What Do You Know About Shell?

Ammonites were shelled cephalopods that died out about 66 million years ago. Fossils of them are institute all around the world, sometimes in very large concentrations.

The frequently tightly wound shells of ammonites may be a familiar sight, but how much do you know well-nigh the animals that once lived within?

What were ammonites?

Before we understood what they were, one of the explanations for ammonites was that they were coiled-upward snakes that had been turned to rock, earning them the nickname 'snakestones'. But ammonites weren't reptiles: they were body of water-habitation molluscs, specifically cephalopods.

An ammonite fossil with a carved snake'south head

Zoë Hughes, Curator of Fossil Invertebrates at the Museum, explains, 'Ammonites are extinct shelled cephalopods. All of them had a chambered shell that they used for buoyancy.'

The grouping Cephalopoda is divided into three subgroups: coleoids (including squids, octopuses and cuttlefishes), nautiloids (the nautiluses) and ammonites.

Ammonites' shells make the animals look almost like nautiluses, merely they are actually thought to be more closely related to coleoids.

'Some of their morphology was closer to that of the coleoid group,' says Zoë. 'We think information technology's more probable that ammonites would have had eight artillery rather than lots of tentacles like a nautilus, though the shell is more similar to that of a nautilus.'

Ammonites were born with tiny shells and, every bit they grew, they congenital new chambers onto it. They would movement their entire torso into a new sleeping accommodation and seal off their old and now also-small living quarters with walls known as septa.

Ammonites looked a scrap like nautiluses only are thought to exist more closely related to coleoids, a group that includes octopuses and cuttlefish © Esteban De Armas/Shutterstock

Zoë adds, 'The ammonite would take lived in one chamber, merely we don't know how often they built a new 1.

'Previously it has been suggested this could have been a monthly occurrence, just there is no evidence for that. Some studies looking at the chemical limerick of the shells - a field chosen sclerochronology - are starting to gain some insight of how long ammonites might have lived.'

Ammonites' growing shells typically formed into a flat spiral, known as a planispiral, although a variety of shapes did evolve over fourth dimension. Shells could be a loose spiral or tightly curled with whorls touching. They could be flat or helical. Some species would begin growing their shell in a tight spiral only straighten it out through later growth phases. There were also some more unusual shapes - the species Nipponites mirabilis, which is constitute in Nihon, is exceptionally rare and looks a flake like a knot.

Nipponites mirabilis ammonites grew in an unusual knot shape, rather than in a typical spiral

While ammonite shells are arable in the fossil record, information technology was only recently that scientists accept found a very rare fossil of the soft parts of an ammonite. However, fossilised testify of ammonite arms is nevertheless to be found.

Until now, a lot of what we know almost ammonites has been inferred based on what we run into in living cephalopods.

How old are ammonites?

The subclass Ammonoidea, a group that is oft referred to as ammonites, offset appeared well-nigh 450 million years ago.

Ammonoidea includes a more exclusive group called Ammonitida, also known as the true ammonites. These animals are known from the Jurassic Period, from nearly 200 million years ago.

Most ammonites died out at the same time every bit the not-avian dinosaurs, at the cease of the Cretaceous Menses, 66 million years agone.

Zoë says, 'We didn't quite lose all of them at the end of the Cretaceous. A few species connected into the Palaeogene in the Western Interior Seaway before dying out.'

The astroid that hitting World 66 million years ago and concluded the age of dinosaurs is also idea to have been responsible for the demise of most ammonites. Image by Donald E Davis courtesy of NASA/JPL-Caltech, via Wikimedia Commons

Why did ammonites go extinct?

At the finish of the Cretaceous Flow, an asteroid colliding with Earth brought on a global mass extinction. A lingering impact winter halted photosynthesis on land and in the oceans, which had a major impact on food availability and was devastating for ammonites.

Nautiloids, nonetheless, which had ancient relatives that lived at the same time as ammonites, survived this mass extinction. It is thought this is in part linked to these groups' preferred water depths.

Zoë explains, 'Nautilus survived probably because it lives deeper in the ocean. Deeper environments were less affected by what was going on in shallow water environments. This is a pattern that can be seen in other groups, aside from cephalopods – fish, for example.'

Ammonites generally died out 66 million years agone, but other cephalopods, such every bit nautiluses, survived © Manuae via Wikimedia Commons (CC Past-SA 3.0)

The size of hatchlings may also take played in the nautiluses' favour as they were larger and would have been less restricted by the size of food bachelor to them.

How many ammonite species were at that place?

Scientists can tell species of ammonites apart through several attributes including shell shape, size, historic period, location, features such as the number and spacing of ribs, defensive spines or shell-strengthening ornamentation.

We know this specimen of Kosmoceras phaeinum is a male from the long prongs, known as lappets, sticking out near the opening of the beat. They might take been used by the male person to hold onto the female during mating, a bit like shark claspers.

Just figuring out exactly how many species have been found so far is a flake catchy.

Similar modernistic cephalopods, ammonites displayed sexual dimorphism, which is the noticeable difference in appearance between sexes. But when ammonite fossils that looked unique were found in the by, they tended to be recorded as new species instead of as the microconch (male) or macroconch (female) of an existing species, as this divergence between the sexes was non yet known about.

Even so, information technology is estimated that over 10,000 species of ammonite - possibly fifty-fifty over 20,000- accept been discovered.

Zoë says, 'Ammonites were quite various and evolved rapidly, and so if you lot sample stratigraphically through rocks, you tin actually see the development and the changes through them.'

How big were ammonites?

Ammonites came in a range of sizes, from just a few millimetres to times bigger, with larger sizes more common from the Late Jurassic onwards.

The largest known species of ammonite is Parapuzosia seppenradensis from the Late Cretaceous. The largest specimen found is 1.eight metres in diameter but is besides incomplete. If it were complete, this ammonite's total diameter could have been from two.5-3.5 metres.

Parapuzosia seppenradensis is the largest known species of ammonite. Image via Wikimedia Commons

Where did ammonites live?

Ammonites lived all around the globe. Like their modern-twenty-four hours cephalopod relations, they were exclusively sea-dwelling. They tended to alive in more shallow seas and may accept had a maximum depth of virtually 400 metres.

What did ammonites eat and what ate them?

Though it would largely have depended on their size, ammonites would likely have eaten similar things to today's cephalopods, such every bit crustaceans, bivalves and fish. Smaller species would probably have eaten plankton. Some other species may accept been scavengers, like living nautiloids can sometimes be.

Ammonites would also have served every bit food for other marine animals. There is evidence of mosasaurs and ichthyosaurs having eaten them, and some fish would likely also take considered them prey.

Ichthyosaurs were amongst the marine animals that would have preyed on ammonites

Why are ammonites of import to science?

Ammonites can be a useful tool for scientists. Because they are and so common and evolved so chop-chop, they are excellent to assistance determine the age of the rocks they were fossilised in.

Much of the Mesozoic aged rock in Europe has been sectioned into 'ammonite zones', where rocks in different areas can be associated with each other based on the ammonite fossils found in them.

Zoë says, 'I've done a few identifications where there are bits of ichthyosaur and an ammonite has too been found, and they need it identifying. If yous can identify the ammonite, you tin really narrow things down. They're a really good indicator for biostratigraphy.'

Some other potential use for ammonite fossils could exist for telling united states of america about how animals responded to climate modify in the past.

Zoë explains, 'Shelled marine animals can assist us expect back into the past at what was going on in terms of climatic change post-obit extinction events. If we take known periods of warming or cooling, we can then infer that into mod climate science.

'Looking at size change will tell you an awful lot. Quite often later on an extinction upshot a lot of shelled animals shrink because they don't have the resources they need to abound. If there isn't the resources to build their shells, it'south a bit of a struggle for them. You see that in a lot of organisms.'

Detect how to make an ammonite out of table salt dough.

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Source: https://www.nhm.ac.uk/discover/what-is-an-ammonite.html