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’.

The relationships of crustaceans and hexapods (Pancrustacea) have been much discussed and partially elucidated following the emergence of phylogenomic data sets. However, major uncertainties still remain regarding the position of iconic taxa such as Branchiopoda, Copepoda, Remipedia, and Cephalocarida, and the sister group relationship of hexapods. We assembled the most taxon-rich phylogenomic pancrustacean data set to date and analyzed it using a variety of methodological approaches. We prioritized low levels of missing data and found that some clades were consistently recovered independently of the analytical approach used. These include, for example, Oligostraca and Altocrustacea. Substantial support was also found for Allotriocarida, with Remipedia as the sister of Hexapoda (i.e., Labiocarida), and Branchiopoda as the sister of Labiocarida, a clade that we name Athalassocarida (=”nonmarine shrimps”). Within Allotriocarida, Cephalocarida was found as the sister of Athalassocarida. Finally, moderate support was found for Hexanauplia (Copepoda as sister to Thecostraca) in alliance with Malacostraca. Mapping key crustacean tagmosis patterns and developmental characters across the revised phylogeny suggests that the ancestral pancrustacean was relatively short-bodied, with extreme body elongation and anamorphic development emerging later in pancrustacean evolution.

Phylogeny of Branchiopoda (Crustacea) has received intensive attention the last decade and a widely accepted full resolution of the relationships between the major branchiopod taxa is close. Various well-preserved fossil branchiopods (e.g., Castracollis, Lepidocaris, Kazacharthra, Leptodorosida) combine characters from major recent taxa and are therefore necessary for a full understanding of branchiopod phylogeny and evolution. Here is presented a morphology-based phylogenetic analysis of Branchiopoda, which combines recent taxa with the most well-preserved and informative fossils. The analysis results in support of the following clades: Branchiopoda, Sarsostraca (= Anostraca + Lepidocaris), Phyllopoda, Calmanostraca (= Kazacharthra + Notostraca), an unnamed clade consisting of Castracollis and Calmanostraca, Diplostraca, an unnamed clade consisting of Spinicaudata and Cladoceromorpha, Spinicaudata, Cladoceromorpha, Cladocera, and Gymnomera. The character support for all clades is presented, which provides a basis for a discussion of aspects of branchiopod evolution.