On the Sharks of the Devonian
“The Golden Age of Fishes” is known for, among many things, its incredible ichthyologic diversity. As life on land began its own rapid radiation of terrestrial forms, including the earliest ferns and the insects that ate them, marine ecosystems flourished below the ocean’s surface. Between 416 and 359 million years ago, the Devonian Period saw the evolution and diversification of the first bony fishes, the armored placoderms, and the cartilaginous sharks—the forebears of the vertebrates that dominate aquatic environments today.
Devonian Earth
The Devonian Earth consisted of two supercontinents, Gondwana and Laurussia, and a deep, global Panthalassic Ocean that covered the rest of the planet. While the placoderms gave the world its first vertebrate superpredator (the 33-foot-long Dunkleosteus, seen below), the sharks gave them a run for their money with an incredible array of species.
Dunkleosteus
One of the earliest and most abundant sharks was Cladoselache, a 6-foot-long predator known for its streamlined body and aquadynamic agility. It is one of the best-known Devonian sharks because of the large number of well-preserved specimens that currently exist in museum collections. During this period, it hunted bony fish and smaller sharks in the oceans that once covered North America. Cladoselache is unique because it lacked the scales and claspers possessed by other sharks. However, it must have been successful because the genus survived 100 million years.
Stethacanthus is another species from the Devonian, famous for its dorsal appendage. Found in North America, this two-foot-long fish resembled modern varieties except for one major anatomical difference. Unlike other sharks (let alone other organisms), male Stethacanthids possessed dorsal fins that resembled upside-down irons covered in teeth (see below).
Stethacanthus, taken from Diving with Sea Monsters
It is believed that these strange headpieces existed for mating displays and were products of sexual selection.
Another two-foot-long shark from places as far away as Kansas and Scotland also lived during the Devonian. This one, called Ctenacanthus (comb-spine shark), is known from fossilized fin spines uncovered in shallow-water marine deposits. It closely resembles modern sharks. Though the earliest traces of this group were dated to the Late Devonian, fossils have been found from the Mesozoic 150 million years later. This indicates considerable evolutionary success, a trait possessed by all other shark species.
Other groups of sharks from the Late Devonian include the bizarre Iniopteryx, or flying shark (see below); the extinct freshwater Xenacanthids (see below); and the Holocephalids (or chimeras) that survive to this day.
Other groups of sharks from the Late Devonian include the bizarre Iniopteryx, or flying shark (see below); the extinct freshwater Xenacanthids (see below); and the Holocephalids (or chimeras) that survive to this day.
Iniopteryx
Xenacanthus
“The Golden Age of Fishes” was short-lived and ended catastrophically with a mass extinction that wiped out more than half of the life on Earth, including the placoderms. Sharks survived and radiated across the planet. The period that followed, the Carboniferous, became “The Golden Age of Sharks”. During the Carboniferous, sharks occupied every niche. Freshwater, saltwater, and brackish varieties were common, and they placed themselves at the top of the food chain. To this day, they remain unchallenged and largely unchanged from the Devonian. Today, 700 species of sharks, rays, and chimeras survive—each one a living testimony of the lineage’s evolutionary prowess.
As human continue to prey on sharks for their fins and for bragging rights, the future of this great predatory fish is threatened. Now, with more than 90% of large sharks depleted from the oceans, we have the power to save these magnificent creatures from extinction—a fate they have avoided for more than 400 million years.
Andrew Blitman -- Author & Artist 
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