The Pygidium

In most trilobites, the posterior thoracic region or abdomen terminates in a shield called pygidium. This may result from the fusion (or lack of separation during growth) of a number of tergites. Extreme variation in size occur for this portion of the carapace, and the extension of the pygidium seems related to the number of thoracic segments. In the example in figure 2, the pygidium is small-the trilobite is called micropygous-but the number of thoracic segments is relatively large (twenty for the example shown). Frequently, the pygidium reaches a size comparable to that of the cephalon {isopygous trilobites), as will be seen from many examples in the atlas. In such cases, the number of thoracic segments is usually small. Macropygous trilobites are provided with pygidium larger than the cephalon. The axial lobe clearly extends into the pygidial region in a great majority of trilobite species, so that the trilobation is usually preserved. It may extend to the posterior margin or terminate earlier. Evidence of the segmentation from which the pygidium was derived is to be found in the frequently observed ribbing of the axial lobe, resembling the articulated axial rings of the thorax. This causes the axial lobe to appear continuous from the thorax through the pygidial region. There are pleural regions resembling thoracic pleurae, sulcated by furrows. When the latter do not reach the lateral margin, a smooth border results. The marginal border is turned under to form a doublure, much as in the cephalic and thoracic regions.The shape and ornamentation of the pygidium may reach extravagant extremes. Marginal spines are often present, which may be related or not to the pygidial pleurae. Occasionally the pygidium may terminate with a long axial spine, as the continuation of the pygidial axis. Exceptionally, as in the Olenellidae, the pygidium is formed of a single tergite, shaped as a true telson.

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