LAUSR

Dinosaur eggshell fragments attributed to the oofamilies Spheroolithidae and Prismatoolithidae and recovered from the latest Cretaceous Kakanaut Formation of North-eastern Russia (Chukotka) constitute one of the northernmost records of dinosaur reproductive behaviors. The high palaeolatitude of the locality (~75° N), as well as the cool near-polar climate, where summer temperatures only averaged 20 °C during the warmest month, dark near-freezing winters and egg incubation that could have lasted several months, raise questions about dinosaur reproductive strategies, particularly in terms of the timing of egg laying. In order to investigate seasonal aspects of Kakanaut dinosaur reproductions, carbonate from eggshell fragments have been analyzed for their oxygen and carbon isotope compositions, along with the oxygen and carbon isotope compositions of apatite phosphate and structural carbonate of associated theropod, hadrosaur and ankylosaur teeth as well as lepisosteid fish scales. Stable oxygen and carbon isotope compositions of eggshells from the Kakanaut Formation together with those of associated adult dinosaur teeth and fish scales reveal differences in mineralization timing between eggshells and teeth and show that eggs were laid at the very beginning of spring when snowmelt drained from nearby highlands. We propose that Kakanaut dinosaurs laid their eggs at the very beginning of spring in order to accommodate an incubation period that lasted several months. This timing would also benefit from mild temperatures and increasing food availability when the eggs hatch, allowing the hatchlings to grow large enough to survive the next winter or perhaps follow adult animals in their migration southwards.