Shark tooth
Overview
 
A shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

 tooth
Tooth
Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...

is one of the numerous teeth of a shark. Sharks continually shed their teeth, and some Carcharhiniformes
Carcharhiniformes
The ground sharks, order Carcharhiniformes, are the largest order of sharks. With over 270 species, carcharhiniforms include a number of common types, such as the blue shark, catsharks, swellsharks, and sandbar shark....

 shed approximately 35,000 teeth in a lifetime. In some geological formations, shark's teeth are a common fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

. These fossils can be analyzed for information on shark evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 and biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

, especially because the teeth are often the only part of the shark to be fossilized, in fact fossil teeth comprise much of the fossil record of the Selachimorpha, extending back hundreds of millions of years.

The most ancient types of sharks date back to 450 million years ago, during the Late Ordovician period, and they are mostly known from their fossilised teeth.
Quotations

Memory can change the shape of a room; it can change the color of a car. And memories can be distorted. They're just an interpretation, they're not a record, and they're irrelevant if you have the facts.

I have to believe in a world outside my own mind. I have to believe that my actions still have meaning, even if I can't remember them. I have to believe that when my eyes are closed, the world's still there. Do I believe the world's still there? Is it still out there? … Yeah. We all need memories to remind ourselves who we are. I'm no different … now … where was I?

Your life is over. You're a dead man. The only thing the doctors are hoping to do is teach you to be less of a burden to the orderlies. And they'll probably never let you go home, wherever that would be. So the question is not "to be or not to be", because you aren't. The question is whether you want to do something about it.

From "Memento Mori", by Jonathan Nolan, as quoted on the DVD

 
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