What’s the coolest shark you can think of? The weirdest? The most fascinating? Chances are, you’re listing in your head anything from great whites to goblin sharks. But what do you know about ancient sharks? Let’s take a look at three weird–but super cool–sharks from way, way back.
Wait, What Is a Shark?
Sharks are cartilaginous fish. Their skeletons are made of the same firm, yet flexible tissue that gives our ears and nose structure. As well, they are cold-blooded and use gills for breathing, much like bony fish. However, unlike their counterparts, many shark species give birth to live young (though there is a range of diversity in reproductive methods).
Sharks are expert swimmers, and their bodies have adapted over hundreds of millions of years to suit this ability. Their unique scales–called dermal denticles–look like teeth and help reduce friction. With rigid fins that don’t bend or flutter, they can maneuver through the water with ease. And, to keep them afloat as they traverse marine environments, sharks have evolved an oily liver.
Of course, a shark is not a shark without its iconic teeth. Arranged in multiple rows, shark teeth are shed and replaced constantly. While more research has to be done, on average, sharks replace their teeth anywhere between every 9 to 36 days. This replacement ability may have played a part in the development of sharks’ strong bite force.
Shark teeth are also highly diverse. We see the triangular, serrated teeth of great white represented in our culture most often, but a range of shapes and sizes exist. Dense, flattened teeth are best used for crushing bivalves and crustaceans on the sea floor; meanwhile, needle-like teeth help sharks grasp and hold onto slippery, wriggling prey.
An Abbreviated History
What we know about the history of these animals comes mostly from all the teeth they’ve left behind; because their skeletons are made of cartilage, they typically don’t fossilize well. Thankfully, shark teeth and scales have stuck around in the fossil record, providing us clues into their unique and long-lived history.
So what exactly do we know? Well, first and foremost, the fossil record tells us that sharks have been around for a long, long time. We get the oldest undisputed evidence of sharks from around 420 million years ago (mya), somewhere towards the end of the Silurian period into the early Devonian. Some evidence suggests they may date back even further–455 mya–to the late Ordovician.
Once they appeared in the Devonian, sharks were initially slow to diversify. However, towards the end of the Devonian and into the Carboniferous, we see what’s known as a radiation in their fossil record. This period of intense diversification is sometimes referred to as the Golden Age of Sharks, with over 45 families of sharks roaming the Carboniferous waters. This was, without a doubt, a time where sharks were at their most diverse. (In comparison, there are around 37 extant families today)
Sharks continued to do well as the Carboniferous gave way to the Permian, but things wouldn’t last. Around 251 mya, the End-Permian Extinction event wiped out around 95 percent of all species on Earth. This included many of the unique and unusual sharks that had come out of the Golden Age. A few groups did persevere, one of which–Holocephali, also referred to as Chimaera–still exists today.
The ancestors of today’s sharks, skates, and rays–the Neoselachians–first appear in the Triassic. Skates and rays appeared somewhere in the mid-to-late Jurassic (~170 mya), flattening out their bodies as they adapted to living on the seafloor. All of the groups in existence today diversified rapidly throughout the Jurassic and into the Cretaceous, passing through the K-T Extinction event. As the world entered the Cenozoic, we saw the rise of giant shark species like Carcharodon carcharias–aka our modern great whites–Cosmopolitodus hastalis, and Carcharocles megalodon.
And to be clear, Megalodon is extinct. I cannot stress that enough.
Today, we’ve got close to 500 species of sharks, skates, and rays, divided into eight distinct orders and 37 families. Each of these species occupies a unique role in our marine environments. In fact, many act as apex predators within their local ecosystems. Over 400-450 million years, these animals has managed to withstand four (if not five) mass extinctions. Today, they are in-force at the top of many aquatic trophic systems.
Some Examples of Ancient Sharks
Sharks have always been a diverse, weird, and fascinating group. In their humble beginnings, they widely diversified as they expanded into new territories and niches. Especially between the Devonian to the end of the Permian, sharks got weird and wild.
It’s important to note that the three ancient sharks highlighted in this piece–and in fact, all members of their lineage up until about 100 million years ago–are technically shark ancestors. The majority of modern shark families didn’t appear until somewhere between the Jurassic and Cretaceous.
Why is this distinction important? Well, the body plan of modern sharks is different than their predecessors–enough so that what we consider a shark today would appear different compared to these ancient species. So when looking at some of our candidates for “Weirdest Ancient Shark,” keep this in mind–it’s an important distinction in the evolution of this group of animals.
Xenacanthus
Xenacanthus were highly successful sharks that lived from the end-Devonian to the end-Triassic (360-208 mya). They could grow to be around three feet in length, and had a slim, elongated body with a low dorsal fin extending down their spine.
These ancient sharks often get compared to eels, and an untrained eye might mistake Xenacanthus as such. They were unlike sharks we know today–as well as other sharks of the time–in a number of ways. They had v-shaped teeth (what we call diplodonts) that helped them catch and devour prey like bony fish and crustaceans. Xenacanthus also lived exclusively in freshwater, something not really seen today.
Xenacanthus also possessed an elongated spine that protruded from the back of its head. Made of bone, this spine’s function is not entirely clear. Given the animal seemed to lack speed, large teeth, and any sort of jaw capacity, scientists believe the spine acted as a defensive mechanism. Some scientists even speculate that the spine was poisonous.
Stethacanthus
Living somewhere between 345 to 280 mya, Stethacanthus was one of the most interesting sharks to ever live. This shark arose in the late Devonian and survived to the early Carboniferous. It’s best known for its very unique dorsal fin. Sometimes referred to as an “ironing board”, this fin had a flat surface covered in large denticles.
A few different explanations for this fin have been proposed. Some scientists believe that, with the denticles on top, the fin resembled the jaws of a massive predator. Or, it may have been a simple way of warding off danger to keep Stethacanthus safe as it swam and ate close to the seafloor. However, the prevailing theory is that the fin was used for mating.
All specimens of Stethacanthus that have been collected have “claspers”–those cartilaginous extensions of a shark’s pelvic fins that males use in delivering sperm to females. Female sharks do not have this anatomical feature. Another shark present in the same stratigraphy, Symmorium, is comprised only of female specimens. This has led scientists to think Symmorium are actually female Stethacanthus, and that perhaps the “ironing board” was used as a sort of docking mechanism. Male Stethacanthus may have leveraged the rough texture of the denticles on top of the fin to securely attach themselves to their female partners.
Cladoselache
Cladoselache is often considered the “first true shark” in the fossil record. They first show up in the late Devonian (370 mya), and their bodies follow the basic fusiform plan. Torpedo-like, Cladoselache had a slightly round head and tapering body with large pectoral fins similar to those of modern sharks. Yet, while they closely resemble modern mackerel sharks, Cladoselache also had a number of unusual features that set them apart.
While they were likely fast swimmers, Cladoselache mostly lacked the signature shark denticles. They did have some around their mouths, eyes, and edges of their fins. Also, their mouths were significantly different; Cladoselache had what is known as a “terminal mouth.” Rather than being “slung under” or suspended below the skull, the jaws of these sharks were situated in the center of their heads. Their jaw joints, as a result, were weak but supported by powerful muscles.
Within those jaws sat Cladoselache‘s namesake feature–their teeth. The name Cladoselache stands for “branch tooth shark.” While most sharks have one tooth per root, Cladoselache had multiple. This unique adaptation helped make them great generalist hunters, going after ray-finned fishes, conodonts, arthropods, and so on.
Among the other odd features of Cladoselache is their relative lack of claspers. This is odd, considering how well-preserved some Cladoselache specimens are. With some specimens, scientists have even been able to internal workings of the animal’s muscles and organs. Therefore, figuring out how Cladoselache reproduced–along with other mysteries about these enigmatic animals–is a mystery still waiting to be solved.
True Evolutionary Survivors (Who Still Need Our Help)
These ancient sharks are just a few of the examples of the incredible, weird, and strange members of this diverse group of animals. They’re hardy and tough survivors of the worst this planet has thrown at them. Today, they’re integral to our marine ecosystems–the very ones nearly half the planet’s current population rely on as a primary food source.
While it’s possible sharks will outlive our species, we’re certainly not doing a lot to help their chances. Because sharks grow and reproduce at slow rates, the damage we’re inflicting on their populations–whether it’s finning, hunting for sport, bycatching, etc.–takes a long time to overcome. We’re currently killing about 100 million sharks a year, and at this rate we’re on track to force some species straight into extinction. As we continue to cull their populations, we’re causing ripple effects in their ecosystems. And, if we sit by and do nothing, those ripples will eventually come back to haunt us.
Interested in learning how you can help shark conservation? Check out the links below!
Atlantic White Shark Conservancy
Society for Conservation Biology
Shark Research & Conservation Program | University of Miami
Header image by Nobu Tamura. Used under CC BY-SA 3.0
November 30, 2018: Article was expanded to include information about the distinction between prehistoric and modern sharks.
What a lovely post! I learned so many shark facts.