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Character States and Evolution of the Chelicerate Claws

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Character States and Evolution of the Chelicerate Claws 345 European Arachnology 2000 (S. Toft & N. Scharff eds.), pp. 345-354. © Aarhus University Press, Aarhus, 2002. ISBN 87 7934 001 6 (Proceedings of the 19th European Colloquium of Arachnology, Århus 17-22 July 2000) Character states and evolution of the chelicerate claws JASON A. DUNLOP Institut für Systematische Zooloigie, Museum für Naturkunde der Humboldt-Universität zu Berlin, Invalidenstraße 43, D-10115 Berlin, Germany ([email protected]) Abstract Outgroups of Chelicerata have an apotele in which two smaller claws insert on a larger median claw. A three-clawed plesiomorphic state is retained in basal Pycnogonida and the Palaeozoic xiphosuran Weinbergina. Modifications or reductions from this pattern are interpreted here as apo- morphic character states. A single apotele element occurs in the crown group Xiphosurida and in the extinct taxa Eurypterida and Chasmataspida. The digitigrade, 'eurypteroid' apotele of Allopalaeo- phonus-like fossil Scorpiones may not be the plesiomorphic condition for the group since the most basal clade, Palaeoscorpius, has an apotele more like Weinbergina and the outgroups. Among the other arachnids Palpigradi retain the most plesiomorphic apotele morphology with three claws on all postcheliceral appendages. Unequivocal homologies between the claws in different arachnid or- ders are difficult to resolve, especially in relation to the complex apoteles seen among the Acari. However, further apomorphic apotele states in arachnids include the development of the empodial region between the claws into a pulvillus in adults of basal Amblypygi, in Solifugae and Pseudoscor- piones and among the mites in the (Opilioacariformes + Parasitiformes) clade, but not in basal Acariformes. Key words: Apotele, claw, ungue, empodium, pulvillus, Chelicerata, phylogeny INTRODUCTION scorpions and xiphosurans. Palaeozoic fossils The terminal element of the postcheliceral of the latter two taxa preserve character states limbs in Chelicerata is called the apotele. This different from extant forms, which probably apotele has been modified in arachnids to form better reflect the ground pattern of these the claws, while in some taxa (e.g. solifuges, clades. The aim of the present paper is to give some amblypygids) the membranous region an overview of apotele morphology which inte- between the claws - the empodium - has been grates the fossil data and to try and identify further modified to form a complex, typically potential synapomorphies for clades within the eversible structure which is usually called the chelicerates. pulvillus. Chelicerate limb morphology, includ- ing apotele character states, has been reviewed MATERIALS AND METHODS by authors such as Barrows (1925), van der Wherever possible specimens of Recent taxa Hammen (1989) and Shultz (1989). However, were drawn from life from material in the col- these authors restricted their surveys to Recent lections of the Muesum für Naturkunde, Berlin, euchelicerates and did not consider the basal supplemented by descriptions in the literature pycnogonid group (sea spiders), extinct taxa as detailed below. Well preserved eurypterids (e.g. eurypterids) or fossil representatives of and trigonotarbids from the Natural History 346 European Arachnology 2000 Museum, London were also examined. Termi- main claw is tiny, and in contrast to the trilo- nology generally follows Barrows (1925) and/or bite condition (see above) the auxiliary claws Shultz (1989). do not insert into the main claw, but arise from the membrane above it (Fig. 1b). The auxiliary RESULTS claws are lost in more derived clades, e.g. Outgroups (Trilobita) Pycnogonidae. The sister group of Chelicerata (including pycnogonids) has not been satisfactorily estab- Xiphosura lished, but Trilobita, and various arachno- Following Anderson & Selden (1997), the class morph or trilobite-like taxa, have emerged as Xiphosura (horseshoe crabs) can be divided potential outgroups for polarising chelicerate into a series of stem group plesion taxa, the characters. Appendage morphology can only synziphosurines, plus a monophyletic crown be determined with certainty in a few fossils group, Xiphosurida. Appendages are rarely showing exceptional preservation. Studies of preserved in Palaeozoic Xiphosura, but signifi- Triarthrus by Cisne (1975) and Whittington & cantly the Lower Devonian synziphosurine Almond (1987), of Agnostus by Müller & Weinbergina opitzi (Fig. 1c) lacks the chelate Walossek (1987) and of Phacops by Bruton & postcheliceral appendages with a single apotele Haas (1999) consistently show an apotele mor- characteristic for living species. Stürmer & phology in which two lateral claws appear to Bergström (1981) redescibed W. opitzi and al- insert into a slightly larger central claw (Fig. though there are inconsistencies in their inter- 1a). These three apotele structures are poten- pretative drawings, their plates (especially tially homologous with the three claws seen in their fig. 7a) indicate a trifurcate apotele with many chelicerates (see below) and provide a all three elements approximately the same size. plesiomorphic condition against which the che- These three elements appear to emerge adja- licerate apotele can be compared. cent to each other (Fig. 1c) and the lateral ele- ments do not insert on the central one as in the Pycnogonida trilobites. By contrast, extant Xiphosura have Most phylogenetic studies have concluded that only a single apotele element in all their post- Pycnogonida are basal chelicerates (Weygoldt cheliceral limbs which forms the movable fin- & Paulus 1979; Wheeler & Hayashi 1998), rep- ger of a distal chela (Fig. 1d). This apparently resenting the sister group of all other chelicer- apomorphic condition arose in the Xiphosurida ates: the Euchelicerata. The palps (limb II) of by the Carboniferous, having been recently de- Pycnogonida are variable within the group and scribed in the fossil genus Euproops by Schultka are reduced or absent in adults of certain taxa. (2000, pl. 1, fig. 2). In Recent taxa this chela is Nevertheless, a distinct apotele in the palp ap- larger in appendages II-V, but rather small in pears to be lacking, at least in adults. By con- appendage VI and is essentially subchelate in trast, the oviger (limb III) - also absent in some appendages III and IV of the extant species taxa - ends in a single claw in most groups Tachypleus tridentatus. where it is present (Arnaud & Bamber 1987). The legs of Pycnogonida (limbs IV-VII) typi- Eurypterida cally end in three claws (Fig. 1b): a main claw In most reconstructions the extinct Eurypterida plus a pair of auxiliary claws (Arnaud & Bam- (sea scorpions) are shown with a trifurcate end ber 1987). The relative proportions of these to the legs. Jeram (1998) referred to the lateral claw elements can vary, even intraspecifically elements as ‘tarsal spurs’ and suggested that (e.g. Helfer & Schlottke 1935, fig. 44). The main homologous structures occur in fossil scorpions claw is usually longer than the auxiliary claws, and that they are in turn homologous with the but see e.g. Ammothea biunguiculata where the lateral claws or ungues of other arachnids (see Dunlop: Evolution of the claws 347 Fig. 1. (a) Triarthrus sp. (Trilobita). Ordovician, after Whittington & Almond (1987). (b) Achelia echinata (Pycnogonida). Recent, from life. (c) Weinbergina optzi (Xiphosura: ‘synziphosurine’). Devonian, after Stür- mer & Bergström (1981). (d) Limulus polyphemus (Xiphosura: Xiphosurida). Recent, from life. (e) Baltoeu- rypterus tetragonophthalmus (Eurypterida). Silurian, after Selden (1981). (f) Diploaspis casteri (Chasma- taspida). Devonian, from original material. (g) Palaeoscorpius devonicus (Scorpiones: stem group). Devonian, after Kjellesvig-Waering (1986). (h) Buthus occitanus (Scorpiones: Buthidae). Recent, from life. All drawings not to scale. below). However, when examined in detail - and Eurypterida (Dunlop in press). Their ap- see e.g. Selden's (1981) study of Baltoeurypterus pendages are poorly known, but Caster & tetragonophthalmus - the apotele of eurypterids Brooks (1956) described an isolated limb with is represented by a single claw. The spinous chelate, distal podomeres, resembling those of elements either side of it, when present, are Xiphosurida (see above). A better preserved derived from the preceding podomere (Fig. 1e) limb described by Størmer (1972) and Dunlop either as fixed spines (see e.g. Selden 1981, figs. et al. (2002) ends in a short, slightly curving 26-29 for Baltoeurypterus) or as socketed spines element with a weakly developed spine on the (see e.g. Clarke & Ruedemann 1912, pl. 28 for preceding podomere. As in Eurypterida there Carcinosoma) and thus their homology with the is no evidence for lateral claws (Fig. 1f) and ungues of arachnids is questionable. In stylon- Chasmataspida should also provisionally be urid eurypterids, which lack paddles and scored as retaining only a single element in which probably represent the more basal taxa their apotele. (S. Braddy pers. comm.), the spines on this po- domere preceding the apotele are either absent Scorpiones or only weakly developed; see e.g. Tollerton Among the best known Palaeozoic scorpions (1989, fig. 9). are the Silurian Palaeophonus / Allopalaeophonus species which have been figured as having Chasmataspida crab-like legs ending in a single, large apotele - Chasmataspida are a rare group of extinct che- Pocock's (1901, p. 295) ‘clawless terminal seg- licerates. Although initially interpreted as un- ment’ - similar to that seen in many eurypterids
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