Using a technique called synchrotron X-ray tomography to image growth rings in fossilised tooth roots, the researchers were able to estimate lifespans, growth rates, metabolic rates, and even the timing of sexual maturity in these ancient creatures.
The study 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 earlier evolving “mammaliaforms” there was comparatively little change through life. 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. However, like mammaliaforms, early mammals still grew more slowly and lived for much longer than today’s small mammals like rats and mice, reaching maximum lifespans up to 14 (mammaliaforms) and 11 (early mammals) years-of-age.
"Our results suggest that the unique life history traits of mammals, like high metabolic rates and extended parental care, evolved gradually over millions of years," explains Dr Ian Corfe.
Dr Ian Corfe, second author of the study, is currently a researcher at the Geological Survey of Finland. He was a member of the Jernvall Evo-Devo group (Department of Geosciences and geography/ Institute of Biotechnology, UH) when the synchrotron X-ray tomography imaging was carried out.
"The roughly 50 million year-long Jurassic period has previously been thought to be key for early mammalian evolution, with increasing diversity of species and body forms probably representing an evolutionary radiation. Our findings show it was an extremely important time in the evolution of mammalian physiology, metabolism, life history and growth patterns too," Dr Corfe says.
Laboratory engineer Heikki Suhonen, department of physics (UH) is a co-author on the paper. Other participants are from Queen Mary University of London, The University of Bonn, The Natural History Museum London, the University of Hull, the European Synchrotron Radiation Facility (France), the Swiss Light Source, the University of Southampton, the College of Osteopathic Medicine (United States), the University of Bristol, and the University of Edinburgh.
Original article: 'The origins of mammal growth patterns during the Jurassic mammalian radiation', in Science Advances.