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Monday, 22 March 2010
Fossils and the History of Life : Part Three
Fossils and the History of Life: Part Three
Okay, I’d better crack on with my round up of S193, Fossils and the History of Life’.
Part Three: Common Phyla in the fossil record
As I mentioned last time, the animal kindom is split into groups called phyla. These are represented in the fossil record and the following is a brief summary of the main points concerning the most important animal phyla represented:- Porifera:
• Cambrian to Recent • Mainly marine though some freshwater • Simple multicellular organisms – no nervous system • Filter feeders • Locally abundant fossils especially in Cretaceous rocks • Found in flint nodules
• Corals • Late Precambrian (Ediacaran) to Recent • Almost entirely marine • Colonial or solitary • 3 groups: 1) rugose (solitary or colonial) Ordovician to Permian; 2) tabulate (colonial) Ordovician to Permian; 3) scleractinian (solitary and colonial) Triassic to Recent. • Have central mouth and stinging tentacles
Note that Sea anemones and jellyfish are also cnidarians, but being soft bodied are rarely found in the fossil record.
Bryozoans: • Also known as ‘Moss animals’. • Ordovician to Recent • Normally marine but also freshwater (uncommon) • Very small colonial creatures living in groups of several to thousands of individuals • Feed through tentacles • Fossils can resemble corals and graptolites (see later)
Brachiopods: • Cambrian to Recent • Entirely marine and benthic (sea floor dwelling) • Consist of 2 parts or valves and shells made up of calcium carbonate mainly though some of calcium phosphate. • Filter feeders • Are common fossil in Palaeozoic limestones and shales.
Molluscs: • Cambrian to Recent • Mainly marine. • Very diverse phylum and most abundant large invertebrates in the fossil record. • Phylum includes: bivalves; gastropods and cephalopods
Brachiopods seem rather similar to bivalves in many ways, but differ in that their ‘plane of symmetry’ passes through both valves rather than between them. Invariably, one valve is larger than the other in a brachiopod hence the common name of ‘lamp shells’.
Gastropods are aquatic, living in shallow marine and freshwater environments.
Cephalopods are entirely marine; have chambered shells and include ammonites, belemnites and nautiloids.
• Cambrian to Recent • Entirely marine are benthic, sessile (fixed to rocks) or vagrant (move around sea floor). • Feed, respire and move using extendible tentacles called ‘tube feet’. • Echinoids commonly has 5 rayed arrangement of plates and tube feet.• The phylum includes sea urchins, starfish (asteroids) and brittle stars (ophiuroids) and sea lillies (crinoids, see below).
Arthropods: • Cambrian to Recent • The largest animal phylum • Found in land and water habitats. • Includes: crustaceans, insects, spiders, and now extinct groups such as trilobites • Have hard outer segmented exoskeleton which is periodically shed to allow growth
• Exoskeleton comprises chitin and possibly strengthened by calcium carbonate or phosphate.
Hemichordates: • Cambrian to Recent • The most important group of hemichordates are the extinct and entirely marine graptolites. These are confined to the palaeozoic era and are therefore an important zone fossil. (see later) These look strangely like ‘saw blades’ where the ‘teeth’ are actually small recesses that house individuals that make up a colony.
• Cambrian to Recent • Possess a notochord through the length of the body • Include the 5 classes of vertebrates i.e. fish; amphibians; reptiles; mammals and birds.
Above: Dinosaur fossil, a juvenile Tyrannosaurus rex, (Cretaceous) locatedat the Burpee Museum of Natural History, Rockford Illinois, USA.
• Extinct groups include the land dwelling dinosaurs (see above); marine ichthyosaurs and plesiosaurs; and the flying pterosaurs. • Vertebrate fossils are rare and usually comprise only fragments.
So that's a brief and way too rapid round up of the common phyla in the fossil record. The phylum 'Chordata' itself could warrant an entire blog on its own - as indeed any of them could. But I'll maybe do that at a later date if time permits. Next time I wil start that journey through time, starting with a look at the mysterious Ediacaran fossils, examples of which were found close to my hoome in the East Midlands, at Bradgate Park. Then I'll get down to the nitty-gritty of the 'Cambrian Explosion'.
All photographs included above were courtesy of 'The Virtual Fossil Museum' web site. This is an excellent resource, providing photographic galleries of the fossil groups and a lot of other information besides. For a closer look click here.