In these bizarre skulls, McCurry saw a mystery. Many aquatic animals, from river dolphins to gharial crocodiles, have evolved long, toothy snouts to help them catch fish. But why did these particular dolphins take their snouts to such an extreme? In a curious twist, these species weren’t all part of the same lineage. Rather, they evolved from short-snouted ancestors on at least three different occasions, all during the Miocene period between 5 and 23 million years ago. “There must have been something going on in their environment at the same time to drive their evolution,” says McCurry.
To work out what that was, he first had to understand how these animals used their snouts. Working with Nick Pyenson, the Smithsonian’s expert on prehistoric whales, McCurry used a medical scanner to create digital models of the skulls of several long-snouted dolphins. He then analyzed those models with techniques that engineers use to measure the strength of beams and girders.
He found that the dolphins could easily have swept their snouts through the water at high speed, stunning fish in the way that modern billfishes like swordfish do. The different species likely used different techniques. Some, like Zarhachis, had flattened snouts, and probably swept the water from side to side at stunning speeds, just like today’s swordfish. Others, like Xiphiacetus, had snouts that were circular in cross section; like today’s marlins, they sacrificed a bit of speed for the ability to attack in any direction.
Crucially, these long-snouted species arose during a time in the middle of the Miocene when ocean temperatures started climbing. In cold water, warm-blooded predators like dolphins have an advantage over cold-blooded prey like fish or squid, because they’re better at maintaining a high metabolism and swimming at high speeds. As the oceans warm, fish can move faster and the dolphins’ advantage disappears. Perhaps some of them regained the upper hand by evolving long snouts that could swiftly sweep through shoals of prey.
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During the mid-Miocene, sea levels also rose, flooding the shorelines and creating a variety of new shallow habitats. This varied coastal world effectively gave dolphins permission to be weirder. Some evolved massive underbites and perhaps used their lower jaws to skim through mud. Others developed walrus-like tusks, which they could have used to extract buried shellfish. And still others, of course, evolved extremely long snouts.
But this extraordinary period of evolutionary experimentation ended when the Miocene gave way to the Pliocene. Temperatures fell, the climate became more erratic, and the planet entered into a series of cycling ice ages. Creatures that evolved during the more stable climatic heyday disappeared. “That explains why we don’t have these weird, long-snouted dolphins around today,” says McCurry.
Studies like these are important because they highlight how the shapes of animals are shaped by their environment, and how much diversity can be lost when that environment changes, says Karina Amaral from the Federal University of Rio Grande do Sul, who was not involved in the study. “At a time when many people insist on ignoring our changing climate,” she adds, this kind of research can help to paint a clear—and concerning—image of the consequences.