For a period of several million years, ancient species of dolphins glided through the seas, looking in many ways similar to today’s toothed whales—with the notable exception of their remarkably long snouts. These odd cetaceans boasted proportionally longer snouts than any other aquatic mammal or reptile, living or extinct; some of their nose-like appendages extended more than 500 percent further than their braincases. Even Matthew McCurry, curator of palaeontology at the Australian Museum who has studied the evolution of long snouts in extant species, finds their skulls “extremely strange-looking.”
In 2015, as a pre-doctoral fellow at the Smithsonian National Museum of Natural History, McCurry decided to take a closer look at these extinct marine mammals. Scientists have known about them for more than 100 years, but no one had pinned down the function of their bountiful snouts. The hypotheses were “largely qualitative and offhand,” says Nicholas Pyenson, curator of fossil marine mammals at the Museum of Natural History. “People said, ‘Oh, the long snout is probably used for stirring up prey in the sediment … [W]hat I would say is that those are adaptational hypotheses, but nothing had really been tested.”
So McCurry and Pyenson set out to do just that. And in a new paper published in Paleobiology, the researchers have put forth a solution to the curious case of the long-snouted dolphin: the creatures, they found, were able to swish their snouts through the water, using them to hit and stun prey, much as swordfish do today.
In their quest to analyze the unique skulls of long-gone cetaceans, McCurry and Pyenson turned to the Smithsonian’s vast trove of whale fossils. “We have so many that have not been looked at that I actually cannot tell you the full extent of the whale fossil records that we possess,” Pyenson says, but estimates that there may be as many as 15,000 in the collection.
The researchers performed computed tomography (CT) scans of the crania of three extinct species (Pomatodelphis inaequalis, Xiphiacetus bossi and Zarhachis flagellator), and casts of two other ancient cetaceans (Parapontoporia sternbergi and Zarhinocetus errabundus). To compare these creatures to animals that are alive today, McCurry and Pyenson scanned two species of river dolphins, which have considerably longer snouts than their ocean dwelling-counterparts, though not nearly as long as their prehistoric predecessors. The researchers also looked at two species of long-snouted fish: the Atlantic blue marlin and the swordfish.
McCurry and Pyenson then analyzed the digital models of the skulls using calculations that engineers rely on to assess the load-bearing capabilities of beams. According to Pyenson, “beam theory” is useful in the study of snouts because it “talks about these objects as they are built to respond to forces: how rigid it is, what kind of stresses are imposed on it.” And the researchers found that dolphins of yesteryear would have had no trouble sweeping their impressive snouts through the water to whack their prey.
Because the snouts of the species varied in shape, they moved their handy appendages in different ways. Some swept them from side to side, others up and down, and still others could move their snouts in multiple directions.
“Imagine a beam like a ski,” Pyenson says, as an example. “A ski flexes well up and down, but not side to side. A pole, which has the same shape distributed, can flex up and down [and] side to side, no problem.”
The researchers were particularly struck by the fact that these animals were not all closely related to one another. Several species appear to have independently evolved exceptionally long snouts, which suggests that something in their environment was driving the change. But what, precisely?
Long-snouted dolphins emerged in the Middle Miocene, a period stretching from 11.6 to 16 million years ago, when the climate was warmer than it is today. Ocean temperatures went up and sea levels rose, creating more near-shore sea floor, which is “a really great habitat for fish and other prey items for dolphins,” Pyenson says. But fish’s escape response gets quicker in warmer waters, making them more difficult to catch. It is possible, the researchers theorize, that dolphins evolved hyper-long snouts during this period to give them an extra advantage during the hunt.
For millions of years, global temperatures remained steady and dolphins with supremely elongated snouts frolicked in warm waters.
“Maybe this is a consequence of what happens when you have that kind of environment stable for several millions of years,” Pyenson theorizes. “These traits get exaggerated.”
But with the advent of the Pliocene era, the climate became more erratic and the abundance of temperate, near-shore feeding grounds fluctuated. With these changes, the long-nosed dolphins vanished. And this raises interesting questions about whether the evolutionary trajectory of extinct dolphins can tell us anything about how dolphins might fare in the current era of climate change.
The story of these ancient creatures highlights how an organism’s environment transforms its appearance, and clearly shows what we stand to lose in terms of biodiversity when an environment changes, zoologist Karina Amaral from the Federal University of Rio Grande do Sul, who was not involved in the study, tells Ed Yong of The Atlantic. And that’s important to consider, especially, “[at] a time when many people insist on ignoring our changing climate,” Amaral says.
What can the evolutionary trajectory of extinct dolphins can tell us about how dolphins might fare in the current era of climate change? McCurry notes that it is difficult to draw definitive conclusions because fluctuations in temperature today are “unprecedented in their cause and speed.” But he does see the study as a “cautionary tale,” and Pyenson adds that looking more closely at ancient whales can provide insight into the future of the Earth’s ocean systems.
“High sea level rise, acidified oceans, warmer oceans—those are all traits of past whale worlds,” he says. “And looking at the fossil record, looking at the biological response of those past worlds, that's going to be really important moving forward.”