Researchers used X-ray tomography technology to image growth rings in fossilized tooth roots. Image source: Science Progress. A recent study published in the journal Science Progress reveals how early mammals grew and developed during critical periods of their long "life history". A research team including Queen Mary University of London used synchrotron X-ray tomography technology to image the growth rings in fossilized tooth roots, in order to infer the lifespan, growth rate, and even sexual maturity time of these ancient organisms. This is the first time that the growth patterns of these early mammals have been reconstructed in such detail. By studying the spacing and texture of these growth rings, people can not only know the growth rate of these animals at different stages of life, but also infer their metabolism and overall life history. New research answers an important question about the life history of early mammals: when did the growth patterns of modern mammals evolve? Previous studies have suggested that the growth of early mammals may have been more similar to modern mammals, but the results of this study challenge previous hypotheses. The team found that the first signs of modern mammalian growth patterns (high growth rates in juvenile animals that stop growing during puberty) originated from the earliest true mammals approximately 130 million years ago. In contrast, early evolved mammals have relatively small changes throughout their lives. However, the growth rate of these animals is still much slower than existing small mammals such as rats and mice, and their lifespan is much longer, with the longest lifespan ranging from 8 to 14 years. The time of this growth rate change, as well as the changes in growth ring structure, indicate when these animals enter puberty and when they reach sexual maturity. The data shows that although existing small mammals reach sexual maturity within a few months of birth, the earliest mammals took several years to reach sexual maturity. Further research has found that this long life history was common among early mammals throughout the Jurassic period. The unique life history characteristics of mammals, such as high metabolic rates and prolonged parental care, have evolved over millions of years. The Jurassic period seems to be a crucial period for this evolution. The team used synchrotron X-ray tomography technology to image tiny growth rings in fossilized dental bone (the bone tissue that connects teeth to the jawbone). These rings are similar to those in trees, but smaller in size. By calculating the number of rings and analyzing their thickness and texture, the research team was able to reconstruct the growth patterns and lifespans of these extinct animals. (New Society)