Animals sporting elongated curved canine teeth have appeared twice in the fossil record, millions of years apart. One lineage, the nimravids, went extinct about in North America about 23 million years ago. Then the saber-toothed tiger vanished about 8,000 years to 10,000 years ago. More recently, saber teeth have also evolved in “true” cats of the Felidae family.

Scientists have long wondered why saber teeth emerged over different times and places, vanished, then re-appeared. A new study in Current Biology fills in some evolutionary gaps.

Scanning Saber-Toothed Tiger Skulls

As part of her doctoral research at the University of Liège, Narimane Chatar and her team collected 99 mandibles and 91 skulls from different eras and continents. They analyzed the specimens with 3-D scanners to catalog how the shapes of the skulls and teeth changed over time.

That meticulous methodology yielded some surprising results. The simplified version of the conventional wisdom about saber tooth evolution has held that it has been considered “convergent”— that is, the saber teeth emerged separately over time in different species. Instead, Chatar’s work suggests more of a slow, steady change.

Read More: How a Fossil Hotspot Reveals Two New Species of Sabretooth Cat

Saber Toothed Shapes

“I think the biggest surprise was the continuum of shape,” says Chatar, who is now a research fellow at the University of California, Berkeley.

In other words, the changes between species came, not necessarily in drastic shifts in teeth type, but in more subtle changes to skull shapes.

She suspects that earlier classifications of cat-like carnivores over-emphasized teeth shape. For instance, some species were categorized as saber toothed, conical toothed, or scimitar toothed. This sort of analysis may have “shoehorned” several species into categories that the rest of their body type betrayed.

“When sampling their whole diversity, we realized it was not that straightforward,” Chatar says. Instead of “convergence,” there appears to be more of a continuum.

The team found some other evolutionary surprises as well. First, saber-toothed species seemed to show faster changes to skull and jaw shapes earlier in their evolutionary history than species with shorter canines — essentially a “recipe” for evolving into saber-toothed feline-like predators, Chatar said in a statement.

The research group also hypothesizes that the “over-specialization” of saber teeth more rapidly led groups with that feature to extinction.

This theory is called a macroevolutionary ratchet. “This phenomenon has been proposed as a potential driver of decline, wherein evolution favors the loss of early generalized forms, leading to the emergence of more specialized, yet more vulnerable, forms later in the history of the lineage,” Chatar says.

Read More: Everything You Need to Know About the Saber Tooth Tiger Extinction

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Before joining Discover Magazine, Paul spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.