Fossil of the Week

Picture an ammonite and you’re probably thinking about something that looks rather like a discus – flat and coiled only in one plane in the typical planispiral shape that’s familiar in most of the ammonoids and nautiloids. Heteromorph ammonites, however, are not “normal” ammonites. In fact, they grow into all sorts of strange shapes and look really weird. The one pictured here [Didymoceras sp., from the Campanian Stage (Upper Cretaceous Series, ca. 80 million years ago) of Bélo sur Tsiribina, Madagascar, PRI K22064, Klose Collection] is relatively tame. It’s still basically a spiral, just a very loose one. Many of the heteromorphs remained quite coiled, just not in one dimension, sometimes ending up looking rather like gastropod shells, while some started growing as standard planispiral shells then de-coiled. Some groups, though, abandoned normal ammonite coiling altogether and came almost totally unraveled. Some became hooked J shapes, some U shapes, or even trombone-like. Still others straightened out completely and looked superficially like belemnites or orthoconic nautiloids. Some of the heteromorphs seem to have just tied themselves in knots!

We tend to think of ammonites as having been speedy predators, propelling themselves through the ocean by jet propulsion. But the heteromorphs don’t look particularly streamlined, and in fact, biomechanical and hydrodynamic studies of models in flume tanks support the idea that these odd ammonites probably weren’t going anywhere fast. Some probably floated around in the open ocean, eating whatever happened to pass by (rather than actively hunting), and some might actually have been benthic, living slow or sedentary lives on the sea floor.

It is not clear what adaptational advantages favored the evolution of heteromorph ammonites, but they are common and occurred in a number of lineages throughout the history of the group. Some of them (such as the straight Baculites that could reach up to 6½ feet, or 2 meters, long) were extremely diverse, so they must have been doing something right. At the beginning of the twentieth century, they were cited as being a wonderful example of a phenomenon dubbed “racial senescence” in which a lineage was thought to go through a period of degeneration as it declines and eventually becomes extinct. The whole idea of racial senescence has since been abandoned, but it is clear that heteromorph ammonites don’t fit that pattern anyway: they were a successful, if strange, part of the ammonite story.

Text by Ursula Smith (reprinted from “Fossil Focus” in American Paleontologist, Summer 2010)