Speaking of Jurassic, the pliosaur was the most fearsome beast the waters saw in those days. Described by one news outlet as a "crocodile on steroids," this creature ran up to 59 feet (18 meters) in length. The roughly 155 million-year-old mammoth featured a human-sized jaw and large teeth that may have been able to take down a Dearcmhara in one bite, as well as huge, melon-sized eyes that gave the animal impressive binocular vision. In the early 2000s, fossils of the beast have been located in England and Norway [source: Morelle].
A short-necked marine reptile, the pliosaur resembles a large croc with longer, paddle-shaped limbs. That made it faster than many of its counterparts, traveling at speeds of up to 6 mph (10 kph). The creature fed mostly on fish, mollusks and other marine reptiles, but paleontologists have also found something else in pliosaur stomachs: dinosaur remains. They don't believe that these beasts actually hunted dinosaurs, but instead probably feasted on discarded remains [source: BBC].
Author's Note: 10 Extinct Exotic Sea Creatures
Don't get me wrong, I think it's pretty safe to say that I wouldn't want to encounter any of these creatures during a dip in the water. I have to say, however, that I've already come face to face with a monster every bit as depraved and fearsome: The dreaded hangover at sea. It's tough to enjoy a nice jaunt out to the ocean under a cloudless Caribbean sky when you're also tussling with Montezuma's revenge.
- Animal Planet. "Dunkleosteus." (Feb. 9, 2015) http://www.animalplanet.com/tv-shows/other/videos/animal-armageddon-dunkleosteus/
- BBC. "Dunkleosteus." (Feb. 9, 2015) http://www.bbc.co.uk/science/seamonsters/factfiles/dunkleosteus.shtml?img1
- BBC. "Pliosaurs." (Feb. 9, 2015) http://www.bbc.co.uk/nature/life/Pliosaur
- Borenstein, Seth. "James Cameron Sees 'Another Planet' in the Deepest Sea." Associated Press. March 26, 2012 (Feb. 8, 2015) http://www.nbcnews.com/id/46860722/ns/technology_and_science-science/t/james-cameron-sees-another-planet-deepest-sea/#.VNe_2i6ulK0
- Bittel, Jason. "Motorboat-Size Sea Monster Found, Sheds Insight on Evolution." National Geographic. Jan. 13, 2015 (Feb. 9, 2015) http://news.nationalgeographic.com/news/2015/01/150113-sea-monsters-paleontology-scotland-animals-science-fossils/
- Chappell, Bill. "Rare And 'Horrific': Frilled Shark Startles Fishermen In Australia." NPR. Jan. 21, 2015 (Feb. 8, 2015) http://www.npr.org/blogs/thetwo-way/2015/01/21/378944897/rare-and-horrific-frilled-shark-startles-fishermen-in-australia?utm_source=npr_newsletter&utm_medium=email&utm_content=20150125&utm_campaign=mostemailed&utm_term=nprnews
- Crew, Bec. "Prehistoric ghost shark Helicoprion's spiral-toothed jaw explained." Scientific American. Feb. 27, 2013 (Feb. 9, 2015) http://blogs.scientificamerican.com/running-ponies/2013/02/27/prehistoric-ghost-shark-helicoprions-spiral-toothed-jaw-explained/
- ESPN. "Shaquille O'Neal." (Feb. 9, 2015) http://espn.go.com/nba/player/_/id/614/shaquille-oneal
- Fossil Guy. "Mosasaur: Facts and Information about the Giant Marine Reptile." (Feb. 8, 2015) http://www.fossilguy.com/gallery/vert/reptile/mosasaur/
- McCormick, Cameron. "Stupendemys: Giant Amongst Mega-Turtles." The Lord Geekington. March 11, 2010 (Feb. 8, 2015) http://cameronmccormick.blogspot.com/2010/03/stupendemys-giant-amongst-mega-turtles.html
- Morelle, Rebecca. "Dorset pliosaur: 'Most fearsome predator' unveiled." BBC. July 8, 2011 (Feb. 9, 2015) http://www.bbc.com/news/science-environment-14061347
- Moss, Stephen. "Another new ichthyosaur in the golden age of Jurassic discovery." The Guardian. Jan. 12, 2015 (Feb. 10, 2015) http://www.theguardian.com/science/shortcuts/2015/jan/12/new-dinosaur-golden-age-jurassic-discovery-skye
- Museum of Unnatural History. "The Kraken." (Feb. 8, 2015) http://www.unmuseum.org/kraken.htm
- National Geographic. "Tylosaurus Proriger." (Feb. 8, 2015) http://animals.nationalgeographic.com/animals/prehistoric/tylosaurus/
- Prehistoric Wildlife. "Stupendemys." (Feb. 9, 2015) http://www.prehistoric-wildlife.com/species/s/stupendemys.html
- Prehistoric Wildlife. "Tanystropheus." (Feb. 8, 2015) http://www.prehistoric-wildlife.com/species/t/tanystropheus.html
- Prigg. Mark. "The megalodon IS extinct: New study rubbishes claim that giant shark still roams the ocean - and says it died out 2.6 million years ago." Daily Mail. Oct. 23, 2014 (Feb. 9, 2015)
- Rincon, Paul. "Monster shark 'kept whales in check.'" BBC. Oct. 23, 2014 (Feb. 8, 2015) http://www.bbc.com/news/science-environment-29743081
- South East Travel Fishing Industry Association. "Living fossil captured." Jan. 14, 2015 (Feb. 8, 2015) http://www.setfia.org.au/item/living-fossil-captured.html
- University of California Museum of Paleontology. "Prolacertiformes." May 2007 (Feb. 8, 2015) http://www.ucmp.berkeley.edu/taxa/verts/archosaurs/prolacertiformes.php
This pelycosaur roamed Earth 298 million years ago and is the oldest-known vegetarian tetrapod with gaps in its teeth. HowStuffWorks checks him out.