The resources here cover a fair bit of ground. We meet both micro and macro fossils (small and big), and also cover in places how these are used to tell us about ages, rates of deposition, and past environments.

The video below provides an overview of what we'll be doing. Some of the videos contain small bits of information specific to the course this created for – those bits you can ignore.


Key points to take away from this video are:

  • These resources cover a number of major invertebrate fossil groups, which are the fossils of greatest utility to a working geologist.
  • A fossil is a geological structure that includes both the remains of a once living organism, and the record of their behaviour in the form of a preserved track or trail, for example.
  • Due to its 19th century origins there are lots of long words in palaeontology. Sorry about that. I'll do my best to introduce them gently.


Let's move right along, and start looking at some of the biology you'll need to understand for everything else to make sense.

We'll be covering the following, important, terms, in this next video:

  • Anterior
  • Posterior
  • Dorsal
  • Ventral
  • Median/axial
  • Lateral
  • Proximal
  • Distal
  • Phylogeny
  • Taxonomy

Perhaps have a quick think before you start about which of these terms you already know, and what you think they mean. After watching the video you can then revisit this lot and see what you got right. If it helps, you may want to make a list of them and include your own definitions. The other vocabulary in the video we'll revisit later.


Key points to take away from this video are:

  • We have specific, anatomical terms for directions in a living organism or fossils.
  • A key split in the animal tree of life is based on the development of the embryo.
  • That tree of life is called a phylogeny. This describes relationships. Taxonomy is a way of categorising, which doesn't always match evolutionary relationships.
  • Living lineages evolve through geological time, and so the fossils present in a rock can help us date it. This is biostratigraphy (for more, check out the biostratigraphy website).
  • Fossils are also really useful for identifying the depositional environment of a rock.

Anatomical directions are easy right?

Right? Perhaps this is true on the organisms we've looked at thus far, but let's dig a little deeper. Take the quiz below, and see if you can get full marks. If you do, you get much respect from me!


The study of how fossils are preserved is taphonomy. In our last video we delve into this a little, looking at what happens between death and fossilisation, and spend a little time looking at different qualities and types of preservation.


Key points to take away from this video are:

  • Organisms make their hard parts from a variety of materials, and the hard parts have many different uses.
  • There are a great many process that might occur to the remains of an organism between death and fossilisation. These include:
    • Decay
    • Transport
    • Fragmentation, abrasion, and disarticulation
    • Flattening
  • There exist sites notable for their fossils (Lagerstätten) which can be concentration, or conservation Lagerstätten (note the capital letter – it's a German noun, and is thus capitalised!).
  • The latter are sites of exceptional preservation: these provide key insights into past life.

Aragonite v.s. calcite

A key thing to note, which will crop up a few times over the course of these websites, is that there are two forms of calcium carbonate (CaCO3) that are common in the animal kingdom - calcite and aragonite. Furthermore, calcite comes in two flavours in the animal world – high low magnesium calcite.

Calcite and aragonite differ in terms of their crystal structure, and also in terms of their stability. A result of the latter point is that, over geological time under the majority of pressure and temperature conditions, aragonite recrystallises to calcite.

This information is here, just so you know it is the case! If you want to learn more (there are lots of implications surrounding changes in sea water chemistry and the evolution of animals), the following paper provides an overview:

Ries, J.B., 2010. Geological and experimental evidence for secular variation in seawater Mg/Ca (calcite-aragonite seas) and its effects on marine biological calcification. Biogeosciences, 7(9), p.2795.

Click here for a free copy of the paper.

Types of preservation

Now, you're sitting reading this on a website, and of course, for these resources to be useful it could be handy to be able to see actual fossils. So for these sites, we have a digital solution. You will get to see lots of really amazing fossils as 3D models in your browser. This is about palaeontology after all. Hence, throughout, I'll be providing you with plentiful fossils to look at – the 3D models can just be loaded in your browser, as manipulated as you so wish!

The 3D models of fossils below represent the different kinds of preservation we may see as geologists. Clicking on any will load a 3D model, and then you can click and drag to rotate it. Give it a shot!

Unaltered fossils

Sometimes the hard parts of an organism are found in rocks unaltered. We can thus find the original materials (often biomineralised, but sometimes tissues) the organism created in life. Below are some examples in the form of shells.

This is a mollusc - a gastropod, called Ecphora quadricostata. It's upper Pliocene in age, and from the Yorktown Formation, Hampton County, Virginia. This shell is made of calcite, and is 37.7 mm in length.

Depending on your computer you may be able to load multiple models at once. To stop any browser issues, though, I recommend closing each when you have finished looking at it. Hit the cross in the top right to do this.

This is a bivalve mollusc (a scallop, in fact : Argopecten gibbus). It is Pliocene in age, and comes from Sarasota County, Florida. Both this, and the above, are from the collections of the Paleontological Research Institution, Ithaca, New York. Specimen width ~6 cm.


When the pore spaces in a porous material get filled with minerals this is called permineralization. This is common in - for example - bone and wood. Common minerals include calcite or silica, and are often sourced from groundwater. The original non-pore space may remain unaltered, or itself be replaced with minerals. A famous example of this form of preservation is found in the form of petrified wood, where the pore space is filled with minerals, and the wood is later replaced - thus no original wood is left. Check out the example below!

Petrified (= permineralized) wood. Cretaceous in age, and sourced from Seymour Island, Antarctica (!) - antarctica was a lot warmer in the Cretaceous. Longest dimension ~9 cm.

Carbonized fossils

This kind of fossil occurs when organisms were buried quickly, and most often when there was little oxygen around. The remains of the organism are two-dimensional carbon films found on bedding planes. They often look black, because they are so carbon rich. The famous Burgess Shale, which you learned about last year, comprises carbonised fossils, but also this is relatively common in plant fossils, especially from the Carboniferous!

These are fossil graptolites - we'll be learning about this group later in the course (species Monograptus clintonensis if you're interested). They are Silurian in age, and were found in the Williamson Shale, Rochester, New York. Maximum dimension of rock ~11.5 cm.


Cool beans. Aren't these fossils nice? Let's look at replacement next - this happens when the original materials in a fossil are replaced with some form of secondary material - often shortly after burial. In this example, pyrite (FeS2) has replaced calcite: this is a pyritized fossil.

A pyritized brachiopod (Paraspirifer bownockeri), Middle Devonian in age and sourced from the Silica Formation of Ohio. Longest dimension ~4.5 cm. Check out that gold coating this is the pyrite.

External moulds

As I mentioned in the lecture, sometimes a fossil can dissolve. The impression it leaves is a mould. Similarly, when you split a rock, on one part you will have the fossil, but you will find a mould on the other bit of rock you split off. If this mould represents the form of the outside, it is an external mould. These are relatively common within concretions - growths of a mineral around a fossil which harden, prior to its complete decay, leaving a void and a mould. A couple of super cool examples are shown below.

This external mould records the shape of an ammonoid in the genus Gunnarites. It's Cretaceous in age, and was found in the Lopez de Bertodano Formation, Snow Hill Island, Antarctica. Diameter of mould ~9 cm. In this case you can also see - if you want to - the shell that formed this mould.

And here's another lovely example, from the Echinodermata:

Now, this is a good age. Here we have the Devonian crinoid Melocrinus williamsi (Ithaca Formation, Cortland County, New York). Here you can see both the unaltered mineralized hard parts, and external moulds of bits now lost. The whole rock is ~29cm long.

Internal moulds

Just sometimes, we get internal moulds. This occurs when, for example, an organism dies, and rots, but its shell was filled with sediment, or a mineral. If, then, the shell eventually dissolves - but what filled it does not - we are left with an internal mould! Sometimes, if you're lucky, you can find an external mould to go with the internal one.

In this gastropod, lucky you, you can see both internal and external moulds! They are labelled for you. This species is called Cassidaria mirabilis and it's another Cretaceous fossil from Snow Hill Island, Antarctica. Gastropod ~6 long.


If an external mould (or gap between and internal and external one) is filled with a substance that then lithifies, what you have is a cast. Cool huh? This reproduces the original form of the structure. So a mould is formed off an original structure, and a cast when a mould is filled.

This is a really famous example of a cast – a fossilised tree stump from Fossil Grove, an ancient forest in Glasgow's Victoria Park (this model is copyright of Historic Environment Scotland; it's around waist height). It is Carboniferous in age, and was formed after an influx of sediment buried the lower parts of trees. That became sandstone, and then the void left when the tree rotted away was filled with sand as well, forming a natural sandstone cast.