Pygidium (Trilobites)
The pygidium , also known as the tail shield or anal shield, is the rearmost body section ( tagma ) of the extinct trilobites . The pygidium of the trilobites was not homologous to the formations of other arthropods and the annelids, also called pygidium , but is an evolutionarily completely independent structure.
anatomy
The body of the trilobite is divided into a head section, which is covered by a non-articulated head shield ( cephalon ), which comprises a non-segmental head lobe and five segments , and a trunk section. The trunk section is further subdivided into the front ( anterior ) actual trunk or thorax and the pygidium behind it (posterior), so that the body as a whole appears to be made up of three sections when viewed from above. The number of segments of the pygidium is variable between the species; a pygidium was present in most, but not all, trilobite species. According to the proportions, micropyge species with a small Pygidium are sometimes distinguished from macropygen with a large one, the size of the head shield being used as a reference; if the head shield and pygidium are about the same size, they are called isopyg . The pygidium, like the trunk lying in front of it, is also divided into three parts in many species into a central, arched axial lobe and two flat pleural lobes on both sides, but in these the integument was without joint sutures. In the living animal , the segments of the pygidium had split legs , the structure of which corresponded completely to those of the thoracic segments, so that the animal only looked split in two when viewed from the ventral side (from the ventral side). The pygidium is defined as the intrinsically immobile trunk section behind the last trunk joint.
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development
Trilobiten developed over a number of, in each case by a molting separate larval stages . With fossils of larvae and calcified, empty larval shells, called exuvia , the development of many trilobite species is easy to understand. The first stage, called protaspis, was not yet divided into head and trunk. A trunk with pygidium could be present from the second larval stage, the Meraspis. Later larval stages, which eventually show all segments of the adult animal, are called holaspis. The course of development was an anamerism or anamorphosis, characterized by a variable number of segments in the larvae. Finally, the adults showed, as a rule, a fixed number of segments typical of the species (hemianamorphosis).
The growth pattern of the segments in the trilobites was different from all living (or recent ) arthropods: New segments were formed at the front edge of the pygidium in a growth zone. These segments, which initially belonged to the pygidium, usually formed a joint membrane during later moults, and then belonged to the movable trunk section. As a result of this development process, the pygidium "migrated" backwards along the trunk, so to speak, so that the pygidium of the larvae was made up of other segments like that of the adults; one often speaks of the "transitory pygidium" of the larvae.
swell
- Riccardo Levi-Setti: Trilobites. University of Chicago Press, 2nd edition 1995. ISBN 978 0226474526
- Nigel C. Hughes, Alessandro Minelli, Giuseppe Fusco (2006): The ontogeny of trilobite segmentation: a comparative approach. Paleobiology 32 (4): 602-627. abstract online
- Nigel C. Hughes (2007): The Evolution of Trilobite Body Patterning. Annual Review of Earth and Planetary Sciences 35: 401-434. doi : 10.1146 / annurev.earth.35.031306.140258