The Global Expansion of the Earliest Dolphins

The dolphin lineage Delphinida represents the most diverse group within the infraorder Cetacea, which includes all whales and dolphins alive today. Among living cetaceans, dolphins alone account for nearly half of all species. Based on my own tally—excluding forms whose taxonomic status remains uncertain—44 of the approximately 90 recognized living cetacean species belong to this dolphin lineage. The extraordinary success of dolphins may be closely linked to one of their early representatives, the genus Kentriodon, which is widely considered to belong near the ancestry of modern dolphins.

The genus Kentriodon first appeared around 20 million years ago and persisted until roughly 11.3 million years ago. Fossil evidence indicates that its distribution was nearly global during this interval. Several valid species have already been described within the genus, and additional material representing yet-undescribed forms has also been discovered. When diversity, geographic spread, and temporal longevity are considered together, Kentriodon clearly stands out as one of the most successful and widespread dolphin groups in the fossil record.

MCAF-MB2 discovery locality

Recently, researchers described a new fossil specimen of Kentriodon from northeastern Italy, catalogued as MCAF-MB2. This discovery is significant for two reasons. First, it represents the first unambiguous specimen of Kentriodon ever found in the Mediterranean region. Second, it is among the oldest fossils attributed to this genus. The find therefore fills an important geographic gap in the fossil record while also providing valuable clues about the origin of early dolphins.

The Origin and Evolutionary History of Kentriodon

Reconstruction of Kentriodon（Image source：Nobu Tamura， CC BY 3.0 ）

At present, the only specimen comparable in age to MCAF-MB2 is Kentriodon pernix from the United States. This similarity suggests that the genus may have originated either in the northeastern Atlantic Ocean or within the Mediterranean region. Additional evidence supports the Mediterranean as a likely center of origin. Phylogenetic analyses indicate that Kentriodon is closely related to Rudicetus, a dolphin discovered in southern Italy. Furthermore, several isolated fossils resembling Kentriodon—often represented by single bones—have also been recovered from Mediterranean sediments of similar age. Taken together, these clues suggest that the Mediterranean basin may have played a key role in the early evolution of this lineage.

Regardless of whether Kentriodon first evolved in the Mediterranean or the northeastern Atlantic, the genus appears to have spread rapidly across the world's oceans. By about 18.8 million years ago, fossils indicate that these dolphins had already reached the waters of Peru. Soon afterward, indeterminate species of Kentriodon appeared around New Zealand between roughly 19 and 16 million years ago. By about 16 million years ago the genus had expanded into the North Pacific. Once established in the Pacific Ocean, Kentriodon diversified quickly into multiple species, and its descendants spread across the western Pacific in a relatively short geological interval. The lineage ultimately disappeared around 11.3 million years ago, when the species Kentriodon nakajimai became extinct.

It should be emphasized, however, that the fossil record of Kentriodon is still fragmentary. Many specimens consist of incomplete skeletal remains, which makes reconstructing evolutionary relationships difficult. As a result, the precise origin of the genus remains debated. For example, one study published in 2019 proposed that Kentriodon might instead have originated in the Pacific Ocean and later migrated into the Atlantic on multiple occasions.

Why Was Kentriodon So Successful?

Although the exact reasons behind the evolutionary success of Kentriodon remain uncertain, several features of these early dolphins may have given them important advantages over other toothed whales of their time.

One key trait is echolocation. At least one species, Kentriodon pernix, appears to have possessed narrow-band high-frequency echolocation, often abbreviated as NBHF. This form of biosonar uses extremely high frequencies that many large predators cannot easily detect. Such signals are thought to function as an acoustic form of camouflage, allowing smaller dolphins to communicate and navigate while remaining effectively invisible to the hearing range of larger hunters.

Although killer whales did not yet exist during the early Miocene, the oceans still contained formidable predatory toothed whales such as Squalodon. These animals were large, powerful hunters that may have occupied ecological roles similar to modern apex predators. The NBHF echolocation system of Kentriodon could therefore have helped these early dolphins avoid detection and reduce the risk of predation.

Another possible advantage involves brain size. Researchers who examined cranial cavities from 36 extinct whale species found that Kentriodon possessed a relatively large brain compared with its body size. In fact, its encephalization levels approach those of some modern porpoises. A larger brain may have supported more complex behavior, including improved communication, learning ability, and coordinated social interactions.

This cognitive capacity could have allowed Kentriodon to form larger and more structured social groups than many other early toothed whales. Group living can provide numerous ecological advantages, including cooperative foraging, improved predator detection, and more efficient use of resources. Such social strategies may have contributed significantly to the competitive success of early dolphins.

Conclusion

During the Miocene epoch, roughly 23 to 5.33 million years ago, dolphins underwent several waves of evolutionary radiation. These events produced an extraordinary diversity of forms and eventually allowed dolphins to surpass other toothed whale lineages in both species richness and ecological variety. Among the most prominent representatives of this early radiation were Kentriodon and its close relatives within the family Kentriodontidae.

The recent discovery of a new Mediterranean specimen provides an important glimpse into this formative chapter of dolphin evolution. It reveals that early dolphins were capable of remarkable geographic expansion and possessed biological traits—such as advanced echolocation and relatively large brains—that may have paved the way for the later dominance of modern dolphin groups.

Author: Bai Leng

Reference:

Nobile, F., Collareta, A., Perenzin, V., Fornaciari, E., Giusberti, L., Bianucci, G. (2024). Dawn of the Delphinidans: New Remains of Kentriodon from the Lower Miocene of Italy Shed Light on the Early Radiation of the Most Diverse Extant Cetacean Clade. Biology.

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