Ammonites, which at first glance sound like some kind of stone, are actually a very famous extinct animal group in the history of the earth, and their historical status is no less than that of trilobites.

Ammonites are cephalopod animals, their modern close relatives are octopus, squid, nautilus and other animals, they also have a shell, and the appearance of the shell looks very similar to the modern nautilus, but paleontologists believe that they are genetically closer to octopus, squid and other subclass animals, so ammonite has "Shelled squidTitle.

Since ammonite fossils usually have only one shell, and the software part is basically not preserved, it is not known what the part of the ammonite that protrudes from the shell looks like.

However, it is now common to assume that cephalopods such as ammonites and squids also possess eight brachiopods, rather than having many tentacles like nautilus.

I've always wondered why previous Chinese paleontologists translated these animals as "ammonite", and some people say that it may be because the shell of ammonite is spiral-shaped, and this spiral has a line of connection, and the shape between the two lines is like the petals of a chrysanthemum, hence the name.

我們現在看到的大部分菊石化石確實都呈現螺旋狀，但其實這種形態是在2億年前的侏羅紀才佔據主導的，它們被稱為True ammonite（True Ammonida），而最早能稱為菊石的動物在4.5億年前就出現了，它們被稱為Ammonite(Ammonoidea)。

△ These are ammonites

The diversity of the shape of the shell of ammonites is striking,It can be described as growing at will, but they did not achieve much, true ammonites - that is, spirals of those ammonites - are the group of epic success in the ammonite family, and their fossils can be described everywhere in modern times.

Since the true ammonites and dinosaurs took the initiative for about the same time, some people call the Mesozoic Era "The land of dinosaurs, the sea of ammonites”。

有趣的是，菊石與恐龍的命運相似程度也體現在滅絕上，兩者都在白堊紀-第三紀滅絕事件之後消失，也就是6500萬年前小行星撞擊地球那次。

The extinction of ammonites is quite incomprehensible, as they have survived three of the worst mass extinctions in the history of the earth, and other cephalopods that are far less successful than ammonites have survived, especially the nautilus, which is so similar to ammonites.Why did the most successful ammonites go extinct?

△ The largest ammonite

Well, the jury is not out at the moment, but there are several hypotheses to explain this!

One hypothesis is that ammonites were already inevitably declining in the early Cretaceous。

Their decline is linked to the rise of other animals with similar ecological niches, such as the bony fish, which now floods the oceans and also rose during the Mesozoic Era.

Because the competition could not be compared to the latecomers, when the mass extinction came, the ammonite "pushed the boat" to give up the ecological niche and eventually became completely extinct.

Another hypothesis is that ammonites live mainly in shallow seas, and they adopt the R-strategy (producing a large number of offspring) and are adapted to rapid growth– This means that they need a lot of food to grow their shells.

When an asteroid hit the Earth and the Cretaceous-Tertiary extinction event occurred, the oceans became acidified and sunlight exposure decreased, which was a fatal blow to ammonite larvae, whose shells could not grow, so they eventually went extinct.

Nautilus survived because they live in the deep sea, grow slowly, and mass extinction events have relatively little impact on them.

What many people find difficult to understand is that there are also some ammonites that have adapted to the deep sea, and they even live in deeper seas than nautilus, why are they also extinct?

In fact, a small number of individuals are not representative of the entire group, and we do not know how many deep-sea nautilus perishes were extinct during the mass extinction, but the remaining nautilus species are deep-sea species.

For example, on the eve of a mass extinction, if there are 25 species of nautilus and 0 species of ammonites living in the deep sea, they share the niche of the deep sea, and when the mass extinction event comes, there are 0 species of deep-sea shelled cephalopods extinct (I see 0% of marine species extinct in the Cretaceous-Tertiary extinction event), then in this case, it will not be extinct with 0.0 species of ammonites and 0.0 species of nautilus, it is more likely that all ammonites will be extinct, and 0 species will be left with all nautilus.

In addition, ammonites did not completely disappear during the Cretaceous-Tertiary extinction period, and some species actually survived the mass extinction event, and after the environment improved, they continued to live and diverge for hundreds of thousands of years before finally becoming extinct.

In the same way, we cannot say that the extinction of ammonites was not due to the K-T extinction event, because a small number of individuals are not representative of the entire taxa, and a small number of ammonites are still extinct after the mass extinction, simply because they are in line with the background extinction rate of marine life.

△ This is also an ammonite, which can be played out of flowers, which should be called twisted into twist flowers more appropriately

There is also a third hypothesis, which is that the diversity of ammonites eventually buried them!

(This is an argument that I accidentally came across when I was writing this article, which was published in 2024 years, but the main purpose of the paper is to argue that ammonites did not decline before the Cretaceous, and to refute the first hypothesis we just mentioned.)

The reason why diversification made them extinct is that the Cretaceous-Tertiary extinction event came too suddenly and too quickly.

菊石在4.5億年前就出現了，它們確實挺過了多次大大小小的滅絕事件，每次滅絕事件它們也都受到了嚴重打擊，但都在後面迅速充斥海洋，並佔據統治級地位。

These extinction events are all spontaneous on earth, and it is a slow process, which gives the ammonite enough time to adapt.

In the Cretaceous-Tertiary extinction event, when an asteroid hit the Earth, in just a few months or even weeks, soot covered the Earth from sunlight, and acid rain began to rapidly deteriorate the marine environment.

Highly diverse ammonites have completely occupied all the ecological niches that their own taxa can occupy before their environmental degradation, which makes it extremely difficult for new species to emergeBecause the new species simply cannot find an ecological niche to adapt to, they must compete with other ammonite species.

The low probability of new species appearing means that it is difficult for their taxa to develop species that can adapt to the environment during the mass extinction, so they eventually become extinct.

last

It should be mentioned that although these assumptions seem to be fine when taken out individually, when you look at them more, there may be some contradictions between the assumptions.

In addition, even if we live in the Cretaceous-Tertiary extinction period, it may be difficult to explain why such a staggering number of taxa as ammonites suddenly disappeared.

So, not to mention that there are only fossils now, and the fossils of ammonites lack the software part, and we can now only guess what they eat.