Megalodon Wasn't the Only Impressive Shark in the Prehistoric Seas

Science
پنجشنبه 18 مرداد 1397 ساعت 21:39
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Smith sonian mag
If ever there were a creature that sparks our primal fear of what resides in the deep, it’s the megatooth shark. Known to experts as Otodus megalodon , this 50-foot-long distant cousin of the modern great white was the largest predatory fish of all time. The exceptionally large specimens had mouths brimming with finely serrated teeth the size of your hand. Everything from a nod in Peter Benchley’s novel JAWS to the big-budget film The Meg has kept our fear of this shark alive, despite the fact that it’s been dead for over 2.6 million years .
All this sensationalism has masked some of what we’ve come to know about this impressive, whale-munching shark. For example, where megalodon came from in the first place. But in order to understand the origins of megalodon, we need to look back to the beginning of sharks.
Putting a finger on the moment sharks became sharks is a little tricky. It’s relatively simple to look at modern fish and separate sharks from other fish, but, the further back in time you go, the fuzzier the dividing lines become. Still, DePaul University paleontologist Kenshu Shimada says, “‘sharks’ as cartilaginous fishes in a broad sense go back roughly 400 million years ago.”
Many of these earliest sharks are only known from scales or teeth. One of the earliest sharks, called Leonodus , is principally known from two-pronged teeth. Comparisons with later, better-known sharks, hints that Leonodus was more eel-like in form than most sharks we’re familiar with today. But the fossil record is not always so spare.
A skeleton found in New Brunswick, Canada helps reveal what these early sharks were like. Named Doliodus problematicus (Latin for "problematic deceiver"), this fish, one of the oldest sharks ever unearthed, had a wedge-shaped head and spines jutting from its fins and underside. “Some modern sharks retain fin spines,” Shimada says, “but the major reduction in the number of spines have made modern sharks less ‘spiny’ in appearance compared to their earliest ancestors.” These spines likely acted as protection from the other jawed-fish that were proliferating in the same ancient seas.
If only more sharks were preserved in such detail. Despite having such a long history—nearly twice as long as that of dinosaurs—“the vast majority of sharks in the fossil record are represented by isolated teeth,” Shimada says. There are some exceptions—the 318 million year old Bear Gulch Limestone in Montana preserves some sharks in delicate detail—but most of what we know about ancient sharks comes from teeth. An entire group of early sharks called cladodonts, for example, are primarily known from strange teeth that feature a long, central blade surrounded by smaller tines. They look like terrible crowns, and were suited to trapping slippery prey rather than cutting.
Even so, we can learn a great deal about the nature of these ancient swimmers from what they’ve left behind. Despite their reputation as “living fossils” that have persisted unchanged, we know fossil sharks took on a vast array of body sizes, shapes, and ornamentations, from eel-like xenacanthids decorated with unicorn-like spikes to the striking Stethacanthus , which was adorned with what looks like a bristly comb atop its head. In fact, finds at places like Bear Gulch help illuminate the behavior of some of these strange forms. One particular fossil found there is of two roughly six-inch sharks called Falcatus . The two seem to have died courting, with the female with her jaws around the male’s spike-like head ornament, perhaps giving us a look at mating habits long ago.
Even familiar species are undergoing revision. “A good example is the gigantic Cretaceous shark Cretoxyrhina mantelli from Kansas,” Shimada says. The fact that the teeth of this shark looked like those of modern mako sharks led paleontologists to propose a connection between the fossil shark and the modern seagoing speed demons. But that’s changed. “A small number of skeletal remains have allowed us to better infer the species’ body size, body form, tooth organization, and even its growth pattern,” Shimada says, revealing that Cretoxyrhina was a unique shark that wasn’t just a prehistoric carbon copy of today’s mako sharks. Stretching to about 23 feet in length, this “Ginsu shark” was a bulkier predator that was more similar in size and lifestyle than today’s great white, only feeding on marine reptiles like mosasaurs and plesiosaurs instead of seals.
Against that background, we can turn our gaze back to megalodon. As with most other ancient sharks, navigating the origins of megalodon is primarily a story of teeth. In the case of this celebrity shark, Swansea University paleontologist Catalina Pimiento says, there is still some debate about specifics, “but I am convinced megalodon belongs to the extinct family Otodontidae,” which is thought to have sprung from an even more ancient form called Cretalamna .
This taxonomic wrangling is important, not just for scientific communication but because determining the closest relatives of megalodon helps inform ideas and debates about where the shark came from and how it behaved. When megalodon was thought to be a close relative of today’s great white shark, for example, much of the great white’s behavior was transposed onto its larger relative. Now that megalodon is further removed from the great white, finding its root among other “megatooth” sharks, paleontologists have to ask new questions about a shark that seemed familiar.
So far as paleontologists have been able to track, sharks recognizable as megalodon evolved about 20 million years ago. That raises the question of what was happening at that time to spur the evolution of such an impressive fish. “There were a lot of environmental changes going on during that time period,” Pimiento says, including a pulse of global warming. This might be related to the rapid evolution of many new marine mammals—the primary food source for megalodon - and allowed such an impressive shark to arise. And it was certainly an imposing presence in the seas right from the start. “The body size of this species did not vary over time,” Pimiento says, meaning megalodon was always a giant. There were other large sharks at the time - the ancestors of the great white were swimming the same seas - but none were even close to the same size as megalodon.
All things considered, being large and in charge worked well for megalodon. The shark swam the seas for over 17 million years, eventually following its prey species into extinction as changes to global temperature and sea level dramatically altered ocean habitats. What we have left now are teeth and some vertebrae that allow us to appraise this massive predator from 2.6 million years' distance.
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