1) Ancient Marine Life

Long before even the first primitive sharks, the oceans were dominated by strange invertebrates. Alien creatures such as Anomalocaris (top predator with shrimp-like mouth parts, grew up to 6 feet long), Orthoceras (& other cephalopods), large sea scorpions and others ruled.

Burgess Shale fauna, Cambrian era (~508 million years ago)

The first sharks appeared either in the Ordovician or Silurian era (~420+ Million years ago (MA), scant evidence, mostly fossil dermal denticles. The first sharks may have been jawless & toothless, like many other fish of the time). But the seas were still ruled by lengthy invertebrates.

Silurian sea scorpion feasting on a cephalopod.

Shark diversity radiated during the Devonian (~420-360 MA) much like the dominant fish of the time- Placoderms. These fish did not survive the End Devonian extinctions (likely caused by Ice Age/ Global Cooling) but were hugely successful. They were among the first jawed fish and most sported heavily armored bodies. In the Late Devonian, the oceans were ruled by 30+ foot long Dunkleosteus, which had massive shearing mouth-gear.

Dunkleosteus feasting while several odd sharks, Stethacanthus (top middle), ctenacanth(?, left) and Cladoselache sharks (right) pick up the scraps.

Sharks such as Cladoselache fairly resembled modern sharks but were scaleless and mainly built for speed and agility (& only grew to 6 feet long).

Before the end of the Paleozoic/largest mass extinction event to date, many other strange sharks lived. Edestus, a large (up to ~20 foot) shark found in Midwest coal measures, had interesting shearing teeth that did were not replaceable. The "buzz-saw shark" or Helicoprion, sported a bizarre tooth whorl and was actually more closely related to Ratfish than sharks.

Helicoprion

Also Helicoprion (most, if not all, fossils found are from the head/jaw though)

Xenacanth/Orthacanth sharks were eel-like, equipped with double-pronged teeth and were hugely successful in freshwater environments (but were also found in marine environments).

Marine crocodiles (geosaurs) that lived during the rise of the first modern sharks, Jurassic period.

For many reasons unknown, Icthyosaur, Pliosaur, and Plesiosaur diversity declined drastically into the Cretaceous, coincidently or not just as modern shark diversity (all modern orders) began to radiate, displacing more primitive Hybodont sharks (that didn't have state of the art features such as better cutting teeth, much more flexible jaw (allowing suction of prey), and stronger swimming muscles for more power, larger brains and improved senses. Hybodonts were pushed into more freshwater niches. By 80 million years ago, formidable lamniform sharks such as Ginsu sharks (highly resembled modern great whites in size/ function) existed. But they were certainly not alone at the top.

Huge marine predators that evolved from monitor lizards on land- the mosasaurs- quickly rose to prominance and overtook sharks in diversity in a hurry (more recognizable as the huge sea monster in Jurassic World, although it is grossly overestimated in size in the film). The largest grew up to 50 or perhaps even 60 feet long and fed on near anything they pleased.

Other large creatures roaming the Cretaceous seas included Xiphactinus- a toothy tarpon known to swallow other members of its species and the largest sea turtles known to date (10-20 feet long).

Late cretaceous seaway (plesiosaur left, Mosasaur middle, lamniform shark bottom, Xiphactinus right)

Of course large turnover occured in the end of the Mesozoic & into the Cenozoic, which wiped out the non-avian dinosaurs, mosasaurs, ammonites (spiral shelled cephalopods) and others, but sharks remained resilient. Lamniforms continued to dominate, with Paleocarcharodon sporting wicked serrations that made many paleontologists think they were ancestral great whites. During the hot Eocene, modern shark, bony fish, marine mammal, invertebrate, and other fauna biodiversity exploded and pretty much shaped the mold for how sharks are today.

In the Cenozoic only whales (& later humans) would challenge sharks as the "top dogs". 50 foot predatory whales dominated ocean seaways around 40 million years ago.

Basilosaurus whales

Archaeocete whales died out after the hot Eocene & the first modern whales (baleen & toothed whales) took their place. Lamniform & Carcharhiniform (Grey) sharks still prospered, but for the first time in history Grey sharks were just as diverse as lamniforms! (& would continue to usurp the ancient lamniforms in terms of diversity).

But in the Miocene, there was enough for all to prosper, large predatory lamniform & carcharhiniforms, and including predatory whales.

Livyatan whale chasing a long-beaked dolphin

Shark & other marine life declined in abundance & diversity during the Mid Miocene disruption, around 14.8 to 14.5 million years ago, whereby temperatures declined, upwelling began to cease and oceanic current patterns changed. Tectonic forces may have also played a role.

Being adaptable, sharks continued to thrive as a group to this day, but are far from the top apex predators in the sea. Orcas & humans can best any large predatory shark, but that does not diminish the spectacular form & function of these predators. Only humans can help save or cause the demise of many sharks that live in the present age.