Page 14 - Mammal (DK Eyewitness)

The evolution of mammals Skull from ANCESTOR?
MAMMAL
above
The cynodonts were
mammal-like
reptiles of the
Triassic period. Their
As far as we know, mammals teeth were not all the
same, as in other
appeared on Earth some 200 million Lower jaw reptile groups, but
were different shapes,
years ago. We know because we have specialized to do certain
found their fossils: bones, teeth, and jobs. This is one of the
mammal’s characteristics,
other parts that have been turned to although some modern species (such as the
stone and preserved in rocks. Since dolphins) have re-evolved teeth that are all the
same, in response to their diet (p. 51).
An early rhino? some of the features we recognize in Species shown: Thrinaxon liorhinus (S. Africa)
living mammals (warm blood, fur, and milk) do not fossilize we
must look for other clues. These must be based on the bones. ONE OF THE FIRST
So another two important features of being a mammal, alive or Embedded in rocks laid down in the middle
Jurassic, in what is now England, is this jaw of
fossilized, are a particular kind of jaw (only one bone in the a triconodont.
lower jaw, not several like the reptiles), and tiny bones in the These creatures
were some of the
middle ear cavity. Mammals did not exactly burst upon the earliest
evolutionary scene. During their first 100 million or so years, mammals, and
they were rat- to
life on land was mainly dominated by huge dinosaurs, while cat-size
pterosaurs flew above and ichthyosaurs swam in the sea. The predators.
Species shown:
first true mammals were probably small, shrewlike creatures Phascolo­
therium
that were active at night and fed by eating insects and bucklandi
stealing dinosaur eggs. As the dinosaurs died out, and (Oxfordshire, England) Lower jaw
embedded in rock
finally disappeared some 65
million years ago, mammals
filled their place.

THE STAGE IS SET Upper jaw
Into this sort of world, populated by
giant fern-like plants, fish, insects,
and reptiles, the first mammals Lower jaw
emerged some 200 million
years ago.
SUCCESSFUL LINE
Freed from the domination of the dinosaurs,
mammals changed rapidly during the Paleocene and
Eocene as evolution “experimented” with new forms.
Some died out. This one did, but its general design
was continued to the present day. It was an early
relative of the horse, from the Eocene epoch.
Species shown: Hyracotherium
vulpiceps (Essex, England)