Jude Sparks accidentally discovered the skull of a long-extinct elephant-like creature while hiking in the New Mexico desert with his parents.

The family were out testing their walkie-talkies when 9-year-old Sparks tripped and fell onto an unusual object protruding from the ground.

"I was running farther up and I tripped on part of the tusk," said Sparks.

"My face landed next to the bottom jaw. I looked farther up and there was another tusk."

Sparks' parents contacted a palaeontologist at New Mexico State University, Peter Houde, to tell him of their son's discovery. The family later took part in excavation of the fossil, which turned out to be that of a creature called a stegomastodon.

"A stegomastodon would look to any of us like an elephant," said Houde.

"For the several types of elephants that we have in the area, this is probably one of the more common of them. But they're still very rare. This may be only the second complete skull found in New Mexico."

The jaw of the stegomastodon alone weighs about 54 kg (120 lb), with the whole skull estimated to weigh about a tonne. Despite its size, the skull is very delicate. The creature it belonged to lived at least 1.2 million years ago, Dr. Houde estimated.

"The upper part of the skull is deceiving. It's mostly hollow and the surface of the skull is eggshell thin," Houde said. "You can imagine an extremely large skull would be very heavy for the animal if it didn't have air inside it to lighten it up just like our own sinuses. That makes the thing extremely fragile and the only thing holding it together is the sediment surrounding it.

A stegomastodon skull. James St. John / Flickr

"In fact when the sediments are removed from the sides of them, they start to fall apart immediately and literally fall into tiny, tiny bits. It has to be done carefully by somebody who knows how to go about doing it. It is a very deliberate process that takes a little bit of time."

For this reason, Houde warns against fossil discoverers to attempt any amateur excavation of their finds themselves, as they may end up with little more than a few handfuls of dust. As it is, the fossil once fully excavated and preserved is expected to be displayed in an exhibition.