‘ve come to this late-night market to observe one of the first crucial steps in the from reptile to mammal story. The development of an amazing feature that gave them a key advantage. But only after dark. The mammals found a niche for themselves not so much in space as in time – at night, when the reptiles are not active.

A simple experiment with two pets that happened to be for sale in the market tonight can demonstrate why this is so.

This is a thermal camera, and it will show a cold body as black or very dark.

Late Night Market
Late Night Market

So, this Lizard which is on the table is cold-blooded, and it appears to be very much the same temperature as the table. Reptiles get much of their energy directly from the sun as warmth.

But there is no sun at night. As a consequence, it’s scarcely got the energy to move.

Warm-blooded puppy
Warm-blooded puppy

This puppy, on the other hand, is very active. And when you look at him with the camera, you can see that his body is very warm indeed.

From Reptile to Mammal

The mammals, very early in their history, developed the remarkable ability to generate heat within their bodies. They became warm-blooded, and they achieved this by driving their metabolism at a much higher rate. But to do that, you need extra fuel, extra food. A reptile like a lizard can go for many days without eating. But if a mammal is denied its food for several days, it will die. So, in order to keep their fuel bills down, In order to move from reptile to mammal, the mammals used a technique familiar to any householder – insulation. They coated their bodies, as this puppy has, with fur.

With warm blood and a covering of hair, Hadrocodium was free to hunt for insects in the cool of the night. But now came a new challenge – to find its way around in pitch darkness. Detailed analysis of Hadrocodium’s skull is revealing remarkable new evidence of a set of ingenious solutions to this problem and key in the move from reptile to mammal.

Professor Zhe-Xi Luo
Professor Zhe-Xi Luo

The clues are tiny and invisible to outside scrutiny. But Prof Zhe-Xi Luo, an expert on early mammals, is using a micro CT scanner to unlock the skull’s inner secrets. X-rays penetrate the rock and pick out detailed fossil structures within. The computer then builds a 3-D model of the bones, and, in particular, the cavity that once held the brain.

Professor Luo is able to identify an area that is clearly much larger than its equivalent in a reptile.

Professor Luo “If you look at the CT scan here, you can tell that, despite a tiny little skull, the brain is enormous. But one of the most striking features of this particular fossil is that it has very large olfactory bulbs.”

Olfactory Bulbs

When you say olfactory bulbs, those are the part of the brain that detects smell.

Professor Luo “Correct. This mammal must have had very refined sensory detection of all kinds of smell, allowing it to be active in the dark of the night.”

This powerful sense of smell would have helped Hadrocodium pick out the scent of the worms and insects it fed on. The scanners have also revealed a radical advance in a second sense that’s vital in the dark. Hearing. The tell-tale clue lies, surprisingly, in Hadrocodium’s jaw.

Reptile Jaw
Reptile Jaw

Professor Luo “One very interesting feature that is so unique about this fossil mammal is… very flat jaw. The surface on the inside of the jaw is perfectly flat. In the primitive, pre-mammalian forms, there are big grooves.”

Grooves like these indicate the presence of two key bones that are attached to the jaw of a reptile. Seen here in green and red. A third bone, coloured blue, transmits sound waves in its ear. In a mammal there has been a truly amazing evolutionary development. The two jaw bones have shifted to form, with the third… the middle ear. This three-bone arrangement opens up a range of higher-pitched frequencies that a reptile cannot hear.

It’s the system we have inherited inside our ears.

Professor Luo “So, in Hadrocodium, we get the earliest indication that the three ear bones so important for our hearing have already originated with this fossil.”

Now ears could pick up the faintest rustle in the undergrowth and guide Hadrocodium to any insects moving nearby. Prof Luo’s analysis has also identified a spectacular advance in a third key sense.

The Scientific View

Professor Luo “It also has very large areas responsible for skin touch.”

For touch?

Professor Luo “That’s right. Mammals have hairs. One of the most important functions of the hair is actually to give us the sensory touch, and this animal has already developed that.”

The use of hairs as touch sensors is perhaps most obvious from the way modern mammals use their whiskers. This brown rat relies on them for finding its way around at night or underground. At the base of each of those long hairs on its nose, there is a nerve receptor. And whenever the hair is touched, a message is sent up to the rat’s brain. It’s not just the whiskers, though. Hairs all over its body are wired up to its nervous system. This creates a sensory bubble, allowing the rat to map the world around it just by using its hairs.

195 million years ago, the hairs of Hadrocodium must have been wired up in the same way. This remarkable little creature now had a whole array of new powers with which to meet the challenges of the night.

A heightening of the senses powered by a growing brain had enabled the early mammals to survive in the shadow of the dinosaurs. And then, they also developed a radical new way of nourishing their young.

We can look for clues to this next crucial step in our evolutionary story from reptile to mammal in Australia. Not in fossils, but in the bodies of two highly unusual creatures that live there.

Related Posts

Vertebrates: the animals with a backbone the Dawn of Mammals
Monotremes: Platypus and Echidna from Australia
Titanotheres the mammal monsters that began as tiny creatures

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From Reptile to Mammal with David Attenborough

2 thoughts on “From Reptile to Mammal with David Attenborough

    • October 22, 2023 at 8:55 am
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      Hi,

      I hope you found the subject matter as fascinating as I did. Evolution is an incredible thing that affects our modern society more than we can comprehend.

      Reply

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