Deep in the bowels of the Natural History Museum, Kate Whittington is standing in front of the skeleton of a 23ft plesiosaur, one of prehistoric Earth’s most fearsome marine reptiles, explaining how it would eat us for dinner, were it still around today.

“Its long neck allowed its head to get a head start on its body,” says the museum’s exhibition and interpretation manager. “So it could sneak up on prey and grab it [with its mouth] before its body and flippers created a disturbance in the water.”

The bones of this immense predator are among the centrepieces of Jurassic Oceans: Monsters of the Deep, an immersive exhibition showcasing fossils, casts and 3D-printed sculptures of the marine creatures that ruled the oceans while dinosaurs roamed the land more than 66m years ago.

As we walk past ancient crocodile-like creatures and colossal squid tentacles, Marc Jones, the exhibition’s curator, is explaining what the world’s waters used to look like and, despite aeons passing, the parallels between ancient oceans and today’s deep blue depths.

“[In the Jurassic era], the sun was slightly dimmer, about 2% less powerful,” he says, “but the planet was much warmer, much more humid, because there was a lot more CO2 in the atmosphere.” That meant there were no permanent ice caps, so sea levels were higher, with more of the planet covered by water, he adds. Indeed, at the beginning of the Jurassic era, nearly all land was joined together in the supercontinent Pangaea, surrounded by a single global ocean known as Panthalassa. “Because that ocean was so vast and slow moving, circulation was limited in many places,” Jones says.

Particularly well suited to these conditions were ammonites, a group of soft bodied, shell-dwelling creatures related to modern cephalopods such as octopus, squid and nautilus. “There’s evidence that squids are doing really well because the oceans are getting warmer,” Jones says. It makes sense, he adds, because “their relatives did really well in this warmer, slightly more stagnant ocean”.

The exhibition also shows how dramatically marine ecosystems have changed over time. In today’s oceans, sharks are among the dominant hunters, but 200m years ago “they were middle predators”, says Jones, as we pass remains of their ancestors lining the walls. “They were very effective hunters, but they would also have been preyed upon by marine reptiles.”

Larger animals lurk deeper in the exhibition, including ichthyosaurs, a family of vicious long-snouted marine reptiles. “Ichthyosaurs probably have the largest eye of any vertebrate animal,” says Jones. “It shows that it had areas that were very developed in processing movement, vision and scent, which reinforces what we know about it being a very speedy predator that relied on vision as one of its strategies.”

A bottlenose dolphin skeleton is on show to demonstrate how similar their body shapes and hunting tactics are to ichthyosaurs. Jones says this is an example of convergent evolution – two species independently evolving similar anatomy.

“Animals that live in similar environments and have evolved to eat similar prey tend to develop the same adaptations to achieve the same goal, but completely separately,” says Jones. “So they’re completely unrelated, but they’ve ended up, through natural selection, evolving the same features to do the same thing, but at completely different times in life.”

Unlike other marine reptiles, which were almost entirely wiped out by an asteroid crashing into Earth at the end of the Mesozoic era, ichthyosaurs are thought to have become extinct much earlier, due to the diminishing availability of prey related to natural changes in the climate.

Ammonites were a “kind of high energy snack for them”, says Jones. “It might be that, as the climate changed and ammonites started to die out, the ichthyosaurs couldn’t adapt fast enough to recover from one of their main food sources declining.”

It is the same climate story depleting marine life today. Ocean warming, acidification, and deoxygenation threatens phytoplankton, the base of a food chain that feeds bigger species. “We’ve added more than 2,000 gigatons of CO2 into the atmosphere in less than 200 years, and that has consequences,” Jones says. “That’s going to affect ecosystems.”

The grand finale of the exhibition centres on the skull of a mosasaur. Known as the “T rex of the sea”, these large predators ruled the oceans in the Cretaceous period, which ended about 66m years ago. “It has these big pointy teeth on the outside, but it also has teeth in the roof of its mouth to help it grip on to prey,” says Whittington.

“When dinosaurs were living on the land, you had all these amazing things living in the oceans, like giant marine reptiles, that we don’t really have equivalents of today,” adds Jones. “We do have saltwater crocodiles and big turtles, but [the role] of predator is dominated by mammals.”

However, this is not the only change to have happened in our oceans. Today, more than 90% of the heat trapped by carbon emissions is absorbed by the ocean, and almost every year since the start of the millennium, a new ocean heat record has been set.

For Jones, looking back offers a stark warning. “There is lots of evidence of the climate changing during the prehistoric era and that being associated with changes in the fauna, the ecosystem and the environments,” he says. “Some of those changes took place over millions of years and yet they still had a big impact on what was alive then and the type of ecosystem that was around. It’s the speed of the changes happening today that is the problem. Many animals can’t keep up.”