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 the temperature fell below freezing. Hatchlings of most of the families found there have turned up in the same beds, which rules out the obvious explanation: these animals were not flying north to breed and leaving. They stayed.

Why it matters

An egg takes months to hatch and the growing season lasts about six. That only works if the parents can hold their body temperature through the dark half of the year. These nests are among the strongest field evidence that dinosaurs were warm-blooded, and unlike bone chemistry or posture, it is an argument anybody can follow.

Dating & uncertainty

The Alaskan and Chukotkan bone beds 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 differ: bone microstructure from Chukotka shows no sign of stress, while Alaskan hadrosaurs do.

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