Jaws of most shark species show little variation over millions of years

Sharks, the ocean’s top predators, have held our fascination for centuries. Their impressive teeth and lower jaws, which allow them to devour a wide variety of prey, are among their most distinctive traits. But how has the jaw shape of these fascinating creatures changed over their long evolutionary journey? 

An international team of researchers, spearheaded by Faviel A. López-Romero of the University of Vienna, has dived into this question, revealing some intriguing findings about these marine predators.

The study, recently published in the renowned journal Communications Biology, highlights that the jaw shape of the most widespread shark species has shown surprisingly little variation over millions of years. Intriguingly, the most significant jaw diversity was observed among deep-sea sharks.

Sharks’ ability to feast on a diverse array of prey is evident in the corresponding adaptations they have acquired over the course of their evolution. These evolutionary advancements have enabled them to colonize almost all marine habitats, with some species even daring to venture into freshwater.

An international, multidisciplinary team of scientists embarked on this research journey. Alongside López-Romero from the University of Vienna, scholars from Imperial College London, Muséum national d’histoire naturelle in Paris, Christian-Albrechts-University in Kiel, and the Naturalis Museum in Leiden also participated.

The research findings underscore the significance of prey, position in the marine food webs, and habitat in relation to the diversity of jaw shapes among shark species. This crucial insight helps decipher the evolutionary triggers behind the differences in jaw morphology associated with various habitats.

First evidence of sharks dates back to 180 million years ago

With an evolutionary lineage extending back to nearly 180 million years, today’s sharks have consistently been key components in the marine realm and its food chains, primarily occupying the roles of meso and top predators. Over these vast spans of time, sharks have adopted a plethora of lifestyles and forms, including bottom dwellers, swift swimmers in the open sea, and even small species inhabiting the deep sea.

The research team conducted a detailed quantitative analysis to examine the possible correlation between jaw morphology and sharks’ lifestyle. They used X-ray computed tomographic scans of the jaws of 90 shark species, preparing 3D reconstructions to estimate the evolution of shark jaw shape over time.

What the researchers found

One of the surprising results was that species-rich groups like requiem sharks displayed low variation in jaw shape, despite being one of the most widely distributed shark groups. Conversely, the most variability in jaw shape was found among deep-sea dwelling species.

“Although sharks from the deep sea are not as extensively represented in the data as reef sharks, they display the most disparate forms seen in our analysis,” explained López-Romero, the study’s first author from the Department of Palaeontology at the University of Vienna.

Deep-sea sharks showcase an array of adaptations, including bioluminescence and a wide variety of feeding strategies, ranging from large bites out of whales to a diet consisting of eggs or cephalopods. Meanwhile, the feeding choices of reef-dwelling species and large top predators in the open sea seem more restricted, mainly preying on fish and even other shark species.

“Of course, many sharks in these environments feed on a large variety of prey with only a few having adapted to a single, specific prey, such as the bonnethead shark, Sphyrna tiburo, which preys almost entirely on hard-shelled crabs, while shrimps and fish are only captured occasionally,” noted Jürgen Kriwet from the University of Vienna.

The evolution of jaw shape also allowed the team to reconstruct the historical changes in jaw morphology over time. 

“Remarkable changes occurred in carpet, sleeper, and dogfish sharks. These changes were probably concomitant with the clear distribution of these sharks in reefs and the deep sea, which noticeably distinguishes them morphologically from other species with larger jaws as seen in the top predators in the open sea,” said López-Romero.

Sharks living in the deep showed most variation

Deep-sea sharks, despite their underrepresentation in the data, presented the most diverse jaw shapes, noted López-Romero. Such diversity likely reflects the diverse feeding strategies of these creatures, which span from chomping down on whales to dining on eggs or cephalopods. As such, their jaws have evolved to be incredibly versatile and adapt to their unique environmental needs.

By contrast, the choices of most reef-dwelling species and the large top predators in the open sea are more limited. These predators mainly feed on fish and occasionally other shark species, restricting the evolution of their jaw morphology.

However, this doesn’t mean that all sharks in these environments have limited diets. “Many sharks in these environments feed on a wide variety of prey, with only a few having adapted to a single, specific prey,” explained Kriwet.

He cited the bonnethead shark, Sphyrna tiburo, as an example, which preys almost exclusively on hard-shelled crabs, with shrimps and fish only occasionally captured.

The research team’s study of the evolution of shark jaw shape provided an opportunity to reconstruct the historical changes in this crucial morphological feature. López-Romero noted that “remarkable changes occurred in carpet, sleeper, and dogfish sharks.” 

He suggested that these changes likely coincided with the clear distribution of these sharks in reefs and the deep sea, distinguishing them from other species with larger jaws found among the top predators in the open sea.

The findings provide an insightful snapshot of shark evolution, illuminating the intriguing link between jaw morphology, habitat, and prey diversity. As scientists continue to delve into the evolutionary history of these captivating creatures, each discovery brings us one step closer to understanding their long-standing dominance in the world’s oceans.

More about sharks

Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head.

They are a diverse group with over 500 known species ranging from the small dwarf lanternshark, a species of only 17 centimeters in length, to the whale shark, the largest fish, which can grow up to 12 meters in length.

Here are some key points about sharks:

Evolution and Classification

Sharks have a long evolutionary history dating back to at least 420 million years. They are divided into eight orders, which include the well-known great white shark, tiger shark, and hammerhead shark, among others.

Anatomy and Physiology

Sharks have a streamlined, torpedo-shaped body that enables them to swim swiftly through the water. They have a cartilaginous skeleton, which is lighter and more flexible than bone. They are equipped with multiple rows of teeth, and when they lose a tooth, another one slides forward to replace it.

Sharks have several adaptations for sensing their environment. They possess a network of special cells that can detect electricity (called the ampullae of Lorenzini) which they use to detect the electrical fields that all living things produce. This helps them locate prey. Sharks also have an extraordinary sense of smell.

Diet and Feeding

Most sharks are carnivorous. They can be top predators, feeding on smaller fish and invertebrates, or they can be filter feeders, like the whale shark and basking shark, which sieve tiny plankton from the water.

Reproduction

Sharks have diverse modes of reproduction. Some species lay eggs (oviparity), while others give live birth. In the latter group, some have a placenta-like organ (viviparity), while others do not and the young feed on yolk or other siblings in the womb (ovoviviparity).

Behavior

Sharks exhibit a wide range of behaviors. Some are solitary, while others, like the scalloped hammerhead, can be found in large schools. Some species migrate over great distances to feed or reproduce.

Conservation

Many shark species are threatened by human activities. Overfishing, both targeted and incidental, is the primary threat to sharks, with millions killed annually for their meat, fins, and liver oil. Habitat degradation and climate change also pose significant threats. As a result, many species are now listed as endangered or critically endangered.

Sharks and Humans

While sharks are often portrayed as dangerous to humans, in reality, shark attacks are rare and typically the result of mistaken identity. Sharks play a crucial role in the ecosystem by maintaining the species below them in the food chain and serving as an indicator for ocean health. They help remove the weak and the sick, and maintain balance with competitors, ensuring species diversity.

As of my knowledge cutoff in September 2021, ongoing research continues to provide new insights into the biology, behavior, and ecology of sharks. They remain a fascinating group of animals that are critical to the health of the world’s oceans.

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