As Rick Bellitti walked along the Charles River Locks this month, he spotted a looming figure in the water below. Probably a piece of driftwood, he figured. Until he saw it swimming slowly toward him.

“He was right there on the surface,’’ Bellitti, a 36-year-old accountant, recalled recently. “A prehistoric, floating dinosaur. Covered in armor.’’

Turns out, Bellitti had happened upon an Atlantic sturgeon, an ancient, endangered species that had not been spotted around the Charles River for as long as anyone could remember.

Bellitti had no idea what the strange creature was, but was certain he had never seen anything like it, so he took pictures with his phone. Once specialists got a look at the sharp-snouted fish, a 3-foot-long juvenile, they were immediately convinced.

“No doubt about the identification,’’ said Tom French, an assistant director at the state’s fisheries and wildlife division. “It’s clear.’’

The confirmed sighting delighted aquatic specialists, who said sturgeon, famed for their caviar and predating dinosaurs, are fighting for their survival.

“The fact that one has been spotted in our area is certainly good news,’’ said Julie Wood, watershed scientist at the Charles River Watershed Association. Wood said no one she has spoken to had ever heard of a sturgeon spotted near the Charles.

The week before the sighting, the federal government listed Atlantic sturgeon as endangered in five areas along the Atlantic, and as threatened in the Gulf of Maine.

“There’s no resident population in the Charles,’’ said Matt Ayer, an aquatic biologist with the state’s marine fisheries division. “I would guess it’s been quite a while since one was officially sighted.’’

Juvenile sturgeon travel extensively and feed in freshwater habitats before moving out to sea as they mature.

In Colonial times, sturgeon were abundant, but overfishing and limited access to freshwater spawning areas reduced their numbers over time. Pollution of the Charles peaked in the last century, according to the watershed association, but decades of effort have turned it into one of the nation’s cleanest urban rivers.

However, it is unclear why the sturgeon has reappeared. For more than a decade, in hopes of boosting the population of Atlantic sturgeon, it has been illegal for fishermen to catch or keep the fish.

Enormous fish that can weigh more than 800 pounds, sturgeon are sometimes spotted in the Merrimack River north of Boston, where they startle fishermen with sudden leaps and thunderous landings.

“When they hit the water, it sounds like a piano dropped,’’ Ayer said.

After taking the pictures, Bellitti e-mailed them to his father, an experienced fisherman. He didn’t know what it was either, but sent it to the New England Aquarium, whose specialists quickly identified it and wanted to know more.

“They said it had really caused a stir,’’ Bellitti said with a chuckle. “It’s like a yeti picture.’’

Bill Bennett, the aquarium’s webmaster, often receives pictures from people who think they have seen something rare. Usually, they haven’t.

But this time, Bennett’s eyes widened as he opened the e-mail, with a subject line that said “What is it?’’

“I recognized it instantly,’’ he said. “When you’ve seen one of these bizarre creatures, there’s not a lot of doubt.’’

Bennett replied that he would check with experts, but was confident what they would say.

“If that’s not a sturgeon, I’ll be dining on my fedora,’’ he wrote.

Bellitti, who often takes pictures on walks, said he was happy to document the fish’s presence. And specialists noted that even fish this large and distinctive can easily escape notice.

“Who knows what’s happening under the water?’’ French said. “This one just happened to be swimming along the surface.’’

Peter Schworm can be reached at schworm@globe.com. Follow him on Twitter @globepete.

© Copyright 2012 Globe Newspaper Company.