Prehistoric sharks, the kings of the sea

There is a widespread belief that sharks are living fossils and that, therefore, their body design has not changed significantly in the last hundreds of millions of years. However, such a notion is completely wrong. Prehistoric sharks developed very different teeth, sizes, shapes and ornamentations that rival in originality. The Stethacanthus, for example, had a mass on their heads that is very similar to a brush. 

For their part, the primitive xenacanthus, which resembled eels, had a long and distinctive spike. The latter appeared more than 360 million years ago and lived almost exclusively in freshwater courses. They had what is known as the terminal mouth - which simply means that the mouth is located in the middle or center of the head - a thin, elongated dorsal fin that stretched along most of the back and a symmetrical, tapered tail. This strange anatomical configuration, rather narrow, could allow them to swim amongst the vegetation that grows on the banks of lakes.

In any case, one of the most striking features of this genus was the aforementioned spine, which started just behind the skull and pointed backwards. Its name is a derivative of this spine, which could be translated as external thorn. Perhaps it was a deterrent against larger predators, although some experts argue that it could have been a poisonous structure, similar to a stingray.

Fossil remains of Xenacanthus have been found in America and Europe, suggesting that it was a widely distributed animal. It was a robust shark, which survived the mass extinction that took place at the end of the Permian period, where 95% of marine species perished. The last examples were seen in what is now India, 206 million years ago. 

The Xenacanthus were living at the same time as other certainly atypical fish, such as those of the genus Helicoprion, which, although they were not exactly sharks, they resembled them. The distribution of the teeth of their lower jaws, which were organized in an astonishing spiral, has baffled scientists for decades. But if there are any strange species, it would be the symmoriformes. The members of this extinct class of chondrichthyes - in these animals, each tooth had a sharp central tip flanked on both sides by small cusps - had relatively short skulls, large eyes, a terminal mouth and an externally symmetrical caudal fin. In the case of males, moreover, the anterior dorsal fin had become a curious and intriguing platform.

This was present in the Stethacanthus. These cartilaginous fish lived in the warm and shallow seas of the late Devonian period, 360 million years ago. Curiously, all known specimens of this genus possess testes, the reproductive organs for males. In all of them, the aforementioned dorsal fin ends up on a flat surface and bristled with large dermal denticles, tiny and sharp protuberances. The top of their heads hold another set of denticles that are directed in the backward direction. 

These unusual characteristics have fascinated paleo-ethnologists, who have proposed different hypotheses about their possible function. The late paleontologist Rainer Zangerl, of the Field Museum of Natural History in Chicago, was convinced that it had a deterrent use. Seen from above, the aforementioned denticles appear to be the open mouth of a much larger fish, the Stethacanthus measuring just over 60 cm². Predators wouldn’t dare to mess with a creature with such jaws.

It has also been proposed that, by stretching the neck and bowing the back, these animals could be attached to the belly of a larger marine animals. They would have been a sort of freeloader hitchhikers, more or less like the current remoras, who use their "sucker disc", which is also a modified dorsal fin to cling to whales, sea turtles, sharks…

Another possibility is that the mysterious brush played some role in the courtship rituals. Maybe it was an element of sexual attraction, and the bigger it was, the more males would appear attractive, or perhaps it had a more aggressive role: they could have used it to fight others with a shove, head-to-head with which they would dispute access to mating areas or sexually receptive female wings.

The genus Symmorium is contemporary and very similar to the “brush” shark. However, its dorsal fin looks normal and is not at all like that of the Stethacanthus. It is also important to note that all found specimens of Symmorium are female. As a result, some researchers suspect that they are in fact Stethacanthus females. If so, the sexual dimorphism among the Symmoriformes sharks would have been greatly accentuated.

The aspect of the prehistoric Lamniformes would not have caught our attention so much. They looked very similar to their relatives who still roam the oceans, like bull sharks and porpoises. They used to have parabolic shaped mouths, with great opening capacity, located under the head. In general, theirs jaws contained many solid and strong teeth. In fact, most of the ancient Lamniformes are known from isolated teeth.

During the Late Cretaceous period, a little over 90 million years ago, the species Cretoxyrhina appeared, whose members could have rivaled the modern white sharks in size, shape and habitat. Unlike the latter, which have serrated teeth, those of the Cretoxyrhina were smooth, and five centimetres long. Paleontologist Michael Everhart, one of the greatest experts on marine fauna in the past, discovered fragments of two of them buried in a set of five vertebrae of an adult mosasaurus. Just as the white shark feeds on seals, the Cretoxyrhina hunted these and other marine reptiles, such as plesiosaurs, as befits a predator at the top of the food chain. Yet the two species are not closely related.

Its apparent similarity with the white shark is the result of convergent evolution, that is, the appearance of independently similar characteristics in species belonging to different lineages. Differences can be seen, for example, in speed, cretoxyrhina were much faster. Humberto Ferrón, a researcher at the Cavanilles Institute of Biodiversity and Evolutionary Biology of the University of Valencia, calculated that they usually moved at 12 kilometres/hour and could reach a peak speed of about 70 kilometres/hour. In comparison, great whites travel at eight kilometres/hour and can reach 43 kilometres/hour at any given time. The short-fin mako shark is the only modern-day shark that could compete with the Cretoxyrhina in a race.

Long after the last of them disappeared, Squalicorax sharks could still be found in oceans, opportunistic scavengers that lasted until the Miocene period, about seven million years ago. Their teeth are similar to those of the current tiger shark; they have a rectangular root from which a blade emerges, finely sawn, finishing in an inclined cusp.

Palaeologist Kenshu Shimada of DePaul University in Chicago, Illinois points out that its diet was very varied, like that of the tiger, suggesting that it was a formidable predator. However, it was also an active corpse-eater. During its many years of study, Everhart has observed that the Squalicorax took advantage of every available food source, whether it be live animals or remains of fish, turtles, mosasaurs, pterosaurs and even other sharks.

However, amongst all the extinct sharks, the most fascinating is undoubtedly the megalodon. This huge carnivore, which could exceed 15 metres in length, had similar teeth to those of a great white shark, however some could be as long as 18 centimetres! Additionally, they are thicker and the tiny folds that give them their distinctive appearance in the form of a saw are smaller and are arranged more evenly. These subtle differences provoke a heated debate among specialists about the origin and evolution of the megalodon. Some include the megalodon in the genus Carcharodon, the same to which the whites belong, and in the family Lamnidae. Others indicate that the similarities between the two are only apparent and that the first should be included in the genus Carcharocles and in the family Otodontidae, which separated from the lineage of the white shark during the lower Cretaceous period. Most seem to agree on this last point.

The fossil record indicates that it was a cosmopolitan species. It probably fed on large prey, such as whales and sea turtles. Unlike the white shark, which attacks its victims from below and bites them in the ventral region, which is softer, the megalodon could use its strong jaws to traverse the hard chest cavity, as if it were butter, and pierce the heart.

Its extinction is also controversial. Until recently, it was thought to have disappeared 1.8 million years ago, as a result of a cooling of the oceans that would have killed the cetacean species it ate. However, the latest studies show that its extinction occurred before then; the megalodon fossils examined in the North Pacific deposits indicate that it all ended for them between 3.6 and 4 million years ago.

Sharks, like all other living things, have had to deal with drastic environmental changes and the extinctions that they often cause throughout their evolutionary history. Large-scale deaths leave many ecological niches empty and available to new species. Each time their environment underwent a radical change, sharks responded with a spectacular flowering of diversity, allowing them to expand and occupy a good number of them.

These explosive increases of variety are called adaptive radiations. The first example of this is found in the events at the end of the Devonian period. The seas of this period were populated by primitive armoured fish called placoderms. 372 million years ago, there was a massive extinction that significantly reduced the diversity and abundance of these animals. The ecological niches vacated by the placoderms would end up being exploited by sharks for the next millions of years.

The first major adaptive radiation occurred during the Carboniferous period, between 299 and 359 million years ago. The freshwater lakes were the paleohabitat of the aforementioned xenacanthus, whose lineage extended until a little more than 200 million years ago; meanwhile, in the seas the mentioned Symmoriformes prospered.

In this period of the Paleozoic era, it was the golden age of the sharks, when they came to outnumber even bony fish, in a ratio of three to two. However, the good times ended at the end of the Permian period, some 252 million years ago, when the so-called Great Death or mass extinction of the Permian-Triassic (PT) occurred. In a geological instant, and for reasons not yet known with certainty, more than 70% of terrestrial vertebrates and virtually all marine species disappeared. Even so, some shark lineages overcame the catastrophe.

The second radiation occurred during the Jurassic period, between 145 and 201 million years ago. In the oceans of the time, the ichthyosaur marine reptiles whose shapes resemble those of the dolphins, plesiosaurs and mesosuchia, animals not very different from crocodiles, chased schools of bony fishes and fleets of ammonites cephalopod molluscs. In this environment and moment modern sharks appeared, although it is not known for sure from which group of ancient sharks they evolved. Palaeologist John G. Maisey of the American Museum of Natural History suspects that the fossil genus Synechodus could play a prominent role in this mystery.

In any case, at the end of the Cretaceous period, 66 million years ago, yet another global crisis wiped out much of the marine fauna. In it the ammonites, plesiosaurs, mosasaurus were lost... However, once again, the sharks survived. The new sharks ended up becoming the main oceanic predators and amongst them, as we know, the megalodon stood out.

According to Catalina Pimiento of the Department of Biosciences of the University of Swansea, United Kingdom, its appearance 23 million years ago coincided with certain environmental changes that favored the rapid evolution of a large number of new marine mammals, of which it fed on. Such abundance made the megalodon a giant from its very origin and its size did not change over time. At that time there were other remarkable sharks, the ancestors of the present great whites swimming in the same seas, but none were so imposing.

It could be said that, from an evolutionary point of view, sharks represent one of the most enduring success stories. Although their cartilaginous skeletons have few hard parts susceptible to fossilization, they have provided scientists with a very rich fossil record. So far, between 2,000 and 3,000 paleospecies of these fascinating animals have been described.

Far from being primitive creatures, modern sharks represent an exceptional group of marine vertebrates who, despite everything, have managed to make the planet a permanent home for them.