Introduction
The ancestral mammal lineage split from reptiles around 325 mya, during the Carboniferous Period of the Paleozoic.
The Triassic Period of the Mesozoic, around 200 mya, saw the first mammals.
All mammals fall into one of three groups: the egg-laying monotremes, pouched marcupials and placentals (like us).
Mammal Ancestors
325 mya, Gondwana was moving closer to Laurasia to form Pangea. It was the last major ice age before the current one. Tropical and sub-tropical regions were home to coal swamps and provided a haven for animal life.
Sea levels rose and fell as the polar ice waxed and waned. Huge trees of the swamps drowned and died forming a layer of coal, replaced by new ones when the seas subsided.
Amniotes had already evolved from primitive amphibians. They were able to lay their eggs away from water. And it was around this time of fluctuating sea levels that the amniotes would divide into two groups: the diapsids and the synapsids.
The distinguishing factor between these groups was the presence of two skull openings in the diapsids versus a single opening in synapsids. Diapsids would later evolve into reptiles; synapsids would evolve into mammals.
Between 303 and 307 mya there was a mass extinction of plant life: the Carboniferous Rainforest Collapse. This was due to a hotter, drier climate.
Having the ability to lay their eggs away from water, the synapsids had a great advantage over amphibians. A group of synapsids called the pelycosaurs dominated, with their differentiated teeth, large jaw muscles and vertebrae connected ina way that reduced the undulating side-to-side motion.
However, as the land became more arid towards the middle of the Permian Period, which followed the Carboniferous, the pelycosaurs declined.
In the tropics and temperate zones, a new type of synapsid appeared, the therapsid. Further advanced than the pelycosaurs, the therapsids:
- evolved more upright limbs so that they were able to stand straighter
- began to raise their metabolism
- started to develop better control of their body temperature and hair!
These changes may have been due to synapsids having to deal with seasonal changes at these higher latitudes.
Dicynodonts, a sub-group of terapsids, were the most numerous vertebrates in most ecosystems throughout the middle to late Permian. These dicynodonts were plant-eaters with beaks in place of most of their teeth.
By the late Permian, cynodonts entered the scene. This group of therapsids were smaller than the dicynodonts and had continued the journey of more upright limbs and an even faster metabolism. In addition, their teeth, jaws, muscles, brains and sensory systems were changing, too.
Making a Mammal
The closest that life ever came to being extinguished was 252-251 mya, between the Permian and Triassic periods. 90% of all species died in the megavolcanoes which erupted for several hundred thousand years.
While the rest of the therapsids declined to extinction, the cynodonts diversified throughout the Triassic. This survival was due to the Lilliput effect: a decrease in body size.
This allowed the cynodonts to:
- hide easier in burrows
- grow quickly
- reproduce earlier
A new sub-group of cynodonts also evolved. These tritylodontids are the closest cousins to mammals, walking fully upright.
There were other animals that survived the end-Permian event: turtles, lizards, crocodiles and the precursors to dinosaurs. And, while the cynodonts went small, crocodiles and dinsoaurs went big.
Many cynodonts may have become nocturnal to avoid these huge predators. Rather than developing their vision, the focus was on scent, touch and hearing. They also became fully warm-blooded.
But controlling body temperature requires more energy which in turn requires more oxygen and food. So how did the cynodonts adapt?
- red blood cells got smaller, allowing more oxygen to be carried
- upright stance and changes to vertebra reduced sideways movement that would compress lungs while walking – now they could walk and breathe at the same time!
- developed a diaphragm to pull air into the lungs
- secondary palate, allowing simultaneous eating and breathing
- structures in the nasal passage to warm the air
- divided jaw-closing muscles into separate strands, giving a more powerful and complex jaw motion
- jaw muscles attached to dentray (the bone holding teeth)
HOWEVER, the joint to the upper skull was weak.
Enter….the MAMMALS with their snazzy new jaw-closing joint between the dentray and squamosal bone of the upper skull. This allowed more powerful bites AND chewing – starting digestion before the stomach which made eating a much more efficient process.
Efficient chewing also required a new characteristic: occlusion (full contact between the upper and lower teeth). But this also meant that mammals had to ditch teeth which continually fell out and regrew as this would make chewing ineffective. Instead, mammals exhibit diphyodonty, two sets of teeth that last a lifetime…hopefully.
These early mammals also had a huge cavity in their skull to make room for a huge globular brain with its bulbous cerebral hemispheres. Two key features of this brain being:
- large olfactory bulbs for smell
- a neocortex, responsible for sensory integration, learning, memory and intelligence
Early mammals, such as Morganucodon, diversified and spread throughout Pangea. And then it started to split up…