Dinosaurs needed to be cold enough that being warm-blooded mattered
Briefly

The early invasion of cool niches... [suggests] an early attainment of homeothermic (possibly endothermic) physiology in [certain species], enabling them to colonize and persist in even extreme latitudes since the Early Jurassic,
Sauropods tended to stay in warmer areas, suggesting the need for sunlight and heat associated with ectothermy, possibly limiting their ability to survive in colder temperatures.
Read at Ars Technica
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