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SPHEROIDAL ENROLLMENT [1 record]

Record 1 1977-09-08

English

Subject field(s)
  • Paleontology
OBS

[Spheroidal enrollment] is the commonest type, characterizing [...] in general most isopygous or macropygous trilobites. In the spheroidal mode, all the thoracic segments have an equal share in enrollment. As the gliding of the thoracic segments began, the articulating half-rings were progressively uncovered and the thoracic axis became increasingly convex longitudinally. At the same time the downward curvature of the distal parts of the pleurae increased and their extremities became more and more imbricated, overlapping each other. Also, the transverse curvature of the cephalon increased slightly by downward bending of the genal regions. When enrollment was complete, the pygidium touched the cephalon and the whole animal attained a subspheroidal shape, the overlapping distal parts of the thoracic pleurae forming a sort of continuous lateral closure.

French

Domaine(s)
  • Paléontologie
OBS

Dans le mode [d'enroulement] sphéroïde [...] tous les segments thoraciques concourent à l'enroulement. On observe généralement : a) une augmentation de la convexité de la tête par incurvation vers le bas de la région génale [...]; b) une accentuation de la courbure verticale des extrémités des plèvres thoraciques et une imbrication de leurs facettes articulaires; c) une incurvation générale du rachis, avec écartement des anneaux axiaux initialement jointifs, grâce au jeu des membranes articulaires, et apparition de tous les demi-anneaux articulaires. Au terme de ces divers mouvements, l'animal a pris une forme sphéroïde; le bord postérieur du pygidium est appliqué au bord antérieur du céphalon [...], ou un peu en retrait et parfois engagé dans un sillon de fermeture [...]; les extrémités distales des plèvres, étroitement imbriquées, constituent une cloison latérale continue abritant les appendices.

Spanish

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