Evolution and systematics of the Chelicerata

Abstract

After approximately 40 years of discussion about the question whether the Arthropoda are a monophyletic or a paraphyletic group, or even a polyphyletic assemblage of unrelated taxa, most morphologists, palaeontologists and molecular taxonomists agree that the Arthropoda are a monophylum. The Euarthropoda are composed of the Arachnomorpha and Mandibulata. Myriapods are usually considered to be mandibulates; however, new molecular data as well as some morphological characters show similarities which the Myriapoda share with the Chelicerata, suggesting that there is no taxon Antennata or Atelocerata. Chelicerata are usually considered to be the sister group of Trilobita or, more correctly, Trilobita branch off from the chelicerate stem line. The first adaptive radiation of the Chelicerata took place in the Cambrian. All extant and some extinct orders were present during the Carboniferous. Two systems are compared. It is suggested that the Chelicerata contain the Pantopoda and Euchelicerata. The Euchelicerata are divided into Xiphosura and terrestrial Arachnida. Scorpiones are considered to be the sister group of all other arachnids, the Lipoctena, and these are further divided into the Megoperculata (Uropygi, Amblypygi and Araneae) and Apulmonata (all other groups). The Acari are tentatively considered to be a monophylum and the sister group of the Ricinulei. However, the Actinotrichida and Anactinotrichida diverged early and therefore have had a long history of independent evolution.