As most of my time recently has been spent sorting through the Vernon Park Natural History collection, mentioned in various previous posts now, and since the bulk of that has been Geology in one form or another up to now, for today’s Find it Friday I am going to take a look at two types of marine fossils; Trilobites and Graptolites.
Trilobites first appeared during the Cambrian and became extinct during the Permian (the Paleozoic era, 542 – 252 million years ago.) They were marine Arthropods; invertebrates, like insects, spiders and crabs. They had joined limbs, external skeletons and segmented bodies. Their exoskeletons were made of Chitin and had lines of weakness which would have allowed for easy splitting during moulting as well as protecting it from predators. Its body structure was divided in to three; the Pygidium (the tail-piece), the Thorax (the body) and the Cephalon (the head).
The general features of a Trilobite included a feeding routine where spine like structures on each limb called Gnathobases manipulated food and moved it forward towards the mouth.
Trilobites varied in size and shape and often had different features depending on where they lived in the ocean, for example, small lightweight trilobites were able to float and/or swim very easily. As well as this, an increased size indicates it was a good swimmer. However, the size of the trilobites eyes is usually the most useful feature in analysing the environment in which the trilobite lived; the eyes are kidney-shaped and compound which consisted of many single calcite crystals. Large eyes suggested the trilobite lived at the base of the Photic Zone (the surface layer of the ocean that receives sunlight), whereas small eyes suggested it lived life in the upper part, the the well-lit part Photic zone. In addition, a trilobite with no eyes would have been a burrowing species, living on or close to the ocean floor.
Graptolites are commonly found fossilised in dark shales. A graptolite fossil is the exoskeleton of a colony of animals whose bodies were housed in tiny Thecae arranged in rows along Stipes. They are free-flowing and were usually found in calm waters, as they were very fragile.
They are commonly found in rocks of Ordovician (440 million years ago) and Silurian age (420 million years ago) and make good Zone fossils (able to be used to date the rock they are in because they existed in a specific time period), because of a variety of reasons, including their rapid evolutionary change, their wide and plentiful distribution, as well as being easy to recognise.
Graptolites were mostly planktonic, possibly floating at the mercy of ocean currents, resulting in the wide spread distribution. Because of their fragility graptolites are rarely found in rocks of higher energy origin than siltstones, even where low energy environments did occur in shelf seas (the marginal seas adjacent to the land), the rapid oxidation of their protein-based skeleton and the activity of predators would have ensured the lack of graptolites remains in the fossil record.
Various species of graptolites evolved, but this sample is of Didymograptus; identifiable by a downward facing stipe and thecae. This is not a feature that was consistent; three main features of graptolite morphology changed as they evolved, they were:
- The number of stipes; the first graptolite, Dictyonema had many stipes, then by Didymograptus the number of stipes had decreased to two and by Climacograptus they had decreased again to one.
- The direction of stipes; the first two types of graptolites had stipes that pointed down but the rest following these has stipes that pointed upwards.
- The shape of the thecae; the first graptolites had theca that were sharp, angular and pointed, but as they developed they became considerably more rounded.
From left to right: Dictyonema, Didymograptus, Dicellograptus, Dicranograptus and Monograptus.