Frankie D. Jackson

Embryos and eggs for the Cretaceous theropod dinosaur Troodon formosus

Abstract Elongate and asymmetric eggs of the oospecies Prismatoolithus levis occur 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 Troodon exhibit long distal segments and radically different hindlimb proportions in comparison to adults. Orodromeus and other small vertebrate remains associated with Troodon egg horizons may represent prey of the adults during egg-laying and brooding. Troodon eggs 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.

A nesting trace with eggs for the Cretaceous theropod dinosaur Troodon formosus

ABSTRACT 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 c...

Nest Structure for Sauropods: Sedimentary Criteria for Recognition of Dinosaur Nesting Traces

Abstract 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.

Comparison of water vapor conductance in a titanosaur egg from the Upper Cretaceous of Argentina and a Megaloolithus siruguei egg from Spain

Abstract We calculated water vapor conductance (a product of eggshell porosity) from the first definitively identified sauropod egg (Megaloolithus patagonicus) from the Auca Mahuevo locality in Argentina. We then compared the results with those from M. siruguei (an egg type long associated with sauropod dinosaurs) from the Pinyes locality in Spain. The 14-cm Auca Mahuevo egg has a thinner eggshell and 47 times fewer pores than the 22-cm M. siruguei specimen. The resulting water vapor conductance (GH2O) of the titanosaur and M. siruguei eggs is 341 and 3979 mg H2O day−1 Torr−1, respectively; these values are two and ten times greater than in avian eggs of comparable size, but lower than in eggs of most modern reptiles. Clutches from Auca Mahuevo typically contain 20–40 eggs; in contrast, M. siruguei clutches from the Pinyes site average nine eggs. The GH2O of M. siruguei exceeds that of the Argentine egg by an order of magnitude, supporting previous inferences of egg burial. The GH2O of the Argentine titanosaur egg closely approximates that of Troodon and some oviraptorid eggs, previously calculated as equal to or two times greater than, respectively, the GH2O of avian eggs of similar size. Higher embryonic growth rates (relative to modern reptiles), especially in some dinosaurs with large clutch mass, may have required incubation in a more open environment, where water conservation represented a more critical factor than in a buried clutch. The lower GH2O calculated for the two megaloolithid eggs is consistent with previous interpretations of nesting mode that are based on site taphonomy and nesting traces. This study indicates that at least some dinosaurs did not fully bury their eggs.

A PHYLOGENETIC ASSESSMENT OF PRISMATIC DINOSAUR EGGS FROM THE CRETACEOUS TWO MEDICINE FORMATION OF MONTANA

Abstract Two 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 Troodon are 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.

Late Cretaceous avian eggs with embryos from Argentina

MARY H. SCHWEITZER1,2, FRANKIE D. JACKSON2, LUIS M. CHIAPPE3, JAMES G. SCHMITT2, JORGE O. CALVO4, and DAVID E. RUBILAR5 1Department of Microbiology, Montana State University, Bozeman, Montana 59717, schweitz@montana.edu; 2Department of Earth Sciences and Museum of the Rockies, Montana State University, Bozeman, Montana 59717; 3Natural History Museum of Los Angeles, Los Angeles, California 90007; 4Museo de Geologia y Paleontologia, Universidad Nacional del Comahue, Neuqu6n, Argentina; 5Instituto de Geociencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile, and Secci6n Paleontologia, Museo Nacional de Historia Natural, Casilla 787, Santiago, Chile

Fossil Eggs and Eggshell from the Lowermost Two Medicine Formation of Western Montana, Sevenmile Hill Locality

ABSTRACT In the last 30 years, the Two Medicine Formation of western Montana has provided a wealth of information about dinosaur reproductive biology. Here, we describe a fossil egg-bearing stratum that occurs approximately 105 m above the base of the formation. This site in the Sevenmile Hill outcrops south of Choteau, Montana, lies immediately above a volcanic tuff and bentonite, dated as 80.0 Ma. Spherulitic eggshells from a quarry at this locality are similar to Spheruprismatoolithus candensus Bray, 1999, which were assigned to the oofamily Prismatoolithidae. However, we refer this eggshell to the Spheroolithidae Zhao, 1979, as Spheroolithus choteauensis, oosp. nov. The quarry also produced Triprismatoolithus stephensi, oogen. et oosp. nov. These symmetrical 30 mm × 75 mm prismatic eggs exhibit three structural layers of calcite and round tubercles on the shell surface. Four additional ootaxa occurred at the First Find Microsite (OTM 99-19): Prismatoolithus hirschi, oosp. nov.; Tubercuoolithus tetonensis, oogen. et oosp. nov.; Continuoolithus canadensis Zelenitsky et al., 1996; and Krokolithes Hirsch, 1985. Spheroolithus choteauensis, T. stephensi, P. hirschi, T. tetonensis, and Krokolithes are unique to the lowermost Two Medicine Formation, whereas C. canadensis may occur elsewhere in the middle and upper strata of the Two Medicine and Oldman formations of Montana and Alberta, respectively. Although poorly represented by dinosaur osteological remains, the lower Two Medicine Formation locality yields egg types that suggest a significant difference in faunal composition compared to the middle and upper portions of the formation.

The First Dictyoolithus Egg Clutches from the Lishui Basin, Zhejiang Province, China

ABSTRACT Three clutches and eight additional eggs are described from the Upper Cretaceous Chichengshan Formation in the Lishui Basin of Zhejiang Province, China. The three clutches contain 6–18 partial or complete spherical eggs, 14–18 cm in diameter. The eggshell thickness and reticulate pore system are identical to Dictyoolithus hongpoensis Zhao, 1994. However, the eggshell lacks superimposed shell units previously reported in this oospecies. The difference in microstructure most likely results from the reliance on thin section analysis in the original study; furthermore, superimposed shell units are inadequately documented in all Dictyoolithus oospecies due to the absence of scanning electron microscopy (SEM) or low (×45) magnification of SEM imaging. Cladistic analysis and the presence of the following characters support a theropod affinity of Dictyoolithus hongpoensis: relatively narrow spacing of nucleation sites, mammillary cones with blocky calcite cleavage, prismatic columns, and parallel laminae within the shell units. We reevaluate previous assignment of Dictyoolithus eggs to the Dinosauroid-spherulitic Basic Type and advocate discontinuation of Basic Type and Morphotype in fossil egg classification.

3-D Modelling of Megaloolithid Clutches: Insights about Nest Construction and Dinosaur Behaviour

Background Megaloolithid eggs have long been associated with sauropod dinosaurs. Despite their extensive and worldwide fossil record, interpretations of egg size and shape, clutch morphology, and incubation strategy vary. The Pinyes locality in the Upper Cretaceous Tremp Formation in the southern Pyrenees, Catalonia provides new information for addressing these issues. Nine horizons containing Megaloolithus siruguei clutches are exposed near the village of Coll de Nargó. Tectonic deformation in the study area strongly influenced egg size and shape, which could potentially lead to misinterpretation of reproductive biology if 2D and 3D maps are not corrected for bed dip that results from tectonism. Methodology/Findings Detailed taphonomic study and three-dimensional modelling of fossil eggs show that intact M. siruguei clutches contained 20–28 eggs, which is substantially larger than commonly reported from Europe and India. Linear and grouped eggs occur in three superimposed levels and form an asymmetric, elongate, bowl-shaped profile in lateral view. Computed tomography data support previous interpretations that the eggs hatched within the substrate. Megaloolithid clutch sizes reported from other European and Indian localities are typically less than 15 eggs; however, these clutches often include linear or grouped eggs that resemble those of the larger Pinyes clutches and may reflect preservation of incomplete clutches. Conclusions/Significance We propose that 25 eggs represent a typical megaloolithid clutch size and smaller egg clusters that display linear or grouped egg arrangements reported at Pinyes and other localities may represent eroded remnants of larger clutches. The similarity of megaloolithid clutch morphology from localities worldwide strongly suggests common reproductive behaviour. The distinct clutch geometry at Pinyes and other localities likely resulted from the asymmetrical, inclined, and laterally compressed titanosaur pes unguals of the female, using the hind foot for scratch-digging during nest excavation.

Porosity and water vapor conductance of two Troodon formosus eggs: an assessment of incubation strategy in a maniraptoran dinosaur

Abstract Using tangential thin sections, we examined variation in porosity and water vapor conductance across two eggs of Troodon formosus, a small (∼50 kg) theropod dinosaur from the North American Upper Cretaceous, testing two hypotheses of egg incubation: (1) full burial within sediments or vegetation and (2) partial burial with exposed upper egg portions. We divided and sampled the eggs in five zones, 1 through 5 from blunt top to more pointed bottom. A geometric model composed of a hemisphere, cone, and paraboloid was used to estimate total and zonal volumes and surface areas. The 138 × 67 mm idealized Troodon egg has a volume, surface area, and mass of 296.4 cm3, 239.23 cm2, and 314.2 g, respectively. Zonal surface areas and volumes highlight the strongly asymmetric and elongate form of the Troodon egg. Geometric modeling provides better estimates of volume and surface area where egg shape diverges markedly from that of a typical bird egg. Porosity varies significantly across both Troodon eggs, with zones 2 and 3 having the largest pores and a majority (70–78%) of total conductance, whereas zone 5 has very low conductance. Total water vapor conductance in the two eggs are 31.85 and 40.62 mg H2O day− Torr−, values 76% and 97% of those predicted for an avian egg of similar size. Low total conductance compares favorably to values in extant birds and non-avian reptiles that incubate in open nests, arguing against full burial incubation. Together with nesting site evidence, low conductance values favor partial burial and incubation by a Troodon adult. Asymmetric egg shape concentrates volume, surface area, and conductance near or at the point of subaerial exposure. Among non-avian dinosaurs, the eggs of Troodon and troodontids are most similar to those of modern birds in having an asymmetric shape, low porosity, no ornamentation, and three structural eggshell layers.