Evidence for heterothermic endothermy and reptile-like eggshell mineralization in Troodon, a non-avian maniraptoran theropod

The dinosaur-bird transition is among the most fascinating events in evolutionary history, but several biological aspects such as changes in reproductive system, nesting strategy, and body temperature are still poorly understood. Dual clumped isotope thermometry (Δ₄₇ and Δ₄₈) can shed light on these biological aspects in fossils. Our results show that eggshells of modern reptiles and birds differ in their isotopic compositions. Interestingly, analyses of eggshells of Troodon, a non-avian theropod, reveal that it retained a slower, reptile-like mineralization to produce its eggs, despite having already evolved the capacity of changing its body temperature (heterothermic endothermy) like modern birds. Our findings also suggest that Troodon possessed two functional ovaries and that their nests were shared by multiple females.