In new research, paleontologists used synchrotron X-ray tomography of annual growth increments in the dental cementum of fossil mammals from three faunas across the Jurassic period to map the origin of patterns of mammalian growth patterns, which are intrinsically related to mammalian warm-bloodedness.
Mammals in a Jurassic forest. Image credit: © Zhao Chuang.
“This is the first time we’ve been able to reconstruct the growth patterns of these early mammals in such detail,” said Dr. Elis Newham, a postdoctoral researcher at Queen Mary University of London and the University of Bonn.
“By studying the spacing and texture of these growth rings, we can not only tell how fast they grew at different stages of life, but also make inferences about their metabolism and overall life history.”
The current study challenges previous assumptions about the growth patterns of mammal ancestors and suggestions that these animals may have grown more similarly to modern mammals.
Instead, the study answers the question posed by similar recent studies of early mammal ancestors: When did the modern mammal life history evolve?
The researchers found that the first signs of the modern mammal growth pattern — high growth rates in young animals that are arrested at puberty — originated amongst the earliest true mammals around 130 million years ago, in comparison with relatively little change through life in earlier evolving mammaliaforms.
However, like mammaliaforms, these animals still grew more slowly and lived for much longer than living small mammals like rats and mice, reaching maximum lifespans anywhere between eight to 14 years-of-age.
The timing of this growth rate change, alongside changes in the structure of growth rings, indicates when these animals underwent puberty, and potentially when they became sexually mature.
“These data suggest that while living small-bodied mammals are sexually mature within months from birth, the earliest mammals took several years to reach sexual maturity, corroborating recent findings for one of our studied animals, Krusatodon,” said Dr. Pam Gill, a researcher at the Natural History Museum London and the University of Bristol.
“We further find here that this long, drawn out life history was common amongst early mammals all the way through the Jurassic.”
“The results suggest that the unique life history traits of mammals, like high metabolic rates and extended parental care, evolved gradually over millions of years,” Dr. Newham said.
“The Jurassic period appears to be a pivotal time in this evolution.”
The researchers used a technique called synchrotron X-ray tomography to image tiny growth rings in fossilized root cementum, the boney tissue attaching teeth to the jaw. These rings are similar to those found in trees, but on a microscopic scale.
By counting the rings and analyzing their thickness and texture, they were able to reconstruct the growth patterns and lifespans of these extinct animals.
“This study is a great example of how new technologies are revolutionizing our understanding of the deep past,” said University of Bonn’s Professor Thomas Martin.
“By peering into these fossilized teeth, we can gain valuable insights into the lives of creatures that lived millions of years ago.”
“It’s been so exciting to be involved in this project,” said Dr. Jen Bright, a researcher at the University of Hull.
“Putting Jurassic fossils into a particle accelerator (synchrotron) and reconstructing the past from it sounds like science fiction, but we can actually do it!”
The findings appear in the journal Science Advances.
_____
Elis Newham et al. 2024. The origins of mammal growth patterns during the Jurassic mammalian radiation. Science Advances 10 (32); doi: 10.1126/sciadv.ado4555
