The fascinating evolution of sharks throughout history is truly captivating. Transporting ourselves back to the extraordinary prehistoric sharks of the Ordovician period, approximately 420 million years ago, one would never have envisioned that their descendants would rise to be the dominant creatures we know today. Holding the title of one of Earth’s oldest inhabitants, sharks made their initial appearance an impressive 455 million years ago, long preceding the existence of dinosaurs, insects, mammals, and even trees. Over time, these remarkable fish bravely faced formidable marine reptiles like pliosaurs and mosasaurs, engaging in fierce battles for survival.
The First Shark-Like Fish
Sharks, as a subgroup of fish, possess unique characteristics that set them apart. Their skeletons are composed of cartilage instead of bone, granting them a distinctive feature. Along with their streamlined and hydrodynamic body shape, they boast sharp teeth and sandpaper-like skin, distinguishing them from other fish.
Unearthing fossil evidence of ancient fish closely related to sharks, we find scales dating back to the Late Ordovician period, approximately 455 million years ago. These fish, known as Chimaera, shared similarities with sharks in being cartilaginous and scaled. However, they lacked the characteristic sharp teeth that sharks possess. Through extensive analysis of present-day sharks, rays, and living chimaeras, scientists have determined that sharks diverged from the rest of the group around 420 million years ago. This conclusion is supported by the absence of chimaera fossils from this specific period. The evidence for this separation is further strengthened by DNA and molecular data obtained from modern sharks and chimaeras thriving in the depths of the sea. Remarkably, this era, around 420 million years ago, also marked the emergence of the first plants from the Earth’s surface.
First Shark Tooth
The earliest discovery of teeth resembling those found in sharks dates back to the Early Devonian period, approximately 410 million years ago. These teeth were unearthed from a fossil of an ancient fish named Doliodus problematicus. Often referred to as the ‘least shark-like shark,’ Doliodus is believed to have originated from a group of fish known as acanthodians, or spiny fins.
Acanthodians, unlike sharks, had a distinct appearance with diamond-shaped scales and spines positioned on the leading edges of all their fins. Despite this dissimilarity, they shared certain shark-like characteristics, including a cartilaginous skeleton, a skull resembling that of a shark, and a jaw structure akin to sharks. Notably, several shark-like teeth found in Doliodus were frequently fused together, adding to its intriguing combination of features that set it apart from traditional sharks.
Apart from some fossilized scales, there is not much in the way of direct evidence for the first sharks. But the first true sharks are believed to have evolved around 420 million years ago during the Ordovician period. (The first tetrapods, creatures with four limbs, did not emerge from the sea until 400 million years ago.).
The most important genus for which large fossil evidence has been found is Cladoselache, considered the first shark, with many specimens found in the American Midwest. As we would expect from such an ancient shark, Cladoselache was quite small and had some shark-like features. It had no scales (except for small areas around its mouth and eyes) and lacked a clasper (specialized mating organ). It disappeared 250 million years ago.
Following Cladoselache, three significant prehistoric sharks emerged in ancient times: Stethacanthus, Orthacanthus, and Xenacanthus. Stethacanthus, which appeared around 370 to 345 million years ago, measured a mere 1.8 meters from snout to tail but already possessed all the defining characteristics of sharks. It boasted scales, sharp teeth, a prominent wing structure, and a sleek, hydrodynamic body form, making it a remarkable predator even at this early stage of shark evolution.
What truly set this genus apart were the peculiar anvil-like structures found on the backs of male individuals. These structures, believed to have been used during mating, added an intriguing aspect to the behavior and reproductive strategies of Stethacanthus. This unique feature highlights the diversity and complexity of prehistoric sharks and provides valuable insight into their evolutionary history.
However, the aftermath of this extinction proved to be a turning point for sharks. They swiftly capitalized on the available ecological niches, and their evolution accelerated, enabling them to rise as the dominant species in the oceans. This rapid diversification gave birth to extraordinary shark species, with Stethacanthus being one of the most remarkable examples.
During this flourishing period, the Carboniferous sharks displayed their incredible adaptability and resilience, solidifying their position as formidable predators in marine ecosystems. Their success during this era played a pivotal role in shaping the course of life on Earth, establishing sharks as enduring and influential creatures throughout geological history.
Orthacanthus, like Stethacanthus, was an equally ancient shark that thrived in freshwater environments. Despite its small size, it possessed a unique combination of features. Resembling the body shape of an eel, Orthacanthus stood out with a peculiar spine protruding from the top of its head. This distinctive feature served a dual purpose: it acted as a deterrent to annoy or deter potential predators, and it also had the ability to secrete venom, adding to its defensive capabilities.
The similarities between Orthacanthus and Xenacanthus are noteworthy too. Both these ancient sharks shared certain characteristics, possibly indicating some common ancestry or evolutionary traits. Their adaptations to freshwater habitats showcase the adaptability and versatility of early sharks, allowing them to thrive in various ecological settings during the Carboniferous period. The presence of such diverse and intriguing species during this era contributed significantly to the richness of the marine ecosystem and the overall evolutionary history of sharks.
Sharks of the Mesozoic Era
During a significant portion of the Mesozoic era, spanning from 251 to 65 million years ago, sharks lived in relative obscurity, largely overshadowed by the dominance of marine reptiles like ichthyosaurs and plesiosaurs. The intense competition from these reptilian predators contributed to sharks’ somewhat subdued presence during this epoch.
However, amidst this competition, one genus of prehistoric shark stood out as exceptionally successful—Hybodus. This remarkable shark emerged around 260 million years ago and persevered until 65 million years ago, showcasing incredible longevity in the evolutionary timeline. Hybodus was truly built for survival, equipped with a variety of adaptive features.
Hybodus boasted two distinct types of teeth—sharp ones for efficiently consuming fish and flat teeth for crushing molluscs. This dental versatility allowed it to capitalize on various food sources, ensuring its adaptability and resourcefulness. Additionally, for self-defense against other predators, Hybodus had a sharp, knife-like structure protruding from its dorsal fin, enhancing its chances of survival in an unforgiving prehistoric world.
What contributed significantly to Hybodus’ success and long-lasting presence was its cartilaginous skeleton, which possessed an unusual hardness and calcareous composition. This robust skeletal structure not only facilitated its thriving existence in the world’s oceans but also played a key role in its frequent appearance in the fossil record. Consequently, Hybodus left a notable mark in both geological history and the evolutionary journey of sharks, with its lineage stretching from the Triassic to the early Cretaceous periods.
The First Modern Sharks
Around 100 million years ago, during the Middle Cretaceous Period, a pivotal moment in shark evolution occurred with the appearance of the first species that closely resemble modern sharks. Among these early representatives were Cretoxyrhina, measuring approximately 8 meters (26 ft) in length, and Squalicorax, with a size of about 4.8 metres (16 ft). Both of these ancient sharks exhibited striking similarities to the “real” sharks we encounter today.
Cretoxyrhina and Squalicorax showcased key characteristics that align with modern shark features, indicating a significant step forward in shark evolution. Their resemblance to contemporary sharks indicates that they have already developed the fundamental adaptations that make sharks formidable marine predators. This period marked an important turning point, as these sharks paved the way for the emergence of a diverse array of modern shark species that have continued to thrive and adapt throughout the ages.
The existence of direct toothprint evidence linking Squalicorax to the hunting of dinosaurs that ventured into its habitat is truly fascinating. It demonstrates the impressive adaptability and predatory capabilities of these ancient sharks during the Cretaceous period, around 100 million years ago.
However, the real showstopper from this era is the recently discovered Ptychodus. This formidable shark lived between 112 and 70 million years ago and left a lasting impression with its sheer size and unique dental adaptations. Measuring around 9 meters in length, Ptychodus was an absolute monster of the Cretaceous oceans.
What set Ptychodus apart were its numerous flat teeth, which were perfectly suited for grinding small molluscs rather than hunting large fish or aquatic reptiles like some of its contemporaries. This specialization in feeding allowed Ptychodus to exploit a particular ecological niche, thriving as a dominant predator of mollusks during its time.
After the extinction of dinosaurs and their aquatic relatives around 65 million years ago, prehistoric sharks gradually evolved into the formidable and efficient killing machines that we recognize today. Fossil evidence from the Miocene period, spanning from 23 to 5.3 million years ago, is predominantly composed of shark teeth. One notable example is Otodus, which lived around 60 to 40 million years ago and was approximately the size of a modern great white shark. However, most of what we know about Otodus comes from its teeth, and paleontologists have reconstructed the body of this massive 30-meter-long shark based on these dental remains.
Among the most renowned prehistoric sharks of the Cenozoic era is Megalodon, which lived between 23 and 3.6 million years ago. Adults of this species measured around 20 meters in length from head to tail and weighed a staggering 50 tons. Megalodon holds the title of being the largest predator to have roamed the world’s oceans, and its diet was incredibly diverse, encompassing prey such as whales, dolphins, seals, giant fish, and even colossal squids. Some evidence suggests that it might have even hunted Leviathan, a whale species of comparable size to Megalodon itself.
Despite its reign as the ocean’s apex predator, Megalodon mysteriously became extinct approximately 2.6 million years ago. The reasons for its extinction remain uncertain, but climate change and the loss of its prey species are among the most plausible explanations. Regardless of its disappearance, Megalodon’s legacy endures, leaving us in awe of the remarkable diversity and scale of life that once inhabited our planet’s ancient seas.