It is well known that body mass and reproductive investment in terms of egg mass, clutch mass or annual clutch mass, are highly correlated in extant reptiles and birds (Rahn et al . 1975 ; Blueweiss et al . 1978 ). In a study devoted to the relationship between body mass and reproductive traits, Werner & Griebeler ( 2013 ) observed that the egg masses of dinosaurs, including theropods, match neither the egg masses of similar‐sized or scaled‐up birds nor those of reptiles, but were in between. However, when considering the clutch size or annual clutch size, the relationship for theropod dinosaur fits the bird model. Whereas Werner & Griebeler ( 2013 ) invoked peculiar reproductive strategies in theropod dinosaurs to account for this conflicting pattern, another explanation might reconcile this apparent discrepancy. As evidenced by the fossil record, oviraptorids, and most likely all non‐avian dinosaurs had two functioning ovaries that allowed them to produce paired eggs in a monoautochronous sequence (Sato et al . 2005 ). Most birds, on the other hand, have one functional ovary and oviduct (the left one, as the right one regresses during development and is nonfunctional in the adult bird) and therefore produce one egg at a time. In terms of reproductive cost, the egg as a ‘unit’ cannot be considered equivalent in modern birds and dinosaurs, but two eggs in dinosaurs would correspond to one egg in modern birds. By multiplying theropod egg mass by two, theropods match the egg mass to body mass relationship of similar‐sized or scaled‐up birds (Fig. 3 ). The relevance of this consideration is supported by the fact that theropod clutch mass (and annual clutch mass) also matches that of similar size or scaled‐up birds, because in a clutch or annual clutch, all eggs are accounted for in both birds and non‐avian theropods, regardless of their timing of laying. Consequently, the egg mass to body mass allometry established for birds should be applied to dinosaurs considering ( M egg ) × 2, as proposed in Equation 2.

One possible way to assess the state of isotopic preservation of the eggshell and embryo δ 18 O p –δ 18 O c pairs is to compare variation among the seven specimens. As endo‐homeothermic animals, oviraptorosaurs maintained a constant body temperature, so the eggshell precipitated at constant body temperature should only reflect the oxygen isotope composition of the mother's body fluids. These body fluids are also transmitted to the egg via swelling or plumping, a process taking place at the beginning of and during eggshell construction (Fertuck & Newstead 1970 ). During incubation, embryo bone precipitates from these fluids. Considering a constant incubation temperature, it should therefore be expected that the bone phosphate δ 18 O p value of the embryo should also reflects the oxygen isotope composition of the mother's body fluids. Consequently, it should be expected that δ 18 O p and δ 18 O c values follow a close to linear relation, keeping in mind that water loss of individual eggs during incubation might add some scattering. Interestingly, Figure 5 shows that the samples IVPP V14723.2, V14723.3 and NHMG10876 have δ 18 O p –δ 18 O c pairs that seem to follow a straight line, whereas GMV2212, IVPP V20182, V20183 and V20184, having the highest δ 18 O c values, seem to be clustered. The modeled incubation temperature for these specimens that would fit both δ 18 O p and δ 18 O c values that are out of the known range for living animals (≫45°C), the estimated δ 18 O ew values being too high relative to the embryo δ 18 O p values. Because embryo phosphate δ 18 O p values of GMV2212, IVPP V20182, V20183 and V20184 are within the range of other specimens, but the eggshell carbonate δ 18 O c values are more positive, we suspect that some exogenous carbonate may have affected the measured eggshell δ 18 O c values, despite the careful cleaning of the eggshells and the microscope observations showing no apparent signs of extensive recrystallization. Pending additional analyses, specimens GMV2212, IVPP V20182, V20183 and V20184 are considered to have been affected by diagenetic alteration, and will not be considered further in the subsequent discussion.

The methodology proposed in this study requires that both the eggshell calcite and embryo bone apatite phosphate preserve their pristine oxygen isotope compositions. To assess the structural preservation of these biominerals, eggshell thin‐sections were inspected under a light microscope. The eggshell is composed of a mammillary layer and a columnar layer separated by a clear boundary. The columnar layer is characterized by undulating growth lines lying parallel to the outer surface of eggshell and visible in radial sections (Fig. 2 ). These features are diagnostic for the oofamily Elongatoolithidae (Zhao 1975 ) and indicate apparently good preservation of the original calcitic structure of the eggshell. Published thin‐sections have also been cut in embryo long bones of specimens IVPP V20182, V20183 and V20184, and show a typical cortex formed by fibrolamellar bone containing numerous elongate to circular vascular canals. This cortex encircles the medullary cavity, a feature observed in all known embryonic dinosaurs (Wang et al . 2016 ). This in turn would support the preservation of the original oxygen isotope composition of phosphate, as these structures cannot result from any diagenetic processes.

Ecological specificities of the oviraptorosaur theropods from Jiangxi

The Nanxiong Formation has yielded at least eight oviraptorid taxa, including Banji long (Xu & Han 2010), Ganzhousaurus nankangensis (Wang et al. 2013), Jiangxisaurus ganzhouensis (Wei et al. 2013), Heyuannia huangi (Lü 2002), Shixinggia oblita (Lü & Zhang 2005), Huanansaurus ganzhouensis (Lü et al. 2015), Nankangia jiangxiensis (Lü et al. 2013, 2016) and Tongtianlong limosus (Lü et al. 2016). According to Equation 2, the body mass of the oviraptorosaurs that laid the Jiangxi eggs ranged from about 36 kg to 140 kg (Table 3), which is comparable to other estimates ranging from 20 kg to 75 kg for some of the Jiangxi taxa (Paul 2016).

Estimated incubation duration from Equation 6 ranges from about 44 to 56 days, close to those estimated using the same equation for Citipati osmolskae (44 days) and Oviraptor philoceratops (40 days; Lee 2016). It is noteworthy that these durations are significantly shorter than the 3‐ to 6‐month period recently estimated for ornithischian dinosaurs using daily incremental growth line count of embryo teeth (Erickson et al. 2017). It is possible that the model developed by Lee (2016), based on modern bird physiological characteristics, gives too short estimates of incubation duration for theropods (for example, the metabolic mass gain parameter p of precocial birds is used to infer theropod embryo mass gain (Lee 2016)). However, the study performed by Erickson et al. (2017) only includes ornithischian dinosaurs that may have retained reptile‐grade reproductive physiology whereas theropods close to the avian lineage (like oviraptorosaurs) may already have acquired bird‐like reproductive physiology traits.

The clutches of oviraptorosaurs are arranged to form a spiral ring of two to three tiers of subhorizontally‐oriented eggs while their centre is devoid of eggs. The adult oviraptorosaur sat in the center of the clutch with their arms surrounding the eggs, in a position comparable to that of brooding birds, as evidenced by at least four specimens fossilized in brooding position (Dong & Currie 1996; Clark et al. 1999). Active brooding behaviour seems to be well supported by the paired δ18O p –δ18O c values of embryo phosphate and eggshell carbonate with predicted temperature of embryo bone formation within the 35–40°C range. As mentioned in the 3 above, sample NHGM10876 fits the range of possible embryo bone formation temperatures if water loss is lowered by two thirds relative to predicted daily loss (Fig. 6). This specimen has the lowest eggshell δ18O c value, which in turn implies a low value of mother body water as well as of environmental drinking water. According to Lazzerini et al. (2016), terrestrial birds display a drinking water‐body water 18O‐enrichment of about +4‰ (at least in ostrich and chicken). Based on the egg water δ18O ew value of −5.7‰ calculated for NHGM10876, a δ18O w value of local surface waters of about −9.7‰ can be estimated. During the deposition of the Nanxiong Formation, the Jiangxi Province was located at a low palaeolatitude of about 20°N (Cogné et al. 2013). Such a low δ18O w value in subtropical to tropical latitudes could be the signature of local rainwater characterized by high amount of seasonal precipitation, such as monsoon rains experienced today, or the influence of high altitude precipitations from nearby mountain ranges supplied to local rivers. Whereas the latter hypothesis is difficult to test due to the lack of regional palaeoenvironmental studies, the low eggshell δ13C values, ranging from −11.0‰ to −9.3‰ (V‐PDB), seems to support a regime of elevated amounts of precipitation. Indeed, the carbon isotope composition of eggshell calcite reflects the δ13C value of the mother's diet with a fractionation controlled by its digestive physiology (Johnson et al. 1998; Angst et al. 2014). For Mesozoic terrestrial vertebrates, the δ13C value of their eggshells ultimately reflects that of local C 3 vegetation consumed either directly (plant eater) or indirectly (predator of a plant eater), the carbon isotope composition of which is controlled by the amount of local precipitation and aridity (Diefendorf et al. 2010; Kohn 2010). Montanari et al. (2013) analysed the δ13C values of oviraptorosaur eggshells (oofamily Elongatoolithidae) from the Upper Cretaceous Djadokhta (−5.2‰ to −4.6‰) and Nemegt (−5.6‰) formations of Mongolia, contemporaneous to the Nanxiong Formation and located at mid palaeolatitudes (~45°N). They interpreted these high values as reflecting local vegetation growing in a semi‐arid desert most likely dominated by gymnosperms (Montanari et al. 2013). With significantly lower carbon isotope composition of carbonate, Jiangxi eggshells and especially the specimen NHGM10876 strongly suggest that these oviraptorosaurs had consumed directly or indirectly local plants growing under wet conditions before laying their eggs. This wet period that could be seasonal was probably characterized by elevated amounts of precipitation as suggested by the low δ18O value of the eggshell. The atmosphere would in turn be saturated in vapour pressure, thus reducing egg water loss during incubation.

The estimated temperature of embryo bone formation range of 35–40°C for oviraptorosaurs falls within the known range for extant birds (Rahn 1991). It implies a slightly lower incubation temperature (due to heat loss during brooding) and an adult body temperature within the range of modern endotherms. According to some stable oxygen isotope analyses and clumped isotope thermometry of bone and teeth phosphate, dinosaurs were warm blooded (Barrick & Showers 1994; Fricke & Rogers 2000) their body temperature being raised and kept at a constant level within the 36–38°C range (Amiot et al. 2006; Eagle et al. 2011). However, the clumped isotope analysis of oviraptorid eggshells of Mongolia gave a formation temperature estimate of 31.9 ± 2.9°C (Eagle et al. 2015). This one exception is quite surprising, oviraptorosaurs being closely related to the avian lineage (Brusatte et al. 2014) and sometimes having even been considered to be derived birds (Maryanska et al. 2002). Further investigation is needed to determine whether these oviraptorosaur eggshell samples constitute an isolated case or if large variation in body temperature existed within the Dinosauria.