Some fossilized species of ammonites grew almost two meters in diameter—human size. No living ammonite grows anywhere near that large. This article tries to explain why ammonites devolved from giant to small. Even their skeleton-key cause of ‘climate change’ is offered only as a passing suggestion. Their remaining hypothesis about an evolutionary arms race with a predator falls apart on inspection:
After a split occurred, P. seppenradensis began to get bigger. The researchers were not able to find any conclusive evidence to explain why they began to grow, noting that it could have been due to events such as changes in climate.
But they did find that many species of mosasaurs began to grow bigger at around the same time. The large marine reptiles are believed to have been the main predators of ammonite. The researchers suggest the reason P. seppenradensis began to grow was because the larger they were the more difficult it was for the reptiles to fit them in their mouth—those that were bigger survived to reproduce.The researchers acknowledge that there is one kink in their theory—prior research has shown that as P. seppenradensis reached its peak size, mosasaurs continued to get bigger. And after a while, P. seppenradensis began to get smaller again for unknown reasons.
A related article on Live Science is more direct about the supposed “selective pressure” to grow big because of predation:
Regarding the former question, the ammonites may have faced an evolutionary pressure to grow because a major predator of the Cretaceous, marine reptiles called mosasaurs, also grew larger during this time, the authors noted. However, although there’s evidence of mosasaurs preying on ammonites, there’s no direct evidence that they interacted with P. seppenradensis, specifically, Ifrim told Live Science. So, for now, this is just speculation.
None of these articles mention the complex organs, brains, behaviors and mathematical precision of these beautiful animals. The article
also notes that giant ammonites appeared at “more or less the same time” on both sides of the Atlantic. The source paper at PLoS One mentions “retrograde evolution” of these complex animals, saying, “the subsequent retrograde evolution of Parapuzosia from the late early Campanian on is clearly unrelated to the further increase in size of mosasaurs.”Q: So does selection pressure of larger predators drive larger prey size?
Q: Or does it drive smaller prey size?
Both—or maybe neither. One wonders what evolution has to do with any of the science here." CEH