Whether it’s fascination, fear, or a combination of both, Why do sharks attack people?sharks capture our imagination. Hollywood hysteria aside, there’s a lot they do to keep our attention. Sharks can be bulls or tigers. They can be mallets or cookie cutters. Some of the weirder ones can even be bioluminescent ghostswhile others that can even reproduce asexually.
But have sharks always been so diverse? What were the earliest sharks and when did some sharks begin to dominate the marine food chain?
Ancient sharks
It depends on how far back you want to go, of course, but some researchers now believe that the group that includes all three living groups of sharks — rays, sharks, and chimaeras — evolved first from acanthodians.
The beginning of sharks: the acanthodia. (Credit: Serafima Antipova/Shutterstock)
“They’re all terribly spiny and small,” says Michael Coates, a biologist at the University of Chicago. “However, they show us the root of the shark world – getting a bony internal skeleton.”
Read more: This 439-million-year-old “spiny shark” is our oldest jawed ancestor
Characteristics of acanthodians
When acanthodia began to evolve, they weren’t the scary sharks we see today. They didn’t have retractable teeth like the great white or the sheer size of megalodon—those traits would take millions of years to develop.
Most of the acanthodia were the size of fingers – just the size of anchovies. They were probably among the first fish to move from the sea floor into the water zone during the so-called nekton revolution during the Silurian and Devonian periods.
As with other organisms adapting to life in open water for longer periods, this upward shift involved many new changes—things like larger eyes and multiple paired fins evolved to stabilize their swimming. The earliest sharks also had fewer bony plates in their skulls and reduced scales.
From Acanthodians to Cimmerians
By the Devonian period, when the Chondrichthyes, which include the crown groups of elasmoclones (rays and sharks) and chimaeras, evolved from acanthodia, they no longer had internal bones, but cartilage, which was metabolically easier to produce and neutrally buoyant.
“You have to improve your propulsion system, you have to improve your sensors,” Coates says.
Other features common to males in this crown group include the pelvic clasp — “effectively a paired penis,” Coates says. Specialized teething systems began to appear, including rapid tooth extraction and replacement.
“It’s a brilliantly simple modular system, it’s very elegant,” Coates says of the tooth system.
Female sharks, in turn, began to develop some of the specialized modes of reproduction we see today in the Paleozoic, which includes the internal fertilization that occurs in all sharks, rays and chimeras.
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An explosion of ghost shark shoots
The anchovy-like era doesn’t last forever for sharks. Sometime before the series of mass extinctions that occurred during the Late Devonian, beginning approximately 372 million years ago, sharks began to evolve into a variety of new and interesting body shapes. Much of the explosive evolutionary radiation during this period occurred among the group that included chimeras or ghost sharks.
Symmoriids
Symmoriids, possibly early offshoots of the evolving chimeroid group, comprise an entire group of fish. They were “strange looking” sharks with strange appendages sticking out of their tips and long tail fins.
Iniopterygii
One group, the Iniopterygians, has wing-like fins unlike any other shark alive today, Coates says. By the Late Devonian, some of these sharks began to grow in size—some to over two meters in length.
Placoderms
But they are not yet the kings of the sea. Dunkleosteus, a genus of armored fish called placoderms reached larger sizes and could easily have been prey for early chondrichthyans.
“You get a sense of who the top predator is out there, and it’s not sharks,” Coates says.
Read more: Why do sharks attack people?
The prolific evolution of sharks
Before the end of the Devonian, sharks continued to increase in both abundance and diversity in a way that they would never increase again. Successive extinctions greatly reduced abundance and diversity, but in the subsequent Carboniferous, shark diversity continued to be dominated by early chimeras and chimera offshoots.
While modern ghost sharks are mostly relegated to the depths of the ocean, back then the order was everywhere, with some species even spending their time in saltwater rather than seawater.
“Chimeras are just incredibly weird,” says Coates.
The earliest elasmoclones—sharks and rays—also began to adopt sophisticated feeding techniques. Some, dating back to approximately 340 million years ago, evolved the ability to suck up food instead of simply swallowing food with their jaws, like modern nurse sharks. “These are the first truly efficient vertebrate suction feeders we have on the planet,” says Coates.
It wasn’t until much later in the early Miocene that the largest of all sharks, the megalodon, appeared, and shark dominance of the sea was complete.
Within a few hundred million years, the group of fish evolved from feeders the size of anchovies to giants up to 20 meters long, according to some estimates.
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