PERMIAN-TRIASSIC MASS EXTINCTION: EVOLUTIONARY ARMS RACE

On the Fossil Huntress Podcast, we wrestled with the question of whether dinosaurs were warm-blooded or cold-blooded. It is an excellent question and there is good evidence on both sides of that debate.

Many dinosaurs stood upright — a warm-blooded trait. They are also the ancestors of birds who are warm-blooded. Dinosaurs often began life with porous bones, moving to denser bones later in life. This is as much a mark of growth rate as it is for the warm-cold debate. 

And, dinosaurs had small brains relative to body size — a trait of our cold-blooded animals. So, which is it? Cold or warm? My money is on the latter, but we'll likely have some time to wait before we have enough evidence to say for sure one way or the other. One thing we do know to be true is that we see a trend of the Earth's animals moving from cold-bloodedness to warm-bloodedness over time. 

What was the driver for that adaptation? One of the drivers looks to be the Permian-Triassic mass extinction event some 250 million years ago. It was a catastrophic event that killed ninety-five percent of all life on Earth. The remaining species were left to fight for survival against an inhospitable planet and one another. The few surviving species found themselves in a turbulent world —repeatedly hit by ice ages, rapid warming and ocean acidification cycles.

Through all of that, two main groups of tetrapods survived; the synapsids and archosaurs, ancestors of mammals and birds. The ancestors of both mammals and birds became warm-blooded at the same time.

Warm-bloodedness, or endothermy, is the ability to regulate your body temperature using your metabolism rather than relying on the external environment. Humans are endothermic. We eat food and wear warm sweaters to guard against the cold. Warm-bloodedness is key for both survival and reproductive fitness.

There is evidence of warm-bloodedness, including a diaphragm and whiskers in the synapsids as far back as the Triassic. This is supported by a more porous bone structure in both synapsids and archosaurs. Warm-blooded animals tend to have highly vascularized bone tissue. Cold-blooded animals have a denser bone structure that even exhibits annual growth rings. 

Dinosaurs show both traits. They start off life with highly vascularized bone which becomes denser as they mature. This move from vascular to dense bone may have more to do with growth rates than to whether the animals were warm or cold-blooded. 

Another factor in warmth is hair. We know that mammal ancestors had hair from the beginning of the Triassic. More recently, we have learned that archosaurs had feathers from 250 million years ago. Archosaurs are a group of diapsids and are broadly classified as reptiles. The living representatives of this group are birds and crocodilians. It also includes all extinct dinosaurs, pterosaurs, and extinct close relatives of crocodilians. 

Medium-sized and large tetrapods switched from sprawling to erect posture right at the Permian-Triassic boundary. As you know, most warm-blooded animals have an erect or upright posture and our cold-blooded friends tend to walk on all fours. 

The mass posture change and early origin of hair and feathers all speak to the beginning of a species arms race. In ecological terms, an arms race occurs when predators and prey compete on an escalated scale for survival. This pressure caused a rapid change in their evolution as their adaptations escalate. 

When we look at our world today, warm-blooded animals populate all areas of the Earth. They have fewer offspring and show intense parental care, taking months or years to care for their young before they become independent. These adaptations give birds and mammals an edge over amphibians and reptiles and we see this in their domination of the ecosystems in our world.

This revolution in ecosystems was triggered by the independent origins of endothermy in birds and mammals. This particular adaptation lives on as these species survive and thrive in an Earth that can be fickle in terms of environmental conditions.

Reference: Benton, Michael J. The origin of endothermy in synapsids and archosaurs and arms races in 
the Triassic, Gondwana Research, School of Earth Sciences, Life Sciences Building, University of Bristol, Bristol BS8 1TH, UKThe evolution of main groups through the Triassic. Image: Nobu Tamura