The Cretaceous Period, the last period in the Age of Dinosaurs, was a time when all
sorts of incredible and fearsome animals roamed the Earth.
Large, carnivorous dinosaurs ruled the land, enormous flying pterosaurs soared in the skies
above, and extraordinary marine reptiles dwelled in the planet’s oceans.
But alongside these fascinating extinct groups, there were many kinds of more familiar animals
- lineages that are still alive today, and that are just as amazing.
One of these groups of animals is the sharks - cartilaginous fish that were around long
before the time of the dinosaurs, too.
During the Cretaceous Period, sharks were abundant and widespread throughout the planet’s
oceans, just as they are today.
They were adapted to fill all sorts of different niches as well, with some sharks even feeding
on dinosaurs - as we explored in our video on Cretoxyrhina during Shark Week 2020.
Other Cretaceous sharks fed on very different things, though.
When you think of shark teeth, you might understandably at first picture sharp, pointed structures
suited for piercing prey.
However, there’s a genus of shark called Ptychodus that has some very strange-looking
teeth indeed, looking almost like pebbles with fascinating ripple-like structures running
across the surfaces.
Clearly, this shark was suited to a very specific feeding style - but what was it?
Well, when the fossilised teeth of this shark were first discovered and written about all
the way back in the 1700s, they were actually interpreted as the remains of the palates
of fish, and so were considered to be the bony structures forming the roof and base
of the mouth.
Specifically, they were thought to have come from porcupinefishes, and later on, during
the 1800s, they were again described as fish palates by some palaeontologists.
These fossils were turning up all over the place, too, from England and Germany to Alabama
and Kansas - indicating that whatever sort of animal they were coming from probably had
a global range.
It was then in the 1830s that these fossils finally received an official scientific name
- the genus was designated Ptychodus, coming from the Greek for ‘folded tooth’ and
very accurately describing the amazing ridges and patterns seen on these structures.
There are many, many species that have been named within the Ptychodus genus, potentially
as many as 27 species depending on who you ask, and the teeth all vary in different ways.
Over the centuries since they were first written about Ptychodus teeth have been found in even
more locations, including many other European countries, South America, Africa, India, Japan,
Australia and elsewhere.
So, going back to that question of what this animal was feeding on - why do the teeth of
this shark look so strange?
Well, when an animal becomes adapted to feeding on very hard prey, such as invertebrates with
hard shells, a common evolutionary convergence that occurs among different lineages feeding
on this prey time is for the teeth to become rounded and dome-like, as this shape is ideal
for crunching through hard substances.
This type of feeding, known as durophagy, is also seen in Mosasaurs, marine reptiles
called Placodonts, and in other kinds of fish, including many sharks and rays.
So the Cretaceous shark Ptychodus was definitely well-suited to grinding down and crushing
its prey, with those rounded, pebble teeth making short work of Cretaceous animals with
hard shells.
In fact, studies of these teeth have shown them to be structurally quite complex, with
three separate layers of enameloid present.
These separate layers would then have stopped any cracks that might form in the teeth when
crunching down on prey from spreading throughout the entire tooth, and the dome shape as well
as the ridges across the surface are also involved in strengthening them.
And they would need to have been strong, as it seems these sharks employed 3-point bending
stresses to crack open their prey, with these stresses occurring both at a large scale across
the entire jaw, as well as at a smaller scale across the individual ridges of the teeth.
But although it’s very clear that Ptychodus was well-suited to eating hard-shelled organisms
such as bivalves, there have also been some suggestions put forward that this is not all
they fed on.
As sharks are members of the chondrichthyans - the cartilaginous fishes - their skeletons
generally do not fossilise well, being composed of cartilage and not bone, and so we are usually
only left with fossil teeth from these fish.
However, sometimes we do find more, and complete shark body fossils are known, though they
are rare.
In the case of Ptychodus, some very nice complete jaws have been discovered in the past, and
by looking at these it’s possible to get a sense of what this animal may have been
like in life.
Studying one such partial Ptychodus jaw and skull fossil found in Nebraska, palaeontologists
found some interesting implications for the palaeobiology of this shark genus.
The arrangement of the teeth within the jaws was fairly elongate from front to back, and
not very wide - unlike in the modern-day myliobatid rays, known as Eagle rays, which are very
specialised in crushing particularly hard prey but have very broad plates of teeth.
This particular species of Ptychodus, Ptychodus occidentalis, also has relatively high-crowned
teeth unlike the flat teeth of these rays.
The palaeontologists therefore took this, along with other lines of evidence, to suggest
that Ptychodus occidentalis might have been more of a generalist predator, and not a specialist
on only hard prey.
In fact, they also found that the upper and lower jaw teeth of this species had an interlocking
mechanism that would have made them ideal for grasping too, so it seems that in some
members of the Ptychodus genus the initial evolution of durophagous teeth was later co-opted
into other functions - a pretty cool example of how evolution works by modifying structures
that are already there as they adapt to new functions.
The researchers also pointed out that the exact shapes and structures of the teeth and
ridges vary quite a bit between different species of Ptychodus, and so presumably every
species was not feeding on the exact same prey, with some perhaps being a bit more generalist
- such as Ptychodus occidentalis - while others may have still been specialists on a particular
type of hard-shelled organism.
Another benefit of finding varying types of Ptychodus teeth is that it allows palaeontologists
to track their evolution.
The question of what sort of shark Ptychodus actually is has been the subject of debate
ever since the teeth of this animal were first recognised as being from a shark, and some
authors have in fact considered them to be a kind of ray, while others classified them
as belonging to an extinct lineage of shark relatives, the hybodonts.
This classification as hybodonts was generally accepted by many researchers for a long time,
as various similarities in the teeth of Ptychodus and hybodonts appeared to support a close
relationship.
In 2015, an apparent intermediate form between hybodonts and Ptychodus was even described,
based on teeth found in Texas.
These teeth showed a sort of ‘missing link’ in their structure between a particular group
of hybodonts and later Ptychodus teeth, and so it was named as Paraptychodus.
However, more recent studies of the teeth and other bones of the crushing shark have
revealed something different, showing that Ptychodus is actually a member of the neoselachians
- the name given to the grouping containing all living, modern sharks and rays.
This assertion is based on the composition of the teeth and the three layers of enameloid
I mentioned earlier, which is a feature only seen in neoselachians.
So, perhaps Paraptychodus is not a missing link after all, and just shows some similarities
to the groups.
Another contested part of Ptychodus palaeobiology concerns exactly how large this shark could
grow.
Well, an amazing discovery made in Spain and published on in 2020 might just answer this
question - and as it turns out, some Ptychodus species were pretty huge.
Previous estimates of different Ptychodus species have ranged from approximately 2 metres
in total length, all the way up to 14.4 metres - however these are of course based on incomplete
jaws and single or few vertebrae, and it’s also important to remember that there are
many different species of Ptychodus that undoubtedly varied in their dimensions.
However, going back to that 2020 study, this paper documents the discovery of a series
of articulated vertebrae all coming from the same individual, and based on calculations
taking into account the dimensions of large modern sharks they estimate a range of 4.3
to over 7 metres for this particular Ptychodus specimen - a pretty sizable fish, then.
Some other interesting insights were gained from the study of these vertebrae, too.
By studying the growth bands of the vertebrae - similar to looking at tree rings to see
the age of the plant - the scientists were able to determine that Ptychodus was quite
a slow-growing shark and that it matured relatively late in its life.
Not only that, but by examining the radius of the initial, birth ring the researchers
could tell that this Ptychodus specimen would have had a total body length of between 65
cm and just over a meter when it was born.
This is quite large for a baby shark, but it’s consistent with the birth sizes of
some modern sharks such as great hammerheads, tiger sharks, great white, basking sharks
and whale sharks - all of which are big-bodied and are viviparous (meaning they give birth
to live young that are supported by a placenta inside the mother’s body) or are ovoviviparous
(meaning the shark babies hatch from eggs inside the mother and then live inside the
womb and feed on an egg yolk sac for a period of time before coming out during a live birth).
So, the palaeontologists infer that Ptychodus, which seems to have given birth to babies
significantly larger than those recorded from egg-laying (or oviparous) sharks, most likely
reproduced by giving live birth, and put a lot of energy and resources into producing
large, developed offspring.
This then led the researchers to propose that Ptychodus had K-selected traits, meaning they
invested a lot into their young but only gave birth to a few babies.
This contrasts with the condition in r-selected animals, which produce many offspring that
are generally less developed but grow faster.
Based on the evidence presented in their paper, it does indeed seem likely that these fish
were utilising the K-selection strategy, and it’s pretty amazing that this sort of palaeobiological
insight can be gained from just a few vertebrae.
So, Ptychodus was a fascinating prehistoric shark.
These animals had some amazing adaptations that enabled them to survive in the highly
competitive oceans of the Cretaceous period, and clearly they were highly successful with
many different species arising over time.
These were pretty large sharks, too, and you certainly wouldn’t want to have been a hard-shelled
bivalve back in those Cretaceous seas.
And one of the coolest things about Ptychodus is that you can go out there and find fossils
of this shark yourself!
If you’re looking in rocks of the right formations, especially in the UK and US but
also elsewhere in the world, you might just be able to collect the distinctive rippled,
pebble-like teeth of these sharks, as although they’re quite rare fossils, sharks do have
a lot of continuously-replaced teeth and they do turn up in Cretaceous strata, such as in
the Chalk of the UK and the Niobrara Formation of the US.
Anyway, I really hope you enjoyed learning about Ptychodus, the giant crusher of the
Cretaceous.
I hope you’re enjoying shark week 2023 so far, too - still to come is a video on how
sharks hold their breath to dive deeper, a look at the fantastically bizarre goblin shark,
a BoneHeads shark special, our video on our trip to find our own fossil shark teeth, and
of course a Megalodon video to finish us off - this time about the giant shark’s competition
with Livyatan.
Thank you for watching, and a big thank you to our Patreon supporters