Babies are not usually found beneath gooseberry bushes or in blankets carried by storks. They have parents. Mammals didn't appear by magic either. We had non-mammalian ancestors. The first mammals are known from the Upper Triassic, but our ancestors were already sprawling around over 320 million years ago. These were early synapsids.

If one walked by now, you'd probably think it was a weird reptile. That wouldn't be a bad description. The main difference was inside the head. These animals had an extra opening in the bone behind the eye. This made the skull lighter and provided some useful space for the attachment of jaw muscles, which allowed for a stronger bite.

Over time synapsids became less reptile-like. They developed several kinds of teeth in the same mouth, and the legs were joined to the body in a less sprawling type of way. At some stage, (probably during the Upper Permian), some synapsids became 'warm-blooded' and had hair. Synapsids were the dominant land animals back then, and came in many shapes and sizes. A plant-eater, Moschops, was five metres long and then one of the largest animals ever to have walked on land, but there were small insectivores as well. The end of that age was marked by the most devastating mass extinction known. Something like 90% of species on Earth died out in a relatively short amount of time, and this included most synapsids. The reasons for this are unclear.

A few groups did survive into the Triassic. One of these, the cynodonts, then flourished. A well-known meat-eater from South Africa is called Thrinaxodon. Its body was mammal-like in many ways. The creature was about 50cm long and pits on its snout strongly suggest it had whiskers. The mouth contained three kinds of teeth for more effective chewing, (incisors, canines and molars), and had a bony separation between the passages for air and food. Unlike reptiles, this animal could breathe and eat at the same time. Its upper body was clearly divided into two regions. The first was protected by the ribcage and contained the heart, lungs and so on. The stomach area had no ribs. This clear division was new. In other animals the ribs still go down as far as the hips. The extra space in Thrinaxodon allowed for the presence of a membrane called a diaphragm, which enabled the lungs to process oxygen more efficiently. All these details point to this animal being 'warm-blooded'. Its legs were positioned pretty much straight below the body and, like all but a couple of mammals; its neck was made up of seven bones, (vertebrae).

A somewhat larger Lower Triassic meat-eater was Cynognathus. It was even more mammal-like. All living mammals have a lower jaw made from a single bone, whereas other animals have multi-boned ones. In Cynognathus, over 90% of the jaw was formed by the main bone. Remains of this hunter have been found in South Africa, Argentina, China and Antarctica. There were also plant-eating cynodonts about, and a few of their fossilized burrows have even been found. These show clear evidence of communal living.

At the beginning of the Upper Triassic, about 225 million years ago, cynodonts were common and varied. However, they then mostly died out. Possible causes for that include the emergence of new animal groups, (especially dinosaurs), and changes in the climate. By the close of the Triassic, there were only a few types of cynodonts left. They shared many similarities and have tongue twisting names.

The largest of these survivors were plant-eaters of about 50cm long. These were tritylodontids and the family carried on until the Lower Cretaceous. They had some meat-eating, rat-sized relatives called tritheledontans. The lower jaws of these animals were peculiar. In all reptiles, the jaw is connected to the skull at the same joint behind the main bone. This was also the case for these trithes. In mammals, there's only one bone in the lower jaw. This means there's also an entirely different joint for keeping it attached to the head. Trithes such as Pachygenelus had that one as well; it had both the 'reptile' and 'mammal' joints.

The third surviving group was very similar, though smaller. The 'mammal' jaw joint was much better developed, and their teeth were even more efficient. The uppers and lowers were neatly matched up, so they worked better as a team. Unlike in other animals the teeth were replaced only once during a lifetime. This group survived for far longer than any other cynodonts. We call them mammals.

Fact File

Moschops
Meaning: 'calf eye'
Place: South Africa
Age: Upper Permian
Remarks: This animal was a large plant-eater of up to five metres in length.

Thrinaxodon
Meaning: 'trident tooth'
Place: South Africa
Age: Lower Triassic
Remarks: Lots of excellent fossils of both adults and young are known for this animal.

Cynognathus
Meaning: 'dog jaw'
Place: South Africa, Argentina, China and Antartica
Age: Lower Triassic
Remarks: This hunter grew to about the size of a wolf. The best remains are from South Africa. Its skeleton is very similar to that of a contemporary plant-eater called Diademodon. Should the head be missing, it's often impossible to tell them apart.

Tritylodon
Meaning: 'three cusped tooth'
Place: South Africa and Lesotho
Age: Lower Jurassic
Remarks: Tritylodon remains are also common. This herbivore was probably a burrower.

Diarthrognathus
Meaning: 'two joint jaw'
Place: South Africa
Age: Lower Jurassic
Remarks: The name refers to the presence of both the 'reptilian' and 'mammalian' joints on the same jaw. Like most early mammals, (Diarthrognathus isn't quite a mammal, but similar), this creature was a small insectivore. The increasing success of mammals may well explain the disappearance of these animals. None are known beyond the Lower Jurassic.