Geologic Timescale
When the Golden Age of Fishes ended 359 million years ago, more than 75% of Earth’s species were eliminated. The Devonian Period, as it was also known, was over. As life rebounded, a new era of abundance—the Carboniferous—had begun.
An early reconstruction of a Carboniferous coal swamp, from Wikipedia
During the Carboniferous Period (359-299 million years ago), global temperatures and atmospheric carbon dioxide levels were higher than they are today. The warmth melted the ice caps and raised the seas by more than 330 feet (100 meters) above modern levels. Oxygen levels were higher too, due to the appearance of the first rainforests on land. These forests, hotbeds of fervid photosynthesis, covered the planet and churned out enormous quantities of oxygen into the atmosphere. To put this in perspective, the air is about 20% oxygen today; 359 million years ago it was 32% oxygen.
Under these tropical conditions, arthropods grew extraordinarily large. There were dragonflies the size of seagulls (ex: Meganeura), scorpions the size of dogs (ex: Pulmanoscorpius), and millipedes the size of pythons (ex: Arthropleura). Amphibians, too, grew larger than their modern descendants. Some of them, like the 5-foot-long (1.5-meter-long) Eryops, lived in freshwater and preyed on giant insects. Others, like the seagoing Stereospondylids, exceeded 20 feet (6 meters) in length and hunted smaller amphibians. The first reptiles also appeared.
Due to the Devonian mass extinction, marine biodiversity changed significantly. Most of the ammonites, trilobites, and reef-building corals went extinct. For the next 100 million years, reef-building occurred in a diminished state. Vertebrates, too, were dealt a heavy blow. More than 44% of backboned creatures died out. Lobe-finned fish were decimated while the placoderms were completely eradicated.
The competition gone, sharks conquered the Carboniferous waters with incredible variety. During this Golden Age of Sharks, 45 families of sharks (compared to 40 today) prowled the oceans, rivers, and seas. Species from the Devonian—Cladoselache, Ctenacanthus, Iniopteryx, Stethacanthus, and others—survived the mass extinction and radiated across the planet. The freshwater Xenacanthids also endured; their remains litter the coal swamps that give the Carboniferous Period its name. A myriad of sharks arose during this time of widespread evolution. Notable examples include the edestids, the hybodonts, and the petalodonts.
The edestids, or “scissor-tooth” sharks, were one such family. Identified by their bizarre saw-like jaws, edestids have confounded paleontologists for decades. Edestus giganteus, the flagship representative, embodied the word “strange”. Its teeth only grew in linear brackets that left the gums exposed. Its jaws were even weirder; they curved outward from the skull. How this creature reached 20 feet in length (let alone fed itself) is still a mystery.
Edestus giganteus, from Wikipedia
The 15-foot-long (4-meter-long) Helicoprion also defies explanation. While the edestid’s upper jaw is standard fare, its lower jaw—a coil of serrated teeth similar to a circular saw—is fascinatingly curious. The whorl’s purpose—and location—is still hotly debated, although many ichthyologists believe the shark used it to crush ammonite shells. For all its eccentricities, Helicoprion was a successful genus; it persisted into the Jurassic 130 million years later.
Helicoprion bessonovi, from Wikipedia
Listracanthus, the feather-spined shark, was more elegant than outlandish. For 80 million years, this enigmatic eel-shaped edestid patrolled the oceans. Its existence is only known from fossilized spines and anecdotal evidence.
Listracanthus hystrix, from Wikipedia
The hybodonts, or “hump-toothed” sharks, also evolved in the Carboniferous. For the most part, they resembled modern species. Their 7-foot-long (2-meter-long) streamlined bodies, augmented by a unique pair of dorsal fins, were designed for quick bursts of speed. However, unlike other primitive sharks, hybodonts possessed two distinct kinds of teeth, an adaptation that increased their predatory potential. Each hybodont also wielded a single, bony dorsal blade that served a role in self-defense. Because of these characteristics, the order inhabited every corner of the world for nearly 260 million years. For reasons unknown, hybodonts petered out in the Cretaceous.
Hybodus, from eavp.org
The petalodonts, or “flattened tooth” sharks, were more like chimaeras than true sharks. Found only in Carboniferous and Permian sediments, this odd, short-lived order puts the edestids to shame in terms of sheer peculiarity. With few exceptions, petalodonts are known entirely from large triangular teeth. The best-preserved specimens (Belantsea Montana of the Carboniferous and Janassa bituminosa from the Permian) revealed them as muscular, leaf-shaped animals that resembled parrotfish. It is widely believed that petalodonts grazed on bryozoans, corals, crinoids, and sponges.
Belantsea montana, from Wikipedia
The Golden Age of Sharks continued unabated for 100 million years. The era’s incredible diversity transcended the ice age that concluded the Carboniferous Period and initiated the Permian Period that followed. At its zenith, sharks outnumbered other fish three to two. However, like the Golden Age of Fishes before it, the Golden Age of Sharks ended with a mass extinction.
Approximately 245 million years ago, over 90% of life on Earth perished. In the sea, 99% of species disappeared. Yet, despite the odds, sharks survive today. Many have no natural predators. Because of this, they mature slowly and produce few offspring. Some, like the whale shark (Rhincodon typus) and the great white (Carcharodon carcharias), give birth to live young. Others, like dogfish (Order: Squaliformes) and stingrays (Order: Myliobatiformes), lay eggs called “Mermaid’s purses”. Unfortunately, these traits—once key to the shark’s success—now threaten its continued survival.
Due to intensifying global fishing efforts, the world’s 700 species of sharks, rays, and chimaeras now face another mass extinction by our hands. Because of a growing demand for their fins, between 26 and 73 million of these great fish are killed annually. More die unintentionally as bycatch from reckless fishing practices. As a result, 90% of the large sharks are gone.
What will happen to those that still remain? Their fate also lies in our hands. Do you care?
Related articles
- Sharks of the Devonian (thewrittenblit.com)
- Shark-finning stance stain on our global reputation (stuff.co.nz)
- Protect Great White Sharks Under California’s Endangered Species Act (forcechange.com)
- Shark Facts (hadnoclue.com)
- Great White Shark Origins Found (news.discovery.com)
- It’s whale shark season once again! (ameramor.com)
- Great whites ‘not from megashark’ (bbc.co.uk)
- What are great white sharks doing when they dive deep? (earthsky.org)
- Shark Attacks in Hawaiian Waters May Be on the Increase Because of Spear Fishermen (hawaiireporter.com)
- Plant Paleoart Through the Ages (blogs.scientificamerican.com)
- Whale Shark Caught In Abandoned Fishing Net Rescued by Divers (Video) (treehugger.com)
I can’t help but ponder Bill Engvall’s observation about some of these fossilized “strange” creatures: what if these aren’t distinct species but rather defective, “goofy-looking fish Like a Dorkfish!”
Parallel, French Bulldogs would not exist were they not perpetually bred by humans. They can’t anatomically reproduce (but are artificially inseminated), have lots of health problems, etc. Natural selection would weed them out. Therefore, are they a true species?
The thing is, though, that these species were very successful, lasting 50 million years or more in the fossil record. They aren’t defective because most of them (except the Petalodonts) survived at least one mass extinction event (including the Permian that wiped out practically everything).