The classification of the trilobite family Cheiruridae has been revised. The work has included visiting the institutions having the major palaeontological collections in this country, and such material has been borrowed for study and photography in Newcastle. The British species of Cheirurids described before about 1950 have been redescribed and figured. The nomenclature of the family at the specific, generic and sub familial levels has been clarified. The subfamilies considered to belong to the Cheiruridae are as follows; Cheirurinae, Cyrtometopinae, Pilekiinae, Sphaerexochinae, Deiphoninae, Artinae and Acanthoparyphinae. The subfamily Hammatocneminae is excluded from the family and probably requires the erection of a new monotypic family. The genus Onycopyge is also excluded and considered to form a new monotypic subfamily of the Encrinuridae. With reference to all available world literature on the family, as complete as possible a list of Cheirurid species has been built up and a phylogeny of the genera is proposed. The distribution of these genera throughout the time range of the family (Upper Cambrian to Middle Devonian) is dealt with; the Upper Cambrian and Tremadocian genera (Pilekiinae) being widespread, the Ordovician genera being provincial in distribution in common with the pattern shown by many other trilobite families at that time, and the Silurian and Devonian Cheirurids having worldwide distribution. A very short preliminary account of the Llandoverian trilobites of the type area is included at the end of the thesis. The preliminary determinations of the trilobites, which are uncommon in these rocks, indicate about 12 genera to be present in the collections made by the author and in that made by Prof. O. T. Jones which is housed at the Sedgwick Museum, Cambridge University.

The arthropods are the most diverse, abundant and ubiquitous phylum on Earth. Five main extant groups (subphyla) can be recognized: Pycnogonida, Euchelicerata, Myriapoda, Hexapoda, and Crustacea. Each group displays a distinctive body plan and a suite of autapomorphies that makes determining their interrelationships difficult. Although a variety of hypotheses have been proposed regarding their interrelationships, just three have frequently been recovered in recent phylogenetic analyses. Rather than representing incongruent topologies these hypotheses represent variations of the position of the root on the same parent topology. The long histories of the major arthropod subclades, which had begun to diverge by, at least, the early Cambrian, means that long-branch artefacts are highly probable. To alleviate potential long-branch attraction and provide a more accurate placement of the root, 214 fossil taxa were coded into an extensive phylogenetic data set of 753 discrete characters, which also includes 95 extant panarthropods and two cycloneuralian outgroups. Preference was given to those fossil taxa thought to occur during the cladogenesis of the major arthropod clades, i.e. the lower and middle Cambrian. An extensive study of material from the middle Cambrian Burgess Shale Formation and the coeval Stephen Formation in British Columbia (Canada) was undertaken. This study focussed primarily on taxa thought to represent 'upper stem-group euarthropods', namely bivalved arthropods and megacheirans ('great-appendage' arthropods), as they will have the greatest utility in polarizing relationships within the arthropod crown-group [= Euarthropoda]. This study includes the description of three new genera and four new species: the bivalved arthropods Nereocaris exilis, N. briggsi, and Loricicaris spinocaudatus; and the megacheiran Kootenichela deppi; and a restudy selected material referred to the bivalved arthropod taxa Isoxys, Canadaspis perfecta, Odaraia alata and Perspicaris dictynna. Results of the phylogenetic analysis and additional perturbation tests confirm the utility of these taxa for polarizing relationships within Euarthropoda and reducing long-branch artefacts. For example, the hexapods were recovered within a paraphyletic Crustacea, a result anticipated by molecular phylogenetic analyses but until now elusive in morphological phylogenies. Perturbation tests indicate that close affinities of myriapods and hexapods, a result common in morphological analyses, is the result of a long-branch artefact caused by the convergent adaptation to a terrestrial habit, which is broken by the addition of fossil material. The phylogeny provides a detailed picture of character acquisition in the arthropod stem group.

All known Lower Palaeozoic proetid trilobites from northern Europe have been examined, and those from the Ordovician of the British Isles and Scandinavia and those from the Silurian of the British Isles and Bohemia are described in detail. Closely related species from other areas are figured for comparison. Members of 23 proetid genera, 8 of which are new, are described and figured. 113 species have been investigated, 36 of which are new. The Lower Palaeozoic proetids have hitherto been poorly understood and neglected, and in this thesis they are comprehensively treated as a group for the first time. With their description and figuring, it is now possible to propose a provisional phylogeny for the earlier Proetidae, and to attempt to seek the origins of many important Devonian genera. Of morphological features, the most useful characters for classification are found in the pygidium. Cephalic characters tend to be less reliable. The rostral plate is always subtriangular or trapezoidal in outline, and in all except one case the connective sutures converge backwards. The preannulus has been found to be restricted to the subfamilies Proetinae and Cornuproetinae. From this study it appears as if at least two main lines of proetid trilobites extend well back into the Ordovician, and there may well be a case in the future for recognising two distinct families. The origin of the Proetidae remains problematical, but they could have their origins in Cambro-Ordovician Hystricurine trilobites, some of which, like the proetids, have a trapezoidal rostral plate.

No description available.

No description available.

No description available.

No description available.

No description available.

Stasis may be operationally defined as the occurrence of little or no evolutionary change during an interval of geological time, and is an important consequence of punctuated equilibria. Studies of stasis in the fossil record of necessity address only morphological stasis, and that only in the subset of phenotypic characters preservable in the fossil record. Stasis in single characters may be recognised in fossil taxa by lack of significant change in mean value through an interval of geological time; stasis in multiple characters may be recognised by overlap in morpho space occupation by taxa where morpho space occupation is calculated by multivariate techniques. No quantitative definition is placed on such stasis because of the lack of comparable data on non-static (i.e. rapidly evolving) taxa to provide the alternative. Proposed explanations for stasis include: developmental and genetic constraints; environment fidelity; selection of generalist phenotypes in fluctuating environments; stabilising selection (including stabilising species selection); developmental canalisation; effects due to population size and distribution. Mean generic and specific durations (in myr.) of trilobites originating In the stratigraphical systems Cambrian-Carboniferous of England, Scotland and Wales are, respectively: Cambrian (4.42, 2.13); Ordovician (10.89, 2.06); Silurian (10.34, 3.54), Devonian (4.19, 1.12), Carboniferous, (14.82,5.74). Distributions of both generic and specific durations are highly positively skewed. Study of the species composition of the longest duration genera (those whose durations exceed the 90% quantile value for the system in which they originated) suggests that species stasis played an important role in the Cambrian and Carboniferous; no clear pattern is revealed for the interval SilurianDevonian inclusive. Chronostratigraphical range charts are presented for species and genera from England, Scotland and Wales. Study of the durations of Ordovician Laurentian genera in relation to their position on the palaeoslope shows that longest duration genera are eurytopic; their wide geographical and environmental dispersal enabled them to avoid localised factors which caused extinction in more endemic genera. Taxonomy-independent phylogenetic and morphometric analysis of selected long duration shape conservative genera from the middle to upper Ordovician and Silurian shows that disassociated mosaic evolution in some characters is abundant in all three, superimposed on an almost invariant body plan. Achatella Delo, 1935 had a duration of about 22 myr. (upper Llanvirn - Hirnantian, time scale of Tucker et al. 1990). Nine species (three new) and one form under open nomenclature have been diagnosed. Calyptaulax Cooper, 1930 had a duration of about 25 myr. (lower Llanvirn - upper Rawtheyan, time scale of Tucker et al. 1990). Two subgenera are diagnosed, each of duration about 20 myr. (time scale of Tucker et al. 1990). The nominate subgenus is well resolved on the cladogram, and five species have been diagnosed. Calyptaulax Abstract. Page ii (Calliops) is unresolved on the cladogram because of a disassociated mosaic pattern of "peripheral" character evolution; ten species have been diagnosed. A sixteenth species could not be assigned to a subgenus. Acernaspis Campbell, 1967 had a duration of about 11 myr. (lower Llandovery - Wenlock, time scale of Harland et al. 1989). Eighteen species have been diagnosed, three of them new. Several stratigraphical samples of Ananaspis Campbell, 1967 have been studied and an hypothesis that this genus arose through neoteny from Acernaspis has been confirmed, although not a further hypothesis that progressive neoteny continued throughout the existence of Ananaspis. Four Ananaspis species have been diagnosed, one of which is new. This does not constitute a complete survey of Ananaspis. The disassociated mosaic pattern of peripheral character states probably reflects differing degrees of developmental canalisation at different levels of phenotypic organisation. The basic body plan is strongly canalised, whereas at "peripheral" levels, less strong canalisation allows emergence of superficial characteristics. This, combined with eurytopic distribution, may keep the taxa adapted to their (various) environments without need for more major evolutionary change.

Many trilobites have cuesta-like structures, known as terrace ridges, on both the dorsal and ventral surfaces of the exoskeleton. Although terrace ridges all appear to have the same basic construction, they are highly variable and several types are known. These structures are poorly understood and there are many, varied and sometimes contradictory theories as to their function, which are discussed herein. Terrace ridge shape variation was explored across Class Trilobita, first qualitatively and then using a novel geometric morphometric technique, extended (landmarkregistered) eigenshape analysis (EEA) (MacLeod, 1999). A database containing details of over 6000 images of trilobite terrace ridges in the literature was compiled from over 450 scientific papers, from which a resource of 1600 scanned images of terrace ridges within the Asaphida was produced. A successful heuristic analysis technique was developed using EEA, analysing approximately 400 of these images. Trends in the variation of simplified terrace ridge arrays on several parts of the trilobite were successfully identified. The analysis of these terrace ridge arraysachieved good taxonomic separation and, in particular, this analysis appeared to separate pelagic and benthic terrace ridge-bearing forms, potentially providing an independent cryptic test for trilobite mode of life hypotheses based on exposed morphologies. Both qualitative and quantitative strands of research contributed to a phylogenetic discussion of terrace ridges across Class Trilobita as well as informing an analysis of the suggested functions of terrace ridges. The mapping of terrace ridge character states clarified patterns of acquisition and secondary loss of terrace ridges across the Class. Secondary losses were suggested to be related to the adoption of specialised feeding behaviours and the development of alternative types of sculpture. Some support was found for theories of frictional interaction and species recognition as roles for terrace ridges from the morphometric analyses.