Making sense of 'lower' and 'upper' stem group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848
The ever increasing number of studies that address the origin and evolution of Euarthropoda whose extant representatives include chelicerates, myriapods, crustaceans and hexapods are gradually reaching a consensus with regard to the overall phylogenetic relationships of some of the earliest representatives of this phylum. The stem lineage of Euarthropoda includes numerous forms that reflect the major morphological transition from a lobopodian type to a completely arthrodized body organization. Several methods of classification that aim to reflect such a complex evolutionary history have been proposed as a consequence of this taxonomic diversity. Unfortunately, this has also led to a saturation of nomenclatural schemes, often in conflict with each other, some of which are incompatible with cladistic based methodologies. Here, I review the convoluted terminology associated with the classification of stem group Euarthropoda, and propose a synapomorphy based distinction that allows 'lower stem Euarthropoda' (e.g. lobopodians, radiodontans) to be separated from 'upper stem Euarthropoda' (e.g. fuxianhuiids, Cambrian bivalved forms) in terms of the structural organization of the head region and other aspects of overall body architecture. The step wise acquisition of morphological features associated with the origins of the crown group indicate that the node defining upper stem Euarthropoda is phylogenetically stable, and supported by numerous synapomorphic characters; these include the presence of a deutocerebral first appendage pair, multisegmented head region with one or more pairs of post ocular differentiated limbs, complete body arthrodization, posterior facing mouth associated with the hypostome/labrum complex, and post oral biramous arthropodized appendages. The name 'Deuteropoda' nov. is proposed for the scion (monophyletic group including the crown group and an extension of the stem group) that comprises upper stem Euarthropoda and Euarthropoda. A brief account of common terminological inaccuracies in recent palaeontological studies evinces the utility of Deuteropoda nov. as a reference point for discussing aspects of early euarthropod phylogeny.
Biological reviews of the Cambridge Philosophical Society