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Discovering the Mosasaurs of the Late Cretaceous

Updated: Apr 6

Mosasaur (meaning "Meuse lizard") were large ancient marine reptiles and the apex predator of the Late Cretaceous oceans and epicontinental seas feeding on fish, turtles, birds and cannibalism. With a global distribution of over 40 genera of mosasaur measuring up to 18 meters in length they were highly adaptable and able to live in both marine and freshwater ecological niches after the demise of the ichthyosaurs, Thalattosuchians or marine crocodiles and plesiosaurs. Mosasaurs belonged to the largest order of reptiles known as Squamata (meaning “scaly”) including lizards and snakes. Mosasaurs evolved from a terrestrial lifestyle to thrive and survive in seas and oceans that were experiencing a severe global oceanic biogeochemical anoxic event and biodiversity loss until their extinction at the Cretaceous–Paleogene (K–Pg) boundary.

A Late Cretaceous Mosasaur
A Late Cretaceous Mosasaur

The first fossil of a Mosasaur skull was found in a Dutch chalk quarry near Maastricht in 1764 and is locally known as the "Maastricht mosasaur". This fossil now resides in the Teylers Museum in Haarlem. The word Mosasaur is named after the Meuse River that runs through the city of Maastricht located in the south east of The Netherlands.

Mosasaurs have a complicated evolutionary tree but research into their fossilised morphology including body form and body structure suggests that their ancestry lays within the suborder of Anguimorph lizards.

These lizards are characterised by having an elongated body, reduced or absent limbs and have a long tail such as the Monitor and Alligator lizards.

The ancestor to the Mosasaur was a small swimming reptile that first emerged in Late Cretaceous epoch fossil record during the Cenomanian Age (100.3 – 93.9 million years) and by the Turonian Age (93.9 – 89.4 million years) the true marine Mosasaur had emerged and was fully adapted to live in the oceans and seas.

The Mosasaurs along with over 75% of all other species of life including non-avian dinosaurs would disappear during the Cretaceous–Paleogene (K–Pg) mass extinction event after a 10 Km wide asteroid called the Chicxulub Impactor smashed into the Yucatán Peninsula located in Mexico 66 million years ago.

Unlike other lizards that are cold blooded or ectothermic the characteristics of the Mosasaur show that they were warm blooded or endothermic and were able to maintain a constant body temperature independent of the environment. Mosasaurs were also viviparous and gave birth to live young in the water rather than laying eggs on land.

Komodo Dragon - Image by Guillaume Marques
Komodo Dragon - Image by Guillaume Marques

An interesting debate is called the Toxicoferan Hypothesis an evolutionary biology concept that explores the origin and development of venom glands in reptiles as an adaptation for hunting and defence.

In some small part this hypothesis reflects on whether the Mosasaur was venomous. The origin of venom is synonymous with the Toxicofera group or clade of scaled reptiles that is thought to also include the ancestor to the Mosasaur.

Did the Mosasaur use venom to subjugate, immobilise and incapacitate its prey along with a devastating bite? did they have venom glands? or were their venom glands vestigial in that they may have been present in some form but had lost their original function?

At this time there is no definite answer but various species of Mosasaurs had a variety of shaped teeth that not only pierced their prey but also cut, tore and crushed.

A distant relative to the Mosasaur is the Komodo Dragon (Varanus komodoensis) the largest extant Monitor Lizard species living today. They have a venomous saliva delivered in their bite that stops their prey's blood from clotting resulting massive blood loss and shock. 

Mosasaur Jaw and Teeth - Image by ian35mm
Mosasaur Jaw and Teeth - Image by ian35mm

The timing in the evolution of the Mosasaur is potentially an interesting factor to their success in becoming an marine apex predator.

At the Cenomanian–Turonian boundary a complex relationship spanning between 94.5 and 93.9 million years was underway where the Earth’s geology, biology and climate had created a greenhouse world where gases in the Earth's atmosphere trapped the Sun's heat.

There was rapid atmospheric and sea warming, the hottest for the Phanerozoic Eon, from vast submarine volcanism and eruptions of large igneous provinces that physically reorganised the oceans and formed many epicontinental or in-land seas.

The tectonic activity led to huge releases of carbon dioxide and this also set the conditions for oxygen depleting phytoplankton to bloom and for ocean acidification to happen.

The outcome was a severe global oceanic biogeochemical anoxic event known as OAE2 or the Bonarelli Event named after the Italian geologist and oil explorationist Guido Bonarelli (1871–1951). This anoxic event occurred between the tropical and temperate latitudes below 60° north and south of the then Cretaceous Period palaeoequator. 

OAE2 disrupted the oceans oxygen and sulphur cycles leading to a marine biodiversity crisis and the mass extinction of approximately 8% of marine families and over 26% of marine genera a taxonomic category of related animals sharing similarities or specific characteristics such as reef-building rudistist bivalves as well as the ichthyosaurs and plesiosaurs.

OAE2 is identified in the stratigraphic record worldwide by a marine deposition of organic-rich black shales.

By the end of the Cenomanian–Turonian boundary global cooling had began as marine circulation patterns and the efficiency in heat transfer began to improve between the tropical and polar regions.

This was now the age of the Mosasaur.

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