One of the most defining characteristics of a mammal is warm-bloodedness. This feature helped the mammals to get out of the shadow of reptiles and spread across all the habitats on the planet. But, a recent study which conducted X-ray scanned several hundreds of mammal fossils; found out that they started out as cold-blooded reptiles.
Because of warm blood, animals like us are able to maintain our body temperature despite changing environment. This also helps us in staying active for longer period of time. However, this was the gift of evolution as it took some time for the mammalian kind to change the nature of their blood.
The fossils were almost 200 million years old. These bones and teeth belong to certain animals which we associate with members because of their specialised chewing teeth and larger brains.
With the help of X-ray investigations, the research team was able to get a glimpse into the growth rings in the teeth and blood flow in the bones.
The research has been published in Nature Communications. With the help of a strong international team of 20 scientists, the estimated life span of the earliest members have also been deduced for the first time.
In order to do so, several fossilised teeth of two mammals from the Jurassic period (Kuehneotherium and Morganucodon) had to be scanned in powerful “synchrotron” X-ray sources in Switzerland and France.
From these high resolution scans, scientists were able to study the cementum to look for annual growth lines. By counting the rings, the estimated lifespan of Kuehneotherium and Morganucodon were found out to be 9 and 14 years respectively.
This is a lot in comparison to the smaller shrew-sized mammals of today, indicating that earlier mammals had a very slow metabolism which resembled reptiles.
On measuring the femur of Morganucodon, the density of limb bones were found to be even higher than those of reptiles. A limb bone is generally proportionate to the amount of sustained activity that an animal is capable of.
This means that small animals tend to have lower activity levels. But the femur indicated that the Jurassic mammal had the ability to carry out sustained activities.
Studying both the lifespan and the activity levels of the ancient mammal together gave the scientists a new insight into how mammals had a much slower life before becoming much active like present day counterparts, thus, becoming completely warm-blooded.