Three genomic-scale analyses expose incompatible placements of horseshoe crabs and different counts of terrestrialization events.
Represented range485–444 Ma
RegionsGlobal living-chelicerate sequence and morphology samples
CladesPycnogonida · Xiphosura · Arachnida · Acari
Claim ledger
topology · contested
Genomic-scale chelicerate analyses using different taxon sets, slowly evolving genes, heterogeneous models and total evidence recover mutually incompatible monophyletic or paraphyletic Arachnida; one versus multiple terrestrializations and Cambrian–Ordovician timing are model-dependent historical reconstructions.