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The repeated discovery of adult dinosaurs in close association with egg clutches leads to speculation over the type and extent of care exhibited by these extinct animals for their eggs and young. To assess parental care in Cretaceous troodontid and oviraptorid dinosaurs, we examined clutch volume and the bone histology of brooding adults. In comparison to four archosaur care regressions, the relatively large clutch volumes of Troodon, Oviraptor, and Citipati scale most closely with a bird-paternal care model. Clutch-associated adults lack the maternal and reproductively associated histologic features common to extant archosaurs. Large clutch volumes and a suite of reproductive features shared only with birds favor paternal care, possibly within a polygamous mating system. Paternal care in both troodontids and oviraptorids indicates that this care system evolved before the emergence of birds and represents birds' ancestral condition. In extant birds and over most adult sizes, paternal and biparental care correspond to the largest and smallest relative clutch volumes, respectively.A vian reproduction differs from that of other vertebrates in the extensive contribution of males to the care of eggs and young. Males participate in parental care in more than 90% of extant bird species (1). By comparison, males contribute to parental care in fewer than 5% of mammalian species and even more rarely among extant non-avian reptiles (1). The origin of this paternal contribution as well as the overall parental care system in ancestral birds remains controversial (2, 3). Maternal care predominates in crocodilians (the closest living sister taxon to birds), and the two major clades of extant birds use differing parental care systems. Neognathes (i.e., galliforms through passerines) typically exhibit biparental care, with females and males variably sharing incubation and care of the young (1-3). In contrast, males of nearly all Paleognathes (ratites, tinamous) incubate and care for the young alone (4). Cretaceous troodontid and oviraptorid dinosaurs share a close ancestry with birds and display some of their reproductive attributes, including multilayered eggshells, asymmetric eggs, and monoautochronic ovulation (5). Additionally, adult Troodon formosus, Oviraptor philoceratops, and Citipati osmolskae have been discovered on top of egg clutches, with some specimens retaining avian-like brooding postures (5, 6). We assessed the parental care system of these dinosaurs with the use of clutch volume-adult body mass models and bone histology data from brooding adults (7).Complete egg clutches for Troodon and the oviraptorids contain 22 to 30 large eggs (7). Total clutch volumes far exceed those of extant crocodilians, more closely matching those of polygamous ratites with similar adult size (Fig. 1). We generated regression models describing the adult (table S2). For birds, paternal care is associated with relatively larger clutch volumes than either biparental or maternal care over most body sizes; biparental care is associated wi...

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.

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