The evolution of mammals is an interesting one. Mammals are a very diverse group- from the smallest Bumble bee Bat, to us humans, to the largest Blue Whale- mammals have ventured to every habitat, even returning to the sea or learning to fly. But where did it all begin? Here I will tell their story, whilst describing some interesting individuals.
Mammals first evolved from a group of ‘mammal-like reptiles’ called synapsids. In fact, all mammals are included in the Synapsida as we have an inferior temporal fenestra in our skulls. The phrase “miss you like a hole in the head” works perfectly here, as it was the loss of other fenestra that allowed this group to evolve! It also allowed new jaw muscle attachment. Synapsids were the largest group of vertebrates in the Permian Age, about 299 to 251 million years ago and some made it through the Great Permian Extinction. However, after this mass extinction, archosaurs truly reigned supreme but synapsids were always there in the background.
Synapsids are generally split into two groups; pelycosaurs and therapsids- the latter being the more advanced. ‘Pelycosaurs’ make up the six most primitive families of synapsids. They were all rather lizard-like, with sprawling gait and possibly horny scutes. The therapsids contain the more advanced synapsids, having a more erect pose and possibly hair, at least in some forms. The low-slung pelycosaurs have given rise to the more erect therapsids, who in their turn have given rise to the mammals.
Archaeothyris and Clepsydrops, the earliest known synapsids, lived in the Carboniferous Period and belonged to the series of primitive synapsids. Archaeothyris was relatively large, measuring 50 centimetres head to tail. It was more advanced than early reptiles and could open its jaws very wide in comparison. Although its sharp teeth were all of the same shape, it did possess a pair of enlarged canines, suggesting that it was a carnivore.
Clepsydrops also had a diet of small animals and insects, as did most early synapsids. It too had advanced jaws but still possessed the need to lay eggs, unlike modern mammals that give birth to live young (apart from the platypus and the echidna). These pelycosaurs, along with Archaeothyris, grew and diversified so much that they became the largest terrestrial animals in the latest Carboniferous and Early Permian Periods, ranging up to 6 m in length! However, appearance-wise, these animals were nothing like the mammals we know today; they had a sprawled stance and they still had reptilian-like skin (but with some ‘fuzz’). The major difference is that they were still cold-blooded and still had very small brains. Some developed sails to help raise their body temperature, like Dimetrodon. Dimetrodon is often mistaken for some sort of dinosaur in pop culture, but it went extinct 40 million years before dinosaurs even existed. Most Dimetrodon species ranged in length from 1.7 to 4.6 metres and are estimated to have weighed between 28 and 250 kilograms. Unfortunately, by the middle of the Late Permian, all of the pelycosaurs had either died off or evolved into their successors- the therapsids.
The more advanced theraspids appeared in the Middle Permian and included meat and plant eaters. Moschops (no, it’s not a Pokémon) lived around 265–260 million years ago and was massive, roughly 2.7-metre-long. Due to the fact that it possessed long-crowned, stout teeth, it is believed that Moschops was a herbivore feeding on nutrient-poor and tough vegetation and probably had to feed very frequently.
Theraspids fell victim to the Great Permian Extinction, with only a few making it through. One synapsid that flourished in the Triassic was Cynognathus-a meter-long predator. It had a particularly large head, 30 centimetres in length, with wide jaws and sharp teeth. Its hind limbs were placed directly beneath the body, but the fore-limbs sprawled outwards in a reptilian fashion. The lack of ribs in the stomach region suggests the presence of an efficient diaphragm, which is an important muscle for mammalian breathing. These creatures were accompanied with archosaurs, (who would give rise to the dinosaurs) who very often overshadowed theraspids.
There are three groups within the Triassic theraspids: dicynodonts were beaked herbivores; cynodonts were more mammal-like and had a range of diets and therocephalians were carnivores with large skulls. Unlike the dicynodonts, which remained large, the cynodonts became progressively smaller and more mammal-like as the Triassic progressed. The first mammaliaforms evolved from these during the Late Triassic. During the Jurassic and Cretaceous, the remaining nonmammalian cynodonts were small, such as Tritylodon. This creature was one of the most advanced groups of cynodonts. They were about 0.3m long and had a diet much like that of a modern-day rodent and even burrowed like rodents. There were large incisors at the very front of their mouth separated by a gap from the rest of the teeth. Even with their mouth closed the incisors would still stick out slightly visible. The legs were directly beneath the body like mammals, unlike the earlier therapsids with sprawling limbs. They still laid eggs but were now warm blooded.
These animals that lived during the Triassic and Jurassic had high metabolic rates, so they had to eat a lot of food. Therefore, they evolved the ability to chew and their teeth developed to assist them with rapid digestion. Limbs, which were previously sprawling to the side, moved underneath the body, allowing them to breathe more easily when moving.
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