On the Golden Age of Sharks

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 Period—had begun.

320px-meyers_b15_s0272b
A Carboniferous rainforest/swamp

On land, the world’s first rainforests emerged. Early clubmosses, conifers, and cycads—some more than 100 feet (33 meters) tall—covered the planet with foliage. Through fervid photosynthesis these dense jungles pumped enormous quantities of oxygen into the atmosphere. As a result, the skies were richer in oxygen than they are today. Because the Earth was warmer and lacked polar ice caps, the seas rose 400 feet (120 meters) above modern levels.

geologic-timetable
Geologic Timescale

Under these tropical conditions, arthropods grew extraordinarily large. There were dragonflies the size of seagulls (ex: Meganeura), scorpions the size of dogs (ex: Pulmonoscorpius), 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 Stereospondyli, exceeded 20 feet (6 meters) in length and hunted smaller amphibians. The first reptiles also appeared.

carboniferous
The Carboniferous Earth, taken from the BBC (http://www.bbc.co.uk/nature/history_of_the_earth/Carboniferous)

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 Xenacanthiformes 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 edestidae, the hybodontiformes, and the petalodontiformes.

The edestidae, 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, is the definition of strange. Its teeth only grow in linear brackets that leave the gums exposed. Its jaws are even weirder; they curve outward from the skull. How this creature reached 20 feet in length (let alone fed itself) is still a mystery.

edestus_imago
Edestus giganteus

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.

800px-helicoprion_bessonovi1db
Helicoprion bessonovi

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 written accounts in scientific journals.

listracanthus_hystrix
Listracanthus hystrix

The hybodontiformes, 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.

cgtpl_hybodus
Hybodus

The petalodontiformes, or “flattened tooth” sharks, were more like chimeras 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) reveal them to be muscular, leaf-shaped animals that resembled parrotfish. It is widely believed that petalodonts grazed on bryozoans, corals, crinoids, and sponges.

belantsea_montana
Belantsea montana

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, 96% 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 shark (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 chimeras now face another mass extinction by our hands. Because of a growing demand for their fins, at least 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 lies in our hands.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s