GEOL2301: Palaeoecosystems

This practical will introduce you different ways that a fossil can preserve. We'll look at a number of (virtual) specimens, and consider their taphonomic history, to get an appreciation of different modes of fossilization.

Remineralization

Let's start by looking at the possible fates of original shelly material. In rocks that haven't undergone much diagenesis, it's possible that the original microstructure – and even original colour – may be retained, as in this Pliocene gastropod (Ecphora quadricostata, PRI 70750; 38 mm long):

• Remember, type f for full-screen view.
  Click the × in the top-right corner when you're done to help your computer render other models.
• Get help with 3D models
• Which biominerals are most likely to be preserved in their original form?

• Stable minerals are more likely to preserve: e.g. low-Mg calcite over high-Mg calcite over metastable aragonite.

  Phosphate has a smaller stability field than carbonate (7 < pH < 7.8; eH < 0), so is less likely to preserve.

  I'm not counting carbon as a 'mineral' for the purposes of this question...

Permineralization is the infilling of spaces in a fossil with a mineral. As space is replaced with mineral, the resulting fossil may feel denser than expected.

The Creteaceous wood below, from Antarctica, has been permineralized in silica; the black colour is likely the original carbon. Specimen size: ~ 90 mm

Recrystalization is when crystal structure is rearranged, despite the original atoms being retained – only their configuration is altered.

The typical example is aragonite transforming to calcite, but dissolution and reprecipitation in situ can lead to original calcite microstructure being overprinted.

• Classify the organism in the thin section as best you can. What was its likely original mineralogy?
• The near-perfect symmetry of the valves suggests a bivalve mollusc. Bivalves can be aragonitic or calcitic; the uniform replacement hints at an originally aragonitic microstructure, but calcite layers could also have been remineralized.

The shell of this brachiopod, Paraspirifer bownockeri (PRI 76880; width: ~ 45 mm), has been replaced with pyrite, now visible as a 'golden' layer encasing an internal mould of the shell.

• What brachiopod class does Paraspirifer belong to?
• It's a spiriferid. Note the strophic hinge line and deep conical wings (handy for housing that spiral lophophore + brachidium).

Casts and moulds

Plaster casts are made by filling a cavity within a mould with liquid plaster. One way to make a mould is to surround a wax original with resin. Once the resin hardens, the wax can be melted away to leave a mould.

Fossils may experience an analogous process. Imagine a calcareous shell settling in a silica-rich sediment. The sediment forms a natural mould around and within the shell.

What happens if the sediment porewaters decrease in pH? First, the shell will dissolve, leaving an empty cavity where once the shell was. If this was dug up, we'd have a mould of the external and (if the shell filled with sediment) internal surface of the shell. Instead, imagine that pH falls still further and pyrite fills the empty space. We now have a pyrite cast of the shell.

Now let's look at some (virtual) fossil material and see how this works in practice.

For each specimen, you should:

• Briefly sketch or describe the material
• Note the original composition of the living organism. How is it preserved now: as a cast or mould? In its original mineralogy, or replaced by something else?
• Estimate how long the organisms were in the taphonomically active zone. What did they experience whilst they were? (e.g. transport, sedimentary energy, burrowing…)
• Reconstruct the diagenetic history (pH, eH)

Gunnarites (Ammonoidea)

Specimen width: ~ 90 mm

• Describe the taphonomy of this specimen
• This is an external mould, preserved within a concretion. The shell itself is not preserved, but you can view the internal mould – the other half of the specimen – below.

Melocrinus (Crinoidea)

Slab height: 290 mm

• Describe the taphonomy of this specimen
• Notice here the original calcitic ossicles in the stalk, and their moulds towards the calyx.

Cassidaria mirabilis (Gastropoda)

Specimen height: 60 mm

• Describe the taphonomy of this specimen
• The shell here has completely disappeared, leaving a beautiful internal mould (1) as well as an external mould (2); notice the shell ornament near the apex of the spire.

Lepidodendron

This carboniferous tree (stump height: 900 mm) heralds from Glasgow. Look carefully for evidence of original carbonaceous material.

• Describe the taphonomy of this specimen
• This is a cast: the carbonaceous material rotted quickly, leaving a mould in the muddy sediment that was later infilled by sand washed in from above. Erosion of the softer muddy sediment left the sandy infill intact.

When the chamber of an organism is only partly filled with sediment, we can obtain valuable information of the original way up. Such gravity-indicating features are called geopetal structures. Note the peloids that worked their way into the replaced shell we saw earlier.

• Which way was 'up'?

• Up. The peloids have come to rest at the bottom of the shell.

  Notice also the peloid-free shadows beneath this shell and the one overlying it. These sediment grains were seemingly introduced some time after the original matrix had been washed out.

• Describe the taphonomy of this specimen, paying particular attention to what has become of the original shell
• This is an example of pyritization. Has the pyrite replaced the original shell? No! Note the gap at each septum that would originally have ben occupied by the shell. Rather, the pyrite is forming an internal mould of the shell.

Brachiopod shell (thin section)

As some of you asked about punctae in session 3, here's a thin section of a brachiopod shell [view full size]. The dark tubes perpendicular to the shell surface are punctae. Note also the impression of the original shell microstructure. Field of view: 2 mm.

• What was the original mineralogy of this brachiopod?
• Phosphatic
• Calcitic
• You may wish to refer to your crib sheet from session 3.
• Linguliform brachiopods are phosphatic, but lack punctae.
• Describe the taphonomy of this specimen
• The original shell microstructure seems to be preserved – consistent with the reasonably good preservation potential of calcite.

Examine the reference specimens of the Wenlock Limestone, and perhaps some of the group slabs. Start to think about their taphonomy.

• How do these fossil assemblages correspond to the original biological community/ies?
• A good answer is likely to address the following points:
  • Is there any evidence of transport? How far might individual specimens have travelled?
  • Are fossils articulated or disarticulated?
  • Has time averaging affected the sample composition?
  • What range of mineralogies can you observe?
  • Have any diagenetic processes affected the sample?
  • What might be missing from the assemblage?
  • Do different Wenlock samples have different taphonomic histories? What are the palaeoecological implications of your answer?

In situ: abandoned quarry workings of the Wenlock limestone