The trilobites ( Trilobita ) constitute a class marine Arthropodes Fossile S having lived during the Paléozoïque ( primary era ) of the Cambrien to the Permien. The last trilobites disappeared at the time of the extinction of mass at the end of Permian the, there are 250 My. The trilobites are well-known because they are certainly the second groups most widespread of fossils after the dinosaurs. In addition, they gave the most diversified fossils: one counts between nine and fifteen thousand species. The majority of them were simple and small marine animals, which filtered the mud to feed.

Morphology

Principal devices

One speaks about “devices” because one distinguishes at the longitudinal trilobites three Lobe S: the rachis ( axial lobe in English) for the median part are surrounded by two pleural lobes, on the right and on the left ( pleural lobes in English).

One also distinguishes another cutting in three parts, from the front to the back. The trilobites are indeed equipped with a body divided into three segments “logic”, namely a head (made up of the two préoraux segments and the first four postoraux segments, these last segments being related ones to the others), a thorax (compound of freely articulated segments) and of a tail (made up of the last two segments dependant between them by the telson . The thorax is undoubtedly very mobile, since many fossils are rolled up on this part of the body.

The trilobites were equipped with a pair of antenna préorales and other members undifferentiated Biramé S. Each exopodite (members of displacement) comprised six segments, following the example other first arthropods. The first segment were covered with a épipodite which was used with breathing and the stroke. The members all were protected on their dimensioned by the pleural lobes, laid out symmetrically around an axial lobe. Contrary to the popular thought, the term of tri lobite comes from longitudinal division between pleural lobes right and left, and not from division in céphalon, thorax and pygidium:

• the head (or Céphalon) includes/understands a central bulge, the glabelle one, as well as the two eyes of the animal. The céphalon is in general broader than the Thorax. Certain species have a Pygidium equal or broader than the céphalon. When the trilobites moult or die, the librigenes (free cheeks) disappear, leaving cranidium (glabelle + fixigene);
• the Thorax is composed of articulated segments (between two and forty-four);
• the tail (or Pygidium) is also composed of segments, but these are welded together.

Although the trilobites were protected only surfaces some, they had a solid Exosquelette. During the moult, this exosquelette was divided into two on the level of the junction head/thorax, so that of many fossil are deprived of one or the other of these parts of the body. Indeed, of many fossils are residues of the moult rather than dead trilobites. In the majority of the groups, one found two joinings (articulations without cartilages and little mobiles) facial on the cheeks, which facilitated the moult. The cheeks in addition were very often equipped with a pair of eyes in the shape of crescent, which could be particularly advanced. In fact, the trilobites were the first animals to develop truths eyes, there is that 543 My.

Some trilobites, like those of the Lichida order, evolved to forms worked out in spines, Ordovicien until the end of the period of the Dévonien. One found such fossils in the formation of Hamar Laghad of Alnif, in Morocco. It should be noted that many forgeries of this type of fossils were produced. Others spéciments were found in the east of Russia and Ontario, in Canada.

According to the New Scientist magazine (May 2005), “some… trilobites… had horns on the head, similar to those of the current Scarabée S. ” While being based on their localization, their form and their size, the London biologist Rob Knell and its colleague Richard Fortey, of the Muséum of natural history of London, has advanced that these horns were probably used as attack force with the trilobites, at the time of engagements aiming at deciding between applicants with a coupling. That would make trilobites one of the very first groups have expressed this behavior, and although the study covered only the family of the raphiophoridae , the conclusions can certainly apply to other groups.

The trilobites varied in the face, between a millimetre and soixante-douze centimetres, with an average between two and seven centimetres. Largest trilobite of the world, Isotelus rex , was found in 1998 by Canadian scientists on the shores of Hudson Bay.

Dorsal morphology

1 facial Joining, 2 Librigène (plays free), 3 Spine génale, 4 Glabelle, 5 occipital Ring, 6 Fixigéne (plays fixes), 7 Eye, 8 axial Lobe, 9 pleural Spines, 10 dorsal line, 11 pleural Furrow, 12 Spine pygidiale

Ventral morphology

Joining rostrale, Plate rostrale, former facial Joining, connective Joining, cephalic Lining, Hypostome, posterior facial Joining, pleural Lining, Lining pygidiale

Lifestyle

The trilobites all were sailors, the ones were Pélagique S swimming and the others benthic, crawling on the bottom.

Nutrition

The majority of the first trilobites such as Olenoides were probably predatory benthic invertebrates, such as worms (Ottoia). Their legs were provided with gnathobases thorny. The worm was extracted from its gallery then crushed or torn by the spines of the legs and powerful the gnathobases, then probably brought between the legs towards the mouth or it will be crushed on the level of the hypostome before ingestion. In shellfish and the insects, all these functions are filled by the former horns specialized in the transformation of food before ingestion. However, the major part of the treatment was carried out in the longitudinal groove médiale between the members with their successions of pairs of gnathobases. The predatory trilobites were to have the hypostomes conterminants firmly attached to the lining frontal, primarily stabilizing the hypostome against the cephalic exosquelette in order to improve the treatment of the prey. It is interesting to note that there is a considerable variation of the size and form of the conterminant hypostome, suggesting many trophic specializations.

Stratigraphic role

These fossils had a vast geographical distribution, with a great quickly evolving/moving number of species, which makes it possible to date the geological formations in a precise way. This is why the trilobites are stratigraphic fossil goods , of which one is used for oneself to create chronological divisions in the Paleozoic one (especially of Cambrien in Dévonien).

In France, one finds trilobites in Languedoc, in the Pyrenees, in Brittany/Normandy and in the Ardennes. Oldest were discovered in the departments of the Herault, the Aude and the Aveyron with specimens dated from lower Cambrien. Cambrien, with the Ordovicien and the Dévonien, the Wales, the Brittany, the Bohemia (Czech Republic), the Russia, the the Appalachian Mountains with the the United States and the Morocco (in particular the area of Erfoud) are some of the areas where the layers of trilobites are among most important and the most diversified in the world. From the Carboniferous , their manpower and their diversity tend to decrease until their disappearance at the end of the Permian one, defining the higher limit of the Paléozoïque ( primary era ).

Place des Trilobites in the animal kingdom

See too

Taxonomic references

External bonds

• '' Trilobite Systematic Relationships ''
• '' Russian trilobite ''

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