Evolutionary Giants Reacting to the Growth of Prehistoric Predators in Ancient Ammonites

The specimen was gathered close to Seppenrade in 1894. Münster’s LWL-Museum for Natural History Image: CI. 100 mm is the scale. Credit: Creative Commons (https://creativecommons.org/licenses/by/4.0/) Ifrim et al., 2021, PLOS ONE

A small group of researchers from Germany, Mexico, and the United Kingdom have discovered evidence indicating that the growing size of the mosasaurs that preyed on a patch of ammonite led to their transformation into giants. The group reports their comprehensive research of 154 specimens of two species of ammonite fish and what they learnt about them in a publication published on the open-access journal PLOS ONE.

A type of mollusc with a coil shell that extincted millions of years ago are known as ammonites. Their characteristic frilled suture lines made them stand out. Previous studies have indicated that the average diameter of an ammonite was not greater than half a meter. However, there is one species that is more common than the others: Parapuzosia seppenradensis, which can reach up to 1.5 or 1.8 meters in diameter. With this new endeavor, the scientists aimed to determine the cause of P. seppenradensis’s enormous growth.

The team’s strategy involved first learning more about ammonites in general. To that aim, they gathered 154 specimens from different institutions, all of which belonged to two species: Parapuzosia leptophylla and P. seppenradensis. They discovered evidence throughout their investigation that P. seppenradensis diverged from P. leptophylla when the latter started to migrate from the coasts of what is now western Europe to the ̘ϻoγȩ of what is now the Americas.7

Following the separation, P. seppenradensis grew larger. The reason they started to grow could have been caused by events like changes in climate, but the researchers were unable to locate any conclusive evidence for this. However, they did discover that a large number of mosasaur specimens started to enlarge at roughly the same moment.

The primary predators of ammonite are thought to have been enormous marine reptiles. The larger P. seppenradensis were harder for the reptiles to put in their mouths, making it harder for them to procreate, according to the researchers, which is why they started to expand.

The researchers admit that their idea has a flaw in that earlier studies have demonstrated that mosasaurs continued to grow in size even after P. seppenradensis achieved its peak size. Subsequently, P. seppenradensis started to exhibit reduced growth for inexplicable reasons.

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