LIFE × CLIMATE

Dinosaurs in the Dark

~80 to 66 million years ago

Phanerozoic · Mesozoic · Cretaceous

Dinosaurs nested north of the seventy-fifth parallel, where winter darkness lasted about four months and temperatures fell below freezing. Hatchlings of most of the families found there have turned up in the same beds, which undercuts the obvious explanation: these animals were not migrating north to breed and then leaving. Most of them, if not all, stayed.

Why it matters

An egg takes months to hatch and the growing season lasts about six. Animals unable to hold their own body temperature through the dark half of the year would have struggled to fit one inside the other, which is what makes these nests such a direct argument for warm-blooded dinosaurs. Unlike bone chemistry or posture, it is an argument anybody can follow.

Dating & uncertainty

The bone beds of Alaska and Chukotka fall in the last twenty million years of the Cretaceous. How hard the winters were is estimated from fossil leaves and tree rings rather than measured, and the two best sites disagree: bone microstructure from Chukotka carries no sign of seasonal stress, while that of the Alaskan hadrosaurs does.

Sources

1. Druckenmiller et al. (2021), Current Biology · Nesting at extreme polar latitudes by non-avian dinosaurs
2. Herman et al. (2016), Palaeogeography, Palaeoclimatology, Palaeoecology · Environmental constraints on terrestrial vertebrate behaviour and reproduction in the high Arctic of the Late Cretaceous
3. Bapinaev et al. (2023), Cretaceous Research · New data on high-latitude hadrosaurid dinosaurs from the Upper Cretaceous Kakanaut Formation of Chukotka, Russia

Earth history as one day

23:34:3725 min 22 s before midnight

if Earth’s 4.54 billion years were compressed into a single day