Making sense of ‘lower’ and ‘upper’ stem‐group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848

Abstract

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.

Fuxianhuiid ventral nerve cord and early nervous system evolution in Panarthropoda

“…The early evolutionary history of the panarthropod postcephalic CNS, however, remains obscure due to the exclusive preservation of brains in most available fossils (8,10,11). Moreover, the unresolved phylogenetic relationships within Panarthropoda complicate accurate reconstruction of the CNS ground pattern (16)(17)(18)(19)(20)(21)(22). In this study, we demonstrate the exceptional preservation of postcephalic neurological features in the early Cambrian fuxianhuiid Chengjiangocaris kunmingensis, an upper stem-group euarthropod (17) from the Xiaoshiba Lagerstätte, South China (23).…”

“…1-3) with a tripartite brain bearing both olfactory and optic lobes (8). Considering the position of fuxianhuiids within upper-stem Euarthropoda (17), this character combination could suggest the evolutionary convergence of either the VNC of Chengjiangocaris with Branchiopoda or the complex brain of Fuxianhuia with Malacostraca. Alternatively, the fuxianhuiid CNS may approximate the complex brain (sans Fig.…”

“…Apart from Euarthropoda (1-5, 9), the presence of segmental ganglia is also characteristic of the CNS in Tardigrada (18,28) (Fig. S5B) and has been used to advocate the sister-group relationship between these clades (16,17,20). Thus, the presence of segmental ganglia in C. kunmingensis provides a minimum phylogenetic threshold for the evolution of this feature in totalgroup Euarthropoda (SI Text).…”

The Fezouata fossils of Morocco; an extraordinary record of marine life in the Early Ordovician

“…A second iconic Cambrian clade represented in the Fezouata biota is the anomalocaridids (Fig. 3a): the Fezouata biota includes at least five forms, some of substantial size (Van Roy & Briggs 2011;Van Roy et al 2014, 2015; Box 2; Table 1). …”

“…It is the first anomalocaridid to show definitive evidence of both dorsal and ventral flaps. Anomalocaridids are potentially critical to understanding the origin of the arthropod biramous limb, sitting, as they do, stemward of the euarthropods but above lobopodians (Budd 1996;Zhang & Briggs 2007;Daley et al 2009;Kühl et al 2009;Cong et al 2014;Vinther et al 2014;Ortega-Hernández 2015;Van Roy et al 2015). Prior to the discovery of A. benmoulai, anomalocaridids had been reconstructed with just one row of flaps along each side of the trunk (e.g.…”