Earlier than people and early primates, earlier than dinosaurs, and even earlier than timber, there have been sharks. Sharks have been round for greater than 400 million years (though how lengthy precisely stays contested). They’ve survived 5 main mass extinctions.
However the sharks of way back usually are not like those we see right this moment. In truth, we nonetheless perceive fairly little about their long-term evolution. Our analysis, revealed right this moment within the journal Present Biology, demonstrates how shark evolution over the previous 83 million years has been pushed by food plan choice and local weather change — resulting in the variety we see right this moment.
Because it seems, being choosy about your prey is a dangerous sport for sharks to play.
When the scales tipped
One of many extra peculiar patterns in biology is for very carefully associated orders of residing animals to have drastically totally different numbers of species. A notable instance is the distinction in species quantity between mackerel sharks (the Lamniformes order) and floor sharks (the Carcharhiniformes order).
Each orders share practically 170 million years of evolutionary historical past, and each have species discovered the world over. Nonetheless, there are solely 15 species of Lamniformes recognized right this moment (together with the nice white shark), in comparison with greater than 290 species of Carcharhiniformes (together with hammerheads, tiger sharks and plenty of species discovered on coral reefs).
However why do some orders of shark thrive, whereas others dwindle? To seek out out, we turned to the fossil file.
The fossil file reveals shark species in prehistoric instances adopted a really totally different sample to species alive right this moment. Earlier than the “age of dinosaurs” ended some 66 million years in the past, on the finish of the Cretaceous Interval, Lamniformes have been extra numerous than Carcharhiniformes.
To analyze this shift, we checked out adjustments within the shapes of shark enamel over the previous 83 million years.
In contrast to their delicate cartilaginous skeleton, shark enamel are made up of a substance known as “enameloid”, making them very onerous. Sharks additionally constantly develop new enamel, which implies their enamel present an nearly steady fossil file.
Fortunately, the shapes of shark enamel additionally present wealthy data on their diets. As an example, a fish-eating shark is more likely to have pointy, slim enamel — typically with a number of cusps to extend its possibilities of catching slippery prey (see the picture of the mako shark under, a predominately bony-fish specialist).
By comparability, a shark that specialises in looking seals is extra more likely to have broad enamel, which can be serrated to assist with reducing. It’s exactly this variation in tooth form which we targeted on in our newest research.
By analyzing greater than 3,000 enamel, we discovered a transparent hyperlink between adjustments in tooth form over time and adjustments within the atmosphere that occurred throughout and after the end-Cretaceous mass extinction — the identical occasion that wiped-out non-bird dinosaurs about 66 million years in the past.
Loads of fish, but sharks may be picky
Through the Cretaceous, when Laminformes have been extra plentiful, many shark species lived in inland seas that have been frequent on the time. One instance was the Western Inside Seaway, which divided North America into east and west “subcontinents”.
Nonetheless, in the direction of the top of the Cretaceous, these inland seas began disappearing. Sea ranges lowered and uncovered total chunks of land. Inland seas are uncommon right this moment (the Caspian Sea is one instance, nevertheless it too is receding).
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The discount in these marine ecosystems led to a big lack of wildlife, together with marine reptiles and cephalopod ammonites (relations of squid and octopus) upon which many Cretaceous Lamniformes preyed.
Consequently, many Lamniformes suffered extinction. However, Lamniformes with extra generalised diets survived the extinction occasion — as did Carcharhiniformes, which additionally are inclined to have extra generalised diets.
Why the meg went lacking
An identical occasion might have occurred just some million years in the past to some of the awe-inspiring lamniform sharks ever recognized: the meg (Otodus megalodon). The meg was the most important predatory shark species to have existed.
Megalodon was really an imposing predator that lived through the Miocene and early Pliocene, roughly 4—23 million years in the past. Primarily based on its tooth form, it seemingly specialised in consuming whales, which have been very numerous at the moment.
Our outcomes present the interval through which it lived was additionally a turning level for Lamniformes, with record-low tooth disparity (a loss within the quantity of form variation).
Though it’s nonetheless troublesome to know why precisely the meg went extinct, it’s seemingly its specialised food plan, which could have included the enormous sperm whale Leviathan melvillei, put it at an obstacle as cooling climates through the Miocene and Pliocene led to adjustments in its most well-liked food plan.
To generalise, it appears specialised diets, akin to that of the megalodon and a few Cretaceous Lamniformes, might have put these species at a better danger of extinction.
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So what does this imply for contemporary sharks?
By learning the abdomen contents of contemporary Lamniformes, we discovered most species are inclined to feed on particular meals teams. The thresher and mako sharks feed totally on bony fish. The basking shark completely eats plankton, whereas grownup nice white sharks feed primarily on mammals.
Since Lamniformes have been far more numerous prior to now, our analysis signifies the low variety of Lamniformes residing right this moment is probably going the results of repeated extinction occasions.
By comparability, trendy and previous Carcharhiniformes are and have been extra versatile of their diets. In addition they benefited immediately from the growth of coral reefs over the previous 50 million years.
Because of vital organic insights supplied by the fossil file, we now have proof dietary specialisation and adaptableness to environmental adjustments seemingly drove shark evolution over the previous 83 million years — resulting in the imbalance in Lamniformes and Carcharhiniformes species numbers right this moment.
However what does the long run maintain? Though it’s onerous to say for positive, the information isn’t nice for Lamniformes. Of the 15 species remaining, 5 are categorised as “endangered” or “critically endangered” by the Worldwide Union for Conservation of Nature and Pure Sources. One other 5 are thought-about “susceptible”.
Lamniformes are additionally principally oceanic species with specialised diets, and are subsequently notably susceptible to persistent overfishing and habitat destruction.
And since our outcomes point out food plan and prey availability underpinned a lot of the variety amongst trendy sharks, we expect it’ll in all probability determine their survival sooner or later, too.