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First things first, a little backstory:

Sometime between the Paleocene and Eocene epochs, there was a mysterious, sudden, dramatic rise in global temperature. This moment in time was known as the "Paleocene-Eocene Thermal Maximum", shortened into "PETM". In just 20 to 50 millennia, the temperature rose by five to eight degrees Celsius, and this heatweave persisted for another 200 millennia (that's just the mean estimate.)

While its impact on terrestrial plant and animal life is well-known, the focus of the question is on how the PETM impacted the marine ecosystems. The warmer the water's temperature, the less oxygen it can hold, so the equatorial waters--which may have been 36 degrees Celsius--were absent of the plankton vital for oceanic food webs. And since water has a low albedo, it absorbs carbon dioxide rather than reflects it. And in an episode as CO2-rich as the PETM, the oceans absorbed so much of the greenhouse gas that they had become acidifed. That, in turn, depleted the supply of carbonates, which many animals relied on to build shells and other structures. Indeed, fossil remains of coral reefs dating from the PETM to several million years afterwards were rare, and anywhere between one-third and half of all the deep-sea species of foraminifera (tiny, planktonic lifeforms) went extinct. Finally, the warmer waters also affected the arrangement of a particular layer called the lysocline:

The lysocline marks the depth at which carbonate starts to dissolve (above the lysocline, carbonate is oversaturated): today, this is at about 4 km, comparable to the median depth of the oceans. This depth depends on (among other things) temperature and the amount of CO2 dissolved in the ocean. Adding CO2 initially raises the lysocline, resulting in the dissolution of deep water carbonates. This deep-water acidification can be observed in ocean cores, which show (where bioturbation has not destroyed the signal) an abrupt change from grey carbonate ooze to red clays (followed by a gradual grading back to grey). It is far more pronounced in north Atlantic cores than elsewhere, suggesting that acidification was more concentrated here, related to a greater rise in the level of the lysocline. In parts of the southeast Atlantic, the lysocline rose by 2 km in just a few thousand years.

So here are the points of departure:

  1. The Paleocene-Eocene Thermal Maximum did happen at the same time as OTL and at the same speed, but it lasted three to four times longer, which lifted the lysocline to a depth of one mile and wiped out the coral reef ecosystem as well as 25% of other marine species.
  2. An additional element is the development of the forest walls of fully marine trees or "megamangroves", trees so tall that they support themselves on buttress roots rather than the familiar stilts. These walls are currently cosmopolitan at tropical and temperate latitudes and stop at a maximum depth of 250 feet. Even though they did debut during the PETM, they did not become this formidable barrier until later in the Eocene (41-37 ma).

And now, using our knowledge of shark genera or species that were around to witness the PETM, could they survive a longer PETM coupled afterwards by the natural building of walls of exclusively marine trees?

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    $\begingroup$ "Since water has a low albedo, it absorbs carbon dioxide rather than reflects it": this needs some re-work. Probably it is not "carbon dioxide" that this sentence is about. $\endgroup$ – AlexP May 26 '20 at 1:44
  • $\begingroup$ Wait, how would raising the lysocline wipe out coral reefs? Most corals only grow in waters of 27 m or less, far above this raised lysocline. $\endgroup$ – user2352714 May 27 '20 at 23:34
  • $\begingroup$ @user2352714 If the PETM were longer than in OTL, that would make the ocean waters warmer, making them more acidic, and corals did not like hot, acidic waters. The depth of the lyscoline is merely tied to the water temperature. $\endgroup$ – JohnWDailey May 28 '20 at 0:36
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As to 1, yes. Sharks survived the K-T extinction, though not every species did and there was a general turnover from Lamniformes to Carcharhiniformes and a huge dieoff of freshwater sharks and rays, and they survived the P-T and T-J extinctions before that. You're going to have to hit the ecosystem a lot harder than just an extended PETM to wipe them out. Indeed, shark fossils are really common in Paleocene sites like the Cannonball Sea of North Dakota right after the K-T extinction, and they seem to have done better in some ways with competition from mosasaurs, other marine reptiles, and predatory bony fishes being wiped out. Maybe some of the more specialized species would die off (like the Potamotrygonidae and maybe the early megatoothed sharks), but that would be it.

As to 2, your "megamangroves" are going to screw things up royally for life around the globe. Mangrove trees in general provide important habitat for many ocean species, providing a home for the smaller juveniles in brackish water before they grow large enough to head out to sea. For example black-tip reef sharks and lemon sharks, among others, depend on mangroves for the survival of their young. A new radiation of mangroves without the stilt roots would be devastating for many, many marine fishes that rely on them for food and shelter. To be honest, it would probably negatively any aquatic species that travels between land and water in their lifetime and lives in tropical climates. If they go to temperate latitudes they might wipe out things like sturgeon, salmon, and eels. It still probably wouldn't get every shark, you have sharks that live in high latitudes or in offshore environments and don't require shallow water nurseries, but your consequences for marine life in general would be much worse than you are probably budgeting for.

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  • $\begingroup$ I clarified on 1. Would that change the first answer in any way? Also, for 2, I asked this question afterwards (worldbuilding.stackexchange.com/questions/177295/…), and the answer I got were Banyan-type prop roots instead of buttresses. Would that change the answer as well? $\endgroup$ – JohnWDailey May 27 '20 at 20:11
  • $\begingroup$ @JohnWDailey Sharks have previously survived at least 3 complete collapses of reef ecosystems in the Devonian, Permian, and Cretaceous. 25% of all marine species dying out is nothing compared to any of these events and most sharks would definitely survive. And if the trees have Banyan-type roots there would be no difference, the banyans would provide the same kind of necessary shelter to most marine life as mangroves. $\endgroup$ – user2352714 May 27 '20 at 23:31

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