Current views on chelicerate phylogeny-A tribute to Peter Weygoldt

Peter Weygoldt pioneered studies of arachnid phylogeny by providing the first synapomorphy scheme to underpin inter-ordinal relationships. Since this seminal work, arachnid relationships have been evaluated using morphological characters of extant and fossil taxa as well as multiple generations of molecular sequence data. While nearly all datasets agree on the monophyly of Tetrapulmonata, and modern analyses of molecules and novel morphological and genomic data support Arachnopulmonata (a sister group relationship of Scorpiones to Tetrapulmonata), the relationships of the apulmonate arachnid orders remain largely unresolved. Three threads may allow us to resolve the recalcitrant phylogeny of arachnids: the careful evaluation of large amounts of molecular data; novel techniques grounded in comparative morphology and evolutionary developmental biology, which add improved homology statements and explain the transition between character states; and new fossil discoveries, which continue to incorporate relevant novel data to the discussion of the relationships among the arachnid orders.