A 90 million-year-old turtle fossil has been discovered in a desert in New Mexico after a museum volunteer noticed a pile of unusual looking rocks.

Jeff Dornbusch, a volunteer with a Truth or Consequences museum, said he first noticed the 'rocks' were slightly different in colour compared to rest of the terrain of brown dirt a few years ago while out hiking.

He alerted the US Bureau of Land Management to let them know what he had noticed and archaeologist James Renn visited the site with him, after which a scientist from the New Mexico Museum of Natural History went to examine the finding, the Las Cruces Sun-News reports.

The team from the museum soon discovered Dornbusch had discovered the remains of a prehistoric turtle that would have lived in a swampy environment almost 100 million years ago.

Researchers found fossilised bone and turtle shell and worked out its age by sampling the surrounding dirt.

The turtle was found just six miles from an area called Turtleback Mountain: "I never really knew this area as a place for marine fossils — shells and stuff in the mountains," Dornbusch said.

The remains have been collected and taken back to the Natural History Museum in Albuquerque where they will be put on display.

Tom Suazo, who prepares fossils at the museum, said the site where the turtle was discovered is part of the Crevasse Canyon, which dates to the late Cretaceous Period.

Explaining what the area would have looked like 90 million years ago, he said: "Basically, what this is a swampy, near-shore environment."