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An Endless Supply of Teeth

One of the most impressive and intimidating features of sharks is those teeth...but shark teeth are even more fascinating and incredible than you might have imagined. Unlike human teeth, which are permanent in adulthood, sharks never stop growing new teeth. A great white shark (Carcharodon carcharias) can have as many as 30,000 teeth throughout its lifetime! So how does this work? How do sharks keep growing new teeth? And why do they do it?


When you look closely at this sandtiger shark you can clearly see multiple rows of teeth in the jaw at once (Copyright: Sophie Maycock)

The Revolving Door

Sharks have evolved to have "revolver dentition" (also known as "polyphyodont dentition"); with multiple rows of teeth present in the jaw at once, arraying backwards on a conveyor-belt-like system. So when a tooth at the front of the jaw is shed naturally, another tooth will pop up from behind to replace it (Fraser et al, 2020).


This means that when shark has to struggle with especially feisty prey item, if teeth are damaged or lost, they can be easily replaced. This is an absolutley vital evolutionary adaptation because it ensures that a shark will never lose all its teeth and be unable to hunt (Fraser et al, 2020).



A Never-Ending Story

But how is it possible that sharks can grow new teeth throughout their lives?


Sharks have a life-long source of "progenitor cells" in the "dental lamina" (a band of skin-like tissue which gives rise to a developing tooth). Progenitor cells are similar to "stem cells"; they are dormant until they are by chemical signalling in the body, at which point they can develop into certain tissue types, like muscle or bone. When a shark's tooth is lost at the front of the jaw, towards the back of the conveyor belt, progenitor cells are activated to grow a new tooth; pushing all the previously grown teeth forwards so that a new tooth pops up to fill the gap (Fraser et al, 2020).



A Beautiful, Unique Snowflake

As if that wasn't seriously cool enough, what is especially interesting about sharks, is that many species have multiple different types of teeth in their mouth at once (known as "heterodonty"). The teeth often change in both size and shape in a gradient from the front to the back of the jaw. This is similar to how humans have incisors, canines and molars; different types of teeth for different jobs (Fraser et al, 2020).


Heterodonty = heteros (Greek) ‘other’ & dent (Latin) 'tooth'


For example, bullhead sharks (Family Heterodontidae), like the Port Jackson shark (Heterodontus portjacksoni) have an extreme shift in tooth morphology along the jaw; with pointed, grasping teeth towards the front and flattened, crushing teeth at the back. As these sharks feed on small fish and shellfish, their front teeth are perfect for grasping wriggling prey and their back teeth can be used to grind down bones and hard shells before swallowing (Jambura et al, 2020).


But shark teeth can also change as they grow! Some shark species have a very different type of tooth morphology when they are young compared to when they are grown. For example, the great white has narrow grasping teeth, ideal for impaling fish when they are young, but older animals have triangular, slicing teeth, more suited to biting chunks out of large prey. This is a perfect evolutionary adaptation for these sharks as they change what types of prey they target as they increase in size; preferring fish, squid and octopi at first, and taking on whales and dolphins later in life (this is known as an "ontogenetic diet shift") (Jambura et al, 2020).


As there are over 500 different species of sharks, which all target different types of prey, there is incredible diversity in tooth morphology throughout the sharkie world! For example, the pointed teeth of the order Orectolobiformes (the carpet sharks), such as the spotted wobbegong Orectolobus maculatus (top left in the image) and the Lamniformes (the mackerel sharks), such as the longfin mako shark Isurus paucus (image bottom left) are clearly different to the teeth of the Squaliformes (dogfish), such as gulper shark Centrophorus granulosus (image top middle). This is due to their contrasting diets (Jambura et al, 2020).




Something Old, Something New

The shape of fossilised shark teeth can also show us how shark morphology has changed throughout time. For example, if we look at the fossilised teeth of †Galeocerdo mayumbensis (image top left), an ancestor of the modern Carcharhiniformes sharks, we can see how this group evolved from a "common ancestor" (a species from the past which evolved in two or more different, related species). This can be very important for taxonomists, as it can allow us to understand how all the different types of modern sharks evolved from a common ancestor as much as 330 million years ago! (Jambura et al, 2020).




References

Fraser GJ, Standing A, Underwood C & Thiery AP (2020). The dental lamina: An essential structure for perpetual tooth regeneration in sharks. Integrative and Comparative Biology, 10.1093/icb/icaa102. Access online.


Jambura PL, Türtscher J, Kindlimann R, Metscher B, Pfaff C, Stumpf S, Weber GW, Kriwet J (2020). Evolutionary trajectories of tooth histology patterns in modern sharks (Chondrichthyes, Elasmobranchii). Journal of Anatomy. 236:753–771. Access online.



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tony
Aug 13, 2020

FASCINATING -- GOOD LINK TO VIDEO

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