Ancient dolphin fossil reveals crucial step in the evolution of echolocation

In a recent publication in the journal Diversity, new revelations about the navigation techniques of toothed whales and dolphins using sound waves have emerged. Despite lacking external ears, these marine creatures employ echolocation – emitting high-pitched sounds that bounce off objects and return as echoes, allowing them to effectively map their underwater surroundings.

The asymmetry in the skulls and soft tissues around the blowhole is crucial for sound production. This “lopsidedness” is a key element in these animals’ ability to echolocate. Simultaneously, a fat-filled lower jawbone acts as a conduit for sound waves to reach the internal ear, enabling directional hearing and helping them discern the origin of sounds.

The evolutionary path that led to the development of this sophisticated “built-in sonar” in whales and dolphins remains a mystery. To shed light on this, a study co-authored by Dr. Jonathan Geisler, professor and chair of anatomy at the New York Institute of Technology, and Dr. Robert Boessenecker, a paleontologist and research associate at the University of California Museum of Paleontology, delves into the analysis of fossils.

Fossils demonstrate asymmetry seen in Xenorophus. Credit: Robert Boessenecker

The researchers examined a substantial collection of fossils, including two ancient species of dolphins within the genus Xenorophus, one of which is previously unknown to science. Xenorophus, a large creature that inhabited the waters of Eastern North America 25–30 million years ago, exhibited external similarities to modern dolphins but possessed interlocking molar-like teeth, reminiscent of ancestral land mammals.

Notably, Xenorophus displayed asymmetry around the blowhole, albeit less pronounced than its contemporary relatives. What sets this ancient species apart is the distinct twisting and shifting of the snout to the left. Previous studies in other ancient whales suggested that this “snout bend” might enhance directional hearing by correlating with the asymmetrical placement of fat bodies in the jaw.

Xenorophus, however, took this a step further. The fat bodies in its lower jaws, functioning analogously to external ears in land mammals, were tilted, amplifying directional hearing. This unique adaptation, akin to the asymmetrical ears of owls, possibly allowed Xenorophus to detect the precise location of sounds in its environment.

The findings suggest that, while Xenorophus may not have been as proficient as contemporary odontocetes in producing high-pitched sounds or hearing high frequencies, it played a pivotal role in the evolutionary history of echolocation. Its pronounced asymmetry, particularly the twisting of the snout and tilting of fat bodies, represents a crucial puzzle piece in understanding how whales and dolphins evolved their echolocation abilities.

Dr. Robert Boessenecker emphasized the significance of Xenorophus’s asymmetry, stating it surpasses that of any living or extinct whale, dolphin, or porpoise. Additionally, the study challenges the prevailing focus on symmetry in evolutionary studies, underscoring the importance of investigating asymmetry in fossils.

Moving forward, the researchers plan to examine other odontocetes, searching for similar snout bends. These future studies aim to determine the prevalence of this feature and provide further insights into the evolutionary trajectory of echolocation in whales and dolphins.

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