PETALUMA, Calif. (KRON) — When a dorsal fin popped up from the Petaluma River this week, some curious onlookers wondered if a shark or dolphin had possibly lost its way.

The aquatic animal’s dorsal fin was shaped like a shark’s, but it swam like a dolphin.

As it turns out, the animal was neither. The Marine Mammal Center confirmed from photographs snapped by witnesses that the Petaluma River mystery creature was a harbor porpoise.

The harbor porpoise’s adventure, far from the ocean, was a rare occurrence.

“It’s definitely a harbor porpoise. Every once in a while, one goes up a river looking for fish. One was seen in Napa up the Napa River. I have never heard of one in the Petaluma River before. That’s a first,” Marine Mammal Center cetacean researcher Bill Keener told KRON4.

One of the confirmed sightings happened Wednesday while the porpoise was swimming near the Petaluma Yacht Club. Keener explained that a harbor porpoise’s dorsal fin is small, dark, and triangular, similar to a shark’s. But the porpoise’s tail revealed its identity.

“The difference is the tail fin. In sharks, the fin is vertical so it sticks up. Porpoises’ are flat and horizontal so you don’t see the tail above the surface,” Keener said.

Porpoises usually stay in the San Francisco Bay. NOAA wrote, “Harbor porpoises have a small, robust body with a short, blunt beak and a medium-sized triangular dorsal fin. The harbor porpoise is a shy animal, most often seen in groups of two or three. They prefer coastal areas and are most commonly found in bays.”

Wildlife experts said so far, there are no plans of turning the animal around to guide it back to the ocean.

“Hopefully this animal is not lost and will go back into the bay,” Keener said.

The Marine Mammal Center is encouraging anyone who sees the Petaluma porpoise to send an alert through their website.