The evolution of sharks

The emergance of sharks 

Sharks can be described quite rightly as a living fossil, after all, records suggest they first appeared 450 million years ago.  Since their humble beginnings they have evolved with mind-blowing diversity ranging from the gigantic Whale Shark Rhincodon typus to the endearingly named Nervous Shark Carcharhinus cautus. It would be impossible to highlight the incredible adaptions and diversity of sharks without mentioning the wonderfully bizarre family of sharks Sphyrnidae which are infamous for being characterised by a flattened hammer shaped head (the structure of which is known as cephalofoils) which has of course led to them being known more commonly as the Hammerhead Sharks. Their unique appearance with eyes further apart than in most shark species allows them to have an enhanced visual range of their surroundings and allows for a further ability to use their sensory organs (known as the ampullae of Lorenzini) to search for food across a greater distance. This is particularly useful when you consider that their favourite prey, stingrays, are often buried on the ocean floor.  

So, it is clear that sharks have evolved in various and sometimes dramatic ways but before we go and explore some of these adaptions over time, we must first consider the emergence of sharks some four hundred and fifty million years ago.  

The first emergence of sharks can be traced back to the Late Ordovician period when sea levels were high, coral reefs began to form, and the Earth’s climate was warm and stable. Fossilised shark like scales have been discovered in Colorado, North America that date back to this period. Whether these scales came from true sharks or their ancestors is debated.  If these scales did come from true sharks, then the lack of evidence of any teeth being found from the same time frame has led to the theory that these sharks were toothless.

The discovery of Doliodus problematicus represented the oldest articulated “shark” skeleton to be discovered dating back four hundred and ten million years and shows the oldest evidence of shark teeth.  Doliodus roamed the waters around what we now know as New Brunswick, Canada.  Once again however there is debate as to whether or not Doliodus is a true shark. Thanks to CT scans, scientists were able to spot spines along the belly of the skeleton. This is a trait belonging to the Acanthodians and this is where it begins to get complicated. While Acanthodians are sometimes referred to as “spiny sharks” they are however the ancestors of the Chondrichthyans also known as the cartilaginous fish which include of course sharks in the subclass Elasmobranchii.  What this means then, is that Doliodus is now considered a transitional fossil – an Acanthodian becoming a Chondrichthyan with evidence of primitive beginnings with diamond shaped scales and plentiful spines in front of their fins. The importance of transitional fossils in telling the story of evolution is enormous and there have been many incredible breakthroughs made in science as a result of them. One of the most famous being Archaeopteryx which provided the link between non- avian dinosaurs and birds. 

The Golden Age of Sharks.

A mass extinction event at the conclusion of the Devonian period spelt the end for approximately 75% of life on Earth and in doing so created an opportunity for sharks to well and truly dominate the world’s oceans following the demise of the Placoderms, Acanthodians and Agnathans. The Carboniferous Period occurred between 360 – 286 million years ago, where at first the climate was warm, later becoming warm and humid around the equator with glaciers forming at the poles, similar to today’s climate. This period was aptly named the Golden Age of Sharks with forty-five shark families present during this time. 

It is perhaps not surprising then with such a diversity of sharks in this time that evolution threw up some more unusual variations of the largely unchanged shark body plan.  One of the most eye catching of these was Stethacanthus, a relatively diminutive shark only growing to approximately the size of modern-day dogfish. Males sported some extravagant head accessories in the form of spiky, upright scales. If this wasn’t enough, they also possessed large, flat -topped dorsal fins. These modifications were not found in females and therefore show clear evidence of sexual dimorphism (an obvious difference between males and females of the same species).  Such eye-catching examples of sexual dimorphism are not found in modern-day sharks.  There were other examples of “experimental designs” during this time and they included Helicoprion which possessed a bottom jaw that has been described as “buzz-saw like” and Falcatus which in another example of sexual dimorphism, had long spines protruding from their backs and over their heads.  

The Carboniferous period wasn’t just a time of opportunity for sharks, it was also the time for great diversity amongst the Holocephali a subgroup of the Chrondrichthyes which consists of Rabbit Fish, Chimaeras and Elephant Fish. Nature is unpredictable and volatile however, and despite the success of marine life to this point, another mass extinction at the end of the Permian period some 252 million years ago saw an estimated 96% of marine life becoming extinct. Temperatures were cooler and the earth was dry during the Permian as the continent of Pangea continued to move northward. Corals began to produce again, after being near-extinct at the end of the Devonian and began making enormous reefs. However, many corals did not survive the mass extinction event that came with the end of the Permian period. The Permian extinction was the largest mass extinction that had ever occurred, and no extinction event since has killed so much of the life on the planet. It is believed that approximately 96% of the species in the seas were disastrously harmed or became extinct. Despite being faced with such adversity, some shark lineages persisted and survived through the Triassic, ultimately leading to a second boom of shark diversity during the Jurassic Period. 

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The Jurassic Explosion
Rise of the Six-Gill Shark.

Having once again survived a mass extinction event the Jurassic period was the time of the second great explosion in shark diversity along with the development of protruding, flexible jaws allowing for bigger prey items to be targeted and an increase in speed.  With the Jurassic period also came the Batoids which include rays and skates and whose pectoral fins would eventually develop into what we now call wings.


The world was changing rapidly during this period, which is why sharks were able to exploit emerging habitats and diversify in terms of characteristics. The climate was warm and stable, but there were major tectonic movements that formed new habitats, corals began producing reefs in warm seas, and there were many types of bony fishes, sharks and rays, providing a range of prey for shark species.  

Many of the families that emerged during the Jurassic period are still present today including the Lamniforms, commonly known as Mackerel Sharks, which include Porbeagles, Salmon Sharks and Great White Sharks.  Despite the enormous sizes some of the sharks found within this family can reach, the earliest known species Palaeocarcharias stromeri was in comparison tiny with estimates suggesting it would not have grown any longer than one metre in length.  They were slow moving and are believed to have hunted small fish in shallow waters.  With such differences in behaviour and physical appearance between the Mackerel Sharks of the Jurassic and the Mackerel Sharks of today it may seem difficult to believe they could be related but the microstructure of the teeth of Palaeocarcharias proves it. Following studies of the tooth composition it is undeniable they share the same unique teeth composition as modern day Lamniforms do. So, there we have it, even some of the most enormous, iconic sharks such as Great Whites started small!

It was not just the Lamniforms making their appearance during the Jurassic Period however, Cow Sharks, known scientifically as the Hexanchiformes family, also came onto the scene. These are the oldest of the modern shark families having been around since the early Jurassic approximately 195 million years ago.  They have been described as offering a look back into the past due to their much unchanged appearance which includes the sixth gill slit for which they get their name, an anal fin, and the location of the dorsal fin which is very close to the tail. 

Cow sharks are notoriously secretive and rarely seen by humans which is likely to be at least in part due to the depths they frequent. These can be as far down as 300 feet off the continental shelves of the Indian, Atlantic and Pacific oceans.  In the case of the Bluntnose Sixgill Shark Hexanchus griseus, it has been observed at 6,000 feet and feeds nocturnally on squid and crustaceans in shallow waters before returning to the depths as the sun begins to rise. The Cow Shark family are just another example of remarkable diversity of sharks and of course their persistence and adaptability in the face of adversity. 

Cretaceous Chaos

The infamous asteroid that marked the end of the reign of the non-avian dinosaurs also severely affected sharks. Although shark species survived the asteroid, the fossil record shows us that only small, deep water sharks survived likely making a living on fish near the sea floor. True to form however, sharks were not to be contained for long and began to evolve into far greater sizes resulting in some of the most iconic species ever to swim the Earth’s oceans. During this time, the changes to the continents continued with the North Atlantic Ocean and the South Atlantic Ocean being formed. There was also a rift between Africa and South America which created a long chain of underwater volcanic mountains, growing as the Atlantic Ocean widened. As they grew, they displaced an enormous amount of water, so much so that sea levels were approximately 100 metres higher than today.

Miocene and the Pliocene

Our final stop on our journey of course is to the Miocene and Pliocene eras. This is the time of perhaps the most famous of all the sharks Otodus megalodon. Between Megalodon’s appearance in the mid-Miocene (16 million years ago) and its extinction in the Pliocene (ending 2.6 million years ago) there were a range of large-scale environmental and climate changes taking place. Continued tectonic movements and orogenic effects eliminated some seaways and altered global patterns of wind and precipitation patterns. This, combined with the widespread cooling that began in the early Oligocene, reduced global temperatures and eventually resulted in a period of major glaciation at the poles by the late Miocene. Glaciation locked up much of the planets water supply in massive sheets of ice, dropping the sea level by 200 metres which subsequently dried up many shallow coastal areas.  

Interestingly, the classification of Megalodon is not as previously thought.  Despite popular belief some of the latest research suggests that Megalodon is not related to the modern day Great White Shark Carcharodon carcharias.  This assumption changed following morphological teeth analysis which showed Great Whites and Megalodon evolved from different lineages and revealed that it is more closely related to the extinct Otodus Obliquus which has previously evolved during the Palaeogene period.  The result of this is Megalodon now fits into family Otodontidae hence the name change to Otodus Megalodon. 

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Megalodon tooth  ©George Wykes 

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Great White shark tooth    ©George Wykes 

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Otodus Obliquus tooth    © George Wykes 

So how big was Megalodon? One of the biggest challenges that face experts is the lack of the fossilised materials available to them, the majority of remains being teeth alone due to the cartilage structure of sharks. This, coupled with sometimes inaccurate jaw reconstructions based on only using the largest teeth rather than include the smaller, lateral teeth as well, led to early estimates by Bashford Dean of the American Museum of Natural History estimating Megalodon could potentially reaching up to 98 feet in length. In his defense however, this estimation was made more than a century ago and both technology and scientific understanding of the species has moved on significantly since then. Nowadays, the most conservative estimates put Megalodon at approximately 52 feet with higher estimates ranging to 59 feet, still absolutely enormous and no doubt terrifying for anything unfortunate enough to cross its path.

The sheer size of Megalodon made it a formidable predator and enabled it to take down an abundance of large whales, dolphins, sea lions and sea turtles (which grew to three times more than modern day species) amongst many other prey items. Surely then, an animal of such size and power would be invincible? Well of course that is not the case and scientists have theories for its demise. These include competition with other predators emerging on the scene, notably Livyatan melvillei, an enormous Sperm Whale that far surpassed Megalodon and some experts have suggested it could even have preyed  upon Megalodon if the opportunity arose. It has also been suggested that climatic changes may have contributed to their eventual experience. The fossil record shows that as the ocean’s temperature cooled, so Megalodon remains grew steadily rarer, eventually disappearing altogether in the late Pliocene. A rather chilling indication of how even the mightiest of animals lack the defences to adapt to climate change, a threat that faces wildlife all around the world in the present day. 

Of course, the Miocene also marked the introduction of the earliest ancestors of the modern-day Hammerhead Sharks. And while they may have survived what Megalodon could not, ultimately leading to the diverse range of sizes we see today, they still find themselves facing challenges no marvels of evolution could prepare them for.

Current day 

Although sharks have survived five mass extinction events, it is estimated that one quarter of all extant shark species are endangered. Sharks have survived a range of threats in the past and survived, but today, their biggest threat may be us. Human induced climate change and its effects, overfishing, pollution and habitat degradation are all major threats that impact shark populations worldwide.

As living fossils, sharks have endured and even thrived throughout many major changes in climate and environment in the past 450 million years. However, the rate of human induced climate change has proved far too fast for some sharks to adapt. With increasing temperatures, the ocean is not only becoming warmer, but also acidifying at a rapid pace. It is believed that as the acidity of the water increases, shark teeth and scales could begin to corrode, which would in turn affect their ability to swim, hunt and feed. 

The indirect effects of climate change are also affecting species today, such as the effect warmer ocean temperatures are having on both sharks and their prey species. Sharks prey on a range of species, however, with fast changing climatic factors, species are dispersing from areas they used to inhabit, and some populations are depleting and unable to reproduce at the necessary rate to survive, limiting the amount of food available.

Another major threat to prey availability is overfishing, but humans are not only overfishing prey species. Shark species themselves are caught in large quantities for either for meat or fins, mostly open-water species such as basking, whale and great white sharks. Deep sea sharks are also at risk, targeted for their liver oil that is used as a moisturiser in the cosmetics industry. Deep sea sharks have evolved to succeed in their environment; however, their slow reproduction rate places them in the high-risk category – if they are killed, they don’t have the ability to reproduce offspring quicker than they are dying.

With depleting population numbers, rates of reproduction of sharks are more important than ever. However, humans have a way of impacting that too. With increasing land use change for agriculture and urban development, there have been major changes in shallow marine areas with destruction of shark breeding, birthing and nursery grounds. These areas are already considered to be the most vulnerable to climate change as they will likely see the biggest fluctuations and extremes in temperature, salinity and ocean currents, but with the added impacts of habitat destruction, as well as an increase in pollution in these areas from run-off, these vital areas are being decimated at alarming rates.

However, not all hope is lost. There is an increasing list of countries that are banning shark finning, Colombia announcing just this year that they will be curbing their part in the international shark fin trade and making efforts to protect their shark species. Mozambique has also banned commercial fishing of several endangered marine animals including whale sharks, manta rays and all Mobula species this year.

 

Unfortunately, many species of shark are migratory, meaning there needs to be a high level of cooperation between multiple countries to ensure their protection. Governments worldwide need to enforce common laws and catch limits if we are to truly protect our oceans top predators.

 

As much as this is a global governmental issue, there are things that the individual can do to help protect our shark species. We can all become more conscious consumers, not consuming shark meat and checking our products do not contain shark materials such as squalene. We can also reduce the amount of seafood we consume, which would reduce the demand and therefore reduce overfishing and bycatch. We can also be thoughtful with how we reduce, reuse and recycle rubbish, particularly plastics and single-use plastics. And lastly, and perhaps most importantly, we can sign petitions, lobby, fight and advocate for the safety and preservation of our sharks and our environment. 

Written by Stephanie Wilmott & George Wykes

Earth

Timeline of Earth

4.5 Billion years ago Earth is formed.

4.5- 2.5 Billion years ago- Archean

 2.5 Billion – 541 Million years ago - Proterozoic 

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Paleozoic Era 

Cambrian 541-485 Million years ago

Ordovician 485-443 Million years ago

Silurian 443-419 Million years ago

Devonian 419-359 Million years ago

Mississippian 359-323 Million years ago

Pennsylvanian 323 -299 Million years ago

Permian 299-252 Million years ago

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Mesozoic

Triassic 252-201 Million years ago

Jurassic 201- 145 Million years ago

Cretaceous 145-66 Million years ago

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Cenozoic

Paleocene 66-56 Million years ago

Eocene 56-33.9 Million years ago

Oligocene 33.9-23 Million years ago

Miocene 23-5.3 Million years ago

Pliocene 5.3-2.6 Million years ago

Pleistocene 2.6 Million years ago- 11, 700 thousand years ago.

Holocene 11,700 years – present day.