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ABSTRACTEggs containing well-preserved skeletons were collected from Bugin-tsav, an Upper Cretaceous locality in the Nemegt Formation, Ömnögov' Aimag, Mongolia. These embryos, found in a weathered nest of eggs, are oviraptorid theropods. Eggshell morphotype is Elongatoolithidae, typical of theropods, including basal birds. Bone histology indicates that all embryos were probably close to hatching, based on the degree of ossification and in comparison with ossification patterns in living birds. Maturity of ossification indicates that oviraptorids hatched at a precocial stage of development. Two of the embryos are relatively large, while the other is 25% smaller, suggesting that size disparity of hatchlings in oviraptorids may be a consequence of asynchrony in egg laying.

The first in situ turtle egg clutch reported from China comes from the Lower Cretaceous (Albian) Liangtoutang Formation in the Tiantai Basin, Zhejiang Province. This clutch originally contained a minimum of 27 eggs, but four eggs separated from the clutch during excavation. The spherical eggs vary from 34 to 52 mm in diameter. The eggshell is 0.7-1.0 mm thick and has straight, narrow shell units with parallel margins and a height-to-width ratio of 2.5-3:1. Two eggs from this clutch previously were used to establish the new oogenus and oospecies Tiantaioolithus jiangi Fang et al., 2003, within the oofamily Testudoolithidae Fang et al., 2003. Based on our examination of all available eggs from this clutch, we propose the following: (1) Testudoolithidae Fang et al., 2003, is a junior homonym for Testudoolithidae Hirsch, 1996; (2) Tiantaioolithus Fang et al., 2003, is a junior subjective synonym of Testudoolithus Hirsch, 1996; and (3) the eggs pertain to a distinct oospecies, namely Testudoolithus jiangi (new combination). A previously unreported, isolated egg from the Upper Cretaceous (stage unknown) Chichengshan Formation in the Tiantai Basin is also referred to Testudoolithus jiangi based on its similar size, shell thickness, and shell unit height-to-width ratio; this specimen thus extends the fossil record of this oospecies into the Late Cretaceous. Taphonomic assessment of the egg block suggests that the egg clutch was buried in the substrate in a manner similar to modern turtles. The large, spherical eggs and large number of eggs in the clutch indicate the eggs were laid by a turtle taxon of large body size. The thick eggshell and sparse pores penetrating the shell indicate adaptation for a relatively arid, terrestrial environment.

ABSTRACTTwo egg types from the Upper Cretaceous Two Medicine Formation of Montana are described and incorporated into a phylogenetic analysis of egg characters. Small (7.5 × 3.5 cm), ellipsoidal eggs with a surface ornamentation consisting of isolated round tubercles represent a new, unnamed ootaxon. The microstructure includes narrow, prismatic shell units with three structural layers. The second egg type, oospecies Prismatoolithus levis, belongs to the theropod Troodon formosus. Although previously described, the presence of a third, external layer had been overlooked. Both eggs display several features typical of avian eggs: narrowly spaced nucleation sites, barrel-shaped mammillae with blocky crystal habit, a squamatic-like texture in the prismatic layer, and a third, structural layer. In addition, the new egg type exhibits a cuticle layer and the eggs of Troodonare asymmetric.Cladistic analysis of 14 fossil and extant taxa using 15 egg and shell characters favors a phylogeny consistent with more traditional analyses based on osteologic or genetic data and supports a theropod dinosaur origin for birds. No single character unambiguously distinguishes the eggs of avians from those of non-avian theropods, and the new Two Medicine egg type is recognized simply as that of a theropod. Results also indicate that resemblances in egg characters among non-avian and avian theropods are largely homologous and imply a high-level of similarity in reproductive physiology. Egg features are phylogenetically informative; better classifications and greater utility of eggs and eggshell will be gained through their phylogenetic treatment.

ABSTRACTElongate and asymmetric eggs of the oospecies Prismatoolithus levisoccur regularly in the Upper Cretaceous Two Medicine Formation of western Montana. These eggs had previously been assigned to the ornithischian Orodromeus makelai, for both juvenile and adult remains are typically associated with these eggs. Reexamination of the embryos shows them to exhibit at least 24 apomorphies of the clades Dinosauria, Theropoda and Paraves. The embryos also display a pneumatic quadrate, closely placed basal tubera, a high tooth count, a metatarsal II much narrower than IV and a strongly constricted metatarsal III, all possible synapomorphies of the Troodontidae. Presence of large basal tubera and a broadly rounded anterior border of the maxillary fenestra permit assignment to Troodon formosus.Most but not all bones appear ossified, suggesting a developmental level comparable to stages 35–38 of avian embryos and a time approaching hatching. Embryos show a consistent level of development from one egg to another indicating synchronous hatching of the clutch. Embryonic Troodonexhibit long distal segments and radically different hindlimb proportions in comparison to adults. Orodromeusand other small vertebrate remains associated with Troodonegg horizons may represent prey of the adults during egg-laying and brooding.Troodoneggs show several aspects either shared or convergent with some birds, and further demonstrate the close relationship of Troodontidae and Aves. These features include: asymmetric egg form, non-branching angusticanaliculate pores, distinct structural differentiation of the mammillary and overlying prismatic layer, barrel-shaped mammillary cones with a blocky calcite cleavage, and prismatic structure visible throughout the second structural layer.

An unusual trace containing eggs of the 50 kg-plus theropod dinosaur, Troodon formosus, represents one of the best preserved dinosaur nests. This unique specimen (MOR 963) represents the actual nest structure and the direct product of Troodon behavior. The trace comes from the Campanian, Late Cretaceous Two Medicine Formation of Montana, and consists of a bowl-shaped depression with an internal area of ~1m2 surrounded by a distinct rim. A clutch of 24 tightly-placed eggs sat in the center and both nest and clutch show bilateral symmetry about a north-south axis. The trace occurs within a moderately well-developed micritic paleosol. A physically and chemically distinct mudstone covered the nest and represents overbank deposition.The nest protected the eggs by creating a suitable micro-environment during the lengthy egg-laying and incubation periods. Clutch and nest size, shape, and symmetry and low organic carbon of the overlying mudstone suggests brooding rather than incubation with vegetative cover, although the latter cannot be ruled out. The nest probably played no role in the post-hatching care of precocial Troodon young.Reproductive traits indicated by MOR 963 show that Troodon possessed plesiomorphies shared with crocodilians (some burial of eggs and lack of egg rotation), apomorphies shared with birds (open nests, exposed eggs, and incubation by a brooding adult), but also at least one unusual feature (steeply-inclined eggs) not found in either extant archosaur group. Some reproductive features typically associated with living birds first evolved within non-avian coelurosaurian theropods like Troodon.

The bone microstructure of three third metatarsals and two tibiae of differing sizes from Troodon formosus was examined. Bone in T. formosus passed through three ontogenetic stages: rapid fibro-lamellar, moderate lamellar-zonal, and slow avascular lamellar growth. Highly-vascularized fibro-lamellar bone accounted for the majority of growth, with adult size possibly being reached in 3 to 5 years. No significant growth occurred after T. formosus reached a body weight of roughly 50 kg (femur: tibia length 310:425 mm). Abundant vasculature together with dense Haversian bone suggests T. formosus had a relatively high metabolism. Other gross morphological features (large brain, large eyes, cursorial adaptations including high tibia: femur ratio) would indicate T. formosus was very active and possibly endothermic.