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Introduction
The brachyuran superfamily Xanthoidea MacLeay, 1838, sensu lato is a large, diverse group in modern oceans, and it has a reasonably good fossil record. Beurlen (1930) first recognized the superfamily (as his subtribus) Xanthoidea containing seven families, Gecarcinidae MacLeay, 1838; Geryonidae Colosi, 1923 Goneplacidae MacLeay, 1838; Grapsidae MacLeay, 1838; Pinnotheridae De Haan, 1833; Potamidae Ortmann, 1896 Xanthidae MacLeay, 1838 sensu lato. In her classification of brachyurans, Guinot (1978) included eight families within the Xanthoidea: Carpiliidae Ortmann, 1893; Menippidae Ortmann, 1893; Platyxanthidae Guinot, 1977; Xanthidae; Panopeidae Ortmann, 1893; Pilumnidae Samouelle, 1819; Trapeziidae Miers, 1886; and Geryonidae. Manning and Holthuis (1981) first suggested that the Geryonidae had close affinities with the Portunidae, and subsequently, Bowman and Abele (1982) placed the Geryonidae within the Portunoidea and recognized four families within the Xanthoidea: Goneplacidae, Hexapodidae, Xanthidae, and Platyxanthidae.
Subsequently, work on the classification of the Xanthoidea has continued in earnest. Jamieson (1993) first suggested that the Xanthoidea was paraphyletic based upon spermatological evidence. Coelho and Coelho Filho (1993) considered the Xanthidae sensu lato to be polyphyletic based upon phenetic and cladistic analysis and divided it into four families: Carpiliidae; Xanthidae including the Menippinae, Platyxanthinae, Xanthinae, and Eucratopsinae; Eriphiidae; and Pilumnidae including the Trapeziinae and Pilumninae. The monophyly of the Xanthoidea was further questioned based upon additional cladistic analyses using adult morphology (Von Sternberg et al., 1999; Von Sternberg and Cumberlidge, 2001; Castro et al., 2004), foregut ossicles (Brösing, 2002), and molecular data (Schubart et al., 2000; Wetzer et al., 2003). Martin and Davis (2001) included as xanthoid families the Carpiliidae; Eumedonidae Dana, 1853; Goneplacidae; Hexapodidae Miers, 1886; Menippidae (= our Eriphiidae, see Davie (2002) and Schweitzer (2003a)); Panopeidae; Pilumnidae; Platyxanthidae; Pseudorhombilidae Alcock, 1900; Trapeziidae; and Xanthidae sensu stricto.
More recently, an additional five extant families have also been included within the Xanthoidea sensu lato: Domeciidae Ortmann, 1893; Pseudoziidae Alcock, 1898; Pilumnoididae Guinot and Macpherson, 1987; Tetraliidae Castro et al., 2004; and Trogloplacidae Guinot, 1986, (Castro et al., 2004; Davie, 2002; d’Udekem d’Acoz, 1999; Ng and Liao, 2002). Schweitzer (2003a, 2005a) recognized three extinct families: Palaeoxanthopsidae Schweitzer, 2003a; Tumidocarcinidae Schweitzer, 2005a; and Zanthopsidae Vía, 1959.
Recent investigations have resulted in the description of new fossil and extant material, and numerous studies have revisited fossil and extant material. Thus, revisions to the systematics and classification of the currently known xanthoid families have been provided for the following families which include extant and extinct taxa: Carpiliidae, Palaeoxanthopsidae, Pseudoziidae, and Zanthopsidae (Schweitzer, 2003a); Eriphiidae, Platyxanthidae, and Tumidocarcinidae (Schweitzer, 2005a); Domeciidae and Trapeziidae (Castro et al., 2004; Schweitzer, 2005b); Goneplacidae (Karasawa and Kato, 2003a, b); Hexapodidae (Schweitzer and Feldmann, 2001); Pilumnidae (Schweitzer, 2000); and Panopeidae and Pseudorhombilidae (Schweitzer and Karasawa, 2004). These studies were performed using cladistic analysis based upon adult morphology or using proxy characters of the dorsal carapace.
Among the various families discussed above and assigned to the Xanthoidea sensu lato, the Eumedonidae has been considered to be one of the subfamilies within the Pilumnidae (Ng and Clark, 2000; Davie, 2002; Poore, 2004); we concur. We follow Schweitzer (2005b) in considering the Trapeziidae and Tetraliidae as a single family, and our analysis showing genera from each group as sisters supports her decision.
Števčić (2005) published a major revision of the brachyuran families including both fossil and extant forms, one the few such major works to do so. Among the Xanthoidea sensu lato,his revision includes numerous new subfamilies and tribes, nearly all for extant genera. He also raised some subfamilies to family status. Because our work has concentrated mainly on the fossil forms, the evaluation of each of the new subfamilies and tribes is beyond the scope of this revision, because most were erected for extant genera. Discussion of the new families and subfamilies erected for extinct taxa occur below in the appropriate systematic paleontology section. Further, such extensive splitting of the families into subfamilies and tribes, often only to embrace a single genus, seems needlessly reductionist. Such reductionism has the benefit of reflecting the variety within the Xanthoidea sensu lato, but it also can serve to obscure relationships among members of the group. In addition, we opt not to use the category 'tribe' as it seems arcane and is generally not used amongst the Brachyura in modern classification schemes.
The above discussion demonstrates that there has been considerable confusion over placement of taxa within the Xanthoidea, especially among those in the fossil record. The goal of the present study is to provide an adult morphology-based phylogenetic analysis of 37 taxa, including representatives of all currently-recognized families within the Xanthoidea, based upon 71 adult morphological characters. We also provide an updated list of the xanthoid genera known as fossils and document the diversity of the Xanthoidea through geological time.
Phylogenetic analysis of the Xanthoidea sensu lato