Origins and early evolution of arthropods

Abstract

Phylogenomics reconstructs an arthropod tree in which a monophyletic Arthropoda splits into Pycnogonida + Euchelicerata and Myriapoda + Pancrustacea. The same chelicerate-mandibulate groups are retrieved with morphological data sets, including those encompassing most taxa known from Palaeozoic Konservat-Lagerst€ atten. With respect to the interrelationships of the three extant clades of Panarthropoda, a sister group relationship between Onychophora and Arthropoda is endorsed by transcriptomics and microRNAs, although this hypothesis forces homoplasy in characters of the segmental ganglia that are shared by tardigrades and arthropods. Cambrian lobopodians, dinocaridids, bivalved arthropods and fuxianhuiids document the successive appearance of characteristic arthropod features in the stem lineage of Euarthropoda (crown-group arthropods). Molecular dating suggests that arthropods had their origin and initial diversification in the Ediacaran, but no convincing palaeontological evidence for Panarthropoda is available until the earliest Cambrian.

Ancestral reconstruction of tick lineages

“…Our confidence that ticks fall within this classification scheme (Eukaryotes, Metazoa, Bilateria, Nephrozoa, Protostomia, Ecdysozoa, Arthropoda, Chelicerata, Arachnida, Parasitiformes, Ixodida) is fairly robust based on morphological and molecular evidence (Dunlop, 2010;Edgecombe et al 2011;Edgecombe and Legg, 2014;Garwood and Dunlop, 2014;Giribet and Edgecombe, 2012;Legg et al 2013;Philippe et al 2009;Shultz, 2007Shultz, , 1990. This phylogenetic placement allows us to make some generalized statements with regard to ancestral reconstruction, with the understanding that there is a gradation of lineage specific conservation.…”

“…While most lineages within the Arachnida are well defined, their relationships to each other are not clear, since morphological and molecular data do not support similar phylogenies (Dunlop, 2010;Edgecombe et al 2011;Edgecombe and Legg, 2014;Giribet and Edgecombe, 2012;Legg et al 2013;Philippe et al 2009;Shultz, 2007). The sister group to the Acari (Acariformes and Parasitiformes) has been considered to be the Ricinulei (Shultz, 2007;Wheeler and Hayashi, 1998), unresolved (Edgecombe and Legg, 2014;Giribet and Edgecombe, 2012;Klompen et al 2007), or the Palpigradi and Pycnogonida (Giribet et al 2002).…”

“…The sister group to the Acari (Acariformes and Parasitiformes) has been considered to be the Ricinulei (Shultz, 2007;Wheeler and Hayashi, 1998), unresolved (Edgecombe and Legg, 2014;Giribet and Edgecombe, 2012;Klompen et al 2007), or the Palpigradi and Pycnogonida (Giribet et al 2002). Additionally, a number of morphological and molecular studies have suggested that the Acari is not monophyletic (Borner et al 2014;Dunlop, 2010;Dunlop and Selden, 2009;Garwood and Dunlop, 2014;Ovchinnikov and Masta, 2012;Pepato et al 2010;Regier et al 2010;Sharma et al 2014;Shultz, 2007; van der Hammen, 1989).…”

“…Ecdysozoa include the Arthropoda, Nematozoa (nematodes), Onychophora, Scalidophora and Tartigrada (Philippe et al 2009). The Arthropoda are invertebrates with exoskeletons, segmented bodies and jointed appendages and subphyla include Chelicerata (Arachnida, Xiphosura and Pycnogonida), Crustacea (crustaceans), Hexapoda (insects) and Myriapoda (centipedes and millipedes) (Edgecombe, 2010;Edgecombe and Legg, 2014). Within this scheme ticks are part of the Arachnida; a distinction shared with other Acari (mites), Amblypygi (tailless whip scorpions), Araneae (spiders), Opiliones (harvestmen), Palpigradi (microwhip scorpions), Pseudoscorpionida (pseudoscorpions), Ricinulei (hooded tick-spiders), Schizomida (split middle whip scorpions), Scorpiones (scorpions), Solifugae (sun spiders) and Thelyphonida (whip scorpions).…”

Opsins inLimuluseyes: Characterization of three visible light-sensitive opsins unique to and co-expressed in median eye photoreceptors and a peropsin/RGR that is expressed in all eyes

“…Limulus polyphemus (hereafter referred to as Limulus) is one of only four extant species of xiphosauran chelicerates, an early branching sister group to arachnids (Regier et al, 2010;Edgecombe and Legg, 2014). Therefore, studies of the structure and function of Limulus eyes may provide insights into the structure and function of the eyes of Euarthropoda (Nilsson and Kelber, 2007).…”