This essay was developed for Week 2 of the AMNH online course Sharks and Rays, part of Seminars on Science, a program of online graduate-level professional development courses for K-12 educators. Explore more sample resources...

Major periods of elasmobranch evolutionary radiation (i.e., periods when large numbers of new and diverse species evolved) occurred during the Paleozoic (544–245 mya) and Mesozoic (245–65 mya) Eras. The last of these radiations, which ended approximately 146–65 mya, is the one that ultimately gave rise to the modern sharks and rays that we see in our rivers and oceans today. Let’s start by considering the origin of sharks and rays in the fossil record, and then we will examine some examples of the diversity that resulted from the evolutionary radiations of the Paleozoic and Mesozoic Eras.

The earliest known fossil remains that might have come from sharks are isolated, small, tooth-like structures that cover most or all of their bodies (microscopic skin denticles). These structures date from some 450 million years ago, in the Late Ordovician rock beds of Colorado, and are similar in form to denticles present in sharks today. We are not certain that these are the remains of sharks. They may be the remains of a type of extinct jawless fish known as thelodonts. Thelodonts had similar denticles and lived at that same time. Future research, and finding a fossil shark species with typical shark features and scales in place, might clarify this question.

This Orthacanthus fossil, found in Germany, dates from the Permian, about 260 million years ago. ©AMNH

Paleozoic Sharks (545-244 mya)

The earliest skeletal fragments of any chondrichthyans date from at least 380 million years ago. New evidence suggests that neurocrania (the cartilaginous “skull”) of the shark genus Pucapampella, from Mid Devonian rock strata of Bolivia and South Africa, may be even slightly older than 380 million years.

The best-known early shark is probably Cladoselache, which lived about 370 million years ago. This was a fast-swimming, predatory shark, reaching two meters in length, known from warm “epicontinental seas” that once covered much of North America. (Epicontinental seas are seas that lie within the borders of a continent rather than around its edges; the Caspian Sea is a remnant of a much larger epicontinental sea that extended over much of Asia.)

This fossil specimen of a Helicoprion tooth whorl (top) comes from the Early Permian (280 mya) near Montpelier, Idaho. With nothing more than the tooth whorls to go on, restorations (bottom) are tentative. Top: ©AMNH, bottom ©Ivy Rutzky.

Sediments deposited in the epicontinental seas that covered North America during the Late Devonian Epoch are now exposed as continuous rock strata in Ohio, known as the Cleveland Shales. These shallow seas may have harbored as many as two dozen species of sharks. For example, the shark Stethacanthus was found in the same marine community as Cladoselache. Stethacanthus was unusual because the males had a large “spine-brush” complex on their backs in place of the first dorsal fin. This large brush may have been used as a signaling apparatus for mating, offering females a choice among males (like bird species where males display for females).

An even slightly earlier fossil shark, known from teeth rather than skeletal remains, is Antactilamna from Antarctica and Australia (and perhaps elsewhere in the southern hemisphere). Antactilamna belonged to a group of sharks, the xenacanths, that lived from about 390–230 million years ago. Antactilamna, like all xenacanths, had conspicuous teeth with two protruding cusps and with a much smaller cusp inserted in between. The fossilized skeletal remains of other xenacanth sharks indicate that they were elongated and somewhat eel-like, with long dorsal fins and prominent head spines or two dorsal fins, each with a small fin spine. These sharks were more common in freshwater environments. Many skeletons have been studied from many localities all over the world, but Orthacanthus from the Permian (286–245 mya) of Texas, North America is one of the best known anatomically. Orthacanthus reached more than three meters in length and possessed massive jaws.

From 360–286 million years ago, in the Carboniferous Period, many additional shark lineages evolved, increasing shark diversity considerably. Some two dozen different families of sharks were established during this period, and some forms were highly distinctive. Take, for example, Falcatus and Damocles, two different genera of sharks with long spines protruding from the head that were directed forward. One limestone slab of Falcatus actually has two sharks, one on top of the other. The shark on the top of the slab, which is devoid of any spine, is actually biting the spine of the shark beneath it. This may be an indication of courtship behavior. Fossil sharks of the order Eugeneodontida and the family Edestidae (for example, Edestus and Helicoprion were also quite distinctive, bearing on their lower jaw large “tooth whorls” positioned vertically in the mouth. These strange adaptations most likely served to cut prey, in a manner that may have resembled a buzz saw slicing wood.

Hybodus was a hybodont shark whose fossils are found in Asia, Europe, Africa, and North America. ©AMNH

Mesozoic Sharks (245–65 mya)

The Mesozoic Era (including the Triassic, Jurassic, and Cretaceous Periods), which extended from 245–65 million years ago, is well-known as the period when non-avian dinosaurs roamed the land. It was also an important time in the evolution of sharks and rays.

One of the predominant groups of sharks were the hybodont sharks. In the oceans and in some fresh waters, hybodont sharks were among the most dominant fishes. Hybodonts first originated some 320–290 million years ago but blossomed in the Mesozoic era. They did not survive past the Cretaceous period, which ended some 65 million years ago.

Hybodont sharks were originally described in the early 19th century from fossil teeth.

These teeth are characterized by having a central, principal cusp with smaller cusps positioned laterally to it. Also, the tooth base is wide and contains openings for the passage of nutritive canals. Such teeth are quite common in Mesozoic strata from many localities around the world. However, more complete specimens are rare.

As a result of research conducted at the American Museum of Natural History by curator of fossil fishes Dr. John Maisey, it is now easier to identify hybodonts on the basis of their skeletal remains. He determined that there are unique skeletal specializations in hybodont sharks. For example, we now know that hybodonts had two dorsal fins, each preceded by a fin-spine with a characteristic shape. The leading (front) edge of the fine spine is rounded, and the spine has smooth ridges or rows of small bump-like tubercles running along the sides. The posterior (rear) edge of the spine carries two rows of tooth-like projections.

Hybodus fossils from New Jersey include isolated teeth and cephalic hooks, like the ones shown here, plus occasional dorsal fin spines. ©AMNH

Fin-spines are present in other sharks as well, but hybodont fin-spines have this unique ornamentation.

Male hybodonts also possessed small spines across the top of the head, previously believed to be teeth, that were perhaps used in sexual selection. Examination of well-preserved skeletal material has shown that just behind the orbits (the space in the braincase where the eyes would be positioned), there is an outward projection of cartilage. Hybodonts also had thick, massive jaws that are different among distinct genera, varying in form according to their specific diets; their teeth varied accordingly.

A very interesting hybodont shark is Tribodus. This extinct shark is from the Santana Formation (from the Early Cretaceous Epoch, some 110 mya) of northeastern Brazil. It is known from articulated (meaning that the bones are attached to one another in a lifelike manner) and somewhat complete specimens, from a locality famous for the extraordinary preservation of its fossils. Specimens of Tribodus have been preserved three-dimensionally, and acid-preparation methods (in which the rock matrix is chemically removed from the mineralized skeleton) have revealed much of the anatomy of this hybodont shark. As with the other genera of hybodonts, Tribodus has dorsal fin-spines and head spines (in males only).

The ridges on the Hybodus dorsal fin spine on the left, from the Jurassic of England, contrast with the tubercles on the Cretaceous New Jersey spine on the right. Both are fragments of spines and the NJ spine is a cast of the original. ©AMNH

Hybodonts went extinct at about the same time that many of the living shark families evolved. As early as 200 million years ago, sharks very similar to modern ones were already present—for example,

Palaeospinax and Hopleacanthus. Many characters, or features, that evolved during the Mesozoic Era have persisted through to modern times and are present in living groups of sharks. For example, calcified vertebrae are a feature of modern sharks that we see in Palaeospinax. In contrast, hybodonts and other groups known only from fossils do not have calcified vertebrae and are therefore not so closely related to modern sharks.

The earliest records of fossilized shark teeth that belong to a living family are from representatives of the Hexanchidae (cow sharks) from about 208–187 million years ago. In contrast, the earliest records of fossilized skeletal remains of sharks that belong to a living family

come from about 150 million years ago. For example, Notodanus, known from this period, is a member of the living family Hexanchidae. Other modern families only became established toward the end of the Cretaceous period.

The Solnhofen limestones of Bavaria, famous for exquisite fossils of Archaeopteryx (one of the earliest fossil birds), provide a glimpse of different sharks contemporaneous with hybodonts in the Mesozoic Era. These limestone quarries contain many chondrichthyans that lived in a shallow, warm coral reef and lagoon community 150 million years ago. Many of these are early representatives of living groups, such as Pseudorhina, angel sharks, extinct cow sharks, horn sharks, catsharks, and extinct relatives of guitarfishes. Protospinax, also found here, is a shark but appears intermediate in form between sharks and rays. This reinforces the widely held theory that rays evolved from a shark ancestor.

The posterior view of the specimens in the previous image clearly shows the staggered, double-row of denticles characteristic of Hybodus dorsal fin spines. ©AMNH

This Paleospinax fossil found in England, dates to the Lower Jurassic (about 180 million years ago). Note: the Hybodus photo and the Paleospinax photo in the discoveryeducation.com link are switched. ©AMNH

The first preserved skeletons of rays appear in the fossil record during the Late Jurassic Epoch, 159–144 million years ago (for example, Belemnobatis, Spathobatis, and Asterodermus), and are very similar to modern guitarfishes (ray teeth are known from even earlier, 206–180 mya). Other Mesozoic fossil localities, such as the Cretaceous rock beds of Lebanon, dating from 97–87 million years ago, have yielded many fine specimens of extinct sawfishes, guitarfishes, and skates, along with some sharks, all of which are essentially modern in appearance.