By Blaise Jones

While the sharks that swim through our oceans today are large and fearsome, like most animal species, their ancestors were even larger. We know this because these ancient sharks left behind clues of their existence: fossils.

Fading Away

All fish are divided into two categories: Chondrichthyes or Osteichthyes. Chondrichthyes are known informally as cartilaginous fish because their skeletons are formed of cartilage, the same flexible material found in countless animals including humans (nose and ears most notably). Sharks, skates, and rays are cartilaginous fish; meaning when they die their skeleton mostly dissolves, leaving no fossil. Osteichthyes are commonly known as bony fish because their skeletons are comprised of small bones that can fossilize.

Finding shark fossils is difficult because their cartilaginous skeletons do not fossilize. Most shark fossils that are found are of scales or teeth, the only non-cartilage parts of their skeleton.

Long, Long Ago

Sharks first appeared in the fossil record 400 million years ago, during a period of time known as the Ordovician Period. That’s more than 200 million years older than the oldest dinosaur fossil found to date. Sharks were some of the first fishes to evolve, and have dominated the seas ever since.

The earliest single fossil officially recognized as belonging to a shark is a series of scale imprints found in Siberia. Belonging to a small genus of shark known as the Elegestolepis, the scale imprint was found to be 420 million years old.

The earliest fossilized shark teeth ever found were from a shark known as the Leonodus, which lived 400 million years ago.

There is a minor debate about the oldest shark fossils. Archeologists discovered an aquatic scale imprint in modern day Colorado that dates back to 455 million years ago. This scale imprint belonged to an unidentified creature that shared several similarities to sharks, but whose scales are different enough from modern sharks that some scientists doubt the animal was a shark, at least by contemporary definitions of a shark.

Big as They Get

If you want to find the real monster sharks of the past, you’re going to have to look a little closer to modern day. The largest shark fossil ever found belongs to the megalodon shark. This monster fish grew up to 60 feet long and weighed as much as 50 tons. The jaws of the megalodon were 3.25 feet (1m) wide, the dorsal fin was 4 feet tall (1.4m), and the tail fin was more than 6 feet (1.75m). That’s bigger than a school bus and larger than any other fish throughout the known fossil record and modern day.

The fossil that introduced us to this extinct mega-shark was one of its teeth, the largest of which was more than seven inches long. For comparison, the largest great white shark tooth ever found was only three inches long. Megalodon first appeared in the fossil record 15 million years ago, which means that they lived alongside their smaller cousins, the great white shark. The megalodon went extinct 2 million years ago, leaving the great white shark as the largest predatory fish since then.

SOURCES:

http://www.elasmo-research.org/education/evolution/earliest.htm

http://news.nationalgeographic.com/news/2012/12/121205-oldest-dinosaur-found-tanzania-science-archaeology/

http://www.fossilguy.com/gallery/vert/fish-shark/carcharocles/carcharocles.htm

“The Encyclopedia of Sharks” by Steve Parker

“Sharks: The Animal Answer Guide” by Gene Helfman and George H. Burgess