伸至蛋殼外表面，并未在靠近蛋殼外表面處融合成層；弦切面上具相互鏈接或獨立的多角形的蛋殼單元；以及不規(guī)則的氣孔道。中國的恐龍蛋化石大多出自晚白堊世地層，僅在遼寧有早白堊世恐龍蛋的報道。新發(fā)現(xiàn)擴展了中國恐龍蛋化石的地質和地理分布，也有可能為圓形蛋科蛋殼結構的起源提供新的認識。

甘肅蘭州-民和盆地，早白堊世，多小枝蛋科，恐龍蛋

Q915. 21

A

1000-3118(2016)01-0079-10

2015-07-17

Xie J F, Zhang S K, Jin X S et al., 2016. A new type of dinosaur eggs from Early Cretaceous of Gansu Province, China. Vertebrata PalAsiatica, 54(1): 79-88

國家自然科學基金(批準號：41172027)資助。