Tag: ancyloceratina

Strange Symmetries #16: Go Home Heteromorphs You’re Drunk

Most ammonites had spiral-coiling shells, but during the Cretaceous a group known as the heteromorphs evolved a much weirder range of forms. Some were straightened, some were hooked, some had helical snail-like shapes, and some even ended up bearing a strange resemblance to paperclips. 

But one of the most bizarre of all was the genus Nipponites, whose ribbed shell looked like a bundle of tangled asymmetrical coils.

Nipponites bacchus lived in what is now Hokkaido, Japan, during the late Cretaceous about 90 million years ago. Around 10cm long (~4″), its shell was less tightly coiled up than its better-known relative Nipponites mirabilis, but these looser whorls were formed in the same way via a series of U-bends in different directions during its growth.

Despite their irregular and ungainly appearance, the unique shape of these ammonites seems to have actually been very hydrodynamically stable. They weren’t fast-moving, but they didn’t need to be, probably spending most of their time floating suspended in the water column catching small planktonic prey from around themselves.

It Came From The Wastebasket #09: Hammering Away At Hamites

Ancyloceratines were a lineage of distinctive-looking ammonites, commonly known as “heteromorphs“, which had unusual uncoiled shells that ranged in shape from near-straight to hooked to helical to paperclip-like to “balls of string“.

Heteromorphs’ strange shells would have created a lot of drag in the water, and they may not have been especially agile swimmers, but they were very hydrodynamically stable and easily maintained neutral buoyancy. Their paleobiology has only just started to be properly understood in recent years, and now most species of these ammonites are thought to have floated suspended in the photic zone and twilight zone of the open ocean, catching small zooplankton from the water around themselves.

What these ammonites were doing obviously worked very well for them, because they were incredibly diverse and successful during the Cretaceous period. They were also the only type of ammonite to persist for a short time after the end-Cretaceous mass extinction, existing as a “dead clade walking” for another half a million years or so before finally disappearing entirely.

An illustration of Hamites attenuatus, an unusually-shaped extinct ammonite. It has a mostly-uncoiled shell shaped like a long sideways U, almost paperclip-like. At the bottom end of the shell it has a squid-like body, with large eyes and ten short webbed arms that are lined with speculative fringes for filter-feeding. It has a pinkish color scheme with a faint iridescent sheen.
Hamites attenuatus

The hamitids were a group of heteromorphs from the mid-Cretaceous (~110-90 million years ago), with their namesake genus Hamites traditionally being used as a wastebasket taxon for anything that that didn’t neatly fit into any other group of similar heteromorph ammonites.

By the late 1990s Hamites had become a mess of multiple different diverse lineages, with over 20 species all lumped together – and this was a problem because the hamitids were the ancestors of several other heteromorph ammonite lineages, and having the taxon in such disarray made studying the evolutionary origins of all those other groups very difficult.

So in the early 2000s attempts were made to clean this all up, figuring out the relationships between the different Hamites species and dividing the genus into multiple new genera.

There hasn’t been much more detailed research on the relationships of hamitids since then – and other groups of heteromorphs are still in need of revision – but it’s a start at clearing the wastebasket, at least.