In July 2023, researchers from the Pelagos Cetacean Research Institute spotted a rare striped dolphin in the Gulf of Corinth with deformed flippers resembling thumbs. The dolphin, part of a unique mixed-species society in the region, was observed swimming and interacting with its pod despite its unusual flipper morphology. Experts believe the deformity is likely a genetic defect acquired in the womb, possibly resulting from constant interbreeding among the isolated population of around 1,300 striped dolphins in the Gulf. The abnormal flipper shape, resembling thumbs, is considered a rare expression of irregular genes, with the defect affecting both flippers. While cetaceans, including dolphins, possess finger-like bones within their flippers, the photographed dolphin's unique deformity suggests a deviation from the typical developmental process, leading to the absence of certain fingers and associated tissue. Despite this anomaly, the dolphin appears to be thriving in its environment.

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https://www.livescience.com/animals/dolphins/extremely-rare-dolphin-with-thumbs-photographed-in-greek-gulf

In July 2023, researchers from the Pelagos Whale Research Center found a distinctive striped dolphin with deformed tufts that resembled thumbs. The dolphin was seen swimming with his group and this appearance defect did not seem to bother him.
Experts believe that the deformity is likely to be a genetic defect, as both of the bundles look like this. Although cetaceans, including striped dolphins, have finger bones embedded in their gills, suggesting the evolution of whales from terrestrial mammals, some of those bones appear to be missing from this particular dolphin. Despite this variation, the dolphin appears to be in good health.

Read more here:
https://www.livescience.com/animals/dolphins/extremely-rare-dolphin-with-thumbs-photographed-in-greek-gulf