FUR AND FANGS-MAMMAL ORIGINS

MAMMAL-LIKE REPTILES

During the Permian, a group of mammal-like reptiles called the pelycosaurs evolved. You are probably familiar with one member of this group, the sail-backed Dimetredon('two long teeth') These animals did not survive the terrible mass extinction at the end of the Permian. That extinction was the largest one, so far to hit Earth. Up to 96% of ALL species died out. There were survivors. They radiated into an empty world and the groups statred to fill the ecological niches left open. The major mammal-like groups are described below.

DICYNODONTS ('TWO TRUE TEETH')

This group scraped through the Permian mass extinction with only one genera intact, Lystrosurus.From this base the dicynodonts spread across much of the early Triassic world.These animals were herbivores. They would have filled a similar ecological niche to such animals as deer and sheep that we see today. They were squat, sturdy animals and their most notable facial feature was their two teeth that stuck out walrus like. Most dicynodonts had only these two teeth and used their beak-like jaws to crop vegetation In spite of this early sucess by the time dinosaurs appeared they were all but extinct. They finally went the way of 99% of species that ever lived, extinct, in the Late Triassic.

CYNODONTS

The cynodonts are thought to be the group that gave rise to mammals at some time during the Late Triassic. This group also survived the Permian mass extinction. The most important change they displayed was in their teeth. They had specialised teeth, just as we do. Incisors, stabbing canines and complicated cheek teeth were present. Such a set of teeth would allow cynodots to chew their food more efficiently than most reptiles. Teeth are very important in mammalian palaeontology, as they are often the best way to tell between species and teeth are often the part of the animal that survives the longest, because they are hard and dense.

The cynodonts consisited of both carnivores and herbivores, foreshadowing the omnivorous diet of some mammals. There is evidence that some cynodonts had whiskers. If they had whiskers then they would have had fur too. Fur is strong evidence that cynodonts were warm-blooded,like mammals.

This image is of a reconstruction of Oligokyphus('small curved animal'), a mammal-like animal of the Jurassic that you can see in Bristol City Museum. The small fragments below the model are bits of bone and jaw, showing how important they are in identifying many vetebrates. Thanks to Rodger Vaughan. For more information on reconstructing fossil animals click here.

MAMMALS

It is often a matter of palaeontologists agreeing, rather than anything definite, that decides where one group starts and another stops. Cynodonts, as we have seen, had many mammal characters. Many of the differences are in soft tissues, which are usually not preserved in the fossil record.

Fortunately, teeth and jaws can help us to decide where fossils belong. The big change from reptiles to mammals is at the back of the jaw. Reptiles have seven bones in the lower jaw. The dentary, which houses the teeth, the articular, which forms the jaw joint and five other bones. Cynodonts have an additional joint in their jaws. Mammals have only one bone in the lower jaw, the dentary. The other bones of the lower jaw have moved into the middle ear to help us to hear.

The mammals appeared in the Late Triassic. Megazostrodon is a good example of these early mammals. It was very small, only about 1.5cm long and weighing 20-30g. It is thought that they were nocturnal. This is because they had larger brains, compared to their cynodont relatives, and the enlarged areas are the one that process sounds and smells. Such features help animals that move about at night. These animals ate insects. They were active at night, because the lizards and dinosaurs were active during the day. This would have allowed them to avoid having to compete with the lizards and avoid being eaten by dinosaurs.

The modern mammal groups, monotremes, marsupials and placentals appeared during the Cretaceous. Little is known of them during this period but they were there, waiting to radiate into the empty niches left as the archosaurs succumbed to extinction.