Geoscience Reference
In-Depth Information
ance of lizards and crocodiles is evolutionary misleading and is
refuted by the careful scrutiny inherent in cladistics; so most evolu-
tionary biologists now classify crocodiles and birds in the same evo-
lutionary group (Archosauria), which is a subset of lizards and all other
reptiles.
Understanding the genealogical relationships between ancient
organisms is crucial for reconstructing the origin of living animals and
plants, as well as for understanding the evolution of their anatomical,
physiological, and behavioral systems. When looking at a particular
characteristic, such as our grasping hand, the explanation of its evo-
lutionary origin would be very different if we assumed that humans
originated from bats rather than primates. If we originated from
bats, our grasping hand would have had to evolve from a structure
used for flying, which would be difficult to imagine, although not
impossible. However, since many other characteristics of our skeleton
indicate that humans evolved from other primates, which use their
hands for holding on to branches and gathering food, the evolution-
ary origin of our grasping hands is easier to understand.
With this concept for reconstructing evolutionary history in mind,
we can trace some of the steps that led to the evolution of the major
groups of dinosaurs by working our way up the branches and branch-
ing points of the dinosaur family tree. The earliest known dinosaurs
appear in the fossil record about 230 million years ago. Some of
them, such as the herbivorous
Pisanosaurus,
and the carnivorous
Herrerasaurus
and
Eoraptor,
were found in northwest Argentina. The
fact that these dinosaurs already had specialized anatomical features
for eating plants and flesh, which distinguish them as members of
specific dinosaur groups, indicates that there must have been earlier
dinosaurs, although we have yet to find fossils of them. The first
question that arises in looking at these and other dinosaurs is, what
makes a dinosaur a dinosaur? In other words, what characteristic
evolved in the common ancestor or very first dinosaur? To find out,
we need to search for a characteristic that is found in all dinosaurs
but not found in other reptiles, including turtles, lizards, snakes,
and crocodiles.
The story of the evolution of dinosaurs basically revolves around
locomotion. The common ancestor of dinosaurs possessed a hip
structure different from that found in other reptiles. All dinosaurs have
