“Never before have we been able to reconstruct the growth patterns of these early mammals in such detail,” says lead author Dr. Elis Newham, a postdoc at Queen Mary University of London who during the study was an Alexander von Humboldt Research Fellow at the University of Bonn, up to March 31, 2024.
For the study, the team analyzed fossilized tooth roots of mammal species from the Early to Late Jurassic periods (200-150 million years ago) found at three separate sites. The finds made in Wales are of some of the oldest known mammalian precursors from the Early Jurassic period, while the fossils found in Oxfordshire, UK are of a very broad array of coexisting early mammals. The fossils from the third site in Portugal date from the Late Jurassic.
Fossil tooth roots X-rayed
The research team studied the fossils using a technique called synchrotron X-ray tomography in which electrons are accelerated to near light speed (unlike regular X-ray imaging). The technique affords several advantages, starting with the fact that the fossils no longer have to be prepared, i.e. cut up into slices, so they can be analyzed whole. Furthermore, images obtained via synchrotron X-ray tomography are of higher quality than images from conventional X-ray microtomography.
Researchers were able to image tiny growth rings in fossilized root cement—the bone tissue that attaches the teeth to the jaw. “The rings are similar to those in trees, but on a microscopic level,” explains Professor Thomas Martin of the Vertebrates – Mammals working group at the University of Bonn Institute of Organismic Biology, who is a senior author of the study. “Counting the rings and analyzing their thickness and texture enabled us to reconstruct the growth patterns and lifespans of these extinct animals.”
The researchers determined that the first signs of the growth patterns characteristic of modern mammals, such as a puberty growth spurt, started emerging roughly 150 million years ago. Early mammals grew much more slowly but lived substantially longer than today’s small mammals, with lifespans of eight to fourteen years instead of just one or two as in modern mice, for example. However, it took early mammals years to reach sexual maturity, again in contrast to their modern descendants which reach sexual maturity in just a few months.
“Our findings suggest that the distinctive life history patterns of mammals, characterized by high metabolic rates and extended parental care phases for example, have evolved over millions of years,” Dr. Elis Newham explains, “The Jurassic period appears to have been a crucial time for this shift.”
