A Theropod Nesting Trace With Eggs From the Upper Cretaceous (Campanian) Two Medicine Formation of Montana

Abstract

A nesting trace preserved in alluvial floodplain deposits in the Upper Cretaceous Two Medicine Formation at the Willow Creek anticline in north-central Montana contains four crushed theropod eggs referable to the oospecies Continuoolithus canadensis. These eggs immediately overlie the lower surface of a 35-cm-long 3 7cm-thick, dark-green mudstone lens, surrounded by reddish-purple mudstone. The long axes of three eggs are parallel to one another and to the lower boundary of the lens, whereas the fourth egg lies at a 30u angle to the others. A thin, 1-cm-thick organic horizon overlies the eggs, suggesting they were buried with some vegetation. Geometric modeling of the slightly asymmetrical C. canadensis eggs yields a volume and mass of approximately 194 cm 3 and 205 g for each egg. This method provides a more accurate estimation for the surface area than allometric equations that are based on modern bird eggs because of the elongate shape of many non-avian theropod eggs. Pore density and water vapor conductance (G H2O) calculated from one egg in the trace and five additional C. canadensis eggs from the Willow Creek anticline vary across three regions. High, moderate, and very low G H2O characterize the equatorial zone, blunt, and tapering poles, respectively. The average G H2O for all eggs exceeds that of an avian egg of similar mass by 3.93, thus supporting sedimentologic evidence of substrate burial during incubation.

Fossil egg and eggshells from the Upper Cretaceous Hell Creek Formation, Montana

“…5A, B) shares similar shell thickness with Continuoolithus canadensis (incertae sedis) reported from the Upper Cretaceous (Campanian) Two Medicine and Oldman formations of Montana and Alberta, respectively (Hirsch and Quinn, 1990;Jackson et al, 2015). However, the ML:CL ratio exceeds that of C. canadensis.…”

“…Ootaxa unique to this locality include Triprismatoolithus stephensi, Turbercuoolithus tentonesis, Prismatoolithus hirschi, and Spheroolithus choteauensis. In addition, four nesting traces are reported from Two Medicine Formation: Maiasaura (Horner and Makela, 1979), Troodon (Varricchio et al, 1997(Varricchio et al, , 1999, and unknown theropods associated with C. canadensis (Schaff, 2012;Jackson et al, 2015) and Montanoolithus strongorum .…”

Eggshell Porosity Provides Insight on Evolution of Nesting in Dinosaurs

“…Egg mass for living species was obtained from the literature ( S3 Table ) and that for fossil taxa/ootaxa was estimated from egg length and breadth using the equation of Hoyt [ 68 ] ( Table 3 ). Although other methods exist to estimate fossil egg mass (see [ 17 ]), they produce results that are consistent (within 10%) with Hoyt’s [ 68 ] method [ 17 , 39 ]. Therefore, for consistency, we applied Hoyt's method to all extinct ootaxa/taxa.…”

“…Elongatoolithid and prismatoolithid ootaxa are attributed to Oviraptorosauria [ 57 , 59 – 64 ] and non-oviraptorosaur maniraptorans [ 46 , 65 ], respectively, based on eggshell microstructure similarities with eggs of known taxonomic identity. The ootaxon Continuoolithus is assigned to an indeterminate theropod based on egg and eggshell morphology [ 9 , 39 , 66 ]. Moa eggshells have been assigned to two small-bodied (female body masses 20–30 kg) species, Pachyornis geranoides and Euryapteryx sp., based on DNA analyses [ 48 ].…”