Today sharks are a source of both fascination and fear for people but one fact about them that is still often overlooked is that they are one of the oldest types of animal on the planet. Also while the sharks of today form a critical part of the oceanic ecosystem they are no way near as bizarre or outright terrifying as some of their prehistoric ancestors.
Early beginnings
The
first indication of sharks in the fossil record dates all the way back
to the late Silurian period, possibly as much as four hundred and
fifty million years ago. Unfortunately the only evidence that exists
is scales from genera such as Elgestolepis,
Mongolepis
and
Polymerolepis,
but these scales are still well enough preserved to
indicate that these fish were sharks.
Shark
fossils become much more common in deposits from the Devonian,
possibly reflecting growing success at survival. A greater number of
sharks swimming in the Devonian waters would after all leave a
proportionately higher amount of fossils. Already some of these
sharks had developed a basic shark profile, with some like
Cladoselache
having a long torpedo shaped body and well developed fins
for fast and agile swimming. However these sharks were still
developing in the shade of other predators that meant they kept small
and relied on speed for both hunting and avoidance of more powerful
hunters like the arthrodire placoderms. However the disappearance of
these predators at the end of the Devonian seems to have opened up the
way for sharks to move beyond their more humble beginnings.
Shark diversification- The
classic and the bizarre body forms
The
Carboniferous & Permian periods saw the sharks evolve into what
are possibly some
of their most bizarre forms. In fact some of these forms were so
strange that they would have made the hammerhead shark that we know
today seem totally normal. Most famous of these is probably
Helicoprion
which had its teeth arranged in a strange spiral ‘tooth
whorl’ that is thought to have extended from its lower jaw. It is
unknown how exactly this whorl was used and upon what prey, but
similar forms include Ornithoprion
and Sarcoprion.
Both these sharks
have semi-circular arrangements of teeth in their jaws, and it’s
possible that they may have been for rasping through prey to saw off
bite sized chunks.
The
mystery of the tooth whorl of Helicoprion may be
connected to another
shark named Edestus,
better known as the ‘scissor-toothed
shark’. Edestus acquired this nickname from the
fact that its teeth
were not shed like in other sharks and the jaws steadily grew
throughout its life. This meant that the inner jaws with their teeth
were constantly pushed forward throughout it life so that its outer
most teeth pointed forwards rather than up and down like other sharks.
If Helicoprion's jaw grew in a similar manner
then it may have
started out small, but steadily curled round as the shark grew.
This might even suggest that rather than being used for hunting it was
a display indicating the maturity of the individual.
One
group of sharks that were very different to those we know today are the
xenacanthids. These sharks developed eel like bodies that were better
suited for navigating the freshwater swamps that were clogged with
submerged debris. Additionally, rather than having dorsal fins they
instead had a single spine that rose up from the back of the head.
This is usually interpreted as being for defence from larger fish as
well as large amphibians.
Some
sharks that did have dorsal fins had highly unusual ones. Briefly
going back to the Devonian period and a shark called Stethacanthus
had
an anvil shaped dorsal fin that had growths of tooth like denticles on
top of it (as well as a similar mat of denticles on top of its
head). This dorsal fin is why Stethacanthus is
sometimes called
the ‘anvil shark’, or ‘ironing board shark’, with theories
for the unusual dorsal fin ranging from a method of intimidating
predators to same species recognition, but it remains hard to be
certain without seeing the living creature. Another shark with an
unusual dorsal fin is the small thirty centimetre long Falcatus
that
had a forwards horizontally pointing point projecting from the top of
its dorsal fin and over its head. However the function of this dorsal
fin is easier to determine with confidence as this dorsal fin only
appears to have been on male Falcatus. This makes
it almost certain
that the dorsal fin of Falcatus was for the purpose
of attracting a
mate with perhaps the most developed spike being the most attractive to
the female Falcatus.
‘Spiny Sharks’
Before
moving on another group of fish similar to the sharks are those
referred to as ‘spiny sharks’. These actually appeared back in
the Silurian period and rose to prominence during the Devonian and
Carboniferous periods. As cartilaginous fish the spiny sharks were
related to sharks, but were not true sharks themselves. Spiny
sharks are so named from the well developed and often numerous spines
that projected from their bodies, presumably for the purpose of
defence as these spines would make it both difficult and painful for a
predator to close its jaws around them. Spiny sharks also appear to
have been among the first fish to actually develop jaws, as earlier
fish similar to Cephalaspis
(which itself is actually known from the
early Devonian period) are often referred to as jawless fish.
Spiny
sharks were successful fish for their day, producing a variety of
forms as well as colonising both sea and freshwater environments.
However it would seem that combined competition from true sharks as
well as bony fish reduced the numbers of spiny sharks to the point that
they disappeared at the end of the Permian, before the start of the
Mesozoic era. It’s also plausible that the extinction event that
marks the end of the Permian may have also been responsible for their
total disappearance.
The Mesozoic
Between
the start of the Triassic and end of the Cretaceous the sharks
continued to thrive, some in forms that are much more familiar to us
today. Hybodus
is probably the greatest success story here with a
worldwide distribution as well as temporal range that stretches from
the end of the Permian through to the early Cretaceous. In addition
to these feats Hybodus also survived the mass
extinctions at the end of
the Permian and Triassic periods, the latter being a particularly bad
one as far as marine life in concerned. Success for Hybodus is
thought to have come from its ability to adapt to different types of
prey thanks to its combination of sharp piercing teeth as well as
rounded crushing teeth.
Other
sharks of the time also appear to be quite modern with Scapanorhynchus
resembling a goblin shark, while Squalicorax
and Cretoxyrhina
both
appearing to be oceanic sharks well suited to a pelagic life. With
more standard forms now established they had almost reached their full
potential with successful body forms that are so perfect for aquatic
life they have barely changed since. Shark diversification did not
come to a complete halt however, as the Mesozoic sharks simply became
better adapted to hunt other marine animals. Modifications now were
along the lines of such things as different teeth for different kinds
of prey such as thin pointed teeth for fish, serrated triangular
teeth for large prey or rounded plate like teeth for crushing shells.
Shark giants - The
megatoothed sharks
People
have always had a particular fascination with large sharks, although
usually they envision the great white shark (Carcharodon
carcharias)
which is the biggest predatory shark in the ocean today, while
others like the whale shark are bigger but are filter feeders of
plankton. However both of these sharks pale in comparison to the
giant sharks that lived from the Oligocene to Pleistocene periods.
The
most famous of these sharks is C.
megalodon, often just referred to
as megalodon, which is now well known all over the world thanks
mainly to it appearing as a star monster in an ever expanding
number of films and stories. Despite its current fame C.
megalodon
is actually late to the party, with it being recognised as a distinct
species as far back as 1843, and teeth documented in scientific
journals as far back as the end of the renaissance. With minimum size
estimates averaging fifteen meters long, C. megalodon
was the
largest and last of the megatoothed sharks, but the ones that came
before it such as Otodus
and C.
angustidenss are also estimated at
being around nine meters long themselves.
Two
problems persist with our understanding of the megatoothed sharks,
the first one being how big they grew. With the exception of a few
vertebrae, only the teeth are known, so size is usually determined
by scaling these in comparison to other sharks that are alive in the
ocean today. Unfortunately to muddle things further there are a
number of different methods of comparison, some such as measuring
slant height of a tooth (the length between the tip and outer edge of
the tooth) to the amount of enamel that is on it, as well as
others. Most of this research has been directed towards C.
megalodon as it has the largest teeth, and therefore was
the largest
shark. C. megalodon is usually compared to the
great white shark as
it is thought to be the closest thing to a C. megalodon
still
swimming in the ocean. Depending upon the size of the teeth and the
method involved estimates for C. megalodon
usually range between
fifteen and twenty meters, sometime more, but most researchers
agree that the science is not exact and is best for just a more general
indication of the total size.
The
second problem with the later megatoothed sharks such as C.
megalodon, C.
auriculatus, C.
chubutensis and C.
angustidens
is exactly which genus they should be placed in. As you may have
noticed these species have been referred to as ‘C.’, and this is
because while many people place them in the Carcharocles
genus,
separate from modern sharks. Other people however place them in the
Carcharodon genus along with the great white shark,
on the grounds of
slightly similar teeth and the fact that the great white is the biggest
predatory shark in the ocean today. Most today seem to lean towards
the Carcharocles placement as while the teeth of
sharks like C.
megalodon, do bear a superficial resemblance to the
great white,
closer inspection reveals a number of differences. What can be said
however is while it is almost certain that the great white shark is not
descended from currently known megatoothed sharks, it may have shared
a common ancestor with them.
Shark fossils
The
most common shark fossils are without a doubt the teeth. Made from
dentine and coated with enamel, teeth are extremely resilient to the
processes of decay and fossilisation. Teeth are often so well
preserved that not only can serrations be clearly seen; they can even
remain sharp enough to cut skin if handled carelessly. Teeth are
usually found individually, sometimes coming away from the
surrounding rock, while other times they are deliberately left on
part of the original rock. Very rarely however teeth are found in
whole jaw concentrations, sometimes even with part of the jaw
preserved.
Teeth
fossils can reveal a lot about a sharks lifestyle and predatory
behaviour, especially when they are compared to similar sharks that
are alive and swimming in today’s oceans. For instance Squalicorax
has triangular teeth with serrated edges for sawing off chunks of flesh
from larger prey, while the narrow pointed teeth of Scapanorhynchus
are perfect for piercing the soft bodies of smaller fish. The rounded
teeth of the ten meter long Cretaceous shark Ptychodus
however show
that it crushed the shells of armoured prey like shellfish and
crustaceans.
To
a lesser extent than the teeth, shark vertebrae are still quite well
known. However because vertebra are made from ossified cartilage they
are more susceptible to decomposition, which means unless a dead
shark can be covered quickly in sediment, the vertebrae can quickly
decompose. This can easily be seen in recovered specimens of
vertebrae which had already partially decomposed when they were
fossilised resulting in incomplete preservation. Sometimes vertebrae
are found still lined up together to reveal portions of the spine, up
to the point where the length of the shark can be reasonably determined.
Much
more valuable yet also incredibly rare are impressions that reveal the
shape and internal structure of a sharks cartilaginous skeleton. The
best known shark impressions belong to Cladoselache
which are so well
preserved that they even reveal impressions of the internal soft tissue
and organs as well as the sharks last meal. This reveals important
information on how similar today’s sharks are to their ancient
forbears as well as their feeding habits.
Other
impressions have revealed body features that otherwise would have gone
unknown to science. Impressions of Stethacanthus
for example revealed
its anvil shaped dorsal fin, the mat of toothy denticles on top of
its head and the cartilaginous tendrils that trail from its pectoral
fins. None of these features could have been described from examining
its teeth alone. Most impressions however are for smaller species of
shark, which unsurprisingly are easier to bury than larger forms.
At best larger sharks that are well above two meters long seem to be
preserved in parts, and usually just areas like the jaw and vertebrae.
Prehistoric sharks as predators
and prey
Although
sharks are popularly dubbed the top predators of the ocean today, in
the past they were just as likely to have found themselves to be the
prey as the predator. Most of the documented shark evolution seems to
have been driven towards becoming more effective predators so that they
could more easily kill a greater variety of animals while living in
waters that periodically saw the emergence of new kinds of predatory
creatures. Some sharks went along the lines of direct competition
with these hunters, such as the pelagic carnivorous sharks that took
what they could get, to specialised forms that focused upon eating
certain kinds of animals or hunting in very specific ways.
The
xenacanthid sharks seemed to have preferred the freshwater swamps and
waterways that would have been overgrown with aquatic plants and debris
from above. The xenacanthids were almost certainly not the only fish
swimming these waters with other fish types providing bountiful prey.
Also in the water were tetrapod amphibians and more importantly
juveniles in their larval stages. It would seem that a good portion
of these amphibian juveniles were restricted to the water because of
the larval stage features that meant they could not crawl onto land to
escape predators. Evidence that larval stage amphibians were prey to
ancient sharks comes from the xenacanthid Triodus
that was found with
the remains of one inside its body. The sinuous bodies of the
xenacanthids meant that they could lurk amongst the submerged debris as
well as work their way into tight areas to search for hidden prey.
Some
of the best indications of prey animals comes from well preserved
specimens of Cladoselache dating from the
Devonian. These fossils are
so well preserved that it is actually possible to identify the exact
kinds of animals that were eaten. In the case of Cladoselache,
the
preferred prey items were by far ray finned fish that Cladoselache
would have easily been able to catch with its streamlined body. The
fossils also show that Cladoselache would feed upon
other animals
including conodonts (primitive eel like fish), arthropods and
what appears to have been another shark.
This
briefly brings up the subject of sharks eating other sharks. Many
sharks are thought to eat other sharks especially if the shark being
eaten is much smaller than the aggressor. Young sharks that are still
in their mother’s body are also known to eat each other before birth as
well. How often cannibalism in sharks occurs is a matter of strong
debate, for as long as food sources are plentiful it may be never.
With reduced food supplies it may become frequent, especially if a
shark is sick or wounded and is giving out the distress signals to
other and healthier sharks which then perceive it to be a prey item.
Back
in the Devonian period sharks would have often found themselves prey to
huge arthrodire placoderms like Dunkleosteus.
However while the
placoderms were the dominant predators, they did not survive for long
as their remains disappeared at the end of the Devonian. This
probably helped bring about the great diversification of sharks in the
Carboniferous period that saw many new feeding designs developed.
Although not the only predatory fish of the time, Sharks probably
played a key role in dominating the late Palaeozoic seas. However the
Permian period would see the beginnings of a new challenge in the form
of small reptiles like Hovasaurus
evolving for aquatic life.
The
Mesozoic era that began at the end of the Permian with the start of the
Triassic and lasting until the Cretaceous, would see the continued
development of marine reptiles in the form of ichthyosaurs,
plesiosaurs, pliosaurs and mosasaurs. Some of these marine
reptiles like the icthyosaurs and plesiosaurs would have been more
competition to the same prey sources as sharks had. The pliosaurs and
mosasaurs however were dedicated killers of large prey and would have
thought nothing about adding a shark to the menu.
Despite
this new competition the sharks continued to hold their own in the face
of these new predators. Some like Hybodus would
become generalists
that could feed upon a variety of different prey types. Evidence for
this comes from the fact that Hybodus had sharp
teeth at the front for
seizing fast prey like fish and squid, while also having blunter
crushing teeth best suited for cracking the tough exoskeletons of
crustaceans.
Other
sharks like Scapanorhynchus would become deep water
specialists. By
staying down in the black, Scapanorhynchus would
have been out of
sight and possibly out of range of the marine reptiles that still had
to surface to breathe air. This behaviour is suggested by its
extended snout that would have contained a larger number of
electro-receptive ampullae that would not have been possible in a
smaller area. These ampullae would have sensed the electric fields of
nearby fish allowing Scapanorhynchus to find prey
without even having
to see it. This exact behaviour is seen today in the Goblin shark
which also has teeth that are almost identical to Scapanorhynchus.
Also the shellfish eating shark Ptyhocodus would
have most probably
lived a benthic life near the bottom where its preferred prey would
have been. This would have seen it living in a position that probably
had limited contact with marine reptiles while feeding on animals that
were too tough for other sharks, such as the giant clam Inoceramus.
One
final strategy of sharks to exist in the same oceans as the massive
marine reptiles was to simply grow bigger themselves. The best
example of this was Cretoxyrhina which was
probably 'the' killer
shark of its day. At seven metres long Cretoxyrhina
was a bit bigger
that the largest officially recorded great white shark (which is a
bit over six meters). Cretoxyrhina also has
the nickname 'ginsu
shark' because its teeth seem to have been able to cut through
anything. This is because not only are Cretoxyrhina
teeth sharp,
they have a considerably large amount of enamel coating making them
just as useful for cutting into bone and shell as they were flesh.
Fossil evidence also shows that Cretoxyrhina went
so far up the
predatory scale that it also included plesiosaurs as well as the
dangerous mosasaurs in its diet.
Popular
fiction sometimes depicts epic battles between sharks and dinosaurs
taking place but unfortunately there is very little evidence to suggest
interaction between these two animal groups. The best recorded
evidence for a shark eating a dinosaur comes from a tooth of the shark
Squalicorax that was found embedded in the foot bone
of a hadrosaurid
dinosaur. The majority of researchers believe that the dinosaur would
have been swept out to sea and drowned, and that Squalicorax
was
simply taking the opportunity of a free meal.
Sharks
proved their resilience again in the extinction sixty-five million
years ago that marked the end of the Mesozoic, the dinosaurs and
their great rivals of the time the marine reptiles. Again the sharks
found themselves the dominant predators, but they would soon be
challenged again, Not only were mammals slowly becoming the dominant
life forms on land they were also expanding into the ocean as evidenced
by Pakicetus
and Ambulocetus.
Sharks
on their part did not face a significant threat from these new kinds of
marine animals, in fact fossil evidence suggests that the sharks
treated the new marine mammals as a wholly new food source. As the
mammals grew bigger, so did the sharks until a new line of giant
megatoothed sharks were swimming the oceans.
The
most famous of the megatoothed sharks was C. megalodon,
currently
known as the largest shark to ever swim the oceans. C.
megalodon
seems to have evolved from a large shark pedigree going back to the
twelve meter Otodus to C. auriculatus, and C.
angustidens. C.
megalodon itself has a small average length of fifteen
meters making
it two and half times larger than the biggest great white shark. C.
megalodon also had the teeth for work on medium to large
sized whales
that it could bite chunks from with relative ease.
This
predation of new animals was not one sided however as the early aquatic
mammals also evolved into predatory whales like Basilosaurus
and
Livyaten.
Both of these whales lived at different times, with
Basilosaurus being much earlier in the fossil
record, but they like
others evolved to hunt other whales. They also would have been
capable of killing other animals including sharks and even challenging
the larger sharks for dominance.
Despite
the presence of giant sharks like C. megalodon
and the increase in
large predatory whales, both types of animals coexisted with one
another due to the abundance of large prey items. However it was
nature that ended the dominance of both kinds of predator with colder
oceans and dropping sea levels that restricted ocean currents and
movement. This caused a drop in prey species that meant the giant
sharks and predatory whales no longer had sufficient food to live.
Today
the only remaining large predatory whales are the sperm whale which
feeds in deep water for animals like squid, and the Orca which can
adapt to hunt different kinds of prey. Sharks have continued to
survive on a similar strategy of being generalist carnivores that adapt
to different prey types. The fossil record shows us that
generalisation is the best insurance against extinction with
specialists such as the bizarre Helicoprion to the
gigantic C.
megalodon not surviving the test of time. In contrast the
generalist
Hybodus survived for over one hundred million years
before it finally
vanished.
Acanthodes
(spiny shark) Acrodus Akmonistion Anodontacanthus Anomotodon C. megalodon (often just called ‘Megalodon’) Carcharocles angustidens Carcharocles auriculatus Carcharocles chubutensis Cardabiodon Cobelodus Cladoselache Climatius (spiny shark) Cretoxyrhina Ctenacanthus Dicentrodus Diplodoselache Edestus Elgestolepis Erquitaia Falcatus Galeocerdo contortus Glikmanius Hagenoselache Helicoprion |
Hemipristis Hybodus Iniopteryx (a chimaera) Leonodus Mongolepis Ornithoprion Orthacanthus (xenacanthid) Otodus Physogaleus Plicatodus Polymerolepis Priohybodus Ptychodus Reginaselache (xenacanthid) Sarcoprion Scapanorhynchus Sphenacanthus Sphenonchus Squalicorax Stethacanthus Triodus (xenacanthid) Tristychius Wodnika Xenacanthus (xenacanthid) |