200 Mya
First Mammals

The mammals known from fossils were small, smaller than a cat. They were probably nocturnal as they were endothermic and could keep out of the way as the enormous reptiles and dinosaurs evolved. It wasn’t until the dinosaurs became extinct that they could develop.

Mammals descended from the earliest Amniotes via a group called Synapsids which are sometimes called ‘mammal-like-reptiles’, but this is incorrect as they have a common ancestor with reptiles and are not descended from them

The transformation from primitive synapsids and the fin-backed Dimetrodon to the weasel-like Thrinaxodon and finally to true mammals is one of the best transitional series in the entire fossil record.

The development of the Mammals involved a number of major changes from their antecedents. From a skeletal aspect, legs became more able to support a creature that was more upright, with the legs supporting the body higher off the ground.The digits on both front and rear limbs rotated to the front rather than the splayed flat feet of reptiles. Think of Lizards and Geckos nowadays. Rib cages became limited to the front part of the body, implying that inhaling was carried out by the Diaphragm rather than expanding the rib cage as reptiles do.

One of the most interesting skeletal changes came in the bones of the lower jaw. In reptiles, ‘hearing’ is done by them picking up vibrations through the lower jaw. Snakes do this when their jaw is in contact with the ground. They cannot hear when they pick their heads up. So much for snake charmers! In Mammals these bones have migrated to the inner ear and become the Malleus and Incus the hammer and anvil that we hear with. This is incidentally reflected in the human embryonic development, where the bones that become in your inner start off as cartilage in the lower jaw, they move and develop as the embryo develops.

Following the evolutionary steps.

It is not known when exactly fur or hair first developed but there are fossils from around 140 million years ago that definitely show underfur and hair.

Similarly, the other main mammalian traits of lactation and Endothermy (warm-blooded) are not possible to determine from fossil evidence, but the acquisition of hair and general body shapes do indicate these.

200 Mya First Mammals

Further reading:

200 Mya
First Mammals