The Fossilisation Process

When a land dwelling vertebrate dies its carcass is commonly disarticulated, this means its limbs are removed, often by predators and scavengers alike. Most of the decomposition of the organic material is done by bacteria that will feast on the rotting flesh that remains on the bones. Some bones are completely stripped clean of flesh and bleached in the sun. Others might be carried off or gnawed by rodents. Sometimes, disarticulated remains are trampled and scattered by herds of animals.


Sooner or later the bones are either destroyed or buried. If they aren’t digested their destruction can come from weathering; this is when the minerals in the bone begin to break down and the bones disintegrate. But the weathering can be stopped by rapid burial, it’s at this point fossils are formed. A body fossil is part of an organism that is buried and a trace fossil is an impression left behind in the ground by the organism.


Bone is made out of calcium-sodium hydroxyl apatite, this mineral weathers easily, this means that the mineral is no longer present once a bone becomes fossilised. This mineralogy can remain intact if the bone doe not come into contact with any fluids during its burial, something that is extremely rare.

It is possible to find tissues of extinct animals. Since bones are porous, the spaces once occupied by blood vessels and nerves fill up with minerals. This is called permineralisation.

Fossil of Archaeopteryx. Image credit Humboldt
Museum Fur Naturkunde Berlin

Pristine fossils can be found in geological lagerstatte, feathers of dinosaurs are known from these lagerstatte. Most famously the early bird Archaeopteryx is known from the Solnhofen lagerstatte in Germany.

Ammonite shells are originally made of aragonite, this is unstable so when fossilisation begins the aragonite becomes the more stable calcite. This calcite creates a cast of the shell and this is what we find today.

Woolly Mammoths and Woolly Rhinos have been discovered mummified in the permafrost in Siberia and Alaska. Soft tissue of a Tyrannosaurus Rex has even been found which allowed palaeontologists to see that the animal was female, within the fossil, red blood cells and connective tissues were found.

Natural mummies have been found in a variety of locations around the world; bog deposits or tar pits, deep inside caves, glacier ice and in the permafrost of Alaska and Siberia. A Woolly Rhinoceros was found mummified after it was covered in salty ground water that essentially pickled the carcass, preventing bacteria and microorganisms digesting the flesh by altering the pH of the environment which means that microorganisms cannot survive in these acidic conditions.

Mummified dinosaurs have been found, good examples of these mummies come from Brachylophosaurus and Edmontosaurus. Leonardo, the Brachylophosaurus that features in the palaeoart post, had skin impressions, muscle impressions that showed an excess of tissue around the neck, even parasites are found on Leonardo.


The permafrost is also effective as the low temperature prevents the bacteria from respiring by removing any moisture on the carcass through freezing. However, once the mummy is excavated the bacteria become active and decomposition begins.

The Difference Between...Rugose, Tabulate and Scleractinian corals

This is going to be a series of posts that look at fossils that are either difficult to

Rugose coral. Image credit: paleo.cortland.edu

differentiate or can be easily confused.

This post will look at three types of corals; rugose, tabulate and scleractinian.

First of all, there is the geological context of these corals, in partcular what age the sediments will be that you find them in. Rugose corals are found in Ordovician to Permian sediments, being wiped out at the Permian extinction. Tabulate corals also have the same range, so this would not be useful if you were trying to differentiate the two using geology. However, scleractinian corals have a younger range, from the Triassic to recent times. giving a feature to differentiate this coral from the other two.

Tabulate coral. Image credit: fossillady.wordpress.com

Scleractinian coral. Image credit: hoopermuseum.earthsci.carleton.ca

When trying to identify if your coral is tabulate or not, look at whether or not your fossil is colonial or solitary. Tabulate fossils are almost exclusively colonial. It's difficult to differentiate Scleractinian and Rugose fossils this way as both have colonial and solitary species.

Scleractinian and Rugose corals both also have well developed septa, so this cannot be used to tell the difference. But again, tabulate corals have weak or absent septa. The tabulae can be used to identify Scleractinians easily as they are usually absent in these corals. They are well developed in tabulate and most rugose possess tabulae.

Symmetry is useful for differentiating Rugose corals from tabulate and scleractinians. The symmetry in rugose is bilateral, meaning that two identical halves can be created, humans are bilaterally symmetrical. However, tabulates and scleractinians have radial symmetry.

Scleractinian skeletons are made from aragonite which is unstable in fossilisation, whereas the tabulate and rugose corals have calcite skeletons.

Summary:
Rugose: Ordovician to Permian. Well developed septa. Bilaterally symmetrical. Colonial and solitary. Most possess tabulae. Calcite skeleton.

Tabulate: Ordovician to Permian. Weak or absent septa. Radial symmetry. Always colonial. Well developed tabulae. Calcite skeleton.

Scleractinian: Triassic to Recent. Well developed septa. Radial symmetry. Colonial and solitary. Absent tabulae. Aragonite skeleton.

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