Trilobites

Abstract

What are trilobites? Trilobites are a fossil group of extinct marine arthropods with a heavily calcified external skeleton that populated the oceans from about 520 million years ago to about 250 million years ago. Their name — meaning three-lobed — derives from the distinction between the elevated longitudinal axis and the flatter regions that bound it. The trilobite body is divided from anterior to posterior into a distinct head region in which the mouth, stomach, eyes, and antennae were located, and a trunk region in which the segmented body construction is more clearly evident. In mature trilobites, the trunk is further divided into the thoracic region, in which the skeletal segments are articulated with their neighbours, and the terminal shield or pygidium within which all segments were rigidly conjoined (Figure 1). The oldest eyes known are the compound eyes of trilobites. Non-biomineralised cuticle is occasionally preserved, having been described for about 20 species. Do they have close living relatives? Their closest relatives have been thought to be horseshoe crabs, spiders and scorpions; trilobites certainly resemble horseshoe crabs in overall body shape. But recent renewed focus on the evolution of the arthropod head emphasises similarities between the trilobite antennae and those of mandible-bearing arthropods, such as myriapods, insects and crustaceans. These mixed taxonomic signals may reflect the fact that, aside from their peculiarities, trilobites did not depart far from the form of the common ancestor of all living arthropods, particularly in the overall low degree of limb and body specialization (tagmosis) and the extended, gradual pattern of development achieved through a protracted series of free-living moult stages (instars). Is there significant diversity within the group? Over 22,000 species of trilobites have been described to date and these display a fantastic variety of form, while retaining a highly conserved basic body plan. Functional analysis of some of the most striking morphological features has yielded elegant adaptive explanations in some cases, but other features still defy satisfactory explanation. For example, the shape and structure of the large calcitic lenses in some trilobites are exquisitely adapted to overcome spherical aberration; however, the function of the striking trident-like projections extending forward from the head remains an enticing mystery. What can they tell us about evolution? Trilobite species provide excellent examples of both prolonged morphological stasis and slow, incremental change during evolution. The convergence of different trilobite lineages upon common adaptive strategies, such as the tendency toward tight enrolment as protection against predators, attests to how biotic interactions influenced evolutionary change within the group. Likewise, repeated patterns of evolutionary radiation in species diversity followed by abrupt diversity crashes, indicate how fluctuating physical conditions, and particularly rapid extinction, played an important role in guiding the evolutionary history of the clade. OK, they are cute, but why should a developmental biologist care about them? Onset of calcification early in ontogeny coupled with gradual, progressive morphological development permits the recognition of growth sequences for over a hundred trilobite species. Striking variation in the number of trunk segments at maturity across ontogeny and phylogeny, coupled with the curious pattern of segment exchange between the pygidium and thorax during early development, provides opportunities to explore the co-evolution of different aspects of segmentation and body patterning. Recent results suggest an evolutionary trade-off between the ability to vary the overall number of thoracic segments and the advantages of regional specialization along the body axis. The striking, general decrease over time in the degree of intraspecific variation in the aftermath of the Cambrian radiation offers an opportunity to assess the roles of regulatory evolution and palaeoenvironmental change in shaping the evolutionary trajectory of the clade as a whole. What else are they good for? Because they evolved rapidly they provide the finest resolution of geological time available in rock strata of Cambrian age and are used as index fossils to date the strata. The spatial distribution of species also constrains ancient geography and patterns of plate movement. Trilobites also look well on bolo ties and other fashion accessories.