Phylogeny, classification and evolution of Silurian and Devonian scorpions

The phylogenetic position of scorpions within the Chelicerata has been the subject of much debate in recent years (e.g. Weygoldt & Paulus, 1979; Bergström, 1980; Van der Hammen, 1989; Shultz, 1990; Starobogatov, 1990), but it revolves around one primary question: are scorpions part of a monophyletic Arachnida, or are they “merostomes” which exhibit a terrestrial grade of existence? Inclusion of data from the scorpion fossil record within phylogenetic analyses of the chelicerates tends to support the latter contention. It is apparent from an examination of the fossil record that several of the supposedly derived characters which have been used to support a monophyletic concept of the Arachnida including scorpions, such as the possession of book lungs, simple ocelli rather than compound eyes, and trichobothria, are absent in some Palaeozoic scorpions. However, the significance of character distributions within the Palaeozoic scorpions is open to question in the absence of a convincing account of scorpion phylogeny. After all, it is possible that the characters in question (and indeed others) have undergone reversal in some Palaeozoic scorpion clades. A cladistic analysis of early scorpion phylogeny, the subject of this paper, is an essential step towards settling the question. There has never been a generally agreed stable classification of the Palaeozoic scorpions. Each worker approaching the problem has performed a fundamental revision of previous schemes. This is partly because of a poor database (both in terms of numbers of specimens and knowledge of their morphology), and partly because early scorpions exhibit a mosaic of morphological characters which defy any obvious “natural” grouping of taxa. There are few characters which can be determined in the majority of early scorpion body fossils, and fewer still which show a range of morphological variation useful in classification and the construction of a phylogeny. Earlier workers (e.g. Petrunkevitch, 1955; Kjellesvig-Waering, 1986; Starobogatov, 1990) have emphasized the significance of particular characters, such as the shape of the abdominal plates, the shape of the sternum, and the structure of the coxosternal region. Their classification schemes depended on topological resemblance of taxa, and their schemes naturally varied according to what the particular author considered to be the “most important” characters. The resulting alternative accounts of scorpion phylogeny are equally discordant. Only Stockwell (1989) has attempted to identify apomorphies which characterize monophyletic clades among Silurian and Devonian scorpions. Phylogeny, classification and evolution of Silurian and Devonian scorpions