Thursday, 26 January 2012

The silurian

The Silurian is a geological period that extends from the end of the Ordovician period (439 Mya) to the beginning of the Devonian period (408 Mya). In Silurian period more than 60% of marine invertebrates died, which is considered to be one of the biggest extinction event.

There were many changes at this time. For example, the melting of large glacial formations, the first terrestrial living organisms,the evolution of vascular plants, the apperance of new species like coral reefs or fish with jaws and fresh water fish. The most characteristic Silurian's fossils are cooksonia, stromatoporoids, conodonts, mollusks, corals, graptolites and  trilobites.

                                                                      Marine life

•    Osteichthyes, which are also called bony fish, are the most aboundant group of vertebrates in the actuality. They appeared in the Silurian period. They have gills and are ectothermic which mean that they were organisms that control body temperature through external means. They can be omnivore, carnivore, herbivore or detritivore.

•    Blastoid, also called sea buds, are extind type of echinoderm, whose fossils look like small history nuts. They were protected by a set of plates of calcium carbonate, called theca. They had long, thin, fine structures called brachioles, which were used to trap food particles and brought them to the mouth and hydrospires as respiratory organs.

•    Eurypterids, also called sea scorpions, were an extinct group of marine arthropods related to arachnids. Its name comes from the Greek word eury- meaning "broad" or "wide" and the Greek word pteron- meaning "wing" becuase of their two appendages for swimming. The largest eurypterids reached three metres long, a huge distance for an arthropod.

•    Trilobites are a famous extinct of marine arthropods that fluorished during the lower Palaeozoic (Cambrian, Ordovician, Silurian, Devonian). They had an easily fossilized exoskeleton. Trilobites had many life styles; some moved over the sea-bed as predators, sacvengers or filter feeders and some swan, feeding on plankton.

•    Tabulate Coral are an extinct form of coral. They are characteristic of the shallow waters of the Silurian. They form colonies build up by hexagonal cells. They had their skeletons composed of a form of calcium carbonate known as calcite. This type of coral occurred in limestones and calcareous shales. Their numbers began to decline during the middle of the Silurian period.

                                                                      Terrestrial life

•    Myriapods are a subphylum of arthropods mainly containing millipedes, centipedes. Their name come from the Greek word myriad (10,000), despite the fact that their number of legs do not reach seven hundreds fifty legs. They were terrestrial and they have a pair of antennae and mandibles and they don't have many blood vessels. The oldest fossils of miriapods are from the late Silrurian, although there are evidences that suggest that they existed in the Cambrian.

•    Trigonotarbida They measured a few millimetres centimetres long and had a segmented abdomen. Some of them were predator on other arthropods and were protected by spines and tubercles. They were similar to arachnids. Both groups have four pairs of legs and a pair of pedipalps. Also they had their body divided into two parts, the prosoma and opisthosoma.

•    Lycopodiophyta appeard in the Silurian period and were one of the first vascular plants. They reproduce by spores and were terrestrial. The main differences with the other vascular plants are that they have a single vascular trace (vein) and the microphyls. Some of this plants have evolved into big trees.

•    Psilophytites are a genus of extinct vascular land plants that lacked true roots and leaves, but had a vascular system within a branching cylindrical stem. They had xylem and phloem and reprodued by spores and breathed through stomata on every surfacephotosynthesized in every tissue exposed to light.

•    Cooksonia is an extinct group of vascular land plant. They were small, a few centimetres tall, and had a simple structure; they lacked leaves, flowers and roots. They were one of the first vascular plants and appeared in the middle Silurian period. They had a simple stalk, that branched a few times. Each branch ended in a sporangium, a structure that expelled espores.

 Antonio and Adrián Calderón

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