There were sharks before humans and early primates, before dinosaurs, and even before trees. Sharks have been around for over 400 million years (though exactly how long is contested). They have survived five major mass extinctions.
But the sharks of yesteryear are not the same as we see today. In fact, we still understand very little about their long-term development. Our research, published today in the journal Current Biology, shows how shark evolution over the past 83 million years has been driven by dietary preference and climate change – leading to the diversity we see today.
As it turns out, it’s a risky game for sharks to be picky about their prey to play.
when the scales are tipped
One of the more peculiar patterns in biology is the very different number of species for very closely related orders of living animals. A notable example is the difference in species numbers between mackerel sharks (Lamniformes order) and ground sharks (Carchariniformes order).
Both orders share an evolutionary history of approximately 170 million years, and species of both are found worldwide. However, today only 15 species of Lamniformes are known (including the great white shark), compared to over 290 species of Carcharhiniformes (including hammerheads, tiger sharks and many species found on coral reefs).
But why do some orders of shards increase, while others decrease? To find out, we turned to the fossil record.
The fossil record shows that shark species in prehistoric times followed a very different pattern to the species alive today. Lamniformes were, at the end of the Cretaceous period, before the “Age of Dinosaurs” ended about 66 million years ago. more Diverse than Carcharhiniformes.
To examine this change, we looked at changes in the shape of shark teeth over the past 83 million years.
Unlike their soft cartilaginous skeletons, shark teeth are made of a substance called “enameloid,” which makes them very hard. Sharks also continually grow new teeth, which means their teeth provide a nearly continuous fossil record.
Fortunately, the shapes of sharks’ teeth also provide rich information about their diet. For example, fish-eating sharks are likely to have sharp, narrow teeth – often with multiple cusps to increase their chances of catching slippery prey (see image of mako sharks below, which are primarily bony-fish specialists). Is).
By comparison, a shark that specializes in hunting seals is more likely to have wider teeth, which can be serrated to aid in the bite. This is precisely the variation in tooth shape that we focused on in our latest study.
By examining more than 3,000 teeth, we found a clear link between changes in tooth shape over time and changes in the environment that occurred during and after the Cretaceous mass extinction – the same event that killed non-bird dinosaurs. It was wiped out about 66 million years ago.
Lots of fish, yet sharks can be picky
During the Cretaceous, when Laminiformes were more abundant, many shark species lived in the inland seas that were common at the time. An example was the Western Interior Sea Route, which divided North America into an east and west “subcontinent”.
However, towards the end of the Cretaceous, these inland seas began to disappear. The sea level dropped and exposed entire parts of the land. Inland seas are rare today (the Caspian Sea is an example, but this too is declining).
Read more: The Caspian Sea is about to fall by 9 meters or more this century – an ecology imminent
Depletion in these marine ecosystems resulted in a significant loss of wildlife, including marine reptiles and cephalopod ammonites (relatives of squid and octopus), which many Cretaceous lamniformes preyed on.
As a result, many Lamniformes faced extinction. On the other hand, Lamniformes with more generalized diets survived the extinction event—as did Carcharhiniformes, which also have more generalized diets.
why did meg disappear
A similar event may have occurred a few million years ago with one of the most awe-inspiring laminiform sharks ever known: Meg (Otodus Megalodon) was the largest predatory shark species to have existed.
Megalodon was actually an impressive predator that lived during the Miocene and early Pliocene, about 4–23 million years ago. Based on the shape of its teeth, it probably specialized in eating whales, which were very diverse at the time.
Our results show the period in which it lived was also a turning point for Lamniformes, with record-low tooth disparity (loss of amount of size variation).
Although it is still difficult to know exactly why the meg became extinct, it is probably its specialized diet, which may have included the giant sperm whale. leviathan melvillek, put it at a disadvantage because its preferred diet changed due to the cold climate during the Miocene and Pliocene.
In general, it seems that specialized diets, such as those of megalodon and some Cretaceous lamniformes, may have put these species at greater risk of extinction.
Read more: Creating a megalodon: the evolving science behind estimating the size of the largest killer shark ever
So what does this mean for modern sharks?
By studying the stomach contents of modern Lamniformes, we found that most species feed on specific food groups. Thresher and Mako sharks feed mainly on bony fish. The basking shark eats exclusively plankton, while the adult great white shark mainly eats mammals.
Since Lamniformes were much more diverse in the past, our research indicates that the low diversity of Lamniformes living today is the result of repeated extinction events.
By comparison, the modern and older Carchariniforms are and were more flexible in their diet. They also benefited directly from the expansion of coral reefs over the past 50 million years.
Thanks to the important biological insights provided by the fossil record, we now have dietary specialization and adaptation to environmental changes—potentially over the past 83 million years—that have led to an imbalance in the numbers of Lamniformes and Carcharhiniformes species.
But what does the future hold? While it’s hard to say for sure, the news isn’t great for Lamniformes. Of the remaining 15 species, five are classified as “endangered” or “critically endangered” by the International Union for Conservation of Nature and Natural Resources. The other five are considered “weak”.
Lamniformes are also a mostly marine species with a specialized diet, and are therefore particularly vulnerable to chronic overfishing and habitat destruction.
And since our results suggest that diet and prey availability underlie much of the diversity among modern sharks, we think this will probably decide their existence in the future as well.