Mosasaur: Giant Cretaceous Sea Reptile

A new mosasaur species has been discovered / Tatsuya Shinmura/University of Alberta

Between 80 million and 66 million years ago, a massive carnivorous marine reptile called Mosasaurus roamed the oceans during the Late Cretaceous period across the globe. The term “Mosasaurus” derives its origin from the Latin word “Mosa,” which pertains to the Meuse River, and the second part is derived from the Greek term “Sauros,” signifying a lizard. Following the Late Cretaceous era, this ocean-dwelling creature vanished from existence.

Mosasaurus boasted distinctive characteristics, such as a sturdy, crocodile-like head, a fin located at the end of its tail, and a hydrodynamic body. It thrived on a diet primarily comprising fish, squid, and shellfish, showcasing its dominance as a top predator during its time.

Key Takeaways: Mosasaur


  • Mosasaurs were reptiles that resembled modern-day monitor lizards, but with long streamlined bodies, paddle-like limbs, and a powerful tail for swimming.
  • Mosasaur species varied in size, with some reaching lengths of up to 18 meters (59 feet) or more. They were among the largest predators of their time.
  • As apex predators, mosasaurs fed on a variety of marine organisms, including fish, cephalopods (like squid), and other marine reptiles. They were skilled swimmers and agile hunters.
  • Mosasaurs, along with non-avian dinosaurs and other marine reptiles, went extinct at the end of the Cretaceous period, likely due to a combination of factors, including an asteroid impact and changing environmental conditions.
  • Some well-known mosasaur species include Mosasaurus hoffmanni, Tylosaurus proriger, and Platecarpus tympaniticus.

An especially noteworthy aspect of Mosasaurus was its remarkable average lifespan of approximately 42 years, which was considered quite long for a reptile during that geological epoch. As a magnificent and formidable predator, Mosasaurus held a significant ecological role within the Late Cretaceous marine ecosystems, thereby leaving an intriguing legacy for paleontologists and enthusiasts alike to meticulously investigate and examine.

When Was Mosazor Discovered?

The fossils of Mosasaur were unearthed in the late 18th century in a Dutch mine, well before humanity had any understanding of dinosaurs, marine reptiles, or the concept of evolution. It was only in 1829 that Gideon Mantell bestowed the name “Mosasaur” upon the discovery, drawing inspiration from the nearby Meuse River, and the name’s significance translates to “Meuse River lizard.” Notably, the discovery of this fossil had a significant impact on scientific thought because Georges Cuvier proposed the radical idea that species could indeed go extinct in response to the evidence. This idea challenged the prevailing religious beliefs of that era.

Prior to the late Enlightenment period, the prevailing belief among educated individuals in the Western world was that all Earth’s creatures were divinely created during biblical times, with no consideration for vast geological epochs. The concept of extensive geological time remained unknown to them. As a result, the Mosasaur fossil was subject to various interpretations and mistakenly associated with fish, whales, and even crocodiles. The closest approximation was suggested by the Dutch naturalist Adriaan Camper, who proposed that it might belong to a colossal monitor lizard. It was Georges Cuvier who ultimately identified Mosasaur as a member of the Mosasaur family, further advancing the understanding of ancient marine reptiles.

Mosasaur Species


The Mosasaur family tree branched out into several species, each equipped with its unique characteristics and adaptations. Let’s explore some of the most prominent members of this ancient reptilian lineage:

Mosasaurus Hoffmanni

Reconstructed skeleton of M. hoffmannii at the Maastricht Natural History Museum
Image: Ghedoghedo.

Named after the German naturalist, Hoffmann, this species could reach up to 17 m (56 ft) in length! With its streamlined body and powerful flippers, it was a swift and agile swimmer.Fossil evidence suggests that it primarily fed on fish and ammonites.

Tylosaurus Proriger

USNM 8898, a T. proriger skeleton which was found with plesiosaur remains in its stomach.
Image: Flickr.

Hailing from North America, Tylosaurus was one of the largest Mosasaur species, measuring up to 12–15.8 meters (39–52 ft) long.

Its elongated snout, equipped with robust teeth, allowed it to feast on a diverse diet of fish, squid, and even other marine reptiles!

Clidastes Propython

Fossil cast of a Clidastes propython skeleton next to some ammonite models at the North American Museum of Ancient Life.
Image: North American Museum of Ancient Life /Ninjatacoshell.

Clidastes stood out with its slender body and paddle-like limbs, a perfect design for swift maneuvering in the water. Clidastes is recognized as one of the smaller species of mosasaurs, with Dallasaurus being the smallest known. On average, Clidastes measured approximately 2-4 meters (6.6-13.1 ft) in length, although the largest individuals could reach up to 6.2 meters (20 feet) in length.

This species mainly preyed on small fish and invertebrates, and its fossils have been unearthed across North America.

Facts About the Mosasaur


Mosasaur Was Bigger Than T. rex

The enigmatic creature featured in films like Jurassic World (2015) remains shrouded in relative obscurity, with limited information available. However, paleontologists, specializing in mosasaurs and dedicated to amassing sea fossils over several decades, have shed some light on its characteristics. Consequently, let us explore the intriguing traits of this majestic being.

Mosasaurs, together with their North American relative, Tylosaurus, are reputed to have attained lengths of 12–15.8 meters (39–52 ft, surpassing even the colossal T. rex dinosaur in size. Although the Tyrannosaurus certainly outweighed them significantly, its maximum length merely reached 14 meters.

It is essential to note that mosasaurs are distinct from dinosaurs, and their size was not universally gargantuan. Certain species were diminutive compared to an average American alligator, with some reaching mere dwarf-like proportions. For instance, Carinodens belgicus, a particular species of mosasaur, reached a fully grown length of only 2–2.5 metres (6.6–8.2 ft).

Like whales, these creatures were air-breathers, primarily inhabiting the ocean’s upper regions. Their presence is believed to have been prominent in areas such as North America and Western Europe, and there have been indications suggesting their existence in the waters around Japan and New Zealand as well.

Mosasaur Had Powerful Jaws

Similar to any aquatic predator, capturing prey underwater presents significant challenges. Mosasaurs, much like present-day snakes, possessed a concealed yet formidable advantage within their jaws, referred to as pterygoid teeth. These teeth were firmly connected to the upper jaw bones and exhibited a distinctive hook-shaped structure. Once a mosasaur successfully seized its prey, escaping for the unfortunate victim proved to be a formidable task. The discreet positioning of these teeth within the gum tissue made them less visually conspicuous compared to their portrayal in the Jurassic World film, rendering them substantially more difficult to detect.

Snake and Monitor Lizards Are Mosasaur’s Closest Relatives

Mosasaurs are known to be extinct reptiles that, much like snakes, possessed extra teeth. This characteristic also makes them related to modern lizards, especially creatures like the Komodo dragon. Consequently, it is believed that mosasaurs might have had forked tongues, just like their modern relatives. However, due to the rarity of soft tissue preservation in fossils, certain details like this remain challenging to ascertain definitively. Researchers endeavor to infer such features based on fossil records and other findings. Moreover, adding to our understanding of these ancient creatures often involves speculation and educated guesses.

Mosasaurs Gave Birth in Water

The weight estimation of a 15-meter-long mosasaur, approximately 5.5 metric tons (6.1 short tons), underscores the immense challenge of giving birth to such a large creature on the shore. Consequently, similar to numerous marine animals, these reptiles adopted the strategy of giving birth underwater, below the ocean waves.

This valuable information regarding mosasaur reproductive behavior was gleaned from the analysis of a Plioplatecarpus fossil. Thriving in both the northern and southern hemisphere oceans approximately 83 to 71 million years ago, this medium-sized marine creature provided crucial insights into the species’ reproductive practices. A fossil, affectionately known as the “mother mosasaur,” was serendipitously discovered in the United States, offering a unique glimpse into how these prehistoric reptiles brought forth new life. Notably, the fossilized remains of an unborn offspring were found preserved near the pregnant reptile’s body, providing compelling evidence of their underwater birthing process.

Mosasaurs Diversified Before Extinction

The exceptional success of mosasaurs is evident from the discovery of their fossils on every continent, including Antarctica. However, approximately 25 million years before their eventual extinction, this ancient family embarked on a path of diversification, giving rise to peculiar and novel forms. While traditional mosasaurs thrived in the open ocean, preying on fish, squid, and smaller reptiles, the emerging species exhibited distinct preferences in their choice of prey.

One such example is the Globidens species, characterized by its round teeth that were specifically adapted to crush hard shells. This adaptation enabled them to target and consume creatures with robust protective coverings.

Another remarkable member of the diversifying family was the Goronyosaurus, which boasted a skull reminiscent of a crocodile. This fearsome creature terrorized the rivers of Africa, establishing a unique ecological niche within its habitat.

Then there was the enigmatic Plotosaurus, sporting a peculiar, fish-like appearance with its slender body. This creature was notably designed for high-speed swimming, likely making it an agile predator in pursuit of its prey.

These fascinating variations within the mosasaur family illustrate their ability to adapt and explore different ecological roles, contributing to their remarkable success before ultimately succumbing to extinction.

How Realistic Is Mosazor in Jurassic World?

The portrayal of mosasaurs in the movie appears to be highly unrealistic and overly acrobatic. In reality, mosasaurs are not akin to killer whales and lack the capability to generate the required acceleration for the leaps depicted in the film. Considering the hypothetical size of the mosasaur in the movie, believed to be around 18 meters in length and weighing approximately 6 tons, executing such dramatic jumps in a shallow pool would be entirely implausible from a scientific standpoint.

Moreover, the film inaccurately presents mosasaurs using their flippers in a manner akin to thrashing around, causing friction in the water. In truth, mosasaurs employ their flippers primarily for stabilization, much like how crocodiles efficiently navigate through water. The misrepresentation of this aspect further adds to the departure from scientific accuracy.

Another notable inaccuracy pertains to the depiction of the creature’s skin as crocodile-like and tough. The reality is that real mosasaurs possessed relatively smooth skin adorned with only small scales.

As filmmakers portray prehistoric creatures on the big screen, maintaining accuracy is of utmost importance. Not only does it help educate the audience about these ancient animals, but it also contributes to an enjoyable cinematic experience. Striving for scientific authenticity ensures that the wonders of the past are presented in a manner that captures the imagination while staying true to the knowledge and understanding we have of these magnificent creatures from the fossil record.

Mosasaur at a Glance


What were mosasaurs_

Mosasaurs were a group of large marine reptiles that lived during the Late Cretaceous period. They were dominant predators in the oceans, resembling modern-day monitor lizards but adapted for an aquatic lifestyle.

When did mosasaurs live?

Mosasaurs lived during the Late Cretaceous period, approximately 70 to 66 million years ago.

Were mosasaurs truly marine reptiles?

Yes, mosasaurs were fully adapted to marine life. They were air-breathing reptiles, but they spent their entire lives in the ocean.

Were Mosasaurs dinosaurs?

No, Mosasaurs were not dinosaurs. They were marine reptiles belonging to the group called Squamates, which also includes modern-day lizards and snakes.

What caused the extinction of mosasaurs?

The extinction of mosasaurs occurred during the mass extinction event at the end of the Cretaceous period, around 66 million years ago. The exact cause of their extinction is still debated, but it is likely related to environmental changes and competition with other marine species.