Sharks are fish of the condrichthyes class. The condrichthyes are a truly ancient class of fish, and are the ultimate survivors. Their fossil record
predates most animals, including all land animals (dinosaurs included), trees, and even insects! They have survived all 5 major mass extinctions,
including the Permian one that wiped out most life on Earth.
Since sharks are made of cartilage, they do not preserve very well. Usually only their dermal denticles (modified scales) and teeth readily fossilize. Because of this,
their origins are fragmentary, yet we have a good idea of when they first appeared.
The First Sharks - Ordovician Beginnings
The first sharks come to us from the Ordovician radiation event. The Ordovician radiation, or more impressively called the “Great Ordovician Biodiversification Event,”
arguably rivals the more famous Cambrian Explosion of the previous time period. During this biodiversification event, Earth experienced an unprecedented explosion of new
classes, orders, families, genera, and species, including the chondrichthyes. It was evolution on steroids.
The first shark-like fossils found were denticles, discovered in the Harding Sandstone of Colorado (Samson et al 1996) and in the Carmichael Sandstone of the Stokes Formation
in Australia (Young 1997). However, it was unclear if these fossils were from actual sharks, or shark like fish.
Later, this dentine that forms the denticles found in the Harding Sandstone was found on the fin scales of the small spotted catshark (Zerina 2008). As
a result it was concluded the world’s oldest sharks were in the Harding and Carmichael Sandstone; however these sharks were never named. In 2012, Sansom et al. described
more shark-like scales from the Stairway Sandstone in central Australia, which is middle Ordovician. This time a new shark genus and species was erected to
name the animal, Tantalepis gatehousei (Sansom et al 2012), which is now officially the world’s oldest named shark.
An interesting thing to note is only dermal denticles, and not teeth, have been found. Jawed fish were just developing at this time, so the first sharks may have had no
teeth or maybe even no jaws! It appears teeth developed later. So, according to the fossil evidence, a shark tooth is simply a modified denticle; albeit a great
big and sharp modified denticle!
This figure shows two shark-like denticles from the Harding Sandstone of Colorado. The age is roughly 455 million years old. These are possibly the worlds oldest shark fossils.
These denticles, like modern shark denticles, have raised areas, or crowns, running across the surface. The center crown is the largest.
The image was taken from a standard microscope (hence, the elevated regions are out of focus).
The scales are about 0.4 mm across.
The Silurian Shark Record
After the Ordovician, the fossil record of sharks just gets better and better. Many more shark denticles have been found in Silurian outcroppings.
Some of the first come from the Upper Silurian Pitchi–Shui Formation in Tuva (Southern Siberia). These fossils are around 420 million years old.
They are described by Karatajute & Talimaa (1973) and are given the genus name Elegestolepis. Since only scales have been found of Elegestolepis,
not much is known about the sharks appearance. Just like in the Ordovician, other than denticles, there have been no other fossils from sharks found… Yet.
The Devonian Radiation of Sharks
The Devonian period (starting around 418 million years ago) not only saw the worlds first tetrapods appear, it also saw a great radiation in all fish.
Fish, including sharks became more common and much more diverse. This is why the Devonian period is called the “Age of Fishes.”
It is not until the Age of Fishes that fossil shark teeth and complete shark fossils appear in the fossil record. The world’s oldest complete shark fossil
is of a specimen called Doliodus problematicus. It is Early Devonian (409 million years old) and comes from the Campbellton Formation in New
Brunswick, Canada (Miller et al 2003).
The best-preserved specimens of complete fossilized Devonian sharks are from the Late Devonian Cleveland Shale. The shark fossils of the Cleveland Shale
near Lake Erie in Ohio are so well preserved that muscle fibers are preserved and even remains of prey in the gut can be studied (Hansen, M.C. 1996, p. 290).
The Cleveland Shale contains a variety of shark specimens. There are several species of the streamlined genera Cladoselache (sketched in the introduction image), as well as other genera such as Diademodus,
Stethacanthus, Ctenacanthus, and Orodus (Hansen, M.C. 1996, p. 290). These Devonian sharks looked very different than extant sharks. They were smaller,
were shaped differently, and some even lived in fresh water. To the untrained eye, they looked more like fish than sharks.
This image shows the late Devonian fossil shark tooth, Ageleodus pectinatus. This specimen measures ~4 mm across.
It is from the famous Red Hill locality in Pennsylvania, and is approximately 364 million years old.
This is actually the earliest occurrence of Ageleodus.
The tooth has a series of cusps forming a comb like structure with a porous root.
These Ageleodus teeth are found in fresh water river deposits, meaning it was a freshwater shark, which
was not uncommon in the Paleozoic.
Shark teeth first appear in the fossil record in the Devonian. Although they may have
developed in the Silurian and have not been found yet.
Carboniferous – The Golden Age of Sharks
The Devonian time period ended with a major mass extinction around 374 million years ago. One of the many casualties of this mass extinction was the Placoderms
(armored fish). Armored fish were the prevailing class of fish; Placoderms included the Devonians top predator, the 20 foot long Dunkleosteus. The extinction of
this top class of predators left a large ecological hole. Immediately afterward, in the Carboniferous period, sharks seized the opportunity and became the top predators.
The Carboniferous (Pennsylvanian and Mississippian epochs) marks the high watermark of sharks. Although the Carboniferous is known for it’s huge coal forests and the
radiation of amphibians, it is also known as the “Golden Age of Sharks.”
Permian – "The Big One” - The Near Extinction of Sharks
Excellent preserved specimens from this Golden Age come from limestone beds in the Heath Shale Formation which is of the Mississippian Epoch (318 million years old)
(Lund & Grogan 2005). A famous limestone bed in this formation is called the Bear Gulch Limestone found in Fergus County, Montana. This lagerstatte contains one
of the world’s most diverse fish assemblages. It includes over 65 species of remarkably well-preserved sharks (Lund & Grogan 2005). Some specimens even have skin
pigments and internal organs preserved (Grogan & Lund 1997).
During this Golden Age, sharks were at their highest diversity; there was a profuse amount of families, genera, and species. They came in all shapes and sizes,
from a few inches to over 20 feet. Some lived in freshwater, while others lived in salt water. Some were beginning to look like modern sharks, while others
took on unique, highly specialized forms. Some examples include the large whorl tooth shark, Helicoprion, which had replacement teeth arranged in spirals.
The shark Belantsea Montana looked more like a fancy tropical aquarium fish than a shark. The entire Stethacanthus genus had a large anvil shaped dorsal fin,
unlike anything that exists today. The Carboniferous was truly the Golden Age of Sharks.
This image shows the Carboniferous fossil shark tooth Glikmanius occidentalis (formerly called Cladodus).
The Glikmanius genera lasted into the Permian.
This specimen was found in the Ames Limestone near Pittsburgh, PA. It is Pennsylvanian in age and is approximately 300 million years old.
Glikmanius has a large central crown surrounded by a series of cusps on either side. The cusps are broken off on one side of this specimen.
This fossil shark tooth is 1.2 cm across, and would be about 1.5 cm if complete.
During the next time period, the Permian (starting around 299 million years ago), some of the more specialized shark genera became extinct.
Sharks were still diverse, just not as diverse as in the Carboniferous. However, this all came to an abrupt end at the end of the Permian.
The end of the Permian is marked by “The Big One,” Earths largest mass extinction EVER!
Mesozoic - Modern Beginnings
This extinction was so large, that it is used to mark the end of the entire Paleozoic Era. Life on Earth looked fundamentally different after this
extinction. The common animals that existed for hundreds of millions of years vanished. After the Paleozoic Era, there are no trilobites, eurypterids,
horn corals, graptolites, or spiny sharks (a cross between a shark and a bony fish). The list goes on and on. Any surviving life was severely reduced
as 80 - 95% of all species became extinct (Benton et al 2004). Causes of this extinction are linked to an abrupt climate change. The specific causes are still
being debated. Whether it was the formation of Pangaea, extreme volcanism, other geologic activities, an asteroid(s), or any combination thereof does not change
the fact that almost all life on Earth was wiped out. Life barely squeaked by this one. Sharks barely survived this extinction event.
The first period in the new Mesozoic Era, the Triassic, is an interesting time period as life on Earth began a slow recovery from the Permian mass extinction.
The few surviving taxa, both on land and sea radiated and diversified, creating new ecosystems. Although mammals make their appearance, reptiles were the main
creatures to dominate these new ecosystems. They dominated the land, via dinosaurs. They dominated the air, via flying reptiles. They also dominated the seas
with a new terrifying kind of animal, the Great marine reptiles. These sea monsters were fierce predators that ruled the Mesozoic oceans. The surviving sharks
took a back seat to these terrifying beasts.
Although sharks took a back seat to the Great marine reptiles, they continued to diversify and took on their modern looking characteristics.
By the end of the second period in the Mesozoic era, the Jurassic, all eight extant shark orders appeared. At the close of the Mesozoic era,
the Cretaceous, a great mass extinction event took out most of the reptiles. The Dinosaurs, flying reptiles, and great marine reptiles all disappeared.
This allowed sharks to, once again, become the top sea predators.
This figure shows a sampling of Cretaceous fossil shark teeth. Notice, unlike the previous figures, these fossil teeth are starting to look like "modern" shark teeth.
The teeth in this image include: Scapanorhynchus texanus (Extinct Goblin Shark), Squalicorax pristodontus (Extinct Crow Shark),
Ptychodus sp. (Extinct Shell-Crushing Shark), and Cretolamna appendiculata (Extinct Mackerel Shark).
All were found near Big Brook, NJ, except for the Ptychodus, it was found in Sherman, TX.
The largest tooth measures 4.5 cm in height.
Cenozoic - The Rise of Giant Sharks
During the new era, the Cenozoic, a new food source appeared. Mammals went to the ocean (whales, dolphins, seals, etc...). This proved to be a new and easy
food source for sharks. As a result, the modern looking sharks grew. This period saw the birth of the world’s largest and most fearsome shark, the Megatooth shark.
This beast originated in the Paleocene with the Otodus genera, and slowly became larger and larger through time until the apex species emerged in the
Miocene and Pliocene, Carcarocles Megalodon. Think of this giant Megatooth shark as a 60-foot Great White! Other sharks during this time period also became super sized. A good
example is the Giant White sharks (Cosmopolitodus). These are non-serrated, larger versions of the Great Whites of today. Luckily, for humans, by the close of the Pliocene,
the large sharks became extinct, and we were left with today’s modern shark fauna.
To learn more about the megalodon shark,
click here to go to the megalodon gallery page
This is a sample of the giant fossil sharks
of the pliocene. Fossil shark teeth include Carcharocles megalodon (Extinct Megatoothed Shark), Carcharodon carcharias (Great White Shark), and Cosmopolitodus xiphodon (Extinct Giant White Shark).
The lower left tooth was found at the Calvert Cliffs of MD, The lower right tooth was found by Black water diving in SC.,
and the rest were found in the PCS Phosphate mine in Aurora, NC.
The largest tooth measures 15.2 cm.
The fragmentary fossil record shows that sharks originated sometime in the Ordovician. The fossil record also shows sharks have continually evolved and changed.
Many shark designs, from freshwater forms, to sharks with bizarre shapes, and even giants, have come and gone. These adaptations have allowed sharks to survive
every mass extinction event and thrive to the present day.
One remarkable thing about nature is that we can go out and find these prehistoric remains. To find your own prehistoric shark fossils,
click here to go to the fossil hunting location page for more information.
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Harpagofututor volsellorhinus (Chondrichthyes, Holocephali) from the Bear Gulch Limestone, Montana, USA. Journal of Paleontology, v. 71: 337-342.
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Lund, Richard & Grogan Eileen. (2005). "Fossil Fishes of Bear Gulch." Saint Joseph's University. March 20, 2012. (http://www.sju.edu/research/bear_gulch/index.php).
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Sansom I.J, Smith M.M, Smith M.P. (1996). Scales of thelodont and shark-like fishes from the Ordovician of Colorado. Nature, v. 379: 628–630.
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Zerina Johanson, Mikiko Tanaka, Natalie Chaplin, and Moya Smith. (2008). Early Palaeozoic dentine and patterned scales in the embryonic catshark tail. Biol Lett 4: 87-90.