This group includes includes clams, oysters, snails, octopus, squid, and others, including a huge list of ancient organisms. Primitive members of the group possess a fleshy, muscular "foot" and a simple calcite shell. The muscular "foot" is not a normal foot at all, and is called that, because it was used in locomotion, just as a snail crawls along on its "stomach foot." As implied, the "foot" in in primitive mollusks was modified during evolution in the varied mollusk lineages.
Snails have their fleshy, sticky, muscular "stomach foot." The group that includes snails is called Gastropoda (Gastro = stomach; pod = foot). Shells of snails are coiled about an axis, in a spiral:
The snail above is shown from above, so you don't see how the shell spirals up and around a central axis.
But in this snail fossil, lying sideways, you see how the shell wraps in a spiral about an axis. This snail shell has little bumpy ornamentation -- as with any group, such ornamentation is important for identification of the species, or broader group, if you can't get that specific.
You are seeing the tip of the iceberg for types of coiling in snails, from low, squat forms, to high-spired ("pointy") forms, and everything in-between.
This a distinctive Paleozoic snail, which has a narrow ridge wrapping around the outside edge of the shell.
As with any fossil group, we often don't get to see full specimens in three dimensions. Often, we have obscure partial pieces. Luckily, for snails, at least, it is easy to spot the characteristic look of a snail in partial or complete cross section, which you often see on broken pieces of rock.
Clams and their relatives have their muscular main part of the body, which many used in burrowing, by projecting it out into the mud and expanding and contracting it -- some clams can burrow fast. Clams, oysters and other fossil groups with two valves, two "halves" of the shell, are members of the group called Bivalvia (Bi = two; valves, or sides).
The diversity within Bivalvia is high. Let's start with a garden-variety clam:
Looks like an assemblage of shells you could find on a modern beach, eh? Sure. Clams from millions of years ago, just like snails and many other familiar animals, generally look the same. After death, the soft tissues holding the two halves of the shell together decay, so the shells come apart (Compare back to brachiopods, for which the musculature arrangement is quite different, so that when brachiopods die, their two shell parts stay together, so we commonly find "whole" brachiopods).
Another, rather plain-Jane clam fossil.
But this more elongate clam shows more pizazz. That elongate shape is an indicator for deep burrowing. This clam had a huge muscular extension that it used to burrow. Clams also can stay down in a burrow, extending out a kind of telescoping tube up to the surface for feeding. The burrowing attitude -- the position of the body -- would have been "up and down," not sideways, as the specimen is shown.
And now for some oysters, which as fossils, in some cases, were much bigger than modern forms. Oysters don't have "mirror image" valves. They have one larger valve and another "capping" valve:
These Cretaceous oysters are the genus Gryphaea , nicknamed "Devil's toenails," which is a rather gross reference to the general appearance and curvature.
This one has a distinctive coarsely "saw tooth" connection between the two valves, which you can't see so well in this specimen, because of the top-down view.
Give me a dozen oysters on the half-shell please. Oh, you have Cretaceous oysters on the menu, specifically the genus Exogyra ponderosa ? Well, in that case I just need one, and could I have an extra plate?
This clam-like oyster relative doesn't look like much, especially when you look closely at the pencil marks drawn on by bored students (This specimen is in an introductory geology lab box). But this Cretaceous bivalve, Inoceramus , is distinctive because it was quite common and got to be quite large, up to a meter or so.
The last bivalve shown here, and certainly not the least, is this rudistid bivalve. Yes, that thing is actually related to a clam. Like the oysters, one valve is larger than the other -- in the rudistids, extremely so. This one is shows the larger valve (the small valve was but a cap), with the quarter at the bottom. Rudistids were Mesozoic only, and formed barrier reefs, along with corals and other reef-building organisms.
The group that includes the octopus and squid is called Cephalopoda (Cephalon = head; pod = foot). Cephalopods started out, way back in the Cambrian, as shelled organisms, but with tentacles and a predatory form of life, seen in the surviving members of the group, such as the octopus and squid. Some fossil cephalopods had straight shells, but most had coiled shells. The shells are coiled, not about an axis like in snails, but within one plane, forming what is called planispiral coiling. Cephalopoda includes several groups, but Nautiloidea and Ammonoidea are very both important groups with many fossils.
We can use the living chambered Nautilus as a guide to the group. This is a complete shell of Nautilus , without the octopus-like soft body that occupied the living chamber (that big opening) during life:
In life, the animal would have been positioned, swimming within the water column, with the living chamber down, but it is shown above as it was photographed sitting on a table. Rotate it in your mind -- and stick an octopus in the living chamber, to get the picture. You see a more upright view below.
If you saw through a Nautilus shell along the plane of the shell, you see that the living chamber is but the last of a series of living chambers, separated by successive secretions of of "back walls." Focus your eyes on the very center of the spiral -- that was the shell when the animal was a baby. As it grew, it needed a larger and larger shell to accommodate its growing body. The animal continuously lays down new calcite along the leading edge of the living chamber (at left, as you view this specimen). On a regular interval, as it grows larger, the animal secretes a new back wall to the living chamber. Notice closely that there is calcite remaining from a narrow tube that, in the living animal, connected the chambers (look at the center of the successive back walls). Through this tube, the animal can regulate the pressure inside the shell to affect movement up and down within the water column. That's really cool, but is only the beginning of our discussion of back walls. We'll refer back to this specimen.
Here is a fossil nautiloid:
First, in viewing the nautiloid fossil above, realize that you are looking at the outside of the shell, after it has been filled up with mud (and buried, and hardened to rock, along with the sedimentary layer that contained it, before somebody dug it out). Second, make out the gently curving lines -- these are the suture lines, the lines made by the contact between the successive back walls of living chambers (present during the animals lifetime, as it grew to become this large). Look back at the sawed-through nautiloid shell above, and focus your attention on the contact made by the back walls and the outside of the shell, and think about how these sinuous (S-shaped) lines of contact might appear as viewed from the outside of the shell. Suture patterns are simple like this in nautiloids -- just a gentle curving, without any complication. You can see this beautiful simplicity in the sawed-through shell above.
Incidentally, nautiloids weren't always coiled like this. During the early Paleozoic, there were straight-shelled forms that got up to 6 meters long!
The nautiloids are Cambrian - Recent. Their cousins, the ammonoids lived from the Devonian to the end of the Cretaceous, when they disappeared with the great extinction at the end of the dinosaur age, along with many other forms of life. The ammonoids started out with a fairly simple suture pattern, similar to the nautiloid simple-sinuous curvature, but with an added zig, zag or two:
Do you see that there is a simple, but "zig-zag" suture pattern? This is called a goniatitic suture, the more primitive type within the history of evolutionary change within Ammonoidea. Often within the course of evolution, given a starting point in some shape of a feature, the shape can become more complicated. One reason for the added zigs and zags is that it makes the shell stronger, perhaps enabling the animal to inhabit deeper water (more pressure on the shell). From the goniatitic suture of the Paleozoic, in fact, we move to more complicated suture pattern, wildly so for some groups, within the Mesozoic. Here are a few example Mesozoic ammonoids:
In this rather small specimen, you don't see a suture pattern -- you can't always see one -- but you do see a bumpy external anatomy, that probably makes this ammonoid identifiable to an expert.
You are also looking at external anatomy of the shell in this specimen. Although these particular specimens are small, ammonoids got up to the size of tractor tires! Yes, because they lived as swimmers or free-floaters within the water column, and could move freely about, sometimes swimming with purpose, perhaps cruising above the bottom looking for prey, and sometimes moving along with the currents. With that lifestyle, there is not a great pressure against getting larger, and many were about the size of truck tires, with a few extremes getting up to tractor tire size.
Likewise, only external shell anatomy is visible in the specimen above, but you get a taste of the wide variety of other features -- in addition to the suture pattern -- that are important for identifying ammonoids. For instance, in the following ammonoid, the coiling, normally restricted to a single plane of coiling, "loosens out," so that coiling is somewhat snail-like:
This is the Cretaceous ammonoid Turrilites , distinctive for its spiral coiling pattern. That's not the culmination of ammonoid shell complexity. That came with the Cretaceous genus Baculites , which not only "loosened up" the coiling as it grew, it even became straight-shelled, in a fashion similar to the straight-shelled nautiloids of the early Paleozoic. The straight shell in adult Baculites is not the most distinctive feature. It is the crazy complications added to the suture pattern! Take a look at this specimen, which shows just a piece of the straight shell, broken along suture positions within the shell:
This most complicated suture pattern is called ammonitic, to signify it as the climax of ammonoid evolution. The shell would have certainly been made very strong by this addition of not just zigs and zags, but zigs upon zigs and zags upon zags. It makes for easy to identify specimens.
Ammonoids are among the most useful index fossils, because of their distinctive features, and evolution into many different lineages and forms, and because their free, ocean living gave them a world-wide distribution, or whole-ocean basin distribution. If you want an example of a good index fossil, think ammonoid.
Don't forget that there was an octopus-like animal living in that shell -- the shells are just the skeleton.