Cephalon & Pygidium Relationships

The Pygidium

last revised 12 November 2009 by S. M. Gon III

The pygidium is a tail piece composed of a variable number of fused segments at the posterior end of a trilobite. While the cephalon of a trilobite is typically wider than the thorax, and one of the single largest major features of a trilobite body, the pygidium can range from extremely small to larger than the cephalon. There are four general categories of pygidium relative size, shown below:


micropygous pygidium much smaller than cephalon	subisopygous pygidium subequal to cephalon	isopygous pygidium equal to cephalon	macropygous pygidium larger than cephalon

Above are four hypothetical examples, with cephalon and pygidium tinted

The relationship between the size of the pygidium relative to the cephalon has some utility in considering trilobite classification. For example, Redlichiida typically have micropygous pygidia, while those of Asaphida are typically subisopygous to isopygous. Thus, these terms appear in the Trilobite Order Fact Sheets when describing the pygidium characters of the trilobite orders. Examining the subisopygous and isopygous specimens below, it is clear how the pygidium is made up of several segments, but these are fused, not articulated as thoracic segments are. Even a micropygous pygidium is typically made up of a few fused segments. The relationship between segments and articulations is discussed further in the ontogeny and development page, where the role of the pygidium in growth and addition of segments is described. In trilobites with larger pygidia, it functions as part of a protective capsule when the trilobite enrolls, typically matching the cephalon more or less precisely.

micropygous
(Redlichia)
Order Redlichiida

subisopygous
(Bathyurus)
Order Proetida

isopygous
(Ogygiocaris)
Order Asaphida

macropygous
(Scutellum)
Order Corynexochida

Above are four real examples of various pygidium sizes among trilobites
©2005 by S. M. Gon III. Created using Macromedia Freehand

In addition to size, the pygidium can be small and simple; or larger, but otherwise similar to the thoracic segments before it (the homonomous condition), or distinguished by modifications such as spines and lobes, or otherwise quite different in morphology from the thoracic segment pattern (the heteronomous condition). The examples below display both homonomous and heteronomous pygidia of various sizes and from a variety of orders. Clearly, with so much variation in size and shape, no single consistent function can be assigned to the pygidium.


micropygous homonomous Bolaspidella Order Ptychopariida	subisopygous homonomous Phacops Order Phacopida	isopygous homonomous Proetus Order Proetida	subisopygous heteronomous Kolihapeltis Order Corynexochida	micropygous heteronomous Cheirurus Order Phacopida	isopygous heteronomous Ceratarges Order Lichida