140 Evolution and the Fossil Record
(or monophyletic) groups are those that are defined on shared derived characters and include all
descendants of a common ancestor.
Let us look at another example closer to home. In older classification schemes, it was
common to lump all of the great apes in the family Pongidae and place ourselves in the
separate family Hominidae. In our anthropocentric arrogance, we always considered our-
selves in a special group and exclude humans from the rest of the animal kingdom. But the
evolutionary relationships of apes and humans are well established (fig. 5.5), and humans
are just one branch among the great apes. Chimps and gorillas are much more closely
related to us than they are to orangutans or gibbons, even though they all share primi-
tive features of long, strong limbs and long hair and smaller brains and long snouts, and
we have diverged the most in our bipedalism and nearly hairless skin and large brains
with small faces. Cladistically speaking, it is invalid to place all the rest of the apes in a
group without humans because that is a “wastebasket” group (like Reptilia without birds)
FIGURE 5.4. Different ways of classifying the same groups of organisms. Traditional classifications (top brack-
ets) emphasize overall similarity and prefer to focus on the great evolutionary radiations of birds and mam-
mals by placing them in their own classes, equal in rank to class Reptilia. A cladistic classification (lower
brackets) only recognizes monophyletic groups, that is, groups that include all descendants of a common
ancestor. “Reptilia,” as traditionally defined, is not monophyletic, because it does not include one descendant
group, the birds. Instead, the natural monophyletic groups include amniotes (all land vertebrates), Reptilia
(only if it includes birds), Sauria (the nonturtle reptiles), Archosauria (the group including crocodilians, dino-
saurs, and birds), and Dinosauria (birds and the nonavian dinosaurs).
Mammalia “Reptilia” AvesMAMMALSTURTLES LIZARDS+ SNAKESCROCO-DILIANS NONAVIANDINOSAURS BIRDSDinosauriaArchosauriaSauriaReptiliaAmniota