Geoscience Reference
In-Depth Information
Dinosaurs are a diverse group of animals. They dominated the con-
tinents for more than 150 million years, during which time dozens of
groups arose and went extinct. Just about any large dinosaur that
lived near the end of the Mesozoic had to be considered as a possible
victim of the ancient catastrophe at Auca Mahuevo. The quickest
way to compile our list of likely victims was to look back through the
evolutionary history of dinosaurs and identify which groups lived in
Patagonia at the end of the Mesozoic. This narrative will also provide
those interested with a brief review of dinosaur evolution and the
method that paleontologists use to reconstruct it.
Since the naming of the first dinosaur in 1824, several hundred
kinds of dinosaurs have been discovered, and more are being found all
the time. A primary goal of paleontology is to understand how dif-
ferent species are related to one another, in other words, to reconstruct
the history of how they evolved. This pursuit is somewhat like study-
ing the genealogy of a family tree. Contemporary paleontologists
use a scientific method called cladistics to reconstruct the evolutionary
history of extinct groups and draw family trees that link groups of
ancient animals and plants. Willi Hennig, a German entomologist,
introduced this analytical method in the 1950s, and subsequent
researchers have refined it over the last forty years.
Cladistics is based on a fairly simple concept. Although life is
diverse, we see a pattern in that diversity when we look for charac-
teristics that are shared by different organisms. This pattern of char-
acteristics can be used to arrange organisms into smaller groups
contained within larger groups. The arrangement of groups within
groups results from evolution, when descendants inherit new char-
acteristics from their ancestors. By studying how these characteristics
are distributed among different animals and plants, we can determine
the order in which different groups evolved and thereby interpret the
sequence of evolutionary history.
The evolutionary relationships among different groups of organ-
isms can be shown on branching diagrams called cladograms. A clade
is simply a group of organisms that includes the first member of the
group, or the group's common ancestor, and all of its descendants. In
essence, branches on the tree represent different clades of animals or
plants, and the branching points on the tree represent common
ancestors that possessed new evolutionary characteristics, which
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