The waters off New Zealand 25 million years ago were home to early baleen whales, megatooth sharks and human-size penguins. Now researchers are adding a bizarre dolphin to the mix that may have used tusklike teeth to thrash prey into submission.

The dolphin’s nearly complete skull was collected in 1998, from a cliff side in the Otago region of New Zealand’s South Island. The specimen ended up in the University of Otago Geology Museum’s collection. Two decades later, Amber Coste, who was completing her Ph.D. in paleontology, stumbled upon the strange skull.

“Mentally, I just couldn’t figure out what could possibly need teeth like that,” Dr. Coste said.

The fossil dolphin’s dentition was unlike anything seen in living cetaceans. While modern dolphins are armed with a snoutful of cone-shaped teeth perfectly calibrated to snap up fish, this creature possessed several large teeth that protruded out of the end of its snout. Instead of tapering downward into fangs, these teeth were splayed out horizontally like the blade of a spade.

In a paper published Wednesday in the journal Proceedings of the Royal Society B, Dr. Coste and her colleagues described the snaggletoothed dolphin as a unique species, Nihohae matakoi. The curious cetacean’s genus, Nihohae, is a combination of the Maori words for “teeth” and “slashing.”

True tusks are defined as continually growing teeth that stick out of the mouth, and narwhals are the only living cetaceans that have them. Nihohae’s protruding teeth were deeply rooted in its skull, which limited their ability to continue growing over the animal’s lifetime. As a result, Nihohae’s teeth are more similar to the tusklike projections on the lower jaws of male beaked whales, which use these teeth to tussle over females.

To determine how Nihohae wielded its tusklike teeth, Dr. Coste and her colleagues examined several of the teeth under a scanning electron microscope. Surprisingly, the researchers found little evidence of wear on them.

This meant it was unlikely that Nihohae used its tusklike teeth to fight rivals for mates. It also cast doubt on another hypothesis that Nihohae used its teeth to sift through the sandy seafloor for prey. “Sand is really abrasive and will absolutely trash your teeth, but there’s literally no scratches on these teeth,” Dr. Coste said.

The teeth’s horizontal orientation also made them a poor tool for snagging prey. While the teeth of other fossil dolphins interlocked to trap fish inside, Nihohae’s splayed-out teeth would not have been useful for that task. They were also nearly flat, so Nihohae would have had a tough time biting into anything.

Because no other dolphin possessed teeth like Nihohae’s, the researchers scoured the rest of the animal kingdom for similar anatomy. This led them to sawfish, which swipe their tooth-studded saws to either injure or stun prey before vacuuming it up whole.

Dr. Coste and her colleagues hypothesize that Nihohae, whose unfused vertebrae probably allowed for a broad range of neck movement, hunted in a similar way, swinging its head to skewer or stun squid and other soft-bodied sea creatures. Then it would swallow the staggered prey whole.

“You can just imagine these dolphins swimming up to a shoal of squid and wildly thrashing their heads back and forth,” Dr. Coste said.

This hunting style seems plausible, according to Robert Boessenecker, a paleontologist at the College of Charleston in South Carolina who was not involved in the new study.

“Given that the teeth in Nihohae are splayed out to the side, that’s pretty good indication that there was lateral movement similar to the sweeping of a sawfish snout,” said Dr. Boessenecker, who has studied other toothy dolphin fossils from both South Carolina and New Zealand.

Dr. Coste hopes that further examination of Nihohae’s tusklike teeth and future fossil finds will shed additional light on the diversity of ancient dolphin hunting techniques.