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450 Million Years of Sharks
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Shark fossil evidence

Paleontologists piece together the history of ancient living things, largely through the discovery and analysis of fossils. Fossils are the mineralized or otherwise preserved bodies or impressions of living things.

Problem is, sharks generally don’t fossilize as well as other animals because their skeletons are made of cartilage, a softer and more flexible tissue. Cartilage isn’t as easily fossilized as is bone.

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Early shark teeth. Illustration by R. Aidan Martin
Fortunately, shark teeth do fossilize well, and sharks produce thousands of teeth in a lifetime. Fossilized shark skin scales and, over time, other body parts have also been found.

Much of the earliest shark evolution and history is known through fossils of teeth and scales that are unique to sharks. Paleontologists are able to find enough clues in a well-preserved fossil to determine much about a shark, including its species, and clues to suggest size and behavior.

 

Earliest prehistoric sharks

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Fossilized scale of Elegestolepis. Illustration by R. Aidan Martin
The oldest fossilized evidence of prehistoric sharks comes from shark-like scales that date to 455 million years ago during the Ordovician Period, in Colorado. Some paleontologists do not agree that these scales are sufficiently shark-like to pronounce them sharks. But there is no disagreement that scales found during the Silurian Period, aged 420 million years, are from sharks. Shark scales from this period have been found in Siberia and Mongolia. The oldest shark teeth are from the Devonian Period, about 400 million years old, found in Europe.  Nothing beyond these scales or the teeth is known about these early sharks.

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Braincase, 380 million years ago. Illustration by R. Aidan Martin
More complete fossil shark remains date to 380 million years old, including a fossilized shark braincase, possibly a xenacanth, found in Australia. Other remains of this or similar species have been found in Antartica and Saudi Arabia. Xenocanth was a fresh water shark.

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Xenacanthus. Illustration by R. Aidan Martin
 
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