Fossils show giant sharks evolved far earlier than expected

Long before modern great whites prowled the seas, a colossal shark cruised the warm shallows off northern Australia. Its story begins on a rocky shoreline near Darwin, where a few heavy disks of fossilized cartilage turned up in weathered stone.

These vertebrae look simple at first glance, but their size and shape tell you that they belonged to a massive early lamniform shark that lived about 120 million years ago. The discovery forces you to rethink how quickly sharks evolved giant bodies and how early they claimed the top spot in marine food chains.

For many years, scientists believed that sharks from the lamniform evolutionary tree were only becoming large toward the end of the Cretaceous. Before this time, any previous lamniforms had been believed not to exceed moderate body sizes and lived in shallow coastal settings.

The Australian fossils push this timeline considerably back, indicating that gigantic predatory sharks were already patrolling shallower nearshore seas by the late Aptian, well in advance of the eventual emergence of other giants later in the Late Cretaceous, such as Cretoxyrhina or the much younger megatooth sharks.

Contemporary lamniforms vary greatly in size from small sand-tiger sharks to massive filter-feeders such as basking sharks. They are found in environments ranging from the tide line to over 3,000 feet below the surface. The fossil record suggests there were similar giants in the past, but due to the quick decay of cartilage, almost complete skeletons are sparse.

Although teeth are frequently reported in the fossil record, vertebrae that are large enough to provide an approximation of the true body size are far less common. This is what makes the Darwin fossils so special. The vertebrae were discovered in the Darwin Formation located at Casuarina Beach.

That rock unit accumulated on a broad marine shelf along the southern margin of the ancient Tethys Ocean—a body of water that preceded the modern South Indian Ocean. Microfossils and fragments of belemnites indicate that it is of upper Aptian age, approximately 120 million years ago, during a time when this portion of Australia was located near 49 degrees south.

This site has yielded a diverse array of marine reptiles, early bony fish, and various sharks. Among these finds were five large vertebral centra measuring between 114 and 126 millimeters wide, together exhibiting a shape very closely associated with an extinct group of sharks known as the cardabiodontids, which were common, large-bodied apex predators in the warm seas during the mid-Cretaceous. These vertebrae are therefore milder in age than previously known cardabiodontid sharks, which were already large-bodied by the geologic time.

The central feature is thick calcified rims, circular faces, and fine concentric ridges. These characteristics are unique enough for researchers to assign them to the lineage of the cardabiodontids. Given that the largest vertebrae of sharks are located in the middle of the trunk, these vertebrae are likely situated in that locality. This means that estimates of the length of the animal will be conservative; the actual animal may be longer.

In estimating the body length and weight of the shark, researchers utilized fossil vertebrae measurements to make comparisons with data available from modern sharks. Researchers used a dataset with 1,912 total individuals from 10 different living lamniform species (for example, sand tigers, threshers, and great whites). A particularly important segment comes with a set of 111 well-measured great whites based in South Africa, for which scholars had vertebral diameters, total length, and body mass measurements.

After plotting the centrum diameter versus total length for the great whites, the relationship showed a positive and deterministic correlation between vertebral diameter and total body length, except that the margin of error was under 1%. Meaning, an apparent vertebra would correspond to a longer and heavier shark, with little uncertainty. Consequently, the scaling relations allowed the research team to use great white models as a proxy for the fossil species, regarding the similar overall body plan and ecological role as a top predator.

In this way, the largest Darwin vertebra, with a maximum diameter of 12.6 cm, would correspond to a shark that was about 7.9 m long, with a plausible range of about 7.2 to 8.6 m, and a weight of around 3.3 metric tons. Even the smallest vertebrae recorded would imply a shark that was over 7.0 m in length and potentially well over 2.6 metric tons in weight.

Similarly, other comparative models gave an overall wide range of possibilities. If one used thresher shark data, for example, the total length increased to above 16.0 meters. The estimation changed to about 4.5 meters if crocodile shark data were used. These upper and lower bounds illustrate how different body types could alter the estimates. However, when these researchers restricted their analyses to species with body shapes more similar to the fossil shark, the estimates returned to the 7-8 meter range, suggesting that, in this regard, the great white model was the most realistic.

The ancient oceans of Australia supported large plesiosaurs, fast ichthyosaurs, and ponderous bony fish. The new shark was undoubtedly a part of this circle of powerful predators. It likely preyed on fish and marine reptiles, ruling the nearshore waters of the southern Tethys while the large pliosaurs ruled almost exclusively in the deeper inland basins. Considering these two lineages of marine reptiles existed but were separate from one another likely suggests some division of habitats, with sharks primarily dominant in coastal zones and pliosaurs thriving in the deeper interior seas.

The time of this shark’s existence is also telling. It provides additional evidence that giant body sizes in lamniforms were established millions of years earlier than previously thought.

Furthermore, it fits with the growing body of work that provides evidence of warm-blooded features and the development of new feeding adaptations in sharks during the Early Cretaceous. The Darwin oceans were cooler than many of the tropical areas at that time. This would favor large, partially warm-blooded predators actively hunting in cold water and capable of covering large distances in search of prey.

The study relied on meticulous measurements, statistical modelling, and a transparent analytical process. Every individual vertebra was measured to the nearest half millimeter, and all modern shark metrics were standardized.

All shark data, measurements, and analyses were transformed to stabilize variance and inspected using multiple models to locate the best fit. The final analysis was shared through a reproducible workflow built using R and Docker that allowed other researchers to reproduce every result.

The study published in Communications Biology involved a collaborative international team of paleontologists, ichthyologists, and imaging specialists from the United States, Sweden, and Australia. Fossils from the Age of Dinosaurs, including the associated shark material, remain on exhibit at the Swedish Museum of Natural History.

Research findings are available online in the journal Communications Biology.

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