Ancient little beast: Although I grow slowly, I live long!

Feng Weimin

Recently, an article published in the international journal Nature stated that the lifespan and development period of early small mammals may be longer than those of modern mammals of similar weight. This view has challenged the traditional understanding of the growth pattern of mammals and has attracted widespread attention from scientists.

Mammals are the most advanced animals in the animal world with the most perfect physiological functions. Compared with other animals, the most prominent feature of mammals is that they are viviparous, and their young are raised by the milk secreted by their mothers.

Mammals appeared on the evolutionary stage slightly later than dinosaurs. In the evolutionary process accompanied by dinosaurs in the Mesozoic Era, mammals have always been a small group, and they could not compete with the "giant" dinosaurs. However, in the Cenozoic Era, many tall and powerful animals emerged, and many small animals were also preserved: for example, the American mole is less than 5 cm long and weighs about 10 grams; the dwarf opossum is 5-10 cm long and weighs about 10 grams.

The life span of modern small mammals is usually between one and three years, such as rodents such as mice and weasels. Due to their small size, these animals usually grow quickly, mature earlier, and have more offspring, but their life span is correspondingly shorter.

The research team of the National Museum of Scotland in the United Kingdom reported that partial bones of juvenile and adult columnar teeth were found in the Jurassic strata of the Isle of Skye in Scotland. After analyzing the growth increments of their teeth, the researchers found that the adults were about 7 years old, and the juveniles were only about 7-24 months old when they died, and were in the process of replacing their deciduous teeth with permanent teeth. Based on the length of the teeth and the circumference of the forelimbs and thigh bones, the researchers estimated that the weight of an adult columnar tooth was only about 58 grams. The degus weighs about 158 ​​grams, and the weight of the juvenile is about 51%-59% of that of the adult. This confirms that this columnar tooth is "slow-growing" and has a longer development period and a significantly longer lifespan than modern mammals of similar weight.

Interestingly, a scientific study published in Nature in 2022 pointed out that mammals became more diverse and larger after the extinction of dinosaurs. Blunt-footed mammals are the earliest known large herbivores that appeared during this period. They grow twice as fast as modern mammals of the same size and have a relatively shorter lifespan.

Looking at the entire evolutionary history of mammals, it is not difficult to find that the evolution of mammals is very magical. As the latest vertebrate to appear but the most dominant force in the contemporary world, mammals were actually quite weak in the Mesozoic Era facing the powerful dinosaurs, but became the king of the biological world in the Cenozoic Era. The body shape of mammals has changed several times. Not only did they have a small body shape for a long time in the Mesozoic Era, but they also became smaller during the extreme thermal event at the turn of the Paleocene and Eocene in the Cenozoic Era. In modern times, with the increase of the greenhouse effect, the body shape of mammals has shown a trend of shrinking again.

In the course of evolutionary history, the body shape and lifespan of large and small mammals have shown different changes, showing co-evolution with the climate environment. This is an important revelation for human understanding of nature and evolution.

(The author is a researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, and the honorary curator of the Nanjing Paleontological Museum)