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We provide a detailed description of the sauropod eggs from the Late Cretaceous nesting site of Auca Mahuevo (Neuquén Province, Argentina), the only eggs that are unequivocally associated with titanosaurid dinosaurs. These eggs are subspherical averaging 132 by 115 mm. Well-preserved specimens display a pronounced eggshell ornamentation that consists mostly of single nodes averaging 0.58 mm in diameter and 0.28 mm in height (base to apex) with internodular values of 0.52 to 0.87 mm. The pore canal network consists of vertical and horizontal canals intersecting at the bases of eggshell units. Vertical canals may fork defining a "Y" pattern and their diameters vary between 0.08 and 0.2 mm. Their surficial apertures of 0.15 to 0.29 mm are funnel like and located between the surficial nodes. In pristine specimens, the eggshell thickness equals 1.31 mm, and radial sections exhibit a single structural horizontal layer composed of juxtaposed shell units consisting of acicular calcite crystals radiating from an organic core. Relying only on taxonomically identified oological material, we regard this character also shared in the innermost layer (layer 1) of Deinonychus antirrhopus, oviraptorid theropods, and observed during an early oogenetic stage in extant bird as primitive for saurischians. The eggshell morphology advocates that these eggs were likely incubated in moist nesting environments, perhaps such as nests covered with vegetal matter. Examination of the South American megaloolithid eggshells reveals that the titanosaurid eggs from Auca Mahuevo are mostly similar to those described as Megaloolithus patagonicus and Megaloolithus pseudomamillare.

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Six egg-filled depressions discovered in the Upper Cretaceous Anacleto Formation (Campanian) of Patagonia, Argentina, and interpreted as dinosaur nests, provide the only known evidence of titanosaurid sauropod nest construction. These nest trace fossils show truncation of sedimentary structures as well as differences in texture between the host substrate and in-filling sediment. Titanosaurid sauropods excavated and laid eggs in open nests rather than burying clutches in sediment. In addition, this paper establishes criteria for definitive recognition of excavated nests in the stratigraphic record.

Our understanding of the evolutionary transitions leading to the modern endothermic state of birds and mammals is incomplete, partly because tools available to study the thermophysiology of extinct vertebrates are limited. Here we show that clumped isotope analysis of eggshells can be used to determine body temperatures of females during periods of ovulation. Late Cretaceous titanosaurid eggshells yield temperatures similar to large modern endotherms. In contrast, oviraptorid eggshells yield temperatures lower than most modern endotherms but ∼ 6 °C higher than co-occurring abiogenic carbonates, implying that this taxon did not have thermoregulation comparable to modern birds, but was able to elevate its body temperature above environmental temperatures. Therefore, we observe no strong evidence for end-member ectothermy or endothermy in the species examined. Body temperatures for these two species indicate that variable thermoregulation likely existed among the non-avian dinosaurs and that not all dinosaurs had body temperatures in the range of that seen in modern birds.

BackgroundThe Cretaceous vertebrate assemblages of Romania are famous for geographically endemic dwarfed dinosaur taxa. We report the first complete egg clutches of a dwarf lithostrotian titanosaur, from Toteşti, Romania, and its reproductive adaptation to the “island effect”.Methodology/FindingsThe egg clutches were discovered in sequential sedimentary layers of the Maastrichtian Sânpetru Formation, Toteşti. The occurrence of 11 homogenous clutches in successive strata suggests philopatry by the same dinosaur species, which laid clutches averaging four ∼12 cm diameters eggs. The eggs and eggshells display numerous characters shared with the positively identified material from egg-bearing level 4 of the Auca Mahuevo (Patagonia, Argentina) nemegtosaurid lithostrotian nesting site. Microscopic embryonic integument with bacterial evidences was recovered in one egg. The millimeter-size embryonic integument displays micron size dermal papillae implying an early embryological stage at the time of death, likely corresponding to early organogenesis before the skeleton formation.Conclusions/SignificanceThe shared oological characters between the Haţeg specimens and their mainland relatives suggest a highly conservative reproductive template, while the nest decrease in egg numbers per clutch may reflect an adaptive trait to a smaller body size due to the “island effect”. The combined presence of the lithostrotian egg and its embryo in the Early Cretaceous Gobi coupled with the oological similarities between the Haţeg and Auca Mahuevo oological material evidence that several titanosaur species migrated from Gondwana through the Haţeg Island before or during the Aptian/Albian. It also suggests that this island might have had episodic land bridges with the rest of the European archipelago and Asia deep into the Cretaceous.

Titanosauria is a globally distributed clade of sometimes extremely large Mesozoic herbivorous sauropod dinosaurs. On the basis of current evidence these giant dinosaurs seem to have reproduced in specific and localized nesting sites. However, no investigations have been performed to understand the possible ecological and geological biases that acted for the selection of these nesting sites worldwide. In this study, observations were performed on the best-known Cretaceous nesting sites around the world. Our observations strongly suggest their eggs were incubated with environmental sources of heat, in burial conditions. Taking into account the clutch composition and geometry, the nature and properties of the sediments, the eggshells’ structures and conductance, it would appear that titanosaurs adopted nesting behaviors comparable to the modern Australasian megapodes, using burrow-nesting in diverse media and mound-building strategies.