Sometimes hard parts are found in rocks unaltered. We can thus find the original materials (often biominarlised, but sometimes tissues) the organism created in life. Below are some examples in the form of shells.
This is a mollusk - 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.
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This is a bivalve mollusk (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 porous materials 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, which you can check out below!
Petrified (= permineralized) wood. Cretaceous in age, and sourced from Seymour Island, Antarctica (!) - antartica was a lot warmer in the Cretaceous. Longest dimension ~9 cm.
This kind of fossil occurrs when organisms were buried quickly, and most often when there was little oxygen around. The remains of the oganism 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.
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.
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. 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 mold.
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 molds of bits no lost. The whole rock is ~29cm long.
Just sometimes, we get internal moulds. This occurs when, for example, and organism dies, and rots, and the shell is 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 mold (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.