In a new paper published this month in the journal Diversity, paleontologists described the fossilized skeletons of the dolphin genus Xenorophus from the Oligocene of South Carolina, the United States.
Xenorophus hunting sea turtles. Image credit: Robert Boessenecker.
Whales and dolphins, which lack external ears, rely on echolocation to navigate and hunt in the dark.
Much like shouting and listening for echoes, these animals emit high-pitched sounds that bounce off objects and reflect back at them, allowing them to map out their surroundings.
Their skulls and soft tissues near and within the blowhole are asymmetrical, meaning that a structure on one side is larger or differently shaped than its counterpart on the other side. This lopsidedness enables the production of sound.
At the same time, a fat-filled lower jawbone conducts sound waves to the internal ear, allowing the animals to locate where sounds are coming from directional hearing.
Yet, how whales and dolphins evolved this sophisticated ‘built-in sonar’ is not fully understood.
In the new research, New York Institute of Technology’s Professor Jonathan Geisler and Dr. Robert Boessenecker from the University of California Museum of Paleontology analyzed a large collection of fossils that included two ancient species of dolphins within the genus Xenorophus: Xenorophus sloanii and the newly-discovered species Xenorophus simplicidens.
These species are some of the primitive members of Odontoceti, the suborder of marine mammals that includes all living echolocating whales and dolphins.
Xenorophus dolphins were large creatures approximately 3 m long that swam the waters of Eastern North America 25-30 million years ago and likely fed on fish, sharks, sea turtles, and small marine mammals.
Externally, they resembled modern dolphins but had several interlocking molar-like teeth, much like an ancestral land mammal.
Similar to today’s odontocetes, Xenorophus had asymmetry around the blowhole, though not as pronounced as their living relatives.
Notably, they also had a distinct twisting and shifting of the snout several degrees to the left.
Previous studies in other ancient whales suggest that this ‘snout bend’ may be linked to the asymmetrical placement of fat bodies in the jaw, increasing directional hearing abilities.
However, Xenorophus took this one step further. The fat bodies in their lower jaws, which functioned like external ears in land mammals, were tilted, further exaggerating directional hearing.
This bending of the snout and tilting of the fat bodies may have been similar to the asymmetrical ears of owls, which can detect the precise location of prey based on their sounds.
The new evidence suggests that Xenorophus, with lesser pronounced asymmetry near the blowhole, may not have been as adept at producing high-pitched sounds or hearing high frequencies as living odontocetes.
However, the researchers were able to determine the location of sounds.
Therefore, Xenorophus likely marked a key transition in the history of how whales and dolphins came to use echolocation.
“While this asymmetry is seen in other ancient whales, Xenorophus display the strongest of any whale, dolphin, or porpoise, living or extinct,” Dr. Boessenecker said.
“In addition, although the blowhole-focused asymmetry in today’s odontocetes can be traced back to Xenorophus and other relatives, the twisting and shifting of the snout is no longer seen today.”
“This suggests that Xenorophus is a crucial puzzle piece in understanding how whales and dolphins evolved their echolocation abilities.”
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Robert W. Boessenecker & Jonathan H. Geisler. 2023. New Skeletons of the Ancient Dolphin Xenorophus sloanii and Xenorophus simplicidens sp. nov. (Mammalia, Cetacea) from the Oligocene of South Carolina and the Ontogeny, Functional Anatomy, Asymmetry, Pathology, and Evolution of the Earliest Odontoceti. Diversity 15 (11): 1154; doi: 10.3390/d15111154
