Introduction and biology

The molluscs have deep roots — they have Precambrian origins, had a diversity of weird forms in the Cambrian, and there are a number of groups that have survived from early in the Palaeozoic through to today. Other groups were very successful, but have now gone extinct. So we'll start here with a quick overview of the biology of the group, their evolutionary history, and their diversity through geological time.


Key points to take away from this video are:

  • Molluscs form a diverse group of Lophotrochozoa that includes slugs, snails, squid, cuttlefish, octopuses, ammonoids and all manner of marine shellfish such as clams, mussels, and oysters.
  • Their body plan has four features:
    • A head with sensory organs and a feeding structure called the radula.
    • A foot that is used for locomotion.
    • A dorsal mantle that secretes the shell.
    • A coelomic cavity with organs in, and a mantle cavity with gills.
  • The group has its origins in the Precambrian, and has diversified from an array of sometimes weird-looking Cambrian animals.
  • The ammonoids, gastropods, and bivalves are all important fossil groups.

Meet your first three molluscs

Below you can find a 3D model for each major group that I introduced in the first video – load the models and take a few minutes to familiarise yourself with their anatomy. This will be useful, because in the next video we're going to be learning the terms for the different bits and how they relate to each other.


Cephalopods include squid, octopodes (or octopuses – choose your poison, but not octopi which is a latin pluralisation of a Greek word...), cuttlefish, ammonoids, and also a group called belemnites. More on this in the next video.

This is a ammonoid from the Triassic Toad Formation, British Columbia (species name is Gymnotoceras beachi). It's a maximum of ~4.5 cm in diameter.


You can normally identify a gastropod by its coiled shell - although not all members of the group have a shell. Gastropods include snails, conchs, periwinkles, limpets and whelks. Plus slugs. Cool, huh?

This is a Palaeozoic gastropod, Worthenia tabulata. It's Upper Carboniferous in age (source rocks are the Vamoosa Formation of Osage County, Oklahoma). Specimen ~3cm long.


Bivalves can be easily identified from their shells – whilst there are two shells in life, often you only find one as a fossil! The group includes clams, mussels, and oysters.

This is a fossil bivalve – Mercenaria mercenaria. It is quaternary in age, and was found in St. Mary's County, Maryland. Specimen ~10 cm wide.


We've met our three groups, so - good news - now we can learn about their morphology! That is going to be the subject of our next video. I also provide three 3D models below the video with all the bits labelled for you.

As before, there are a number of terms we can't really avoid. Sorry. Blame the 19th century scientists. The words that it would be really good for you to get to know are:

  • Siphuncle
  • Mantle
  • Protoconch / Phragmocone
  • Septa / suture line
  • Apex
  • Aperture
  • Pallial line
  • Pallial sinus
  • Adductor scar
  • Umbo


Key points to take away from this video are:

  • Cephalopods have chambered shells, which are separated by septa. Where these hit the shell you get suture lines.
  • We can tell between three major ammonoid groups – the goniatites, ceratites, and ammonites, on the basis of these suture lines.
  • Belemnites are often found as bullet-shaped fossils. They are actually closely related to squid and octopodes.
  • Gastropods have a twist in their body, and also a spiral shell.
  • Bivalves are common as fossils, and their hinges, and traces of the mantle/muscle attachments, are very valuable in taxonomy within the group.
  • Beware rudists. They're not corals.

That's a lot of words

It's important to be able to communicate with other palaeontologists/geologists about fossils, so you'll need to be at least familiar with some of these phrases. As I've introduced three groups, and they all have slightly different words, the three models below have these bits of the anatomy labelled for you. Please do use these to help you get on top of that pesky vocabulary!


As well as looking at the labels, check out the suture lines on this amazing fossil where the outer shell has been polished off. This is typical of the complex suture of the ammonites.

This is an ammonite cephalopod - Cleoniceras besairiei. It is Cretaceous in age. Maximum diameter ~12 cm.

By contrast, here you can find a goniatite.

This, right here, is a goniatite ammonoid Imitoceras rotatorium. It is Lower Carboniferous in age, and was found in rocks from Floyd County, Iowa. Specimen ~8.5 cm max diameter.

Gymnotoceras beachi, which you met above, is actually a ceratite, if you want to contrast it with these two!


Here is a gastropod, with lots of bits labelled for you!

This is a fossil gastropod called Naticarius plicatella. Note that inside its aperture, you can see a thing called the operculum (trap door) which is not normally preserved. This particular critter is Pliocene in age (it's from the Pinecrest Beds of Sarasota County, Florida, FYI). Specimen ~3.5 cm long.


Bivalves are often found not with two shells, as shown above, but as a single shell - as you can see here. In particular the inside surface provides lots of clues to their taxonomy. We are not covering this in depth here, but if you ever work with these, you'll get to know the inside of their shells really well.

This is a fossil bivalve – Mercenaria mercenaria. It is quaternary in age, and was found in St. Mary's County, Maryland. Specimen ~10 cm wide.

Remember I promised you that diagram summarising how the morphology of bivalves reflected their mode of life? Well, here it is! Bear this in mind when looking through fossil collections as I ask you to do in the next section.

Image from (and copyright) Benton, M.J. and Harper, D.A., 2020. Introduction to paleobiology and the fossil record. John Wiley & Sons.

There's a whole world of cool molluscs out there.

Molluscs are a bit hard to pin down, because they are so varied, and there are so many of them, that it is really hard for me to give you a proper overview in one lecture, or indeed one website. Check out, for example, the fossils below:

Two mystery fossils.

Maximum length of specimen (not including surrounding rock matrix) is approximately 5.5 cm.

Specimen ~21 cm long.

What are these?

Your mission, should you choose to accept it, is to find more information of these fossils by exploring the impressive range of molluscs that are available in the brilliant Digital Atlas for Ancient Life. Answer the questions below to unlock an upbeat message from a famous face. A good way to find this out is to browse the collections, and thus, on the way, learn about the range of molluscan morphology!

Why should we care?

As with the trilobites, we will finish by looking at why these molluscs are of great use to geologists, and what they look like in rocks!


Key points to take away from this video are:

  • The cephalopods tend to survive fairly well as fossils, as do gastropods.
  • Bivalves are often fragmented, and found concentrated into layers.
  • Bivalves are useful for telling us the way up of a rock, and also its environment of deposition.
  • Ammonites, in contrast, are really excellent index fossils. This is because they are:
    • Rapidly evolving.
    • Easy to identify.
    • Have a wide geographic distribution.
    • Facies independent.
    • Common (they are abundant as fossils).
  • These are key attributes for any really good index fossil in biostratigraphy!

If you would like to get a bit more experience at seeing molluscs in thin section, have a play with the virtual microscope below! This, courtesy of the UK Virtual Microscope and the Open University is a thin section of a famous building stone: the Purbeck (called a Marble in the building industry, actually a Limestone). These rocks are found in Dorset, and are chock full of whole, and fragmented,gastropod shells, embedded in a micrite. There are even some geopetal structures in there - see if you can find one.

To view either of the two spots marked under PPL and XPL, just click on the icons. The slider on the bottom zooms; scale at top.