80 million years ago, a mollusc measuring 1.7 meters in diameter propelled itself through the waters of the Atlantic in its shell. At that size, the species Parapuzosia seppenradensis was the largest ammonite in the oceans, and one of the largest mollusks known. Now researchers understand a little better why the animal may have grown so large.

In this regard, a team of researchers from Germany analyzed the fossils of 154 ammonites from various times during the Cretaceous period. 11 of these fossils belonged to more primitive and smaller species of these mollusks, mainly the P . leptophylla .

According to the research, available in the journal PLOS One, the giant species of this mollusk had a much wider distribution along the Atlantic Ocean.Fossils of P. seppenradensis can be found both off the coast of Mexico and in the U.K. Whereas the smaller ancestors of these giants were restricted to Europe during most of the Cretaceous.

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It turns out that during later Cretaceous periods, the P . leptophylla (smaller and older species) were able to leave Europe and migrate to the Gulf of Mexico, for example.

In this sense, the team believes that these giant ammonites probably evolved from their smaller relatives, P . leptophylla Thus, the P. seppenradensis developed such a large size after the spread of its ancestor.

However, it is not yet known exactly why the P. seppenradensis An interesting hypothesis, however, is that their predators also grew a lot. And they were really giants.

Relation of the mosasaurs to this giant ammonite

80 million years ago, the planet's continents were teeming with tyrannosaurs, triceratopsids and sauropods. However, the ocean was just as teeming with giants as the land. Mosasaurs were the major predators of the seas during the Cretaceous period, so researchers believe they influenced the size of the Ammonites.

It turns out that mosasaurs appeared more or less 92 million years ago, forming a highly diverse group of marine reptiles (mosasaurs were not dinosaurs). These animals, therefore, could have sizes up to 18 meters long.

However, the first mosasaurs were smaller and the species grew over time. What happens is that the time when mosasaurs grew the most coincides with the time when the P. seppenradensis have reached giant sizes.

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It is worth commenting that there is evidence that mosasaurs fed on ammonites, although there is no data that they preyed on the P. seppenradensis in specific.

Still, the mosasaurs may have created selective pressure for the Ammonites to grow so much. That's because an Ammonite that didn't grow enough was easily eaten by a giant mosasaur. In other words, genes for smaller animals were decreasing in frequency, giving prevalence to larger animals.

Still, this is only a hypothesis. As stated, there is no direct evidence that mosasaurs actually ate this species of ammonite. However, the research shows a possible interpretation for the emergence and evolution of these large shell mollusks.

The research is available in the journal PLOS One.