Today's oceans are pretty frightening, as deep within them inhabit barracudas, sharks, super-squids, and many other unknown marine animals. But despite the alarming creatures in the present-day ocean, none compare to the colossal sea monsters that existed in the past.

These include enormous reptiles, monstrous sharks, and whales that were extremely carnivorous and so massive that humans would hardly satisfy their appetite. The fossilized remains in rocks prove their existence in the distant past they left and now scientists are studying them. Below are the top 10 bizarre prehistoric marine animals that would be seen as sea monsters today if they existed.

Habelia Optata

Habelia optata was a miniature monster, measuring up to 1.6 inches (4.1 centimeters) in body length. These small sea predators had heads that resembled helmets and unsettling mouth extensions designed for capturing and tearing apart their victims.

The Royal Ontario Museum reported that fossils of H. optata go back roughly 505 million years to the Cambrian era or about 538.8 million to 485.4 million years ago, and are now available in British Columbia, Canada.

Helicoprion

Helicoprion, also known as "buzz saw sharks," were a type of fish resembling sharks with teeth that looked like the edge of a buzz saw due to their spiral jaw. According to the Australian Museum, they lived in Earth's oceans from the Devonian to the Triassic period and could grow up to 25 feet long, which is larger than the largest modern great white sharks.

Ichthyosaurs

Ichthyosaurs were a diverse group of reptilian predators that evolved around 250 million years ago and became extinct about 90 million years ago. They had bodies resembling dolphins or fish but with a more menacing appearance. While some species were as small as 1 foot long, others were giants that lived during the late Triassic period, with one fossilized jawbone from the UK estimated to belong to an 85-foot-long ichthyosaur, nearly as long as a blue whale, as per Live Science.

Lyrarapax unguispinus

During the Cambrian period, a claw-faced sea monster named Lyrarapax unguispinus existed, which was unlike anything present in today's oceans. This apex predator was a strange arthropod that grew up to 3.2 feet long, with a claw-shaped appendage on its head to grasp prey.

Mosasaurs

Mosasaurs were a group of marine reptiles that roamed the oceans and were known to eat almost anything, including other mosasaurs. They went extinct along with the nonavian dinosaurs, with the Mosasaurus hoffmanni species estimated to have grown up to 56 feet long based on the findings of a 2014 study.

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Placodonts

As Live Science reported, placodonts were turtle-like marine reptiles that lived during the Triassic period and had front teeth for plucking shells and mollusks and flat-crushing plates for chewing. They were found in Europe, the Middle East, and China; and they had incredibly efficient buck teeth.

Plesiosaurs

Plesiosaurs, which had bodies like boats and four flippers, were a group of marine reptiles that lived across the world's oceans from the Triassic period until they went extinct alongside the non-avian dinosaurs at the end of the Cretaceous period. The group consisted of both long-necked plesiosaurs, similar to the ancient Loch Ness monster, and short-necked plesiosaurs, which had massive heads.

Saccorhytus coronarius is a wrinkled, sac-shaped animal that lived around 500 million years ago during the Cambrian period and was found in microfossils in China. A previous report of Live Science revealed that these animals do not have an anus and were possibly predators that lived in seafloor sediment, but their behavior is not well understood. Scientists believe that they are related to penis worms and mud dragons.

Sea scorpions

The Yale Peabody Museum of Natural History describes sea scorpions as ocean-dwelling arthropods resembling modern-day scorpions that terrorized the seas for more than 200 million years until they went extinct at the end of the Permian period, and some were enormous compared to scorpions living today.

Tanystropheus hydroides

Tanystropheus hydroides were ancient marine reptiles with incredibly long necks, which could stretch up to 10 feet (3 meters) in length, a 2020 study revealed. These marine animals live in the Tethys Sea around 242 million years ago, with their small heads on the front and tiny bodies making them ungainly and awkward, similar to long-necked plesiosaurs.

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