The earliest dinosaurs were probably carnivorous, bipedal animals less than two meters long and weighing about 10 kilograms. From these small beginnings evolved thousands of different dinosaurs species. These included the largest land animals ever to live on Earth, as well as the largest bipedal animals known to have existed. Birds—the only living dinosaurs—represent an equally diverse array of shapes, sizes, and behaviors.
The patterns of dinosaur evolution are only now being deciphered by paleontologists. At their heart lies the phylogeny of dinosaurs, essentially a family tree of every dinosaur species. This tree represents the pattern of evolution throughout dinosaur history. Paleontologists can use this pattern to study the changes that have occurred in dinosaurs over vast stretches of geologic time.
One of the most dramatic of these evolutionary changes occurred in body
size. From their small ancestors, some dinosaurs reached sizes exceeding 35
meters in length and 50 tons in weight. In fact, most dinosaurs were relatively
large—the average size of a Mesozoic dinosaur was about 100 kilograms,
quite large compared with the average size of a Cenozoic mammal (about two
to five kilograms). The earliest dinosaurs were among the smallest. Aside
from birds, dinosaurs rarely evolved to small sizes. Instead, they appear
to have followed what is often called “Cope’s Rule,” the
maxim that most animals tend to get larger over time. Not only did some dinosaurs
reach immense sizes, but nearly every group of dinosaurs got larger over time.
The earliest ceratopsians, pachycephalosaurs, ornithopods, and thyreophorans
were all small compared with their descendants. The one significant exception
occurred in predatory dinosaurs. Although some theropods were quite large
(reaching five tons or so), many evolved to become very small, culminating
in the lineage leading to the first birds.
Although the first dinosaurs were carnivores, two groups (sauropodomorphs
and ornithischians) developed
the ability to process plant material. The earliest dinosaurian herbivores eventually
produced a diverse array of descendants,
with well-developed abilities to consume vegetation. In
early ornithopods, for example, the individual teeth show wear facets that
indicate they were
being used to grind food in the mouth, probably aided by thin cheeks. Later
species show the development of more and more teeth, packed so tightly that
they formed a single large surface for cutting and grinding food.
Similar “dental batteries” are also found in ceratopsians, although
like ornithopods their earliest species had much simpler chewing systems.