The Cretaceous period has garnered significant attention due to a series of major global events, including widespread volcanic activity, oceanic anoxic events, superchrons, biotic radiation, and mass extinctions (Jones and Jenkyns, 2001; Bralower et al., 1994; Cronin et al., 2001; Walliser, 1996; Leckie et al., 2002). However, most studies have focused on marine strata and fossil records from the Cretaceous. In contrast, research on terrestrial Cretaceous systems remains limited, except in regions such as northeastern China, where studies have achieved relatively higher resolution (Xi et al., 2019). In other areas, critical environmental-biological events in terrestrial Cretaceous settings are often difficult to define due to challenges in stratigraphic correlation or incomplete fossil records (Zhao et al., 2013).

Dinosaur eggs are a common fossil type in Upper Cretaceous terrestrial strata. They exhibit diverse microscopic shell structures and nesting patterns, and have been discovered in various sedimentary environments (Horner and Makela, 1979; Cousin et al., 1994; Mikhailov et al., 1994; Zhao, 2003; Sander et al., 2008). These fossils provide critical insights into regional stratigraphic correlation, paleoenvironmental reconstruction, and paleoclimate studies (Zhao et al., 2015). In recent years, research on dinosaur egg fossils, their burial chronologies, and associated environmental contexts has emerged as an important approach to understanding the terrestrial “Cretaceous World” (He et al., 2013; Zhao et al., 2017; Han et al., 2022).

Abundant Late Cretaceous dinosaur egg fossils were discovered in Qinglongshan (Qinglong mountain), Yunyang District, Shiyan, Hubei Province (Zhou, 1998). Through conservation efforts, thousands of these eggs are now semi-exposed in situ within their original stratigraphic layers. Most eggs exhibit minimal deformation, preserved as intact three-dimensional structures. Their unique arrangement and well-defined nesting patterns indicate limited post-depositional disturbance, while the continuous egg-bearing section reveals dozens of fossilized egg layers. A preliminary study by Zhang et al. (2018) classified the majority of Qinglongshan eggs as a single oospecies, Placoolithus tumiaolingensis, which belongs to the oofamily Dendroolithidae—a group characterized by highly porous egg-shells with branched eggshell units. However, the dinosaur species responsible for laying these eggs remains unidentified. Thus, Qinglongshan represents an understudied dinosaur nesting site, offering critical insights into dinosaur reproductive behavior and the Late Cretaceous climate, environmental dynamics, and biotic evolution in China’s interior regions.

Dinosaur egg dating generally relies on indirect methods, as fossilized eggs often lack sufficient radioactive isotopes for direct dating. Constraints are achieved by dating volcanic rocks or ash layers around the eggs using U-Pb or K-Ar/Ar-Ar methods to determine the age of zircon and feldspar. Relative dating methods like biostratigraphy are also used, comparing other fossils in the same rock layer with global stratigraphic profiles to estimate the eggs’ age. Paleomagnetic dating helps identify the stratum and constrain the eggs’ age (e.g., Li et al., 2010 used magnetostratigraphy to show that the Xixia Basin dinosaur eggs are no later than 83 Ma). However, these methods have indirect errors, such as volcanic eruptions occurring before or after the eggs, or surrounding minerals being affected by later geological processes.

Similarly, dinosaur bone dating faces similar challenges. But advances in carbonate U-Pb absolute dating have enabled in-situ dating of dinosaur bones. Qi et al. (2024) used LA-ICP-MS to U-Pb date early diagenetic calcite in Jurassic sauropod bone cavities from the Sichuan Basin, South China, obtaining an age of 165.3 ± 3.6/5.6 Ma, consistent with the maximum depositional age of 165.8 ± 1.0 Ma from detrital zircons in surrounding rocks, indicating rapid post-mortem diagenesis.

Nevertheless, the absence of reliable chronostratigraphic data for the egg-bearing horizons at Qinglongshan has significantly hindered regional stratigraphic correlation and sedimentological analysis. To address this gap, our study employs LA-MC-ICP-MS U-Pb dating on calcite samples recently identified within the dinosaur eggs. The results aim to establish a robust chronological framework and enhance the comprehensive understanding of the temporal and environmental context of this significant dinosaur egg assemblage.

The east-west trending Qinling Orogenic Belt traversing central China has under-gone three major tectonic evolutionary stages since the Neoproterozoic: ocean-continent plate subduction orogeny, continent-continent plate subduction-collision orogeny, and intracontinental orogeny (Zhang et al., 2019). From the Late Jurassic to the Cretaceous, the Qinling Orogenic Belt experienced extensional deformation that propagated from its core to the margins. Localized extension along pre-existing NWW-trending faults formed a series of faulted basins such as Xixia, Xichuan, and Yunyang (Figure 1). These basins were filled with thick, predominantly coarse-clastic deposits comprising alluvial fan, fan-delta, and lacustrine facies sediments (Liu and Zhang, 2008).

The Qinglongshan area, located in the western part of the Yunyang Basin, is dominated by red terrigenous clastic continental deposits primarily composed of alluvial fan facies. These deposits unconformably overlie Precambrian metamorphic rocks. Guan et al. (1997) subdivided this red-bed sequence into three lithological units from bottom to top (Figure 1D). The lower unit is distinguished from the middle and upper units by its composition of breccias, breccia-bearing siltstones to fine sandstones, and abundant dinosaur egg fossils, the dinosaur eggs collected in this study were obtained from this stratum. Zhou et al. (1998) assigned the lower unit to the Late Cretaceous based on dinosaur egg fossil studies, correlating it with the Gaogou Formation in the Xichuan (Xixia) Basin. According to Guan et al. (1997) and Zhou et al. (1998), the Yunyang Qinglongshan Formation and the Gaogou Formation in the Xixia Basin are equivalent. Pan et al. (2007) identified the Xixia Basin as the Late Cretaceous Coniacian (89.8–86.3 Ma) through dinosaur egg fossils, small dinosaur bone fossils, and some fossil fragments of bivalves, gastropods, ostracods, charophytes, plant pollen, and plant fossils. However, the parataxonomic validity of reported oogenera such as Faveoloolithus, Dictyoolithus, and Prismatoolithus has been questioned (Zhao et al., 2015), and issues persist in the subdivision of Dendroolithidae (Zhang et al., 2018).

The carbonate samples in this study were collected from a calcite-filled dinosaur egg fossil preserved at Site No. 1 of the Tumiaoling Dinosaur Egg Fossil Locality in Qinglongshan. The sample consists of monomineralic calcite with well-developed crystals and coarse grain size. The sampled egg is embedded within breccia-bearing siltstone of the lower unit and forms part of a large clutch containing 28 eggs. The morphological characteristics of these eggs and the clutch align closely with Placoolithus tumiaolingensis as described by Zhang et al. (2018). The sampled oblate-spheroid egg shows consistent morphology with others in the clutch, displaying regular spacing without evidence of compressive deformation (Figure 2A). Two additional eggs in the clutch also exhibit calcite infilling. While the sampled egg suffered partial shell damage during collection, the other two calcite-filled eggs retain nearly intact external shells and contain no mixed sediments or shell fragments internally. In contrast, the remaining 26 eggs in the clutch show varying degrees of shell fragmentation, with some retaining only the lower approximately 1/2–3/4 of their shells. Their interiors are filled with breccia-bearing siltstone containing shell fragments, lithologically identical to the host rock (Figures 1A, 2B). These observations suggest that the calcite crystallization process occurred under conditions of intact eggshell preservation, likely postdating or contemporaneous with the fossilization of the dinosaur eggs. The sampling criteria were as follows: the outer surface of the dinosaur eggshell was highly intact, and inside the egg, there were almost no other mixed sediments or eggshell fragments except for calcite crystals, mainly based on purity and homogeneity. In addition, since the dinosaur egg preservation site has been built into a museum, we only collected two fragments from a broken dinosaur egg according to the above principles (Zhang et al., 2018).

The emended oogenus Placoolithus is characterized by the following unique combination of characters:oblate egg shape; an equatorial plane circular or sub-circular with long axis of 120–170 mm and short axis of 117–159 mm (Figure 2); eggshell thickness of 1.31–2.40 mm. Moreover, symmetric or asymmetric branches of eggshell unit usually appear in the middle part of the eggshell, and occasionally near the outer surface of the eggshell (Zhang et al., 2018). The eggshell units of the eggs usually contain two symmetrical branches. Most of the eggshell units branch in the middle part of the eggshell, while the others divide near the inner surface of the eggshell. Sometimes, one of the two branches subdivides into two smaller branches near the outer surface of eggshell (Figure 3).

LA-MC-ICP-MS carbonate U-Pb dating was performed to obtain the absolute age of the dinosaur eggs in the Isotope Laboratory of the Institute of Global Environmental Change, Xi’an Jiaotong University, using the method similar to that described in Wang et al. (2022), Wang et al. (2025), and Niu et al. (2025). The egg fossil samples were fixed in a 25 mm epoxy round target and polished to about 15,000 mesh. Note that the cross section of the eggshell should be exposed. Deionized water ultrasound was used to remove possible surface contamination on the target (Figure 4).

The samples and reference materials (RMs) are put into the S155 sample cell (Laurin Technics 155, aerosol dispersion volume <1 cm³), then laser denudation is performed using the 193 nm ArF LA system (RESOlution LR), and the generated aerosols go directly to the Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS, Neptune XT) for measurement. Glass reference materials NIST616 and NIST614 are used to adjust the signal and determine the parameters (see Table 1 for details). Every 8 samples were interspersed with NIST614 glass and carbonate RM samples of known age (Nuriel et al., 2021; Hill et al., 2016; Wang et al., 2022; Wang et al., 2025), for instrument signal drift and matrix effect correction, respectively.

The Iolite software (Paton et al., 2011) is used for background deduction and downhole effect correction, as well as calculations of the isotope ratios, errors, correlation coefficients and other information required for the Tera-Wasserburg diagram (Tera and Wasserburg, 1972). Carbonate RM samples ASH15 (Nuriel et al., 2021) and SB19-2 (Wang et al., 2022; Wang et al., 2025) were used to determine the correction coefficient of U/Pb ratio and U content. U-Pb age results were calculated using IsoplotR (Vermeesch, 2018). The final reported error of age and (²⁰⁷Pb/²⁰⁶Pb)₀ are 2SE, including the dispersion of sample points and the error of glass and carbonate RMs. To ensure the precision and reproducibility, standard ASH15 and SB19-2 was performed as unknowns in the course. And the ages we got (2.954 ± 0.022 Ma, 1.099 ± 0.009 Ma) are within uncertainty of the published results (2.965 ± 0.011 Ma and 1.099 ± 0.011 Ma). We adopted a test-while-selecting approach. We analyzed possible proper spots by checking Pb isotope ratios, especially ²⁰⁷Pb/²⁰⁶Pb in real-time during testing, and chose test points near areas with low ²⁰⁷Pb/²⁰⁶Pb ratios or similar features. Compared to U content, we deem the radiogenic Pb proportion more crucial.

The microscopic characteristics of the eggshell and the white part are obviously different (Figure 4, Figure 5A). The white part is relatively pure calcite. The eggshell showed a mixture of secondary calcite and primary carbonate minerals, and its structure is similar to Zhang et al. (2018). It may be that the pores in the eggshell were subsequently filled with calcite. U-Pb LA-MC-ICP-MS tests were conducted on both the white and eggshell parts. The results (Figure 5B) showed that the white part contained a small amount of Pb, which basically did not contain U. The eggshell part had a certain amount of U (ca. 0.5 ug/g) and therefore detectable radiogenic Pb, among which the calcisfilled part of the eggshell contained little or no U, and the signal fluctuated greatly, which may be caused by the surrounding pollution.

Considering the U and Pb concentration, we only calculated the U-Pb age using points in the eggshell with relatively stable signals. The U-Pb dating results are shown in Figure 5C. The 2 eggshells got same ages within the error, finally we use all the data together to get the combined age of 85.91 ± 1.74 Ma (Figure 5C).

The best dating results were obtained from biogenic calcite, whereas abiogenic calcite yielded poor results. This might be due to the pore structures in abiogenic calcite, which enhance permeability and deviate from a semi-closed system (Figure 6). The following figure shows the laser spot positions for the S2 fragment sample. The orange square spots, which provide consistent ages, are located on the dark calcite visible under a microscope and are primarily biogenic calcite resulting from the biomineralization of amniotes. These correspond to the colored ellipses in the right-side S2 of Figure 5. The orange box spots correspond to the uncolored ellipses in the right-side S2 of Figure 5. The two-half - filled squares correspond to the two colorless ellipses close to the sides of the line in Figure 5.

In this study, we only obtained a small number of samples, but achieved very good results. What we can confirm is that a large amount of data are distributed along the regression line (Figure 5), rather than being scattered on both sides, which proves that there is no significant loss of U. Moreover, we conducted repeat tests on two sets of samples and obtained consistent ages within the margin of error. This fully demonstrates that, at least on the dinosaur eggshells we collected, this age is very reliable and is highly consistent with the stratigraphic age.

Qinglongshan area of the Yunyang Basin in Hubei Province, central China, is rich in dinosaur egg fossils. In this study, we employed LA-MC-ICP-MS U-Pb dating on two parts of a single dinosaur eggshell and obtained reproducible age results. The data from the biogenic calcite indicate that the dinosaur eggs in the Yunyang Basin date back to 85.91 ± 1.74 Ma, placing them in the Late Cretaceous period. This era was marked by a significant global temperature decline. The application of U-Pb dating to dinosaur eggshells may represent a major advance in the study of dinosaur evolution.