The Earth Story

Zone Fossils (Index Fossils) The recognition and use of zone fossils is fundamental to biostratigraphic correlation. Fossil groups that were (i) rapidly evolving, (ii) widespread across different facies and biogeographic provinces (facies are the rocks that represent a particular life environment), (iii) relatively common, and (iv) easy to identify make the ideal zone fossils. In the Early Paleozoic macrofauna, graptolites are the closest to being ideal zone fossils, whereas during the Mesozoic, the ammonites(http://on.fb.me/1LYL0Lm) are most useful.

What have fossils ever done for us?

Palaeontology has provided the inspiration for many great works. But just what is it that palaeontologists actually do with fossils once they’ve got them out of the ground? Of course, you can clean them up and show them off in a museum, and this is the side of palaeontology that the vast majority of the public sees. Nothing is quite as eye-catching as a dinosaur, and the beauty of ammonites is undeniable. This is the realm of curating, though and although it’s the dream job of many palaeontologists, there are only so many museums in the world. Instead, many palaeontologists work on micro-fossils.

As the name suggests, micro-fossils are fossils which are especially small. Whilst they can be spectacular to look at, their main use is for biostratigraphy. Biostratigraphy is the use of fossils to divide rock sequences into time zones. This is done using what are known as index fossils: fossils that are abundant, wide-ranging, and went extinct relatively fast.

Graptolites, an enigmatic group unique to the Palaeozoic, are a fantastic example of this. These organisms can be found in almost all deep-water deposits in the Ordovician and Silurian. Many individual species last for little more than a million years, with some having ranges as short as 300,000 years. Though an incredibly long time from a human perspective, in geological timescales it’s simply the blink of an eye.

But why do we want to know how old rocks are, and how do we know that? Knowing the age of rocks is important for multiple reasons; for example, oil deposits in some areas of the world occur within shales of certain ages. While radiometric dating is an incredibly useful technique for both igneous and metamorphic rocks, it is not a technique that can be reliably used with sedimentary rocks. As such, other methods need to be found. Microfossils can be extracted from boreholes with very little risk of damage and, by determining the biozone of the species, can be used to determine rock age. Microfossils like these can be extracted from rocks sampled through boreholes and matched with microfossils from other areas; matching index fossils would mean the rocks have the same age!

Biozone ranges are determined by radioactively dating overlying and underlying igneous deposits. This gives a maximum and minimum age for the deposit, though the boundaries are difficult to define beyond this. By finding a good biozone defining species entirely within the deposit, it can be correlated across the world with deposits that have not had the good fortune to be bounded by volcanic layers. Once you have a matching age, you can do all sorts of other comparisons, such as figuring out where shorelines were or where units might have generated resources.

Image Credit of various Graptolites - http://geolmag.geoscienceworld.org/cgi/content-nw/full/147/2/253/FIG2 Read More - http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8447431 - http://www.stratigraphy.org/upload/bak/bio.htm - http://the-earth-story.com/post/90435946398/whos-been-writing-on-the-rocks-although-theres

Ossicle This ring-shaped feature is a piece of calcium carbonate that, about 150 million years ago, was part of a Sea Cucumber. These animals are actually echinoderms – the same phylum as starfish and crinoids, but with a completely different body plan. They are often elongate and mostly fleshy tissue – hence their nickname as “Sea cucumbers”. They secrete a tiny internal skeleton in the form of these tiny, calcite “ossicles” (the term for very small parts of a skeleton or very small bones like those of your ear). In sea cucumbers, the tissues and muscles attach to these tiny rings, helping to provide support for the whole body. When the organism dies, those ossicles are released as mineralized “microfossils” – this one is about half a millimeter across and comes from a limestone from the Tethys Seaway, today found in the Czech Republic. It was photographed using an electron microscope -JBB Image credit: Petr Hykš https://flic.kr/p/DSpxju Reference: http://tolweb.org/Holothuroidea

Radiolarians These amoeba like plankton construct elaborate shells, and have been around since the Cambrian. Since they evolve quickly and have a rapid turnover of species, they are a vital diagnostic micro fossil used to establish the age of rock formations that cannot be dated by means of radioactive isotopes. Their exoskeletons are made of silica, and form the ooze that forms below the depth where calcium carbonate is dissolved in the ocean. Magnification is 160x and a microscopic technique called differential interference contrast was used to image them. Loz Image credit: Wim van Egmond http://www.radiolaria.org/

The Bitter Springs Chert Formation

The Bitter Springs chert formation is located at the North East of the Amadeus Basin in Central Australia. This formation is made up of dark limestones and finely laminated layers of black chert (a fine-grained sedimentary rock). This chert is known as chalcedonic (fibrous translucent white/grey quartz).

This particular formation is of special interest as despite the fact its 850 million years old it contains very well preserved Proterozoic (2500 to 542 million years ago) fossils. These fossils include 30 species of microfossil (<4mm), including cyanobacteria (bacteria that gain energy through photosynthesis), fungi and dinoflagellates (single celled micro-organism with no skeleton and largely photosynthetic). These are so well preserved that in a thin section under a microscope 3D morphology can be seen. Due to this fact it can be seen that cyanobacteria have always been morphologically similar to how they appear today. The formation also shows stromatolites (as seen in the picture, layered structures made up of sedimentary grains which are trapped by sticky mucous layers secreted by cyanobacteria) and proves they are the product of cyanobacteria.

This formation also contains evidence of early eukaryote (cells containing a nucleus and organelles such as mitochondria and chloroplast) cell fossils. These cells have preserved some structures inside them such as their nuclei (contains cell’s genetic material).

~SA Picture: http://bit.ly/1PuUmg8 by Daderot Paper: http://bit.ly/1EUZYOb by J. William Schopf Further reading: http://bit.ly/1PuUc8C

What have fossils ever done for us? Palaeontology has provided the inspiration for many great works. But just what is it that palaeontologists actually do with fossils once they’ve got them out of the ground? Of course, you can clean them up and show them off in a museum, and this is the side of palaeontology that the vast majority of the public sees. Nothing is quite as eye-catching as a dinosaur, and the beauty of ammonites is undeniable. This is the realm of curating, though and although it’s the dream job of many palaeontologists, there are only so many museums in the world. Instead, many palaeontologists work on micro-fossils. As the name suggests, micro-fossils are fossils which are especially small. Whilst they can be spectacular to look at, their main use is for biostratigraphy. Biostratigraphy is the use of fossils to divide rock sequences into time zones. This is done using what are known as index fossils: fossils that are abundant, wide-ranging, and went extinct relatively fast. Graptolites, an enigmatic group unique to the Palaeozoic, are a fantastic example of this. These organisms can be found in almost all deep-water deposits in the Ordovician and Silurian. Many individual species last for little more than a million years, with some having ranges as short as 300,000 years. Though an incredibly long time from a human perspective, in geological timescales it’s simply the blink of an eye. But why do we want to know how old rocks are, and how do we know that? Knowing the age of rocks is important for multiple reasons; for example, oil deposits in some areas of the world occur within shales of certain ages. While radiometric dating is an incredibly useful technique for both igneous and metamorphic rocks, it is not a technique that can be reliably used with sedimentary rocks. As such, other methods need to be found. Microfossils can be extracted from boreholes with very little risk of damage and, by determining the biozone of the species, can be used to determine rock age. Microfossils like these can be extracted from rocks sampled through boreholes and matched with microfossils from other areas; matching index fossils would mean the rocks have the same age! Biozone ranges are determined by radioactively dating overlying and underlying igneous deposits. This gives a maximum and minimum age for the deposit, though the boundaries are difficult to define beyond this. By finding a good biozone defining species entirely within the deposit, it can be correlated across the world with deposits that have not had the good fortune to be bounded by volcanic layers. Once you have a matching age, you can do all sorts of other comparisons, such as figuring out where shorelines were or where units might have generated resources. - Dale  Image Credit of various Graptolites - http://geolmag.geoscienceworld.org/cgi/content-nw/full/147/2/253/FIG2 Read More - http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8447431 - http://www.stratigraphy.org/upload/bak/bio.htm - http://the-earth-story.com/post/90435946398/whos-been-writing-on-the-rocks-although-theres