Looking at a modern whale, it’s hard to imagine these creatures once walked on land. Of course, back then they looked a little different, but as a recent photo from a necropsy revealed, whales have hung on to a few of their land-based traits, including a rather haunting hand-like appendage. Beneath a whales’ flipper isn’t the paddle-like anatomy you might imagine, but instead a pentadactyl limb consisting of five finger-like bony protrusions. Or, as we prefer to call it, GHOST HAND.

The spooky photo was posted to Twitter by Curator of Herpetology and Assistant Professor of Vertebrate Zoology at the Natural History Museum of Denmark, Dr Mark D Scherz. If you’re thinking whales don’t fall under the herpetology umbrella, you’d be right, but working so closely with researchers handling specimens at the drop of a hat comes with its perks.

“The day before my first day, I got a message from the Collections Manager for Herpetology and Mammalogy, Daniel Klingberg Johansson, that a whale had washed up and would be dissected the following day,” Scherz told IFLScience. “These rare events spark a frenzy of activity in the museum, as various researchers and assistants work together to prepare and take data on the animal."

The pentadactyl limb is actually present in an enormous variety of animals, demonstrating that they shared a common ancestor who evolved to have the limb before it went off and explored some customizations. The whales’ freaky “fingers” are therefore actually a rather beautiful demonstration of the quirks of evolution.

“Evolution is a tinkerer,” said Scherz. “A repurposing of an existing structure is easier and more likely than the production of a whole new structure 'from scratch'. When the tetrapods (four-legged animals) emerged from the primordial seas, it just so happened that the most successful lineage had five fingers and toes.

“Flippers have evolved repeatedly in various lineages of mammals and reptiles, each time in a different way; the fundamental structure is the pentadactyl limb, but the specific structure [of the limbs] differ very strongly.”

In case you’re wondering, performing a necropsy such as this is a combination of tough and delicate work. Handling delicate organs calls for care if you’re to retain the practice’s academic value, but you’re still working with a whale, and carving away at blubber and muscle is no small feat.

The whale in this particular dissection is at time of writing thought to be a Sowerby's beaked whale, Mesoplodon bidens, which is rare in the region but not unheard of. However, the species is very hard to distinguish from a rarer species which has never been recorded in Danish waters (where the animal beached) known as the Gervais’ beaked whale, Mesoplodon europaeus.

As Daniel Klingberg Johansson explained, stranding and physical data on either species is of much interest to science.

“So when a cetacean strands, especially the bigger and rarer species, we have a lot of institutes in Denmark that are interested in samples for all different kinds of research, but standard is samples for the veterinarians for potential cause of death and general health of the specimen," he told IFLScience. "Then the museum keeps a tissue sample for DNA in the future and the skeleton for the collection.

"One institute looks in the organs for different kinds of environmental pollutants and another takes samples from the intestines to investigate the microbiome. Two other institutes looks into the form and function of the muscles, where one wants samples of the swimming muscles and the other wants to look at the heart. There has been no results [on this whale] yet."