Canoeists saw more than the usual floating leaves, small fish, and trees on the shoreline as they made their way down the North River near Pembroke and Hanover on Wednesday: They discovered a dead 6-foot-long, 75-pound Atlantic sturgeon.

The ancient species is often compared to a dinosaur and is listed as endangered on most of the East Coast, including Massachusetts. Coming across a mature specimen, especially a dead one, is unusual, said Tony LaCasse, spokesman for the New England Aquarium.

The canoeists called the National Oceanic and Atmospheric Administration law enforcement division, which contacted other agencies, including the New England Aquarium. The aquarium’s animal care facility held the carcass overnight, and biologists began inspecting it Thursday.

The fish is so far estimated to be at least 14 years old and is an adult female, measuring an inch short of 6 feet, he said. Usually, younger sturgeons are seen at the mouth of the Merrimack River, but LaCasse said the location of this mature one raises a lot of questions.

He said that Atlantic sturgeon dominated major coastal rivers in Colonial times but that during the Industrial Revolution the population plummeted.

The fish are bottom feeders that eat crabs, mollusks, and worms.

Full-grown specimens “don’t come up dead very often,” he said, since it takes about 14 to 22 years to be able to reproduce.

LaCasse said the Harvard Museum of Natural History has already been promised the carcass by the state.

“There aren’t a lot of specimens in the record,” LaCasse said.