Episode 88 – Evolution of Teeth

Listen to Episode 88 on PodBean, iTunes, YouTube, or your preferred podcatching method!

We have them, you have them, and vertebrates throughout Earth history have had them. The history of vertebrate life, the study of vertebrate paleontology, and the myriad ways we have to discuss and describe living and fossil animals and their ecological roles are tied tightly to the Evolution of Teeth.

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The Tooth

Teeth are structures found in vertebrate animals, typically in or on the bones of the jaws, though sometimes other sections of the mouth and throat. Though many invertebrates have also evolved tooth-like structure, true teeth are exclusive to bony animals.

The most familiar teeth – like our own – are made of three materials: enamel, the hard mineralized structure that covers the exposed crown of the tooth; dentine (or dentin), the softer material that makes up the bulk of the tooth interior; and cementum, which covers the root, the part of the tooth that sits within the gums and anchors the tooth in place. In mammals like ourselves, the center of each tooth usually has a pulp cavity with nerves and blood vessels that feed the dentine.

Across vertebrates, the layout and arrangement of these materials come in many varieties.

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You might recognize this – it’s a human tooth, which is a fairly standard model for most mammal teeth. Photo by Blausen.com staff (CC BY 3.0)
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Pictured here is a cross section of an elephant tusk, which is made of ivory, a special form of dentine. Photo by Hannes Grobe (CC BY-SA 2.5)

The Whole Tooth

The earliest vertebrate ancestors with tooth-like structures you’ll often hear about are conodonts. Their “teeth” might be the earliest examples of mineralized skeleton in vertebrates, though they appear to be only convergent with true teeth, and therefore might not be quite as informative as we used to think when it comes to vertebrate tooth evolution.

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Though they were jawless, conodonts had mouths full of sharp tooth-like structures. They lived from the Cambrian to Late Triassic. Photo by Irengharibyan (CC BY-SA 4.0)

The origins of true teeth in vertebrates seem to be related to tooth-like structures called odontodes which can be seen among various fish on the pharyngeal gill arches or as toothy scales (dermal denticles) on the skin.

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Left: A goldfish’s pair of pharyngeal gill arches with teeth. BiotopAG (CC BY-SA 3.0). Right: Close-up of the dermal denticles of a shark. Pascal Deynat (CC BY-SA 3.0)

Inspired by these structures, researchers have developed two competing theories for tooth origin  which are still debated today. The “inside-out” concept holds that pharyngeal odontodes are the precursors to teeth, later migrating to the mouth; while the “outside-in” model holds that teeth derived originally from skin denticles.

The “outside-in” idea is supported by discoveries of dermal denticles in early jawless fish such as thelodonts and ostracoderms, as well as the elasmobranchs (sharks and rays) and early jawed placoderms who also had some of the earliest mouth-teeth (though the date of the earliest known teeth is debated), and also by genetic and structural similarities between mammal teeth and fish scales. For these reasons, this theory is generally the more popular of the two.

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A fossil, saw snouted ray called Schizorhiza had denticles along its snout that grew in a similar way to the teeth in the mouth. This showed how dermal denticles could begin to develop like teeth. Photo by Gyik Toma (CC BY 2.0)

And Nothing But The Tooth

Once true teeth evolved, they diversified into an incredible variety of forms, so paleontologists have developed a long list of terms to help us describe them all.

There is variety in the way teeth attach to the jaws. Some fit into sockets (thecodont), others are fused to the jawbone surface (acrodont), and some attach to the inside wall of the jaw (pleurodont).

Some teeth are monophyodont, meaning the animal only gets one set of teeth, as opposed to the more familiar diphyodont condition of two sets (we have this!) and the ever-impressive polyphyodont condition of multiple or even continuous tooth replacement (think sharks and crocodiles).

Dentition can also be homodont – meaning the teeth are mostly uniform throughout the jaw – or heterodont, coming in a variety of shapes or sizes.

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Humans, like many mammals, start off life with a set of deciduous teeth, or baby teeth, which are later replaced by our permanent adult teeth. In this image, you can see the permanent teeth developing in the jaw of a child, ready to erupt to replace the baby teeth. Photo by Henry Vandyke Carter / Public domain.
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This is the skull of a Burmese python. Its exhibits pleurodont, polyphyodont, homodont dentition. It also has teeth on five different sets of bones in its jaw! Photo by us!

Mammals like ourselves are famously heterodont, with four standard tooth types: incisors, canines, premolars, and molars. Throughout mammal evolution, this arrangement has shifted and adjusted, so we use dental formulas to describe and track tooth arrangement in different mammal groups.

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Dental formula of a dog. This simple fraction-looking number represents the dentition of an animal and can be easily compared between groups. The “R” in the figure indicates a specially modified pair of teeth called the carnassials, a unique feature of carnivoran mammals. Photo modified from Walter Heubach / Public domain.

Mammal teeth are diverse not just in their arrangement and placement, but also their shape. In particular, the occlusal surface (the chewing surface) of molars and premolars can be particularly diverse. the bumps (cusps) on the crowns of these teeth can be bunodont (rounded and separate, like ours), lophodont (arranged into long ridges, like elephant teeth), selenodont (crescent-shaped, like in deer), secodont (blade-like, as seen in cats and dogs), and more. Teeth can also be short-crowned or high-crowned. All of this variety often relates to diet and evolution, and can be especially important for paleontologists trying to understand tooth evolution and ancient ecology.

For paleontologists, teeth are invaluable resources. They fossilize well; they’re abundant; they can help us infer diet through physical and chemical analysis; they can indicate the age of individuals based on tooth development and wear; and they even give us insight into how our own mouths work.

More on Teeth

If you’d like to explore the incredible diversity of teeth even further, check out these links:

Non-technical:
Dinosaur teeth
Evolution’s Bite: A Story of Teeth, Diet, and Human Origins
How Teeth Became Tusks, and Tusks Became Liabilities

Technical:
Evolution and Function of Dinosaur Teeth
Fracture in teeth—a diagnostic for inferring bite force and tooth function

Also check out these videos about teeth evolution:
How did teeth evolve? (Ted-Ed)
YOUR INNER FISH | The Evolution of your Teeth (PBS)
How Human Teeth Evolved From Ancient Fish (Seeker)
Where Do Teeth Come From? (It’s Okay to be Smart)

 

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