Disparity is a measure of the range or significance of morphology in a given sample of organisms, as opposed to diversity, which is expressed in terms of the number (and sometimes ranking) of taxa. At present there is no agreed definition of disparity, much less any consensus on how to measure it. Two possible categories of metric are considered here, one independent of any hypothesis of relationship (phenetics), the other constrained within an evolutionary framework (cladistics).The Early Cambrian radiation was clearly a period of significant morphologic and taxonomic diversification. However, we question the interpretation of its first generation products as numerous body plans at the highest level. Four phenetic and two cladistic measures have been used to compare disparity among Cambrian arthropods with that in the living fauna. Phenetic methods assessing character-state variability and the amount of morphological attribute space occupied yield similar results for Cambrian and Recent arthropods. Assessments of disparity within a taxonomic framework rely on the identification of particular characters that delineate higher level body plans. This requires a phylogenetic interpretation, a cladistic investigation of hierarchical structure in the data. Both sets of arthropods fall within the same major clades, and within this cladistic framework the amount of character-state evolution in the two groups is comparable. None of these methods identifies markedly greater disparity among the Cambrian compared with the Recent taxa.Although measures of disparity are applied here to a consideration of the Cambrian radiation, the metrics clearly have a much wider potential for estimating macroevolutionary trends independently from existing taxonomic frameworks. Geometric morphometry is ideal for measuring morphological variety at lower taxonomic levels, but it requires the recognition of homologous landmarks in all the forms under comparison, or the identification of entire homologous structures. Conventional phenetics has much wider application as it can operate on data coded as discrete homologous character states (this facility is also a requirement of cladistics), which are a more appropriate basis for comparing disparity in markedly dissimilar forms.

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ABSTRACT. We briefly review the various types of feeding habits in marine arthropods, and suggest that the trilobites adopted a range of different feeding strategies. We show that much of the variety of trilobite exoskeletal development, particularly in the cephalon, can be explained as a response to the adoption of specific feeding modes. We regard the primitive mode as having been predatory/scavenging, both from morphological grounds and by out-group comparison, but this habit had a long subsequent history in the group. Predators/scavengers included those trilobites with rigidly braced and attached conterminant or impendent hypostomes, which often developed posterior forks or rasps used by the animals for manipulating prey after it had been grasped by the 'gnathobases'. Advanced predators often acquired expanded anterior glabellar lobes which are associated with the ingestion of bulky food; concomitantly, the largest trilobites of all had predatory morphology. Associated trace fossils are of the Rusophycus type in which impressions of limb bases and rarely the hypostome can be seen. Detritivors were derived from predators by detachment of the hypostome from the doublure in natant mode; it is significant that the hypostome of such feeders exhibited little change thereafter. The typical detritivor morphology is of the 'generalized' ptychopariid type, common in outer shelf habitats, with rectangular or tapering glabellas and small to moderate overall size. It is suggested that in some species the hypostome may have functioned as a 'scoop' directly to aid ingestion of sediment. Trace fossils of Cruziana semiplicata type have been associated with sediment ploughing in this feeding mode. Filter feeders evolved a vaulted cephalic chamber of trinucleimorph type, and elevated thoraces, often flanked by extended genal spines. Where it is known, the hypostome is curved up inside the cephalic chamber, within which sediment stirred into suspension by the limbs was sorted for edible particles. Filter feeding trilobites are typically small, and are uncommon outside muddy habitats. Bean-like Rusophycus are the associated trace fossils. In trinucleids ingress of the feeding current was alongside the thorax and out through the fringe pits. The combination of different feeding modes with adaptation for different prey and/or particle sizes goes some way to account for the variety of trilobites cohabiting in a single site (alpha diversity). We do not claim that the model accounts for all morphological variation displayed by the group.

The faunal criteria for the recognition of the relative separation of old continents are: 1, the contrast between shallow-(not deep-) water faunas off opposite shorelines; 2, the recognition of deeper-water facies surrounding the edges of continents; and 3, the recognition of the disposition of ancient climatic belts. These criteria are defined and used to assess the relative position of the various parts of what is now Britain in the Palaeozoic. The Iapetus Ocean is well characterized by faunal differences from the Cambrian until the lower Ordovician, but from Caradoc times onwards these differences dwindle, until by the latest Silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds. In the early Ordovician, S British faunas are comparable with those from Bohemia, France and elsewhere to the S, indicating connection with Gondwanaland, and these faunas differ from those in the Baltic area, a distribution which we attribute to a true ocean, Tornquist's Sea, between N and S Europe. Tornquist's Sea probably closed in the upper Ordovician, and from Ashgill times onwards the S British and Baltic faunas remained similar. During the middle and late Silurian there is evidence of increasing faunal distinction between Britain, the Baltic area and North America on the one hand and southern Europe and Gondwanaland on the other, indicating an opening Rheic Ocean which appears to have persisted until late Carboniferous times.

An analysis of the range of morphology among arthropods demonstrates that disparity among living arthropods is similar to that in Cambrian arthropods. The range of morphological design resulting from the Cambrian "explosion" has been overestimated, reflecting a tendency to separate as "problematic" taxa that cannot be accommodated in the classification on the basis of the living biota. Problematic taxa are largely an artifact of an inadequate taxonomy. Special evolutionary processes may not be necessary to explain the early radiation of the metazoans.

The Anthropocene, an informal term used to signal the impact of collective human activity on biological, physical and chemical processes on the Earth system, is assessed using stratigraphic criteria. It is complex in time, space and process, and may be considered in terms of the scale, relative timing, duration and novelty of its various phenomena. The lithostratigraphic signal includes both direct components, such as urban constructions and man-made deposits, and indirect ones, such as sediment flux changes. Already widespread, these are producing a significant ‘event layer’, locally with considerable long-term preservation potential. Chemostratigraphic signals include new organic compounds, but are likely to be dominated by the effects of CO 2 release, particularly via acidification in the marine realm, and man-made radionuclides. The sequence stratigraphic signal is negligible to date, but may become geologically significant over centennial/millennial time scales. The rapidly growing biostratigraphic signal includes geologically novel aspects (the scale of globally transferred species) and geologically will have permanent effects.

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Cambrian arthropods are now well known, but there has been little agreement on how they contribute to an understanding of arthropod phylogeny. Fossils have either been lumped together as "trilobitomorphs" or, more recently, have been the subject of speculation invoking a multiple polyphyletic origin of arthropods. Cladistic analysis of characters of Cambrian and living representatives (excluding Uniramia) shows that trilobites and chelicerates are relatively advanced compared with "crustaceans," and there are doubts whether the latter constitute a national group. An undue emphasis on singular autapomorphies of problematic fossils has obscured these relationships in the past. "Trilobitomorphs" were simply an artificial taxon based on shared primitive characters. The arthropods that evolved during the Cambrian radiation show no more apparent morphological diversity than do the living groups. The evidence of wellpreserved problematica is critical to understanding the nature of this radiation and the affinities of the groups that remain today.