Tuesday, 14 December 2010
Fossils- Part 4: Trilobites
Trilobites are members of an extinct class of arthropods called the Trilobita. They lived during the Palaeozoic Era having evolved around 545 million years ago at the beginning of the Cambrian. They continued to thrive throughout the Cambrian and Ordovician Periods when thousands of species evolved. They gradually declined before becoming extinct at the end of the Permian ~ 250 million years ago during one of the big 5 Mass Extinctions.
Trilobites are the oldest fossilised organisms to show evidence of eyes (visible in several of the photos above and below). The eyes were similar to the compound eyes seen today in insect species, such as flies, and in many species the lens were made of calcite. Some species had large eyes with many lens and could see in many different directions whereas other had smaller eyes with fewer lens and more restricted vision. Some species were blind.
Trilobites are an important group of marine arthropods and were one of the first animals to evolve an exoskeleton (external skeleton) to product the soft body parts. They grew by moulting continuously throughout their lives and growing a new larger skeleton.
Trilobites (tri-lobed) are named because their body is divided into 3 lobes lengthways (a central axis with a lobe on each side) and there are also 3 parts across the body - the cephalon (headshield), thorax (trunk) and pygidium (tailpiece).
Usually only the exoskeleton is found fossilised but in certain parts of the world, for example, the 530 million year old Burgess Shales in the Canadian Rockies, thousands of well preserved trilobites which include soft body parts, such as antennae and legs, have been found.
Most trilobites were 2 - 10 centimetres in length but some such as agnostids are only a few millimetres whereas others grew to almost one metre.
Most species lived on or near shallow sea floors but some swam at the surface above deep oceans and a few species could survive low oxygen levels living in deep ocean water.
Trilobites are amongst my favourite fossils and the next two photos show my favourite.
Elrathia lived at the bottom of outer shelf seas and the specimen seen below was found in 550 million year old Cambrian rocks in Utah, USA. This species is easy to buy as it is one of the few trilobite species to be mined commercially.
Proteus, below, was found in 360 million year old Devonian rocks in Morocco.
You can just make out the trilobite in the middle right of this photo embedded in the rock
Some species of trilobite curled up in a ball to protect themselves from predators - it was hard to photograph this one curled up but I hope you get the general idea!
And finally, "Spot the Trilobite". This fossil is just a faint impression in the rock showing why it is so hard to spot some fossils! It was found in Lower Cambrian rocks in a quarry in Sutherland, Scotland. I did try and photograph this through a magnifying glass and a hand lens as you can make out far more features but the photos didn't really work.
Trilobites are very useful to geologists. They can be used in relative dating of rocks and in correlating the age of sedimentary rocks in different locations. Trilobites can be used as indicators of past environments as different species lived at different depths in the oceans and seas. They can also be used to reconstruct the position of land masses and oceans in the past.
According to the British Geological Survey Earthwise Fossil Focus Leaflet on Trilobites, these fossils have been popular with people for a long time. The Pahvant Ute Indians living in Utah wore Elrathia kingi (see photo above) fossils as necklaces believing they were lucky charms. A species of Calymene trilobite is so regularly found in Wenlock limestones in Dudley, West Midlands, that it has gained local names - The Dudley Bug, The Dudley Locust and the Dudley Insect. Many similarities have been noted between trilobite tails and butterflies or bats worldwide and in South Wales, it was believed the tails were petrified butterflies and they became connected to tales of Merlin - the Arthurian Wizard.
Richard Fortey, who was Senior Palaentologist at the Natural History Museum, London,until he retired a few years ago, is an expert on trilobites and has written a brilliant book, called appropriately "Trilobites!", all about these fascinating animals. He has also written several brilliant and very readable books about earthscience, fossils and the history of life on Earth.