Shark jaws show little change in shape over millions of years, study finds

A team of international researchers, led by Faviel A. López-Romero from the University of Vienna’s Department of Paleontology, has delved into the evolution of shark jaw shape. Their findings, published in the journal Communications Biology, shed light on the minimal variation in jaw shape among the most widespread shark species, while revealing that deep-sea sharks exhibit the most diverse jaws.

Sharks are renowned for their distinctive lower jaws, adorned with formidable teeth. These jaws enable sharks to prey upon a wide range of animals, establishing them as apex predators in the ocean. This broad prey spectrum is reflected in the evolutionary adaptations that sharks have developed over millions of years. These adaptations have allowed them to thrive in diverse marine habitats, with certain species even venturing into freshwater environments.

The international research team, consisting of scientists from the University of Vienna, Imperial College London (UK), Muséum national d’histoire naturelle (Paris, France), Christian-Albrechts-University (Kiel, Germany), and Naturalis Museum (Leiden, The Netherlands), investigated how the shape of shark jaws has changed throughout evolution. Their results highlight the influence of prey, trophic levels, and habitat on the diversity of jaw shapes among shark species. These findings help unravel the evolutionary factors driving variations in jaw morphology linked to specific habitats.

Sharks have a long evolutionary lineage, with some taxa dating back as far as 180 million years. Throughout this extensive history, sharks have played a crucial role in marine ecosystems, occupying high trophic positions as meso- and top predators. Moreover, sharks have adopted diverse lifestyles and forms, ranging from bottom-dwelling species to fast-swimming open-ocean hunters, and even some of the smallest inhabitants of the deep sea.

To explore the potential relationship between jaw morphology and the lifestyle of sharks, the researchers conducted a quantitative analysis using X-ray computed tomography scans of the jaws from 90 shark species. They then created 3D reconstructions to estimate how jaw shape has evolved over time.

Surprisingly, the results revealed low shape variations among the jaws of highly diverse groups, such as requiem sharks, despite their wide distribution. Conversely, the most variable jaws were found among deep-sea species. López-Romero, the first author of the study, explains, “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.”

Deep-sea sharks exhibit various feeding strategies, including bioluminescence, as well as predation on large prey such as whales, eggs, or cephalopods. On the other hand, sharks found in reefs and the open ocean’s top predators have more limited options, primarily preying on fish, and occasionally other shark species.

“While many sharks in these environments feed on a wide variety of prey, only a few have adapted to a single specific prey, such as the bonnethead shark, Sphyrna tiburo, which primarily preys on hard-shelled crabs, occasionally capturing shrimps and fish,” adds Jürgen Kriwet from the University of Vienna, another researcher involved in the study.

Studying the evolution of jaw shape also enabled the researchers to reconstruct the changes that have occurred over deep time. López-Romero concludes, “Remarkable changes occurred in carpet, sleeper, and dogfish sharks. These changes were likely associated with the clear distribution of these sharks in reefs and the deep sea, which significantly sets them apart morphologically from other species, including the larger-jawed top predators in the open ocean.”

Source: University of Vienna

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