Mammal ancestors became warm-blooded in burst of Late Triassic evolution

Our ancestors may have become warm-blooded more suddenly than previously realised.

A new study, published in Nature, suggests that ancestors of mammals known as mammaliamorphs abruptly went from being cold-blooded ectotherms to warm-blooded endotherms, rather than experiencing an extended period of change.

This extremely fast burst of evolution would have rapidly redefined how these animals lived, transforming our ancestors into more active creatures that lived a wide range of lifestyles.

Dr Romain David, a researcher at the Museum who co-led the study, says, 'Until now, the general consensus was that the acquisition of endothermy in the mammalian lineage has been a gradual process, taking place over millions of years from the end of the Permian and through the Triassic, but our results suggest it has actually been very abrupt.'

'We found that the body temperatures of non-mammaliamorph synapsids were around 24-29⁰C, similar to extant lizards, but jumped by 5-9⁰C in mammaliamorphs around 233 million years ago, during the Late Triassic. This pushes the date for the evolution of endothermy later than anticipated.'

'It suggests that earlier forms of synapsids, such as the anomodonts or non-mammaliamorph cynodonts, may not have been as active or warm-blooded as we thought they were.'

When did mammal ancestors become warm blooded?

The point at which animals began to regulate their own body temperature is an important one in the evolution of life.

By controlling their temperature separately from the environment, mammals and birds are able to be active at any time of day in a range of environments, as well as being able to perform energetic behaviours such as flying.

However, the exact timing of this transition has remained uncertain. While it is relatively simple to judge whether a living animal is warm or cold-blooded, it is much more difficult when an animal is extinct.

Characteristics such as the presence of an erect gait or fibrolamellar bone, a type of bone associated with fast growth, have been used in the past to make estimates of when warm-bloodedness evolved. However, these features can be affected by other processes, making their results ambiguous.

Different studies using these methods have given a period of around 60 million years across the Permian and the Triassic when endothermy may have evolved. The new study proposes that the structure and function of the inner ear might be able to narrow this down.

'The inner ear contains structures known as the semicircular canals, which are crucial for an animal to keep its balance and navigate safely,' Romain explains. 'This is achieved using a fluid called endolymph, the viscosity of which is related to its temperature. It is a bit like when you heat honey, causing it to become more fluid.'

'We anticipated that the functional morphology of the canals could be linked to body temperature. Fishes generally live in colder environments than terrestrial animals and so are known to have particularly large semicircular canals, as would be expected to compensate for the more viscous endolymph.'

Preliminary tests carried out by Romain and co-lead author Dr Ricardo Araújo on living species suggested that the functional morphology of the canals was linked to the temperature of an animal. These models were then applied to fossils to estimate the body temperature of these creatures when they were alive.

What evolutionary changes led to warm-blooded mammal ancestors?

The new technique suggests that mammaliamorphs had body temperatures of around 34⁰C, similar to living mammals such as the duck-billed platypus, following a sudden increase of between 5-9⁰C in the Late Triassic.

This would have allowed them to be more active than today's cold-blooded animals, but less so than modern endotherms.

The scientists attribute this jump to an increased ability of mammal ancestors to generate body heat, and to trap it with structures such as hair and fur.

'Living mammals generally rely on brown adipose tissue as their primary heat generator, but they possess another heat-generation pathway which uses a protein known as sarcolipin,' Romain says. 'As the brown adipose pathway is seemingly absent in monotremes, it is possible that the sarcolipin pathway may have come first. There is suspicion this pathway is also used in birds.'

'It's not impossible that either the sarcolipin pathway or fur appeared in earlier synapsids, but these characteristics were likely combined for the first time in mammaliamorphs, leading to endothermy.'

Control of body temperature is also associated with greater control over development, and around the same time as endothermy is estimated to have evolved, mammaliamorphs also started to develop larger brains and more complex nervous systems.

While looking inside the ear to figure out body temperature works for mammals and their ancestors, it is not easily applicable to birds and their relatives. This is because rather than changing the shape of their semicircular canals, the ancestors of birds changed the composition of their endolymph instead as their body temperature increased.

As the endolymph has long since been lost, it is impossible to directly assess its composition, and how it changed over time in the bird lineage. This means that while the researchers would like to assess when bird ancestors such as the dinosaurs became warm-blooded, it will require further research.

The scientists behind the study hope to refine their technique further to investigate further groups, such as the plesiosaurs, in the future.

Dr Kenneth Angielczyk, the senior author on the paper, says, 'The origin of mammalian endothermy is one of the great unsolved mysteries of palaeontology. Many different approaches have been used to try to predict when it first evolved, but they have often given vague or conflicting results.'

'We think our method shows real promise because it has been validated using a very large number of modern species, and it suggests that endothermy evolved at a time when many other features of the mammalian body plan were also falling into place.'