---

Prehistoric Sharks: Shark History and Evolution Through Time

Sharks are fish that belong to the class Chondrichthyes, a group that includes sharks, rays, and chimaeras. These cartilaginous fish are among the oldest surviving vertebrates on Earth. Their fossil record stretches back farther than most animal groups, including all land animals (dinosaurs included), trees, and even insects! Sharks have survived all five major mass extinctions, including the devastating Permian extinction that wiped out most life on Earth.

Because sharks are made mostly of cartilage instead of bone, they do not fossilize very well. In most cases, only their teeth and dermal denticles (modified scales) are commonly preserved as fossils. As a result, the earliest history of sharks is fragmentary. Even so, paleontologists still have a good understanding of when the first sharks appeared and how they evolved through time.

---

The First Sharks - Ordovician Shark Evolution Begins - 450 Mya

The first sharks appear in the fossil record during the Ordovician Period around 450 million years ago. This was a time of incredible evolutionary change known as the Ordovician Radiation, or more formally, the "Great Ordovician Biodiversification Event." In many ways, this event rivals the more famous Cambrian Explosion of the previous period. During this time, Earth experienced a dramatic increase in new classes, orders, families, genera, and species of life, including the earliest chondrichthyans. Evolution was moving at an incredible pace.

The oldest shark-like fossils discovered so far are dermal denticles (tiny skin scales). These fossils were found in the Harding Sandstone of Colorado (Samson et al. 1996) and in the Carmichael Sandstone of the Stokes Formation in Australia (Young 1997).

Later studies found that the dentine structure in the Harding Sandstone denticles closely matched the fin scales of the modern small-spotted catshark (Zerina 2008). Because of this similarity, scientists concluded that the fossils from the Harding and Carmichael Sandstones likely belonged to some of the world's earliest sharks.

In 2016, Andreev et al. reexamined several Ordovician dermal denticles, including specimens from the Harding Sandstone. They also concluded these fossils belonged to chondrichthyans and named them Solinalepis gen. nov.

Then, in 2012, Sansom et al. described additional shark-like scales from the middle Ordovician Stairway Sandstone in central Australia. These fossils were named Tantalepis gatehousei (Sansom et al. 2012), which is currently considered the world's oldest named shark.

One especially interesting detail is that only dermal denticles, and not teeth, have been found from these earliest sharks. Jawed fish were still evolving during this time, so the first sharks may have lacked teeth, or possibly even jaws altogether. The fossil evidence suggests that shark teeth evolved later and were essentially modified dermal denticles - just much larger and far sharper ones!

---

The Silurian Period - Early Shark Evolution and a Scant Fossil Record - 443 Mya

The Silurian Period began around 443 million years ago, and the shark fossil record from this time is still very limited. Aside from a possible shark fin spine called Sinacanthus wuchangensis, most Silurian shark fossils consist only of isolated dermal denticles.

Some of the first Silurian shark denticles were discovered in the Upper Silurian Pitchi-Shui Formation of Tuva in southern Siberia. They were described by Karatajute and Talimaa (1973) and assigned to the genus Elegestolepis. Since then, extensive research in Mongolia and other parts of Asia has revealed more of these primitive chondrichthyans, which were eventually grouped into the order Mongolepidida (Karatajute-Talimaa et al. 1990).

During the 1990s and 2000s, many additional chondrichthyan denticles were discovered across Asia and even in Canada. Known genera from this time now include Elegestolepis, Mongolepis, Teslepis, Sodolepis, Shiqianolepis, and Pilolepis (Andreev et al. 2016).

Although these fossils are tiny and fragmentary, they show that early sharks were already spreading across ancient oceans during the Silurian. Some of these denticles, along with the possible fin spine, are shown in the image below.

---

The Devonian Radiation of Sharks - The Age of Fishes - 419 Mya

The Devonian Period began around 419 million years ago and marked a major turning point in vertebrate evolution. Not only did the first tetrapods begin moving onto land, but the oceans also experienced an incredible diversification of fish, including early sharks. Because of this explosion of aquatic life, the Devonian is often called the "Age of Fishes."

It is during the Devonian that the fossil record of sharks truly begins to expand. For the first time, paleontologists start finding fossil shark teeth and complete shark skeletons instead of isolated dermal denticles alone. One of the oldest known intact shark fossils is Doliodus problematicus (NBMG 10127), shown above. This Early Devonian shark lived around 400 million years ago and was discovered in the Campbellton Formation of New Brunswick, Canada (Miller et al. 2003).

Doliodus is especially important because it displays a mixture of shark characteristics and features seen in acanthodians, an extinct group of primitive fish sometimes called "spiny sharks" (Maisey et al. 2017). This suggests that early shark evolution was more complex than scientists once believed.

Some of the best-preserved Devonian shark fossils come from the Late Devonian Cleveland Shale near Lake Erie in Ohio. These fossils are so exceptionally preserved that paleontologists have even studied preserved muscle fibers and stomach contents from the sharks themselves (Hansen, M.C. 1996, p. 290).

The Cleveland Shale contains a remarkable variety of prehistoric sharks. Among the most famous is Cladoselache, a streamlined early shark often featured in paleoart and museum displays. Other genera found here include Diademodus, Stethacanthus, Ctenacanthus, and Orodus (Hansen, M.C. 1996, p. 290).

These Devonian sharks looked very different from modern sharks. Many were smaller, had unusual body shapes, and some species even lived in freshwater environments. To an untrained eye, several of these early sharks looked more like strange fish than the sharks we know today. Below are some fossil examples from the Devonian Period.

---

Carboniferous Sharks - The Golden Age of Sharks - 359 Mya

The Devonian Period ended with a major mass extinction around 374 million years ago. One of the biggest victims of this extinction event was the placoderms, a group of armored fish that had dominated the world's oceans for millions of years. These armored fish included some of the Devonian's top predators, such as the massive 20-foot-long Dunkleosteus.

With the decline of the placoderms, a huge ecological opportunity opened in the oceans. During the Carboniferous Period, sharks rapidly diversified and rose to become the dominant marine predators. Although the Carboniferous is famous for its vast coal forests and the evolution of giant amphibians and insects, it is also widely known as the "Golden Age of Sharks."

Some of the best-preserved shark fossils from this period come from the Heath Shale Formation of Montana, which dates to the Mississippian Epoch around 318 million years ago (Lund & Grogan 2005). Within this formation is the famous Bear Gulch Limestone lagerstatte in Fergus County, Montana, one of the most diverse fossil fish deposits in the world.

The Bear Gulch Limestone contains more than 65 species of exceptionally preserved prehistoric sharks (Lund & Grogan 2005). Some fossils are so detailed that traces of skin pigments and even internal organs have been preserved (Grogan & Lund 1997).

During this Golden Age, sharks reached an incredible level of diversity. They ranged in size from just a few inches long to more than 20 feet. Some species lived in freshwater environments, while others thrived in the open ocean. A few were beginning to resemble modern sharks, but many evolved into bizarre and highly specialized forms unlike anything alive today.

One famous example is Helicoprion, the strange "whorl-tooth shark" whose replacement teeth formed a spiral-shaped tooth whorl. Another unusual shark was Belantsea montana, which looked more like a colorful tropical reef fish than a shark. Members of the genus Stethacanthus possessed a bizarre anvil-shaped dorsal fin covered in tooth-like denticles, a feature unlike anything seen in modern sharks.

The Carboniferous Period truly was the Golden Age of Sharks, a time when prehistoric sharks evolved into some of the most unusual and diverse forms in vertebrate history.

---

The Permian Period - The Near Extinction of Sharks - 298 Mya

The Permian Period began around 298 million years ago. By this time, some of the highly specialized shark groups that flourished during the Carboniferous had already begun to disappear. Sharks were still diverse and widespread, but they were no longer as abundant or varied as they had been during the earlier "Golden Age of Sharks."

Then came the greatest mass extinction event in Earth's history: the end-Permian extinction, often called "The Big One." This catastrophe was so severe that it marks the end of the entire Paleozoic Era.

Life on Earth changed dramatically after this extinction event. Many groups of organisms that had dominated the oceans for hundreds of millions of years vanished completely. After the Paleozoic, there were no more trilobites, eurypterids, horn corals, graptolites, or the strange spiny sharks, an extinct group of fish that shared features with both sharks and bony fish. The list of losses goes on and on.

Scientists estimate that roughly 80 to 95 percent of all species became extinct during this event (Benton et al. 2004). The causes are still debated today. Possible explanations include massive volcanic eruptions, rapid climate change, the formation of the supercontinent Pangaea, ocean chemistry changes, asteroid impacts, or some combination of these disasters. Whatever the exact cause, the result was nearly total ecological collapse. Life on Earth barely survived, and sharks themselves came dangerously close to extinction.

One of the better-known sharks from the Permian is Orthacanthus, a large freshwater shark with an eel-like body shape. Complete fossils of this unusual prehistoric shark have been discovered in Germany, including the specimen shown above.

---

Mesozoic Sharks - The Beginnings of Modern Sharks - 252 Mya

The Mesozoic Era began around 252 million years ago with the Triassic Period, a time when life on Earth was slowly recovering from the devastating Permian mass extinction. The few surviving plant and animal groups rapidly diversified and began forming entirely new ecosystems both on land and in the oceans.

Although the first mammals appeared during this era, reptiles became the dominant animals of the Mesozoic world. Dinosaurs ruled the land, flying reptiles controlled the skies, and enormous marine reptiles dominated the oceans. These marine reptiles included powerful predators such as ichthyosaurs, plesiosaurs, and later the giant mosasaurs. Compared to these massive sea reptiles, sharks took a secondary role in Mesozoic marine ecosystems.

Even though sharks were no longer the dominant ocean predators, they continued to evolve and diversify throughout the Mesozoic Era. Many began developing the body shapes and features associated with modern sharks. By the end of the Jurassic Period, all eight living shark orders had appeared in the fossil record, marking the rise of truly modern shark lineages.

The Mesozoic Era came to a dramatic end 66 million years ago during the Cretaceous-Paleogene extinction event. This mass extinction wiped out the non-avian dinosaurs, flying reptiles, and the giant marine reptiles that had ruled the oceans for millions of years.

With many of their major competitors gone, sharks once again rose to prominence in marine ecosystems. The stage was now set for the evolution of many of the modern sharks that inhabit the oceans today.

---

Cenozoic Sharks - The Rise of Giant Sharks - 65 Mya

Cenozoic Sharks - The Rise of Giant Sharks - 65 Mya

The Cenozoic Era began around 66 million years ago after the extinction of the dinosaurs and giant marine reptiles. During this new era, mammals rapidly diversified, and some groups eventually returned to the oceans. Whales, dolphins, seals, and other marine mammals became widespread throughout the world's seas, creating an entirely new food source for sharks.

With abundant prey available, many modern shark groups increased in size. The Cenozoic became the age of giant predatory sharks, including the evolution of the famous megatooth sharks.

The megatooth shark lineage first appeared during the Paleocene Epoch with species belonging to the genus Otodus. Over millions of years, these sharks gradually evolved into larger and more powerful apex predators. This evolutionary trend eventually culminated during the Miocene Epoch around 23 million years ago with the appearance of Otodus megalodon, better known simply as Megalodon.

Megalodon was the largest shark that ever lived and one of the most formidable predators in Earth's history. At lengths estimated to exceed 60 feet, it was essentially a gigantic version of a modern great white shark, capable of hunting large whales and other marine mammals.

Other sharks also reached enormous sizes during the Cenozoic. One example is Carcharodon plicatilis, or the broad form C. hastalis, sometimes called the "Giant White Shark." These sharks resembled oversized great whites but possessed large non-serrated teeth.

Many of these giant sharks disappeared during extinction events near the end of the Pliocene around 2.5 million years ago. The causes are still debated, but changing ocean temperatures, shifting ecosystems, and declining marine mammal populations likely played important roles.

To learn more about the Megalodon shark, visit the Megalodon Facts page.

---

Conclusion:

The fossil record shows that sharks first appeared during the Ordovician Period more than 450 million years ago. From that time forward, sharks have followed a long and complex evolutionary path, diversifying into a wide range of forms. They have included everything from small freshwater species to bizarre deep-sea sharks and massive predators like Megalodon.

Rather than being unchanged "living fossils," sharks have continuously evolved and adapted to shifting oceans, changing climates, and multiple mass extinction events. Many ancient shark lineages have come and gone, while others evolved into the modern sharks we see today. This long history makes sharks one of the most successful vertebrate groups in Earth's history.

One of the most exciting parts of studying sharks is that their prehistoric remains can still be found today. Fossil teeth, scales, and occasional skeletons continue to reveal new details about ancient oceans and shark evolution.

To learn more about finding your own prehistoric shark fossils, visit the Fossil Hunting Locations page.

---

References / Works Cited

Andreev, P., Coates, M. I., Karatajute-Talimaa, V., Shelton, R. M., Cooper, P. R., Wang, N.-Z., & Sansom, I. J. (2016). The systematics of the Mongolepidida (Chondrichthyes) and the Ordovician origins of the clade. PeerJ, 4, e1850. http://doi.org/10.7717/peerj.1850

Benton M.J., Tverdokhlebov V.P., Surkov M.V. (2004). Ecosystem remodelling among vertebrates at the Permian-Triassic boundary in Russia: Nature, v. 432: 97-100.

Grogan, E.D. & Lund, R. (1997). Soft tissue pigments of the Upper Mississippian chondrenchelyid, Harpagofututor volsellorhinus (Chondrichthyes, Holocephali) from the Bear Gulch Limestone, Montana, USA. Journal of Paleontology, v. 71: 337-342.

Hansen, M. C., (1996). "Phylum Chordata-Vertebrate Fossils," in Fossils of Ohio, edited by R. M. Feldmann and Merrianne Hackathorn. Ohio Division of Geological Survey Bulletin 70, pp. 288-369.

Karatajute-Talimaa VN, Novitskaya L, Rozman KS, Sodov Z. (1990). Mongolepis-a new lower Silurian genus of elasmobranchs from Mongolia. Paleontologicheskii Zhurnal. 1990;1:76-86.

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).

Maisey, J. G., Miller, R., Pradel, A., Denton, J. S.S., Bronson A., & Janvier, P. (2017) Pectoral Morphology in Doliodus: Bridging the 'Acanthodian'-Chondrichthyan Divide, American Museum Novitates Number 3875 :1-15. 2017 https://doi.org/10.1206/3875.1

Miller, R. F., R. Cloutier, and S. Turner. (2003). The oldest articulated chondrichthyan from the Early Devonian period. Nature, v. 425: 501-504.

Qu, Qingming & Zhu, Min & Zhao, Wenjin. (2010). Silurian atmospheric O2 changes and the early radiation of gnathostomes. Palaeoworld. 19. 146-159.

Sansom, I.J. & Davies, N.S. & Coates, M.I. & Nicoll, R.S. & Ritchie, A. (2012). Chondrichthyan-like scales from the middle Ordovician of Australia. Pelaontology, 55 (2): 243-247.

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.

Tajute-Talimaa V. (1973). Elegestolepis grossi gen. et sp. nov., ein neuer Typ der Placoidschuppe aus dem Oberen Silur der Tuwa, Palaeontographica, A. 143: 35-50.

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.

---

---