Paleontologist Adrian Hunt found a puzzle inside an odd collection of small bones strewn about the landscape near Tucumcari back in the mid 1980s.

There were spiky bones that looked a bit like armor, except they had sutures across them, which is something generally only found in skulls. And there were strange bits of skeleton that looked sort of like they were part of an animal's hip.

He didn't realize it then — in fact it took about 20 years — but it turned out he was looking at the missing link.

Of turtles, that is.

"There's a locality near Tucumcari that's rich in small and bizarre animals, and these bones were part of that," said Hunt, former director of the New Mexico Museum of Natural History and Science. "We talked about the bones being part of a turtle back then, but it just wasn't totally convincing to anyone. We didn't know what they were."

There's a long-standing debate about how turtle shells first evolved, said Spencer Lucas, interim director of the museum and its paleontology curator.

Turtles are very distinctive, because they are the only animal with their spinal cords fused to the inside of their shells, Lucas said.

"There's nobody out there with vertebrae fused to the shell that isn't a turtle," Lucas said.

Some scientists thought the fusing of shell and bone happened when the ribs grew thick and platelike and eventually merged with each other to make a shell. Others thought a thick shell evolved first and then grew into and merged with the turtle's ribs.

And a third hypothesis, called the thin-shelled model, suggested that armor plates on the back of the animal's skin grew into one another and merged, eventually growing into a shell.

"This thing fits the thin-shelled model perfectly," Lucas said. "This thing is the missing link. We didn't realize it at first, but it turns out this fossil solves a more than century-old debate about how the turtle shell originated."

It turns out the bone that looked like part of a hip was actually a spinal bone that had fused with the animal's shell. And the spiky sutured bones appear to be armor plates that have grown together, just like the thin-shelled model suggested, Lucas said.

The 215-million-year-old bones also matched the timeframe, called the Triassic, when turtles were evolving new defenses against predators, Hunt said.

"Back then there were all these weird experiments with animals — and this Triassic turtle is just part of that," Hunt said.

Still, speculating is one thing, but once Hunt, Lucas and others developed a theory of what the bones were after collecting more samples from the site in the mid 1990s, they showed them to one of the world's foremost experts on prehistoric turtles, Walter Joyce, a professor at Yale University.

Joyce, after looking at the bones, agreed with them.

"He was the one to realize that these fossils solved the debate," Lucas said. "When we realized that this was definitely a turtle, we all became very excited."

The group of scientists recently published a paper on the subject in the Sept. 19 issue of the Proceedings of the Royal Society, with Joyce as the lead author.

The bones are now on permanent display at the museum's new Triassic Hall, along with some casts that visitors can touch.

Finding a solution to the mystery of turtle evolution is certainly satisfying, and the answer actually seems the most logical in hindsight, Lucas said.

"It's the more pedestrian explanation of how things evolved," Lucas said. "It's easier to wrap your head around a bunch of little armor plates coming together — especially considering how common armor plates were at that time — than it is to think of a huge mass of ribs somehow coming together."

And beyond answering the riddle, the Triassic turtle also displays some other interesting differences from modern-day turtles, he added.

"Triassic turtles had neck armor, and they couldn't retract their heads like modern turtles can," Lucas said. "They hadn't evolved those muscles yet, so they developed armor to protect that exposed area."

The discovery also points to the importance of continued fossil collecting at sites like the one near Tucumcari, Hunt added.

"It's incredibly exciting that not only are there all these pieces of bone there, but that they all turned out to be part of the same animal," Hunt said. "It just shows that it's important to continue collecting every weird thing that you find."

Contact Sue Vorenberg at svorenberg@sfnewmexican.com.