The first mammals were like reptiles in terms of metabolic rate

The first mammals on Earth grew slowly and lived for a long time, like modern reptiles. This is the conclusion reached by British paleontologists, whose work was published in the scientific journal Nature Communications.

The first mammals appeared at the end of the Triassic period, 220-210 million years ago. Outwardly, they resembled small shrews and weighed only 4-5 grams.

In the past, scientists believed that the key traits of mammals, including their warm-bloodedness, arose at about the same time. We found that despite their complex brains and advanced forms of behavior, these animals lived long and leisurely, like reptiles, and not quickly and briefly, like modern mammals.

Alice Newham, one of the authors of the work, paleontologist at the University of Bristol in the UK
Paleontologists from the University of Bristol in the UK and the University of Helsinki in Finland first studied the tiny, pinhead-sized teeth of two of the earliest mammals, Morganucodon and Kuehneotherium, living in present-day South Wales 200 million years ago, in the early Jurassic using powerful synchrotron X-rays.

The structure of the teeth of the most ancient primitive mammals known to science says that the oldest human ancestors were small creatures and ate insects and fruits. For a long time, researchers believed that mammals began to “grow” and occupy new ecological niches after the extinction of the dinosaurs. However, recent studies show that this began to happen 20 million years before the fall of the asteroid that destroyed the dinosaurs.

Having studied the annual rings of the cement of the roots of the teeth, which are deposited every year, like the annual rings of trees, scientists have calculated that morganucodone lived up to 14 years, and cuneotherium – about nine, and not a year or two, as previously thought.

This discovery came as a big surprise to the researchers. In the past, paleontologists believed that warm-bloodedness and the associated high level of activity at any time of the day were one of the most ancient and important features of mammals, thanks to which these animals occupied those ecological niches that cold-blooded reptiles could not claim.

Newham notes that this does not necessarily suggest that morganucodons and cuenotherium were not warm-blooded. But this suggests that the ancient relatives of modern mammals were significantly inferior to them in the level of activity. How and when ancient mammals became more like modern mammals remains to be seen.