The taxonomy, phylogeny and palaeobiogeography of the trilobite families pliomeridae and encrinuridae: reconstructing the Ordovician world using evolving lineages

The phylogeny of the Family Pliomeridae and the Ordovician representatives of the Encrinuridae was investigated using cladistics. This resulted in the diagnosis of seven subfamilies; five of pliomerids (including one erected herein) and two subfamilies of encrinurid: the Cybelinae and Encrinurinae. Some 75 genera and 12 subgenera are also diagnosed on the basis of cladistic analysis of over 180 relatively complete species. This facilitated the placement of numerous incomplete or anomalous species such that the overall analysis allowed the systematic placement of over 300 species including 159 species of pliomerid and 149 species of encrinurid. The phylogenies produced by this analysis are robust, being resistant to disruption by the addition of incomplete taxa and in showing a close match between branching order and stratigraphy. A recurring evolutionary pattern is demonstrated. Very short initial periods of rapid diversification of clades, accompanied by reduced levels of character burden, were followed by steady ramification of the newly developed subclades. This pattern of development is observed to be independent of scale, occurring at all levels of systematic hierarchy. As a result many taxa are expressly paraphyletic which may be the norm in cladistic analyses. The taxa studied provided an excellent geographical and temporal spread for the investigation of Ordovician palaeobiogeography; the Pliomeridae being particularly useful of the early Ordovician, the Encrinuridae for the middle and late Ordovician. The palaeobiogeography was investigated by first elucidating the phylogeny of the clades and subclades through time and then assessing the palaeogeographical distribution of the taxa in the light of their evolutionary relationships. This palaeobiogeographical assessment was made both qualitatively and by means of geographical character optimisation and calculated gain/loss ratios. The two approaches supported each other. In addition to the detailed terrane-by-terrane results for each subclade, the following recurring patterns were observed: During the early Ordovician, Laurentia was juxtaposed to the eastern margin of Gondwana and close to areas of South America. Towards the end of the early Ordovician, Laurentia and Gondwana diverged, and did not subsequently re-approach each other during the Period although the Argentine Precordillera was situated in a position between Laurentia and Gondwana in the Llanvirn. This supports a modified version of the model of Dalziel for the early Ordovician in which Laurentia and Gondwana are close, but not in contact, but that of Scotese & McKerrow and others for the mid- and late Ordovician. Throughout the Ordovician oceanic islands provided stepping stones for faunal migration between the main continental plates