Introduction to the Dinosaur Problem
This module is based on a single question and two concepts. In answering the question, students will arrive at various interpretations of these major concepts.

The Question: Why did the dinosaurs become extinct?

The Concepts: The extinction of the dinosaurs

• tells us something about Earth as a home, and
• shows us something about science as an inquiry process.

The Question of Dinosaur Extinction
Extinction, or the death of all members of a distinct species, is an almost unavoidable part of living on Earth. Based on studies of fossils, paleontologists find that most species appear, flourish, and then disappear from the fossil record on a typical time scale of several million years (there are a few exceptions). Roughly 99.9% of all the life forms that ever inhabited our planet are now extinct. Stated another way, there are probably about ten million different species living on Earth today, but during Earth's 4.6 billion-year history roughly ten billion species have evolved into existence, lived for awhile, and then died.

Extinction of individual species occurs more or less continually (the so-called "background extinction rate"), but every now and then, a massive extinction event takes place in which many different species of plants and animals all disappear at the same geologic instant. The dinosaurs disappeared in the Cretaceous-Tertiary extinction event, the second-largest extinction event in Earth's history. They had ruled Earth for 180 million years, their fossils dominating rocks formed during the Mesozoic, and then between one rock layer and the next, they disappeared. And the dinosaurs were not alone: many others, including plesiosaurs, icthyosaurs, cephalopods, echinoids, families of foraminifers and phytoplankton--about one fourth of all known families of animals--disappeared with them. What happened? The formation of new rock is a slow process, so the actual duration of the extinction event may have been as long as a few million years (still but an instant in geologic time), or as short as a few days or months. If the extinction event lasted a few million years, then the disappearance of the dinosaurs was a gradual phenomenon related to processes operating over periods which are long in comparison to human experience. If the extinction event was very short, then they disappeared in some global catastrophe.

So, what did happen? This question, like all others in science, must be decided on the basis of evidence. We have given the students short descriptions of a number of possible causes of extinction. It is up to them to search through the possible causes, examine all the evidence they can find in this module or other sources, and then argue for a single cause or a combination of causes that may have led to the extinction of the dinosaurs. Understanding what happened to the dinosaurs is not only intellectually interesting, but understanding the causes of the extinction of the dinosaurs may help Homo sapiens to avoid the same fate in the future.

The concept of Earth as a home, as discussed earlier, is important for two reasons:

1. Knowledge about Earth makes our lives safer and more efficient, and,
2. A sense of stewardship about our earthly home and those other life forms with whom we share it encourages us to care for it so that our descendants can survive and enjoy it also.

The concept of science as inquiry is also important. The Earth Science Explorer provides opportunities to engage in problem-based learning activities that develop abilities and understanding of some aspects of the inquiry process. As students examine explanations based on scientific information and evidence provided through the Dinosaur Floor, they identify and analyze several alternative explanations for the extinction of dinosaurs. In this way students begin to recognize the relationship between explanation and evidence. They also see how scientific inquiry is guided by knowledge, observations, ideas, and questions.

Pedagogical research indicates that students tend to center on evidence that confirms their current beliefs and concepts (i.e., personal explanations). They ignore or simply fail to perceive evidence that does not agree with their current concepts. Teachers of science must challenge those personal explanations and provide scientific explanations as alternatives. The dinosaur module meets several National Research Council's Standards and presents an excellent opportunity for teachers to address these challenges.