How Vertebrates Got Teeth... And Lost Them Again

Life in the oceans of the late Silurian Period, around 420 million years ago, could be tough for a fish like Lophosteus.

So tough, in fact, that it needed skin made of teeth to protect itself from things like giant sea scorpions.

If you’re picturing skin covered in human teeth - just stop, that’s terrifying These “skin-teeth” were really more like bumps made of dentine, which is the hard tissue found underneath the enamel of your teeth.

And they’re part of a paleontological mystery.

Which came first, teeth on skin or teeth in mouths?

The “outside-in” hypothesis proposes that teeth started as skin bumps and then migrated into the mouths of our ancient bony fish ancestors.

The “inside-out” hypothesis points to teeth starting in the mouth first.

Wherever they came from, I can’t stress enough what an evolutionary gamechanger teeth were.

Suddenly, capturing prey and turning it into bite-sized chunks got a lot easier.

These ancient fish teeth would eventually be the raw materials for tiger canines, snake fangs, and our own chompers, after diversifying and adapting to many different diets over millions of years.

Yet, as revolutionary as teeth were, they would go on to disappear in some groups of vertebrates.

But why?

Because in each case, other feeding structures evolved to take their place -- it looks like, in the game of evolution, a beak often beats teeth.

Today, tetrapods - including mammals, birds, reptiles, and amphibians - eat all kinds of different things, from meat to plants to bugs.

But, the earliest tetrapods were all carnivorous predators.

The earliest tetrapod cousin known, a fish-like animal called Tungsenia, lived around 409mya in China The rows of small teeth in its lower jaw, coupled with fangs, may have allowed it to hunt other small, sometimes armored fish that were found in the same formation.

Then, beginning around 400 million years ago, lobe-finned fish were starting to move onto land, bringing teeth into the terrestrial realm -- and their teeth were starting to change, too.

For example, the 365 million-year-old tetrapod, Icthyostega, had curved upper teeth, smaller lower teeth, and a flexible neck that probably made it a formidable predator.

It may have been the first vertebrate to hunt terrestrial prey, supported by its four legs and strengthened ribcage.

Around 50 million years later, the reptiles that emerged during the Carboniferous Period were still terrestrial carnivores.

Found in what’s now Nova Scotia, the 20-cm-long Hylonomus fed on insects and snails, using rows of sharp teeth on both its top and bottom jaws.

And although teeth would eventually disappear in some vertebrates, first they would diversify.

As animals got more specialized during the Permian Period, so did their teeth.

Both amphibians and reptiles radiated into new niches, which included new diets.

And their teeth evolved into different shapes and unique combinations, like canines and incisors.

By about 300 million years ago, vertebrates sported specialized teeth for the first plant-eating lifestyles, capitalizing on the explosion of land plants.

These were creatures like Edaphosaurus, one of the earliest known tetrapod herbivores.

It may have cropped tough plants with its row of serrated-tipped teeth.

And the peg-like teeth on the roof of its mouth could have broken down those bites using a forward and backward chewing motion.

But, in spite of how well specialized teeth worked for herbivory, plant-eating would actually lead to the loss of teeth in another vertebrate group: the weird mammal relatives known as dicynodonts.

This group included the 1-meter-long, pig-like Lystrosaurus.

Other than a pair of tusks that it might’ve used for things like defense, competition, or burrowing, Lystrosaurus was toothless.

But it did just fine feeding on the seed ferns and lycophytes that were common during the Triassic, no teeth required.

Instead of teeth, it had a beak kind of like a turtle’s that could have torn off pieces of vegetation to be ground up against the roof of its mouth.

And it clearly worked!

In fact, the dicynodonts became the most common group of land vertebrates during the Triassic.

And this wasn’t the only time beaks would beat teeth -- and it wasn’t just a strategy that worked for herbivores.

A 228-million-year-old transitional fossil of a turtle found in what’s now China had a beak as well as teeth.

Scientists think that it used its beak to dig in the mud for food, probably invertebrates, which it chewed up with a combination of conical and blunt teeth.

By the Late Jurassic Period, turtles that had acquired beaks lost their teeth completely.

The last known turtle with teeth lived about 160 million years ago, and retained teeth just on the roof of its mouth.

All modern turtles are toothless - instead, they mainly rely on their sharp beaks made of keratin to munch food.

And this pattern of tooth loss in favor of beaks was also repeated in birds!

Their theropod dinosaur ancestors had teeth, but all modern birds have beaks, despite their range of diets from carnivorous to herbivorous.

Like that transitional turtle, there were even transitional birds with both teeth and beaks.

The seabird Ichthyornis, dated to between 100 million and 70 million years ago had 20 pointed teeth in each jaw as well as a beak-like jaw tip.

Why birds evolved to be toothless in the first place is still an open debate, but we do have a hypothesis about why all modern birds are this way.

Leading up to the end-Cretaceous extinction, teeth were going strong in the ancestors of birds - the maniraptorans.

In fact, they got more specialized in shape during the last 18 million years of the Cretaceous.

But, it was the maniraptorans with beaks, not the ones with teeth, that survived the extinction event.

Paleobiologists hypothesize that they exploited one of the only viable food resources in the post-asteroid conditions - seed banks in the soil.

These early, beaked birds that made it through were likely seed eaters.

And while it’s difficult to pin down what drove the repeated shifts from teeth to beaks, the transitional fossils that have both structures suggest that there was a shift in selective pressure.

The adaptation of a beak that made eating easier potentially weakened the selective pressure for teeth.

And making teeth requires an organism to spend energy and resources on them - if beaks needed less of either of these, that might be a reason they’d be favored, no matter what kind of diet an animal had.

The result was that some vertebrates evolved to ditch the teeth and use beaks for feeding instead.

Another weird thing is, a beak isn’t the only innovation that can beat teeth - it looks like other secondary feeding structures can, too.

And maybe the most unexpected toothlessness happened in a lineage of purely carnivorous animals - adult amphibians!

Many amphibians today, such as salamanders, still have teeth.

But, toads are carnivorous and entirely toothless.

And most of their frog relatives have reduced teeth, found in their upper jaws only.

And, to talk about how that happened, we need to back up a little bit.

Alongside turtles and the ancestors of birds that were losing teeth in the Jurassic Period, so were some amphibians.

By the Late Jurassic, frogs like Notobatrachus in Argentina had few if any teeth.

And frogs never developed beaks.

So, what secondary feeding structure could have driven them towards toothlessness?

SLINGSHOT TONGUES.

Which are arguably cooler than beaks.

Fossils of a 99-million-year-old toothed amphibian called Yaksha, shows a bone at the base of the tongue that can be activated by accelerator muscles to slingshot the tongue forward.

Yaksha was part of a group of now-extinct amphibians that first piloted this feeding style.

Instead of teeth, most modern frogs and toads use ballistic tongues to snag insects.

While their poor fossil record makes it difficult to track when the lineage of toads became totally toothless, molecular evidence suggests it was between 60 million and 46 million years ago, well after the turtles and birds went toothless.

And molecular family trees indicate that toothlessness has evolved independently more than 20 times in frogs.

Many of the toothless vertebrates today actually still have tooth genes, which have been developmentally turned off, showing that they came from toothed ancestors.

So, since teeth first evolved in vertebrates more than 400 million years ago, they have vanished multiple times across different groups.

And in each case, another feeding structure evolved and overlapped with teeth.

As the new adaptation took hold, the adaptive value of teeth decreased.

And, eventually, the tooth genes got turned off through mutations that made them no longer functional … and teeth vanished.

So, in a game of evolutionary rock-paper-scissors, it looks like both beaks and ballistic tongues beat teeth.