In this episode we discuss sharks and their evolutionary history. Sharks are well know today as the top predators in many of the world’s ocean environments, but this wasn’t always the case. We’re covering the features that define a sharks and taking a look at some of the most interesting cases along the 400 million year story of shark evolution.
Today there are two dominant groups of fish in the ocean; the Osteichthyes (bony fish) and Chondrichthyes (cartilaginous fish). As both names suggest, the distinction between the groups mainly has to do with what their skeletons are made from. Bony fish have skeletons of bone like our own, and these days they are the most common group of fish. Cartilaginous fish instead have a skeleton made almost completely out of cartilage, though their teeth are dentine. They include a wide variety of members, ranging from filter feeders to bottom dwellers to large predators.
Sharks are an incredibly diverse groups with over 500 modern members. Pictured here from left to right and top to bottom; spiny dogfish (Squalus acanthias), Japanese sawshark (Pristiophorus japonicus), whale shark (Rhincodon typus), Australian angelshark (Squatina australis), great white shark (Carcharodon carcharias), scalloped hammerhead (Sphyrna lewini), frilled shark (Chlamydoselachus anguineus), and horn shark (Heterodontus francisci). Image by LittleJerry from Wikimedia Commons.
Chondrichthyes are broken into two sub-classes, the Elasmobranchii and Holocephali. Holocephali contains the group known as the chimeras (also called ghost, rat, and spook fish). These are medium to small predators that swim slowly in deeper waters. Today they are fairly rare, though at one time they were quite dominant in the oceans. The Elasmobranchs include the more recognizable groups of Selachimorpha (sharks) and Batoidea (skates, rays, and sawfish). Among sharks there are 13 orders, nine extant and 4 extinct.
Carcharhiniformes
Often known as the ground sharks, this group includes the requiem sharks, catsharks, and hammerhead sharks. They typically have an elongated snout and nictitating membrane over the eye. Many of the sharks in this group lack a spiracle and must swim to breathe. Pictured here is one of their more famous members, the tiger shark (Galeocerdo cuvier). Image by Albert kok from Wikimedia Commons.
Heterodontiformes
The bullhead sharks are typically smaller bottom dwelling sharks. Many display spikes in front of each dorsal fin, a feature they share with some fossil cousins but not many modern sharks. Pictured here is the Port Jackson shark (Heterodontus portusjacksoni). Image by Mark Norman form Wikimedia Commons.
Hexanchiformes
This group includes the frilled sharks and cow sharks. They are distinguished by having more than the typical five gill slits found in most sharks. This along with their skeleton makes them very similar to more ancestral sharks. Only seven species survive today. Pictured here is the broadnose sevengill shark (Notorynchus cepedianus). Image by Peter Southwood from Wikimedia Commons.
Lamniformes
Known as the mackerel sharks. This is not the largest group but it has some very notable members, including all the species of mako, thresher, goblin, basking, megamouth, and great white sharks. These give live birth and maintain a higher body temperature than the water they swim in. Pictured here is a shortfin mako shark (Isurus oxyrinchus). Image by Mark Conlin from Wikimedia Commons.
Orectolobiformes
The carpet sharks are named for the ornate patterns many are known to have. This group includes such members as the bamboo sharks, wobbegongs, and whale sharks. Seen here is the common nurse shark (Ginglymostoma cirratum). Image by Dr. Mathew Gilligan from Wikimedia Commons.
Pristiophoriformes
The sawsharks heavily resemble their relatives the sawfishes. You can identify these sharks since their gills are visible from the side and they possess barbels on the rostrum. Like sawfish, use their saws to slash at prey that hides in the sand or vegetation. This is a drawing of a longnose or common sawshark (Pristiophorus cirratus). Image by Van Diemonian from Wikimedia Commons.
Squaliformes
Often called dogfish, this is a diverse group of mostly deep water sharks, including the smallest shark, the dwarf lanternshark (Etmopterus perryi), at less than 8 inches. On the other hand, the Greenland shark (Somniosus microcephalus) (pictured above) is quite large, with individuals known to get about 15 feet long, rivaling the great white in size. Image by Justin from Flickr.
Echinorhiniformes
This group only includes two species, the bramble (Echinorhinus brucus) and prickly (Echinorhinus cookei) sharks. They got these names due to the thorn-like dermal denticles found on their skin. There is some debate as to whether or not this order should actually be grouped within the squaliformes. Seen here is an artist reconstruction of a bramble shark. Image by Arthur Bartholemew form Wikimedia Commons.
Squatiniformes
The aptly named angel sharks are specialized benthic ambush predators. Their body is flattened like a ray but the mouth is positioned forward to grab prey. Pictured here is the sand devil or Atlantic angel shark (Squatina dumeril). Image by D Ross Robertson from Wikimedia Commons.
EXTINCT SHARKS
Cladoselachiformes
Known from the Late Devonian to Early Carboniferous, these are often considered the earliest predecessors to what we know as modern sharks. They had elongated bodies and spines on both dorsal fins. Here we see a fossil of Cladoselache fyleri. Image by James St. John from Wikimedia Commons.
Symmoriida This is actually an extinct group of chimera that likely gave rise to modern shark ancestors. They were some of the dominant predators during the Devonian and Carboniferous. Many of their members appeared very “shark-like” and were notably weird.
Falcatus falcatuswas a small member of this group that showed clear signs of sexual dimorphism. The males (pictured above) had a large forward facing spike that the females lacked. Image by Tommy from Wikimedia Commons.
Stethacanthus is another symmoriid sporting strange head gear. But it is unclear what the purpose may be for this “mohawk.” The reconstruction here is by DiBgd form Wikimedia Commons.
Xenacanthida
These long-bodied shark ancestors swam mostly in freshwater habitats from the Carboniferous to Triassic. They we notable for the long dorsal spine and ribbon-like fin down the body. This spine can be seen clearly on the left of this fossil of Xenacanthus. Image by Citron from Wikimedia Commons.
Hybodontiformes
Finally true sharks, the hybodonts were unusual for their conical teeth. They were very successful, surviving from the Devonian all the way to the Miocene. This is a fossil of Hybodus fraasi. Image by Haplochromis from Wikimedia Commons.
History of Sharks
As you will often hear, shark have a long history. It start with the acanthodians, or spiny sharks. While not true sharks it was likely from this group that the first shark ancestors evolve. They were common from the Silurian to Permian.
These early fish were superficially shark-like in appearance. Though they had cartilaginous skeletons they also had bony bases in their fins that tended to contain long spines. This fossil of Diplacanthus striatus shows off the spines. Image by James St. John from Flickr.
The first shark-like fossil we see are actually only a scale from Silurian deposits in Siberia and a teeth from the Devonian. It is here in the Devonian, 419 to 358 million years ago, that we see the first ancestors to the elasmobranch lineage.
Antarctilamna, an early shark from the Devonian, was likely a freshwater shark like the xenacanthids The name means the “lamnid shark of Antarctica.” It sported two pronged teeth and a spine on the dorsal fin. This image by DiBgd form Wikimedia Commons.
It’s not until the Jurassic that we see more modern features in sharks start to appear. More efficient tail fins and a more mobile set of jaws allowed sharks to take a more dominant role in the oceans. By the mid-Cretaceous, we finally have modern-looking sharks and many of the modern lineages swimming the oceans. After the Cretaceous sharks become the top predators in the ocean and in the Miocene the largest shark known evolves, C. megalodon.
Today
Sharks today are still among the top predators in many ocean ecosystems, but they’ve started having to deal with a new apex predator. Human hunting and side-effects are causing shark numbers in many areas to plummet. Certain groups are being pushed dangerously close to extinction.
It would be a shame to lose such a long-lasting group now only due to misunderstanding and fear.
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If you enjoyed this topic and want more like it, check out these related episodes:
The Common Descent Podcast 
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