It is difficult to overestimate the importance of arthropods in the history of life. In terms of diversity and abundance, modern arthropods embody the ultimate success of the metazoa. Their prominence stems from an innovative, durable body plan, allowing mobility and adaptability for almost any environment in the earth's biosphere. Arthropods have been able to exploit an enormous range of lifestyles, becoming a conspicuous biotic component of many marine and terrestrial settings.

There are three major groups of living arthropods: Crustacea, Chelicerata, and Uniramia. To these may be added a fourth, the extinct trilobites. Determining the relationships of these higher taxa is difficult. A variety of approaches has led to different conclusions. At one extreme is the suggestion that the interrelationships of the major arthropod groups cannot be resolved, that they are quite distinct and are more appropriately treated as separate phyla (Manton, 1977; Schram, 1978). At the other is the view that while some basic arthropod characters — the hard exoskeleton, jointed appendages etc. — might have been acquired convergently, a more parsimonious approach is to treat them as monophyletic (Patterson, 1978).

Trilobites are the most diverse of extinct arthropod groups, being known from several thousand genera; many more are discovered each year. They range in age from near the base of the shell-bearing Cambrian to high in the Permian. Because many trilobites evolved quickly, they have been widely employed in stratigraphy; in the Cambrian they are possibly the most important stratigraphical fossils. This has been a mixed blessing because some experts studying the group have tended to place stratigraphical utility foremost in their taxonomic methods. Stratigraphical boundaries have become taxonomic boundaries. This might not matter for stratigraphy, but it does matter for the other kinds of paleobiological studies which have recently become the center of attention. How, for example, can one study extinction, unless the groups extinguished are natural, monophyletic groups? The extinction of an arbitrary phylogenetic segment at a stratigraphic boundary tells us nothing.

Arthropods are currently abundant and diverse in many environments, with terrestrial insects alone accounting for at least 70% of all extant animal species (Barnes, 1980). The fossilization potential of arthropods, however, is low. With the exception of trilobites, ostracods, and decapods, most arthropod exoskeletons are weakly mineralized and contain abundant organic material. Multielement construction and ecdysis (shedding of exoskeleton during life) introduces the problem of disarticulation and transportation. Therefore, preservation of most arthropods as body fossils requires exceptional circumstances such as rapid burial or anoxic conditions.

Sepkoski (1981a,b, 1988) has characterized the Cambrian fauna as unique, both in terms of taxonomic composition and in environmental distribution of taxa. Compared with the rest of the Phanerozoic, the frequency of mass extinction in the Cambrian must also rank as a distinctive feature which had a profound impact on macroevolutionary patterns. Three well-documented extinctions occurred in the Upper Cambrian of North America (Figure 1; Palmer, 1979; Westrop and Ludvigsen, 1987) and are best expressed in the trilobite faunas. Possible older extinctions may be present at the top of the Olenellus Zone and near the base of the Bolaspidella Zone (e.g., see Palmer, 1982) but more data are required.

Trilobites, like most arthropods, grew through a series of discrete increments (known as instars). The moulted sclerites accumulated in the sediment and were potentially preserved. This mode of growth provides us with a tangible record of morphological attributes that characterize a given stage in an individual's ontogeny, and a record of changes that occur between successive stages.

Since Barrande (1852) first illustrated a trilobite larva, aspects of trilobite ontogeny and early development have received a great deal of attention (see Beecher, 1895; Størmer, 1942; Whittington, 1957; Hu, 1971; Chatterton, 1980). Much of this literature, however, is purely descriptive and very little has been done to incorporate these works into a biological synthesis. During the past several decades a great deal has been learned about the role of larval ecology in monitoring biogeographic distributions, cohort survivorship and taxonomic longevity among modern marine invertebrates. This growing body of knowledge has provided the basis for many new insights regarding patterns of extinction and survivorship and macroevolution evident within the fossil record (see Jablonski, 1986; Jablonski and Lutz, 1983).

Phylogenetic Uncertainty.— Invertebrate zoologists have long debated the relationships of biramous-limbed Crustacea to other groups of arthropods. Haeckel (1866) recognized two groups of arthropods on the basis of respiratory anatomy. The Carides included the crustaceans, trilobites, xiphosures, and eurypterids; and the Tracheata, included the arachnids, insects, and myriapods. Work on the onychophorans (Mosely, 1874) indicated a link between annelids and the terrestrial myriapod-insect line, but this left the origins of the aquatic carides unresolved. Lankester (1881) demonstrated that the xiphosures were allied to the arachnids rather than to the crustaceans. These and other studies led Haeckel (1896) to revise his position and propose two separate lines of arthropods: one of primarily aquatic groups with primitively biramous appendages (crustaceans, trilobites, and chelicerates); and the other of primarily terrestrial groups with uniramous limbs (onychophorans, myriapods, and insects). Haeckel's new arrangement emphasized the convergent origins of trachea in the arachnids and uniramians. Korschelt and Heider (1890) preferred a monophyletic scheme with the two evolutionary lines united by a pre-onychophoran, “protostracan,” ancestor. By modern standards, however, this might indicate that the arthropods are a morphological grade, rather than a true clade.

To a large extent, a chitinous external cuticle is the distinguishing feature of arthropods (Barnes, 1980). Many of the other features considered characteristic of arthropods, such as growth by molting or the jointed appendages, are either directly or indirectly tied to the possession of the cuticle (Cisne, 1974; Neville, 1975; Grasshoff, 1981).

When did arthropods first invade the land? Circumstantial evidence based on trace fossils (Retallack and Feakes, 1987) suggests that large animals, possibly arthropods, may have been present on land and burrowing in soils in the Late Ordovician, but this idea is as yet unsupported by body fossil evidence.

Since the late Paleozoic, insects and arachnids have diversified in the terrestrial world so spectacularly that they have become unquestionably the most diverse group of organisms to ever inhabit the planet. In fact, this 300 million year interval may appropriately be referred to as the age of arthropods. What is the origin and history of terrestrial arthropods? How is arthropod diversity maintained on land? In this rhetorical context we will discuss (1) the degree to which terrestriality is found in arthropods, (2) the physiological barriers to terrestrialization that arthropod clades confronted, (3) the historical record of arthropod diversity on land based on paleobiological, comparative physiological and zoogeographical evidence, and (4) some tentative answers to the “why” of terrestrial arthropod success. We are providing a geochronologic scope to terrestriality that includes not only the early history of terrestrial arthropods, but also the subsequent expansion of arthropods into major terrestrial habitats.

Most paleontological textbooks deal with tracheates and chelicerates in only a cursory way because of their putatively poor fossil record. However, recent investigations into the paleobiology of these groups reveal that the fossil record is not only more extensive than previously assumed, but provides a wealth of information regarding both broad and detailed patterns of evolution of the two most diverse subphyla on the planet. Tracheata, including insects, entognaths and the various myriapod groups, are the most diverse subphylum. Insects alone are the most diverse class of animals known, outnumbering the combined species level diversity of all other animals. The Chelicerata, composed of the eurypterids, xiphosurids, arachnids and pycnogonids, are the second most diverse subphylum, with the diversity of arachnids exceeding all classes except for the insects. Consequently, not only does the evolution of tracheates and chelicerates provide an interesting story in itself, but these groups also provide us with insight into more general aspects of the evolutionary process that are of interest to the general evolutionary biologist as well as to the arthropod specialist.

Crustacean phylogeny, long the focus of intense speculation, has undergone a renaissance in the last decade. This has been largely propelled by the use of cladistic analyses on the part of several workers, though there are still many who prefer more subjective, evolutionary systematic approaches.

Studies of paleobiogeography have changed markedly in recent decades transforming a once static subject into one which now has great potential as a useful counterpart to systematic and ecological studies in the interpretation of the geological history of organisms. This has resulted, in large part, from the emergence of plate tectonic models which, in turn, have been used as the bases for extremely sophisticated paleoclimatic modeling. As a result, paleobiogeography has attained a level of precision comparable to that of the studies of paleoecology and systematic paleontology. It is now possible to consider causes for global patterns of origin and dispersal of organisms on a much more realistic level than was previously possible.