Thalassomedon haningtoni

Last updated: May 27, 2024
Verified by: AZ Animals Staff
© Nobu Tamura (, CC BY-SA 3.0 , via Wikimedia Commons

The length of the Thalassomedon's neck was up to half its body length


Thalassomedon Scientific Classification

Scientific Name
Thalassomedon haningtoni

Read our Complete Guide to Classification of Animals.

Thalassomedon Conservation Status

Thalassomedon Locations

Thalassomedon Locations

Thalassomedon Facts

fish, cephalopods and smaller reptiles
Main Prey
fish and cephalopods
Fun Fact
The length of the Thalassomedon's neck was up to half its body length
Biggest Threat
Bigger marine reptiles
Most Distinctive Feature
Thalassomedon had an extremely long neck
Distinctive Feature
Two pairs of flippers for swimming
The warm tropical waters of the western inland sea
Favorite Food
Fish and cephalopod
North America

Thalassomedon Physical Characteristics

2.8 tons to 4.4 tons
10m - 12m (32.8ft - 39ft)

View all of the Thalassomedon images!

Share on:

Thalassomedon is a genus of marine reptiles that lived in North America during the Late Cretaceous period. This giant creature belongs to an order of marine reptiles known as the plesiosaurs, characterized by their extremely long necks. Its closest relative is Elasmosaurus, another long-necked plesiosaur. Both reptiles belong to the family Elasmosauridae. Thalassomedon is known from a single species, Thalassomedon haningtoni. Just six specimens have been found so far. 

Description and Size

Thalassomedon translates to the ruler  of the sea and it had no competition until the Late Cretaceous period.

translates to the ruler of the sea and it had no competition until the Late Cretaceous period.

©Ryan Somma / CC BY-SA 2.0 – Original / License

The genus name is derived from the Greek words “Thalassa,” which means “sea,” and “medon,” which means “ruler.” The name translates as ruler of the sea, a reference to the massive size of this plesiosaur genus. 

Thalassomedon is the largest member of the family, Elasmosauridae. However, compared to the larger plesiosaur genera like Mauisaurus, it is a medium-sized reptile. Based on average estimates, this reptile was about 35.6 feet long. However, judging from the size of the animal’s skull, scientists think it might have been larger, with a potential to grow to lengths of up to 38 feet.

Based on the proposed length of 35.6 feet, experts estimate that Thalassomedon haningtoni would have weighed about 9,000 pounds. Like other plesiosaurs, the Thalassomedon had an extremely long neck. The neck alone was up to 19 feet long with up to 62 vertebrae.

The skull was relatively small. It was roughly 19 inches long and had massive rows of teeth, with each one measuring up to two inches in length. Thalassomedon moved with the aid of four pairs of giant flippers like a giant turtle would. Each flipper was about 4.9 to 6.6 feet long. Scientists also found stones in its stomach, and many have theorized that these stones were used as ballasts to aid floatation in the water. 

Diet What Did Thalassomedon Eat?

It was a carnivore that fed predominantly on fish. However, the reptile’s diet may have also included cephalopods and smaller reptiles that lived in the same habitat. The massive teeth of this plesiosaur were adapted to crushing the exoskeleton of shell-prey. Experts also think the stomach stones were used as gastroliths to aid digestion.

Habitat — When and Where It Lived

This plesiosaur genus lived on the North American continent. Thalassomedon lived about 95 million years ago in an ancient sea known as the western interior seaway. This was a Cretaceous sea that split North America into two. The western interior seaway was shallow, only about 600 feet, but had abundant marine life. The prehistoric sea was warm and tropical. 

Threats And Predators

The massive size of the Thalassomedon meant it would have thrived unimpeded in the western interior seaway for several years. There were no direct predators or even major competition. However, as time progressed into the Late Cretaceous, new predators began to rise that competed with Thalassomedon for food. Some of the most notable predators of this period include the Mosasaurs and Tylosaurus

Discoveries and Fossils — Where It was Found

R. L. Landberg discovered the first and only known species of the Thalassomedon genus in 1939. He unearthed the fossil in the Baca County of Colorado. The find consisted of short and deep cervical vertebrae numbering about 62. The fossil belonged to an adult, and no pectoral or pelvic bar was present. The name haningtoni was assigned to the type specie by Welles in 1943. Fossils have also been found in the Belle Fourche Formation in Montana. 

Extinction — When Did Thalassomedon Die Out?

Thalassomedon dominated the western inland seaways for several million years during the early stages of the Late Cretaceous. However, their dominance started to dwindle over the years. The disappearance of this marine reptile coincides with the rise of new predators like the Mosasaurs and Tylosaurus. This suggests that the Thalassomedon probably died off because they could not keep up the competition. 

Similar Animals to The Thalassomedon

Similar reptiles to the Thalassomedon include: 

  • Elasmosaurus: This is a closely related plesiosaur genus that lived in North America during the late Cretaceous. With a length of 34 feet, it was slightly smaller than the Thalassomedon
  • Tylosaurus: This was a marine reptile that also lived during the late Cretaceous. It was slightly larger than the Thalassomedon and was also a better-adapted apex predator. 
  • Mosasaurus: The Mosasaurus was a massive marine reptile that lived in the Late Cretaceous and outlived the Thalassomedon
View all 166 animals that start with T

Share on:
About the Author

Abdulmumin is a pharmacist and a top-rated content writer who can pretty much write on anything that can be researched on the internet. However, he particularly enjoys writing about animals, nature, and health. He loves animals, especially horses, and would love to have one someday.

Thalassomedon FAQs (Frequently Asked Questions) 

When did Thalassomedon live?

Thalassomedon lived during the early stages of the Late Cretaceous period. It appeared in fossil records about 99.6 million years ago and lived till the Turonian Age.


How big was Thalassomedon?

On average, the Thalassomedon was about 35.6 feet long. However, according to some estimates, it might have been up to 40 feet with an average weight of up to 9,700 pounds. It had a long neck measuring up to 20 feet, about half of its entire length.


Is Thalassomedon a dinosaur?

No, the Thalassomedon was not a dinosaur. It was a plesiosaur, which is a type of marine reptile. Although it lived around the same time as the dinosaurs, Thalassomedon was not related to them.


Where did Thalassomedon live? 

Thalassomedon lived in the western inland seaway. This was an inland seaway that divided the continent of North America into two.

Thank you for reading! Have some feedback for us? Contact the AZ Animals editorial team.


  1. Wikipedia / Accessed October 26, 2022
  2. Prehistoric Wildlife / Accessed October 26, 2022
  3. Wikipedia / Accessed October 26, 2022