We May Finally Understand Why These Gargantuan Mollusks Got So Huge

Monsters roamed Earth in the late Cretaceous (around 80 million years ago).
It's not just titanosaurs and tyrannosaurs. Even small animals can be super-sized. This was the time when the size of a marine mollusk reached its peak, with the largest ammonite species reaching 2.5 meters (8.2 ft) in length.

Scientists have long sought to determine why this ammonite is so large.

After studying fossilized remains from 154 ammonites in a variety of sizes, a team of scientists led by Christina Ifrim, a paleontologist at Heidelberg University, Germany, thinks they may have an answer: how to avoid being eaten or drowned by the fearsome aquatic mosaurs.

Evolutionary pressure is an interesting phenomenon. This happens when a population is subject to stress that can affect its reproductive success. Individuals with the best traits to survive stress are more likely than others to reproduce and to pass those traits onto their offspring.

One example of evolutionary pressure is the development by microbes of antibiotic resistance.

Predation is an important driver of evolutionary pressure. There are other factors that can influence evolution, such as disease and climate. It can be difficult to determine which factors played a role during the evolution of a species if there is not enough information.

Information has been scarce for the giant ammonite Parapuzosia Seppenradensis. Because of the limited fossil record, it's difficult to make comparisons with other Parapuzosia ammonites.

Ifrim and her collaborators examined all known specimens of Parapuzosia seppenradensis and Parapuzosia septophylla in their new paper. They also included historical fossils and a wealth of new fossils from Mexico, England, and Mexico.

This includes 11 specimens from the rarer P. seppenradensis and 142 of P. leptophylla. The 154 specimens were a range of sizes from 0.1 to 1.8 meters. This allows for the most comprehensive study of the entire development cycle of P. Seppenradensis.

The team determined that P. seppenradensis was derived from P. leptophylla based on the study of these animals. This happened after the latter had spread from a limited habitat near the coasts of western Europe to the western Gulf of Mexico.

P. seppenradensis evolved from its ancestor just over 80 million years ago and became a larger animal. P. leptophylla's largest size was 0.7 meters. This is still a large animal, but not as big as its descendant.

Although it's difficult to give a definitive answer to why the creatures grew in size, the team was able make some guesses. While there may have been some climate shifts that played a part in the shift in size, they are not sufficient to account for the whole thing.

The ammonites were growing in size at the same moment as another creature, the aquatic reptile genus Mosasaurus. These powerful predators of Cretaceous seas were well-known to prey upon ammonoids. Their rapid growth in the Late Cretaceous coincided closely with that of P. seppenradensis.

It's logical. If you can't wrap your fingers around a shell it will be difficult to eat the delicious mollusk within.

This is not the best answer. While mosasaurs grew, P. seppenradensis began to shrink. Researchers said that this clearly had nothing to do with mosasaurs. It may take further analysis to determine how closely the two species might have been interrelated.

Based on the current information, it seems to be the best match.

Researchers write that they have not found any other short-term trend in the global environment that is associated with the ammonite's increase in size across the Santonian/Campanian border.

The research was published in PLOS ONE.