A Cambrian–Ordovician Terrestrialization of Arachnids

Abstract

Understanding the temporal context of terrestrialization in chelicerates depends on whether terrestrial groups, the traditional Arachnida, have a single origin and whether or not horseshoe crabs are primitively or secondarily marine. Molecular dating on a phylogenomic tree that recovers arachnid monophyly, constrained by 27 rigorously vetted fossil calibrations, estimates that Arachnida originated during the Cambrian or Ordovician. After the common ancestor colonized the land, the main lineages appear to have rapidly radiated in the Cambrian-Ordovician boundary interval, coinciding with high rates of molecular evolution. The highest rates of arachnid diversification are detected between the Permian and Early Cretaceous. A pattern of ancient divergence estimates for terrestrial arthropod groups in the Cambrian while the oldest fossils are Silurian (seen in both myriapods and arachnids) is mirrored in the molecular and fossil records of land plants. We suggest the discrepancy between molecular and fossil evidence for terrestrialization is likely driven by the extreme sparseness of terrestrial sediments in the rock record before the late Silurian.

Diversification dynamics of total-, stem-, and crown-groups are compatible with molecular clock estimates of divergence times

“…Last, we explored the impact of nonarbitrary diversification rates by fixing the speciation and extinction rates based on the constant rate birth-death model fitted to the eukaryote timetree by Hedges et al (12). As our work and that of Budd and Mann (8) are predicated on constant rates of speciation and extinction, these values are potentially more appropriate than those estimated from analysis of arthropod clades [see (13)], which are likely to be higher than the underlying diversification rate, if such a rate exists. Here, the speciation rate was 0.073 speciation events per species/million years; extinction rate was 0.070 per species/ million years.…”

De novotranscriptome sequencing of the northern fowl mite,Ornithonyssus sylviarum, shed light into parasitiform poultry mites evolution and its chemoreceptor repertoires

“…To compare the differences of IR and GR gene numbers among various mite and tick species, we compared two species of poultry mite chemoreceptor genes to those of three other Mesostigmata species and those of three additional Acari and an insect. Species are depicted with evolutionary relationships and divergence times based on Lozano-Fernandez et al (2020) and Howard et al (2020), with the timescale axis representing millions of years before the present. PCG: protein coding genes; ND: not determined; Mbp: million base pair.…”

The visual pathway in sea spiders (Pycnogonida) displays a simple serial layout with similarities to the median eye pathway in horseshoe crabs

“…Currently, one of the most contentious issues is the position of the marine Xiphosura. While some recent phylogenomic studies support their traditional placement as sister group of monophyletic terrestrial Arachnida [ 7 – 9 ], others recover them nested within the arachnid taxa [ 6 , 10 – 12 ], thereby indicating multiple marine-terrestrial transitions within Chelicerata or a reconquering of marine habitats by xiphosurans. Putative morphological support for xiphosurans nested within arachnids has been—among others—derived from their adult visual system, which shares similarities with several arachnopulmonate taxa (scorpions, whip spiders, whip scorpions) [ 27 – 29 ].…”

“…While a basal split of the chelicerate lineage into Pycnogonida (sea spiders) and Euchelicerata is now firmly established [ 5 – 7 ], the interrelationships within Euchelicerata are still matter of considerable debate. Recently, one of the most contested issues concerns the position of the marine Xiphosura (horseshoe crabs), which is either placed as sister group of all terrestrial euchelicerates (Arachnida) [ 7 – 9 ] or recovered well-nested within these terrestrial euchelicerate taxa [ 6 , 10 – 12 ]. A nested position would have considerable consequences for our understanding of euchelicerate evolution, implying either multiple marine-terrestrial transitions or alternatively a reconquering of marine habitats by horseshoe crabs.…”