Earlier than people and early primates, earlier than dinosaurs, and even earlier than bushes, 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 will not be like those we see in the present day. In truth, we nonetheless perceive fairly little about their long-term evolution. Our analysis, revealed in the present day within the journal Present Biology, demonstrates how shark evolution over the previous 83 million years has been pushed by weight-reduction plan choice and local weather change — resulting in the variety we see in the present day.
Because it seems, being choosy about your prey is a dangerous recreation for sharks to play.
When the scales tipped
One of many extra peculiar patterns in biology is for very intently associated orders of dwelling animals to have tremendously completely 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 almost 170 million years of evolutionary historical past, and each have species discovered the world over. Nevertheless, there are solely 15 species of Lamniformes recognized in the present day (together with the good 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 search out out, we turned to the fossil report.
The fossil report reveals shark species in prehistoric instances adopted a really completely different sample to species alive in the present day. 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.
Not like their delicate cartilaginous skeleton, shark enamel are made up of a substance known as “enameloid”, making them very arduous. Sharks additionally constantly develop new enamel, which suggests their enamel present an virtually steady fossil report.
Fortunately, the shapes of shark enamel additionally present wealthy info on their diets. For example, a fish-eating shark is prone to have pointy, slim enamel — usually 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 searching seals is extra prone to have broad enamel, which can be serrated to assist with slicing. 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 setting that came about 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 might be picky
Throughout the Cretaceous, when Laminformes have been extra considerable, 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”.
Nevertheless, in the direction of the top of the Cretaceous, these inland seas began disappearing. Sea ranges lowered and uncovered complete chunks of land. Inland seas are uncommon in the present day (the Caspian Sea is one instance, but it surely too is receding).
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The discount in these marine ecosystems led to a major lack of wildlife, together with marine reptiles and cephalopod ammonites (kin 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 analogous occasion could have occurred only a few million years in the past to some of the awe-inspiring lamniform sharks ever recognized: the meg (Otodus megalodon). The meg was the biggest predatory shark species to have existed.
Megalodon was actually 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 doubtless 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 doubtless its specialised weight-reduction plan, which could have included the enormous sperm whale Leviathan melvillei, put it at a drawback as cooling climates through the Miocene and Pliocene led to adjustments in its most popular weight-reduction plan.
To generalise, it appears specialised diets, equivalent to that of the megalodon and a few Cretaceous Lamniformes, could have put these species at a larger danger of extinction.
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In the present day’s species
So what does this imply for contemporary sharks?
By finding out the abdomen contents of recent 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 dwelling in the present day is probably going the results of repeated extinction occasions.
By comparability, fashionable and previous Carcharhiniformes are and have been extra versatile of their diets. In addition they benefited immediately from the enlargement of coral reefs over the previous 50 million years.
Due to necessary organic insights supplied by the fossil report, we now have proof dietary specialisation and adaptableness to environmental adjustments doubtless drove shark evolution over the previous 83 million years — resulting in the imbalance in Lamniformes and Carcharhiniformes species numbers in the present day.
However what does the long run maintain? Though it’s arduous 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 Assets. One other 5 are thought-about “weak”.
Lamniformes are additionally largely oceanic species with specialised diets, and are due to this fact notably weak to persistent overfishing and habitat destruction.
And since our outcomes point out weight-reduction plan and prey availability underpinned a lot of the variety amongst fashionable sharks, we expect it would in all probability determine their survival sooner or later, too.