The Trilobita

3.1 Introduction to Trilobite Morphology

The general description of the distinctive features of the Arthropoda given in chapter 2 applies to the Trilobita as well. Here, however, the picture is less complete than for the living arthropods, since, of course, our evidence must be based only on the fossil record. Several details of the anatomy and physiology of trilobites are inferred by analogy with what is known about the character of arthropods in general. As much as this analogy may be useful, however, it is important to notice that the evolution of trilobite morphology and life habits differed significantly from that of most arthropods. Throughout their long survival, trilobites preserved a relatively primitive body structure, characterized by serial homology manifested in the repetitive appearance of nearly identical segments and by a low degree of tagmosis. This generalized, primitive arthropod morphology is encountered in only a few modern descendants. By contrast, instead, most arthropod groups progressed toward special adaptation of groups of segments to perform functions such as locomotion, food gathering, defense and aggression, respiration and others. As we shall come to understand, the limited specialization of arthropod design in trilobites may be consistent with a predominantly nonpredatory life-style and with humble food-gathering habits.

Trilobites were, on the average, small animals, two to seven centimeters long, with extremes ranging from three millimeters to about seventy centimeters. They lived in the sea, for their remains are always associated with those of marine animals (corals, brachiopods, cephalopods, and so forth).

The commonly preserved portion of the body of trilobites is the dorsal shield, or carapace, made of sclerotized and mineralized protein. Most of the ventral part of the exoskeleton is generally not preserved, and we must infer that it consisted of a cuticle made of nonmineralized protein. As a rule, all soft parts as well as appendages are also missing in the fossil trilobite. There are, however, exceptional forms of fossiliza-tion in which most of the soft pans have been preserved, and these few examples have given a powerful insight into the detailed anatomy of these extinct arthropods. We shall deal with such details in section 3.2, limiting this introduction to the more apparent characteristics of trilobite morphology and life habits.

For the description of trilobite morphology and nomenclature we shall refer to the reconstruction of a trilobite in figure 2, representing the dorsal view of the carapace of Paradoxides gracilis (Boeck), a beautiful trilobite from the Middle Cambrian of Bohemia. Shown in plate 1 is the photograph of the original specimen, on which the reconstruction has been based. We recognize immediately the bilateral symmetry of the trilobite body, a characteristic that is shared with a large majority of animal groups, collectively described as Bilateria. Here the strongly convex axis takes the name of axial lobe and the two adjacent regions are called pleural lobes. The pleural lobes are separated from the axial lobe by two axialfurrows. It is from this longitudinal trilobation (separation into three lobes) that the name "Trilobita" originated and not from the longitudinal subdivision of the body into the three regions of the cephalon, the thorax, and the pygidium, as is often erroneously supposed. The latter subdivision is in fact a characteristic that trilobites have in common with most arthropods. The reader interested in a more comprehensive coverage of the topics, which by necessity are mentioned only briefly in this context, is referred to theTreatise on Invertebrate Paleontology (Moore 1959).

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