Ammonites Were Doing Fine Until the Asteroid Hit

I must confess, I think asteroids and I think of movies like Deep Impact or Armageddon! Scientists think that an asteroid like the ones that appeared in the Hollywood blockbusters struck Mexico 66 million years ago and led to the extinction of the dinosaurs. It now seems they may not have been the only ones that were wiped from our planet. Ammonites are marine mollusks that flourished for 350 million years but they were wiped out too. Some research suggests they were struggling in North America but thriving in other parts of the world. 

Ammonites lived during the Mesozoic era and are related to modern day squids and octopuses. They had coiled spiral shells that were divided into chambers which were used to regulate buoyancy and their movement through the sea. Their fossil remains have been found across the planet on beaches up and down coast lines. The shells somewhat resemble the Fibonacci sequence (where consecutive numbers are added to produce the next; 0 and 1 becomes 1, then 2, 3, 5, 8 and so on) and it is this in part that has fascinated palaeontologists about the creature. 

Along with the dinosaurs, the ammonites were wiped from Earth 66 million years ago when a massive asteroid struck the Earth near what we now call the Yucatan peninsula in Mexico creating the stunning feature; the Chicxulub crater. It’s not just the dinosaurs and ammonites that are thought to have been taken to extinction but a total of 75% of all species are thought to have vanished from Earth in the cataclysmic event. It is thought the asteroid that struck Earth was 10 kilometres in diameter and released energy equivalent to billions of atomic bombs. 

Chicxulub crater in Mexico. Credit: Wikipedia/NASA

Palaeontologists have argued that the ammonites were already declining and that their extinction was unavoidable around the end of the Cretaceous period but new research published in Nature Communications shows that they may not have been so close to extinction after all. The paper by lead author Dr Joseph Flannery-Sutherland and team from Bristol University reveal that the evolution of the ammonites ahead of the asteroid impact was really quite complex. 

Using fossils alone to unravel the way a species like ammonites changed over time is difficult. According to Dr Flannery-Sutherland ‘The fossil record tells us some of the story, but it is often an unreliable narrator. Patterns of diversity can just reflect patterns of sampling, essentially where and when we have found new fossil species, rather than actual biological history.’ He goes on to explain that by analysing just the late cretaceous ammonite fossil record as though it was the full story is why the wrong conclusion has previously been drawn. It is more complex. 

The team created a new database of all fossils collected to date, using museum collections, university samples and any specimens available rather than just rely on previously published papers. This helped the team to build a more complete picture from source data. 

The database allowed the team to understand how ammonite extinction rates and speciation rates (how quickly a species gives rise to new species) varied across the world. If extinction was underway during the late cretaceous period then extinction rates would be greater than their speciation rates everywhere. Instead, across a wide range of geographies, the extinction and speciation rates varied considerably. Possible causes for the variation may have been merely environmental factors like ocean temperatures and sea level to predators and competition among ammonites themselves.

Source : Ammonites’ fate sealed by meteor strike that wiped out dinosaurs

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