Metriorhynchus

Streamlined Body and a “Moderate Snout”

The greatest characteristic of Metriorhynchus is its appearance, specialized for aquatic life and far removed from terrestrial crocodiles.

Transformation into a Fish-like Form

The heavy armor and scales seen in terrestrial crocodiles disappeared, and their bodies became smooth to reduce water resistance.

The heavy armor and scales disappeared, and the body became smooth to reduce water resistance.

Their limbs changed dramatically from legs for walking into “fins (paddles)” for paddling water, with the hind flippers being much more developed than the front ones.
Furthermore, the tip of the long tail bent downward, supporting a membrane of skin that formed a fish-like “vertical tail fin.”

Origin of the Name and Skull

The scientific name means “moderate snout” in Ancient Greek.
This is derived from the fact that its snout was neither extremely long nor extremely short compared to related species.
Additionally, the skull was built to be very lightweight, which is believed to be an adaptation for buoyancy control and quick movements underwater.

Hunting in the Jurassic Seas and Weaknesses

Metriorhynchus was an excellent hunter in the seas of that time.

Diverse Diet

It caught fast-moving fish with the sharp teeth lining its long, narrow mouth.
It also used the characteristic jaw strength of crocodiles to crush ammonites shell and all, or to attack other marine reptiles.
It has also been suggested that they may have jumped to catch pterosaurs flying near the sea surface.

Predation Risks

On the other hand, they also had weaknesses.

Lung Breathing

They needed to surface frequently.

Size

With a total length of about 3 meters, it was small compared to the giant plesiosaurs of the same era.

It is surmised that their swimming speed was not that fast, so escaping from large predators would have been difficult. However, given their wide habitat range, they likely had some kind of survival strategy.

The Biggest Mystery: “Eggs” or “Babies”?

The most debated topic regarding Metriorhynchus is its “reproduction method.”
Archosaurs (dinosaurs, crocodiles, birds) are fundamentally creatures that “lay eggs on land,” but what about these animals that had completely adapted to the sea?

Theory 1: Sea Turtle Style (Oviparity)

It was once thought that they came ashore onto sandy beaches only to lay eggs, like modern sea turtles.

Theory 2: The Amazing “Viviparity” Theory (Dominant)

In recent studies, the view that “it was difficult to support the body and walk on land” has strengthened due to the structure of the hip bones and fin-like limbs.
If they could not come ashore, it means they had acquired “viviparity,” giving birth to live young directly in the sea.
While viviparity in archosaurs is an evolutionary hurdle so high it is considered a “miracle,” it is garnering attention as evidence that they had achieved complete aquatic adaptation.