The impact of fossils on arthropod phylogeny

Abstract

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.