First warm-blooded mammals appeared about 233 million years ago

(ORDO NEWS) — An international team of scientists has shown that warm-bloodedness probably appeared in mammals in the late Triassic period, about 233 million years ago.

The researchers came to this conclusion by analyzing the structure of the semicircular canals of the inner ear of mammals and their ancestors.

Warm-bloodedness, or endothermicity, is the body’s ability to maintain a constant body temperature (31-45 ° C) due to metabolic activity, regardless of the ambient temperature.

This essential feature of mammals and birds allows them to occupy a variety of ecological niches, survive in cold conditions, remain active for a long time, make long-distance migrations, and move quickly.

Earlier, Naked Science wrote about a study whose authors suggested that warm-bloodedness may have first emerged over 300 million years ago in the ancestors of all amniotes (reptiles, birds, and mammals).

However, understanding when warm-bloodedness first appeared in mammals remains a mystery, as most fossil evidence is ambiguous.

Now an international team of scientists has hypothesized that mammals became warm-blooded around 233 million years ago. The results of the study are published in the journal Nature.

First warm blooded mammals appeared about 233 million years ago 1
The size of the semicircular canals differed between warm-blooded and cold-blooded synapsids

Scientists came to this conclusion by analyzing the structure of the semicircular canals of mammalian ancestors. This is a part of the vestibular apparatus, formed by the bony labyrinth of the inner ear, where the membranous labyrinth is located, filled with fluid – endolymph.

Receptors of the semicircular canals respond to endolymph vibrations caused by head movement. It is necessary for navigation, balance, perception of space and coordination of movements.

The viscosity of the endolymph depends on the body temperature of the animal. The transition to warm-bloodedness would reduce the viscosity of the endolymph, which would negatively affect the function of the semicircular canals.

While an increase in behavioral activity would probably require an increase in their effectiveness. Therefore, there may have been morphological changes in the semicircular canals during this transition.

Scientists studied how the semicircular canals changed in 56 extinct synapsids (mammals and their ancestors belong to this group). The soft tissues of the membranous labyrinth have not been preserved in fossils, but are very important for understanding biomechanics.

To recreate the structure of the inner ear of fossil mammals, the authors of the work evaluated the ratio of bone and soft tissues of modern animals.

The results of the study showed that about 233 million years ago, the structure of the semicircular canals in the Mammaliamorpha group changed dramatically: they became narrower and decreased in size.

Probably, it was then, in the Late Triassic period, that mammals acquired warm-bloodedness. Contrary to previous speculation, the ancestors of modern mammals prior to the Mammaliamorpha were most likely cold-blooded.

The Late Triassic was accompanied by climate instability, and warm-bloodedness may have evolved as an adaptation to these changing conditions.


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