Gastropods vs. coiled (nautiloid) cephalopods
Both modern gastropods and cephalopods (squids) with hard shells have coiled shells. How can we tell these very different types of creatures apart as fossils if the animals inside are not preserved? First, we can look at the symmetry of the coiling. Most gastropod shells are coiled out of a single plane, which is termed conispiral coiling. In contrast, Nautilus, the modern cephalopod with a coiled shell, is coiled in the same plane, which is termed planispiral coiling. If you look at the Nautilus shell with the aperture opening facing you, both sides of the shell are the same size and shape, which is termed, bilateral symmetry. In contrast, if you look at most gastropod shells with the aperture opening facing you, and imagine a plane of symmetry through the middle of the shell, the right and left sides of the plane are not the same, which means they are not bilaterally symmetrical.
Although most gastropods are conispirally coiled and lack bilateral symmetry, some forms, such as Bellerophon, are planispirally coiled and have bilateral symmetry. Hence coil symmetry, by itself, may not always be able to distinguish between gastropods and coiled cephalopods.
Another way to distinguish gastropods and coiled cephalopods is to look inside the shells (where possible). Some fossils are naturally broken and provide views of the inside of the shell. In other cases, a fossil site may preserve many samples, and if there is a question concerning a gastropod or cephalopod identification, one of the samples can be broken to see inside the shell. The inside of a gastropod shell is a hollow twisting tube. In contrast, the inside of modern Nautilus (and ancient coiled cephalopods) is divided into chambers by thin shell walls called septae, which is why they are sometimes referred to as “chambered” Nautilus. Nautilus also has a thin twisting tube which passes through each chamber, called a siphuncle. If a coiled fossil has chambers inside its coil, or a siphuncle, it is a coiled (nautiloid) cephalopod.
Lastly, some types of shell ornamentation can be used to distinguish coiled cephalopods and gastropods. Some ancient nautiloid cephalopods are ornamented with more complex patterns (see ammonites and goniatites), which are not found on the outside of gastropods. If these patterns are observed, it is easy to identify the fossils as coiled cephalopods.
Usually coiling, the presence or lack of internal chambers, and some knowledge of the kinds of fossils known to have been found in a rock unit are enough to easily distinguish between gastropods and cephalopods. Many factors, however, can complicate these distinctions. Distinguishing between nautiloid cephalopods and gastropods is most difficult for small fossils and fossils which have been compacted in shales. Compaction of shells during burial and fossilization can result in shells which are only slightly conispirally coiled (out-of-plane of symmetry) appearing as if they are planispirally coiled. Also, when coiled fossils are preserved in surrounding matrix rock, the whole shell is not always visible, and some views may hide the true coiling or symmetry of the shell.
Content and graphics by Stephen Greb, Kentucky Geological Survey