Hypothesis of homeothermy evolution on isolated South China Craton that moved from equator to cold north latitudes 250–200 Myr ago

• A hypothesis is put forward that animals can develop homeothermy only in a climate that progressively cools for millions of years.
• South China Craton cooled while drifted from equator to high northern latitudes of the Paleo-Tethys ocean, 250–200 Myr ago.
• When it collided with the North China Craton, the already cold adapted animals had spread toward northern China and adjacent Siberia.
• The Liaoning oasis delivered modern birds since it remained too cold for poikilotherms to enter and warm enough for homeotherms to evolve.
• After the Cretaceous–Paleogene extinction event killed the most of poikilotherms, homeothermic animals from northern Asia have inherited the Earth.

Based on avian and mammalian fossils found in the northeastern Chinese province of Liaoning and physiological traits linked to homeothermy, a hypothesis of evolution of homeothermic animals is proposed. It is based on the importance of muscle function in cold environment, as a strong selection pressure that favors endothermic metabolism during periods of cold climates. The presented hypothesis postulates that in progressively cooling environment, animals will develop thermal insulation, increased basal metabolism if food is available, and torpor when food is scarce. Since late Permian, Triassic and Cretaceous global temperatures were high, an exceptional place that gradually became cold was needed for the homeothermy evolution.South China Craton is here proposed as a plausible candidate for that role since it drifted across the Paleo-Tethys ocean, from equator to high northern latitudes in a journey that lasted from 250 to 200 Myr ago. After this small continent collided with North China Craton some 200 Myr ago, the already cold-adapted animals had spread to large, mostly empty spaces on the North China Craton, due to their evolutionary advantage of making active living in the cold environment. The most advantageous early homeothermic animals went further north to the cold Liaoning to start an oasis that delivered modern birds during next 50 Myr. Modern mammals possibly evolved somewhere in the cold vicinity. This made Liaoning and similarly cold places the cradles of early birds and early mammals since for the following millions of years these places remained too cold for poikilotherms to enter and warm enough for homeotherms to dwell, until the Cretaceous–Paleogene extinction event and subsequent global cooling that diminished poikilotherms. Homeothermy was probably even more important as a survival advantage in cooler climates of Paleogene, when mammals and birds became dominant animals.This interpretation is probably supported by a recent report that a small primate Archicebus achilles dated ~55 Myr ago was found in the more south central China Hubei Province, while Hadrocodium wui, extinct mammaliaform that lived 195 Myr ago in what is now the Yunnan province in southwestern China. The final confirmation of the proposed scenario would be to find somewhere in South China avian fossils older than those found in Liaoning, or to find the cradle of early mammals somewhere in Northern China.