The Mesozoic was 'the Age of the Dinosaurs'. When alive, between about 230 and 65 million years ago, those dinosaurs shared the world with a wide variety of other animals. For example, the earliest known birds flew by roughly in the middle of that enormous time span. There were also pterosaurs in the sky and various reptiles with strange names in the sea; ichthyosaurs and plesiosaurs among them. What less people know is that there were mammals on the land. Two-thirds of mammalian history is Mesozoic.

The first mammals are not quite as old as the earliest dinos but they also have strange, scientific names. One of the best preserved is called Morganucodon. As prehistoric 'monsters' go, this was a 10cm titch which lived during the Upper Triassic and Lower Jurassic. Judging by its pointy teeth, Morganucodon must've eaten insects. The way the ears are built shows it had a good sense of hearing, so it may well have been most active at night. Although very small, this critter spread across much of the world. Fossils have been found in Europe, China and North America.

Considering it was a mammal, Morganucodon looked a bit strange. If one were to scuttle by now, you'd notice its legs stick out somewhat to the sides and there's no sign of any external ears. The inside of the ear was even odder. All living mammals have three small bones in there to help them hear. Morganucodon also had these bones but two of them were at the back of the lower jaw, rather than in the ear. In part, it listened with its mouth which was wired for sound! Talking of mouths, the pointy teeth were less complicated and less effective than in modern mammals.

Various types of old fashioned mammals followed on from Morganucodon, and died out sooner or later. The most widespread are known as multituberculates. The name refers to the many cusps (tubercles) on their cheek teeth. Multis, as their fans often call them, lived rather like our rodents do today, but they weren't closely related. There's no sign of any mice or rats in the Mesozoic. Whatever the causes of the famous extinctions 65 million years ago, they don't seem to have bothered the multis. These animals survived for about 30 million years longer, and were slowly replaced by Mickey Mouse and relatives.

Virtually all mammals of the Mesozoic were small or tiny, though most living ones are too. A few 'giants' are known, but none were bigger than middle-dog-size. One of the largest found so far was a meat-eater called Repenomamus from Liaoning in China.

Liaoning's now widely known for the quality of its fossils. 125 million years ago, this place was home to feathery dinosaurs and primitive birds. There were also some other great mammals including a couple named Sinodelphys and Eomaia. Both were small, 15 - 20cm in length. The skeletons show they were good climbers, unlike Repenomamus. Very unusually, their fossils come complete with impressions of fur; the oldest preserved hair in the world. This doesn't mean earlier mammals were baldies. Things such as hair and feathers almost never fossilize, but Liaoning is a freaky place.

These two small animals are important for other reasons which involve a couple more strange words. Sinodelphys belongs to a mammal group called Metatheria. This includes marsupials such as koalas and Skippy the Bush Kangaroo. I know those animals come from Australia, but their oldest known close relative was Chinese. Later, marsupials reached every continent on Earth, including Antarctica. It hasn't always been covered in ice.

Eomaia is perhaps even more important, but I'm biased. It's the earliest known eutherian. That group includes placental mammals such as horses, puddy cats and people. Neither of these fossils will ever be as famous as Tyrannosaurus rex. They're about sixty times smaller and 55 million years older. T. rex died and left no descendants. Eomaia died too. However, it (or something much like it) led to you. The scientific word for that is Megacool!

Fact File

Morganucodon
Meaning: 'Morgan's tooth'
Place: Western Europe, China & North America
Age: Upper Triassic - Lower Jurassic
Remarks: Many jaws and other remains have been found in Wales. Finds from Yunnan include skulls and skeleton material.

Repenomamus
Meaning: 'reptile-mammal'
Place: Liaoning, China
Age: Lower Cretaceous
Remarks: The skull-length of this mammal reached over 10cm, which is unusually large for the Mesozoic. At least seven skulls have so far been recovered, (as of 2003), and several partial skeletons have also been found. Repenomamus could easily have preyed upon small dinosaurs and other animals.

Sinodelphys
Meaning: 'Chinese delphys'
Place: Liaoning, China
Age: Lower Cretaceous
Remarks: Sinodelphys is known from an almost complete specimen described just before Christmas in 2003. It's an ancient relative of marsupials. This is shown by various features, but especially the construction of the wrists, ankles and teeth. The word delphys is often used for marsupials and their near relatives. I'm not sure why. It literally translates as uterus.

Eomaia
Meaning: 'dawn mother'
Place: Liaoning, Chinese
Age: Lower Cretaceous
Remarks: This genus is known from another stunning fossil. Before it was described in 2002, the oldest direct evidence of mammalian hair dated back to about 60 million years ago. Eomaia is 65 million years older still. Both she, Sinodelphys and Repenomamus lived at the same place and time. Five further types of mammal have so far been recovered from Liaoning.