NIPPONITES: THE AMMONITE THAT TIED ITSELF IN KNOTS

Nipponites mirabilis photo by Masahiro Miyasaka

Of all the unlikely body plans, Nipponites take the cake! Just look at the twists and turns in these great photos captured by Masahiro Misaka. 

Imagine slipping beneath the surface of a Cretaceous sea. Above you, sunlight breaks into shifting ribbons. 

Tiny particles drift through the water, catching the light before disappearing into blue.

For our imagined encounter, we are travelling to the ancient seas of what is now Japan. Somewhere ahead, a small shape comes into view.

At first, it looks like a piece of tangled cord.

Then you notice the ribs along its surface. The looping curves. The extraordinary shell that seems to have doubled back on itself after forgetting where it was going.

Meet Nipponites mirabilis, an ammonite with a shell resembling a ball of wool after a kitten has finished with it.

If you have ever struggled to untangle a necklace, you may feel an immediate connection.

Nipponites mirabilis photo by Masahiro Miyasaka

Most of us picture an ammonite as a handsome spiral: a shell curling neatly around itself, each successive whorl following the last. 

Nipponites takes that familiar outline and sends it wandering through three dimensions.

Ammonites were marine molluscs belonging to the cephalopods, the broader group that includes living squid, octopuses, cuttlefish and nautiluses. 

Their chambered shells helped them remain buoyant. The animal occupied the outer living chamber, adding to its shell as it grew.

Some ammonites developed shells that departed dramatically from the familiar flat spiral. These are called heteromorphs. Among them, Nipponites is particularly good at making people stop, lean closer and say, “Surely that cannot be right.”

Yet there it is.

Its shell raises a delicious palaeontological puzzle. How could an animal carrying something so apparently awkward stay balanced in the water?

As you might have guessed, some very good folk decided to try and answer that question. 

In a study published in 2020, David J. Peterman, Tomoyuki Mikami and Shinya Inoue investigated Nipponites mirabilis using digital reconstructions. Their models examined 14 stages of growth, allowing them to explore how the animal’s balance changed as its shell developed.

The results suggested that it could achieve neutral buoyancy throughout each stage. In other words, its shell need not have condemned it to life on the seabed.

N. mirabilis (left); Ainoceras kamuy (right)

The modelling also suggested a fascinating possibility: its unusual form could have helped it turn around a vertical axis. 

The researchers proposed a relatively low-energy lifestyle, slowly rotating while searching for planktonic prey.

Picture that for a moment.

A little living sculpture, suspended in seawater, gently turning as the surrounding ocean carries potential meals within reach, rather like a rolling buffet. Our imagined animal has become rather more graceful.

There is another surprise hidden in those bends. Nipponites began with a more conventional open coil before developing alternating U-shaped turns. Its growth followed a recognisable pattern. The apparent jumble had underlying order.

Of course, a model cannot show us an actual feeding animal. The proposed pirouetting lifestyle remains an interpretation, and many details of its behaviour are unknown.

That uncertainty is part of the pleasure.

Look again at the fossil. Follow a curve with your eyes. Watch it turn away from you, return and disappear behind another loop. Then imagine water filling the spaces between those bends.

What seemed cumbersome in a display case becomes a question about movement. What looked like a tangle becomes a record of growth.

Fancy reading their paper? You can find more here:

Peterman, D. J., Mikami, T. and Inoue, S. (2020). The balancing act of Nipponites mirabilis (Nostoceratidae, Ammonoidea): Managing hydrostatics throughout a complex ontogeny. PLOS ONE, 15(8), e0235180.