About 125 million years ago, herds of inguanodontians browsed in a conifer-filled river valley in what is now the Isle of Wight, off of southern England. Iguanodontians are strange animals to human eyes. When they thrived, they could grow as large as a bus, had beaklike mouths, and hands with a spiked thumb on one side, and a long, dexterous pinky, with hoof-like middle fingers.

They’re also some of the earliest fossils to be recognized as dinosaurs. Paleontologist Richard Owens coined the term ‘dinosaur’ in the 1840s to describe iguanodontians and other creatures dug from the cliffs of southern England. Since then, the huge herbivores have been found across Europe, in rocks spanning six million years. But despite that long history, paleontologists only recognize two species of iguanodontians.

Now, according to an analysis of an overlooked iguanodontian skull excavated in 1996, there may be a third type.

The skull itself was found along with an exceptionally large and well-preserved predatory dinosaur, and Jeremy Lockwood, the first author of the research and a PhD candidate at the UK’s National History Museum and the University of Portsmouth, says that “this sort of overshadowed it. People just said, oh it’s just another iguanodon, put it in a box.”

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On closer inspection, however, the skull stood out. Other skulls have straight, horse-like snouts, but this one, Brighstoneus simmondsi, has a big lump around the nostrils, like a much chunkier alligator. And when the researchers looked in its mouth, it had the wrong number of teeth.

It’s quite likely that there was lots of real-world diversity that we just can’t see from the fossils, says Lockwood. That’s compounded by the fact that most iguanodontian fossils are fragments out of ocean cliffs. “There’s lots of reasons we might miss stuff on the Isle of Wight. We rarely get the opportunity to do a proper nice investigation…. We tend to suddenly find that a cliff falls down, and there are bones scattered all over the beach, and the tide’s coming in. It’s usually winter, it’s rough weather, and you scramble around and do what you can.”

So paleontologists need to piece together an image of a species from bits of different skeletons—a little like trying to sort out the pieces from two similar puzzles. Figuring out the fine distinctions between species is crucial for seeing how animals evolved, moved across the landscape, and adapted or died out as the environment changed. Imagine trying to understand the colonization of North America without being able to tell the difference between bison and cattle.

There have been previous efforts to split iguanodontians into a range of species, but most haven’t stood up to scientific scrutiny. “There has always been a tendency to potentially artificially exaggerate the number of ‘species,’”says David Norman, an iguanodontian expert at Cambridge University who wasn’t involved in writing the paper, “In the sense that any fresh discovery has the potential to be unique and therefore something new to science.”

In this case, though, Norman says that he agrees with the findings. “Yes, a new taxon seems to have been established here and it increases the range, or variety, of these types of dinosaur,” he says. That makes sense, he thinks, because the animal is so widespread across Europe, over millions of years.

“I think there’s a feeling that we should be reviewing a lot of stuff that was looked at 30, 40 years ago, because we’re seeing things in new ways these days,” says Lockwood. It may turn out that there are new species hidden away.