Understanding animal terrestrialization, the process through which animals colonized the land, is crucial to clarify extant biodiversity and biological adaptation. Arthropoda (insects, spiders, centipedes and their allies) represent the largest majority of terrestrial biodiversity. Here we implemented a molecular palaeobiological approach, merging molecular and fossil evidence, to elucidate the deepest history of the terrestrial arthropods. We focused on the three independent, Palaeozoic arthropod terrestrialization events (those of Myriapoda, Hexapoda and Arachnida) and showed that a marine route to the colonization of land is the most likely scenario. Molecular clock analyses confirmed an origin for the three terrestrial lineages bracketed between the Cambrian and the Silurian. While molecular divergence times for Arachnida are consistent with the fossil record, Myriapoda are inferred to have colonized land earlier, substantially predating trace or body fossil evidence. An estimated origin of myriapods by the Early Cambrian precedes the appearance of embryophytes and perhaps even terrestrial fungi, raising the possibility that terrestrialization had independent origins in crown-group myriapod lineages, consistent with morphological arguments for convergence in tracheal systems.This article is part of the themed issue ‘Dating species divergences using rocks and clocks’.

With about 500 known species worldwide, the large brachiopods are a relatively small group of primitive crustaceans. With few exceptions they live in temporary aquatic systems that are most abundant in arid and semi arid areas. As many regions remain unexplored and as especially the number of species in clam shrimps and tadpole shrimps is underestimated due to difficult identification, the species list will increase with future surveys. The Branchiopoda are monophyletic, but inter-ordinal relationships, as well as many evolutionary relationships at lower taxonomic levels are still unclear. Ongoing molecular studies will more accurately depict species diversity and phylogenetic patterns. With the exception of some anostracan families, most families are not restricted to the northern or southern hemisphere or specific zoogeographical regions. Large branchiopods are used for the assessment of the quality and function of temporary wetlands. Due to the reduction in number and quality of temporary wetlands, several species became endangered and are red listed by the IUCN.

The phylogenetic relationships of branchiopod crustaceans have been in the focus of a number of recent morphological and molecular systematic studies. Although agreeing in some respects, major differences remain. We analyzed molecular sequences and morphological characters for 43 branchiopods and two outgroups. The branchiopod terminals comprise all eight ''orders''. The molecular data include six loci: two nuclear ribosomal genes (18S rRNA, 28S rRNA), two mitochondrial ribosomal genes (12S rRNA, 16S rRNA), one nuclear protein coding gene (elongation factor 1a), and one mitochondrial protein coding gene (cytochrome c oxidase subunit I). A total of 65 morphological characters were analyzed dealing with different aspects of branchiopod morphology, including internal anatomy and larval characters. The morphological analysis resulted in a monophyletic Phyllopoda, with Notostraca as the sister group to the remaining taxa supporting the Diplostraca concept (''Conchostraca'' + Cladocera). ''Conchostraca'' is not supported but Cyclestheria hislopi is the sister group to Cladocera (constituting together Cladoceromorpha) and Spinicaudata is closer to Cladoceromorpha than to Laevicaudata. Cladocera is supported as monophyletic. The combined analysis under equal weighting gave results in some respects similar to the morphological analysis. Within Phyllopoda, Cladocera, Cladoceromorpha and Spinicaudata + Cladoceromorpha are monophyletic. The combined analysis is different from the morphological analysis with respect to the position of Notostraca and Laevicaudata. Here, Laevicaudata is the sister group to the remaining Phyllopoda and Notostraca is sister group to Spinicaudata and Cladoceromorpha. A sensitivity analysis using 20 different parameter sets (different insertion-deletion [indel] ⁄ substitution and transversion ⁄ transition ratios) show the monophyly of Anostraca, Notostraca, Laevicaudata, Spinicaudata, Cladoceromorpha, Cladocera, and within Cladocera, of Onychopoda and Gymnomera under all or almost all (i.e., 19 of 20) parameter sets. Analyses with an indel-to-transversion ratio up to 2 result in monophyletic Phyllopoda, with Laevicaudata as sister group to the remaining Phyllopoda and with Spinicaudata and Cladoceromorpha as sister groups. Almost all analyses (including those with higher indel weights) result in the same topology when only ingroup taxa are considered.Ó The Willi Hennig Society 2007.Branchiopoda comprise approximately 1180 described species of mainly freshwater dwelling crustaceans (Adamowicz and Purvis, 2005). It includes taxa with a general, archaic morphology very similar to that of some Cambrian and Devonian crustacean fossils (Anostraca, Notostraca, Spinicaudata), as well as taxa that are highly specialized for predatory, scraping, parasitism, and advanced filtration such as the water fleas (Scourfield

The adult male of Cyclestheria hislopi, sole member of the spinicaudate conchostracan clam shrimp family Cyclestheriidae and a species of potential phylogenetic importance, is described for the first time. Several previously unknown features are revealed. Among these are (1) the morphology of the dorsal organ, which is roughly similar in shape to the supposedly homologous structure in other clam shrimps but bears a relatively large, centrally located pore unique to the species; (2) an anterior cuticular pore presumably leading to the ‘internal’ space surrounding the compound eyes, and thereby homologous to the same pore in other clam shrimps and in the Notostraca; (3) the spination and setation of the antennae and thoracopods, and (4) the mature male first thoracopods (claspers). The male claspers are paired and essentially equal in size and shape on right and left sides of the body. The second pair of thoracopods are not modified as claspers, a situation different from all other spinicaudate families but shared (plesiomorphic we propose) with the laevicaudatans and most cladocerans. The claspers bear a field of special spine‐like setae on the extremity of the ‘palm’; this setal type, previously unrecognized, is unique to Cyclestheria. The palm of the clasper also bears two palps (one very small), as in other conchostracan species (both laevicaudatans and spinicaudatans). The movable finger of the clasper, modified from the thoracopod endopod, bears a row of long setae along its outer extremity, also unique. Cyclestheria exhibits a mixture of characters, some unique and others typical of the Spinicaudata (Conchostraca). Cladoceran clasper types are briefly reviewed. as are the claspers in the Spinicaudata and Laevicaudata (Conchostraca). Morphology of the clasper of Cyclestheria shows typical spinicaudate characters. It is suggested that claspers on the first thoracopods may be a synapomorphy for the Conchostraca and the Cladocera. The possible role of Cyclestheria or a Cyclestheria‐like ancestor in cladoceran phylogeny is briefly discussed in light of recent suggestions (Martin and Cash‐Clark, 1995) of cladoceran monophyly and possible ancestral relationships with this genus. Some possibilities concerning the phylogenetic position of Cyclestheria–either as a sister group to the rest of the Spinicaudata or as a sister group to the Cladocera—are discussed.