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Trilobites Ic TRILOBITES IC Les Trilobites constituent un groupe d'Arthropodes marins Division longitudinale Division transversale Glabelle ayant vécu au Paléozoïque (-541 à -252 millions d'années). Ils Œil doivent leur nom à leur corps divisé en trois lobes. Les Trilobites Joue libre sont les premiers Arthropodes munis d’un exosquelette dur en plèvre Céphalon , Joue fixe calcite. Mais, seule la surface dorsale étant protégée par un plèvre bouclier minéral, les appendices ne sont pas minéralisés et donc , Épine rarement conservés. La longueur moyenne des trilobites est génale comprise entre 3 et 10 cm. Cependant les taxa millimétriques Thorax Anneau sont abondants et une espèce (Acanthopleurella stipulae, du axial Trémadoc (Silurien) ne dépasse pas 1,5 mm. Les espèces de plus de 30 cm de long sont rares et ne sont présentes que dans Épine Lobe latéral gauche latéral Lobe quelques familles. La plus grande espèce, Uralichas riberoi de Lobe axial droit latéral Lobe pleurale Pygidium l’Ordovicien ibérique, pouvait atteindre 66 cm. rachis La taille relative du pygidium par rapport au céphalon per- met de distinguer 4 groupes de Trilobites. 1. Micropyge : à pygidium très petit. 2. Hétéropyge : à pygidium plus petit que le céphalon. 3. Isopyge : à pygidium et céphalon de taille identique. 4. Macropyge : à pygidium plus grand que le céphalon. Agnostida Isopyges de petite taille (6 à 7 mm) avec deux ou trois segments thoraciques. : Asaphida Plus de 20% des espèces connues, très diversifié mais unifié par une ontogenèse similaire. Corynexochida Isopyges à céphalon circulaire, Leiostegiina avec épine terminale, Illænina effacement de la trilobation. Harpetida Micropyges de moyennes à grandes tailles, 12 segments ou davantage. Lichida Carapace très souvent ornée d’épines et de tubercules. Odontopleurida Micropyges à isopyges caractérisés par une carapace ornée d’épines et de tubercules, 8-13 segments. 10 ordres 10 Phacopida 11-13 segments, glabelle élargie, Calymenina 10-11 segments glabelle rétrécie, Cheirurina tuberculeux. Proetida Micropyges et isopyges, yeux de petites tailles, thorax 8 à 10 segments en moyenne. Classe des TrilobitaClasse des Ptychopariida Le céphalon présentes des pointes génales, thorax >8 segments. Redlichiida Micropyges avec un très grand nombre de segments thoraciques, parfois plus de 60. Morphotypes : adaptations au milieu, indépendants de la phylogenèse. Lisses Épineux Micros 1 à 10 mm Atheloptiques Pélagiques Olénimorphes À bord perforé Asaphida Marrolithus bureaui Groupe de l’espèce Ampyx priscus en file indienne (Maroc) Citons encore le genre crozonaspis (Phacopida) qui signifie littéra- Certains trilobites ont la faculté de lement bouclier de Crozon. s’enrouler pour se protéger, c’est le cas Crozonaspis struvei (référence), - chauveli, - dujardini, - incerta, du genre Phacops (Phacopida). - kerfornei, - morenensis, - morenensis mayensis, - rouaulti. Chez certaines espèces, le pygidium Autres genres également présents dans la Presqu’Île de Crozon : vient s'imbriquer parfaitement dans le Phacopida Colpocoryphe lennieri, Colpocoryphe grandis, Morgatia céphalon (coaptation), ce qui rend l'en- hupei, Calymenella bayani. roulement encore plus efficace. Phacops du Maroc Asaphida Marrolithus bureaui, Onnia seunesi. Encyclopédie générale du savoir relatif et de la connaissance absolue. www.astrosurf.com/pablanchard 01.12.2019 .
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