Early mammals had slower growth rates and lived longer 

Researchers have collaborated to explore the unique growth and development patterns of early mammals from the Jurassic period. 

By examining growth rings in fossilized tooth roots, paleontologists have been able to determine the lifespan, growth rates, and even the timing of sexual maturity in these ancient creatures. Their findings have been published in the journal Science Advances.

“Never before have we been able to reconstruct the growth patterns of these early mammals in such detail,” said Elis Newham, the lead author of the study and a postdoctoral researcher at Queen Mary University of London. During the study, Newham was also a research fellow at the University of Bonn.

Mammals during the Jurassic 

The experts focused on fossilized tooth roots from various mammal species dating from the Early to Late Jurassic periods (200-150 million years ago) discovered at three distinct sites. 

The fossils found in Wales are among the oldest known mammalian precursors from the Early Jurassic, while those from Oxfordshire, UK, represent a diverse range of early mammals that coexisted. The third site in Portugal provided fossils dating from the Late Jurassic.

Synchrotron X-ray tomography

To analyze the fossils, the team employed synchrotron X-ray tomography, a technique where electrons are accelerated to near light speed, offering several advantages over traditional X-ray imaging. 

This method allows the fossils to be analyzed intact, without the need for slicing, and produces higher quality images than conventional X-ray microtomography.

“We use synchrotron x-ray tomography of annual growth increments in the dental cementum of mammaliaforms (stem and crown fossil mammals) from three faunas across the Jurassic to map the origin of patterns of mammalian growth patterns, which are intrinsically related to mammalian endothermy,” explained the researchers.

Microscopic growth rings 

Using this advanced imaging technique, the experts were able to observe microscopic growth rings in the fossilized root cement, the bone tissue that connects teeth to the jaw. 

“The rings are similar to those in trees, but on a microscopic level,” said senior author Thomas Martin from the University of Bonn’s Vertebrates – Mammals working group. 

“Counting the rings and analyzing their thickness and texture enabled us to reconstruct the growth patterns and lifespans of these extinct animals.”

Jurassic mammals grew more slowly

The study revealed that the first signs of growth patterns typical of modern mammals, such as puberty growth spurts, began to appear around 150 million years ago. 

Unlike today’s small mammals, early mammals grew much more slowly but had significantly longer lifespans, living eight to fourteen years instead of the one or two years typical of modern small mammals like mice. 

However, early mammals also took much longer to reach sexual maturity, often requiring several years, compared to the few months needed by their modern counterparts.

“Although all fossils studied exhibited slower growth rates, longer life spans, and delayed sexual maturity relative to comparably sized extant mammals, the earliest crown mammals developed significantly faster growth rates in early life that reduced at sexual maturity, compared to stem mammaliaforms,” wrote the study authors. 

Distinctive life history patterns of mammals

The experts noted that the estimation of basal metabolic rates (BMRs) suggests that some fossil crown mammals had BMRs approaching the lowest rates of extant mammals. 

“We suggest that mammalian growth patterns first evolved during their mid-Jurassic adaptive radiation, although growth remained slower than in extant mammals.”

According to Newham, the 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. “The Jurassic period appears to have been a crucial time for this shift,” said Newham.

According to the study authors, the the mid-Jurassic radiation of crown mammals occurred coevally with the acquisition of the modern mammalian growth strategy. 

“However, physiological evolution followed a long-fuse pattern, with considerable developments remaining between the earliest crown mammals and extant species.”

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