Aspecies-Level Supertree for Stylophoran Echinoderms
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A species−level supertree for stylophoran echinoderms MARCELLO RUTA Ruta, M. 2003. A species−level supertree for stylophoran echinoderms. Acta Palaeontologica Polonica 48 (4): 559–568. The construction of supertrees from smaller, character−based cladograms permits simultaneous inclusion of a large num− ber of taxa in a single analysis, summarizes patterns of relationships from many independent data sources, and highlights areas of conflict to be targeted by character matrix studies. The method is applied for the first time to stylophoran echinoderms (cornutes and mitrates). Published cladistic analyses of this problematic group are used to build a supertree of 77 species. Key areas of stylophoran phylogeny to be addressed by future studies include the systematic placement of several mitrate−like cornutes, the affinities of peltocystidan and lagynocystid mitrates, and the position of such bi− zarre−looking taxa as Diamphidiocystis and Lobocarpus. A strict consensus of 72,278 equally parsimonious supertree so− lutions shows cornutes to be paraphyletic relative to mitrates. Lobocarpus is either one of the most derived cornutes or the sister taxon to all mitrates, in agreement with its chimaera−like combination of cornute− and mitrate−like features. Chinianocarpos is basal to peltocystidan rather than to mitrocystitidan mitrates. Nanocarpus, an almost bilaterally sym− metrical taxon previously allied to cornutes, is nested within mitrates as sister taxon to mitrocystitids plus anomalo− cystitids. Diamphidiocystis and Lagynocystis cause loss of phylogenetic resolution among derived mitrocystitids and basal anomalocystitids. Key words: Echinodermata, Stylophora, Cornuta, Mitrata, source trees, supertree, strict consensus, parsimony. Marcello Ruta [[email protected]],Department of Organismal Biology and Anatomy,University of Chi − cago, 1027 East 57th Street, Chicago, IL 60637, USA. Introduction 1999; Lefebvre 2001; Martí Mus 2002). Despite much recent progress and unusually fast rate of data accumulation (Mooi Stylophorans are a palaeontological conundrum. They are 2001), there is a dismaying lack of agreement about the among the most problematic of all extinct groups of metazo− higher−level systematic position of stylophorans. As Ubaghs ans. Their zoological affinities as well as morphological po− (1994: 3) aptly put it, “La diversité de ces opinions et les cri− larity have been debated for over thirty years, but no consen− tiques, souvent pertinentes, addressées à chacune d’elles (y sus has emerged (for a detailed review of the controversy, see compris la mienne) … , suggèrent que le problème de la vraie Jefferies 1986). Irreconcilable phylogenetic and morpho− nature des stylophores reste posé”. functional interpretations of these fossils as echinoderms A revision of stylophorans and of their putative allies (Ubaghs 1968, 1975, 1981; Philip 1979; Jollie 1982; Kolata among the diverse array of Palaeozoic, echinoderm−like and Jollie 1982; Paul and Smith 1984; Parsley 1988, 1991, groups (Paul and Smith 1984; Sumrall 1997) are timely to 1994, 1997, 1998, 2000; Kolata et al. 1991; Sumrall 1997; make sense of patterns of character distribution in primitive Lefebvre et al. 1998; Ruta 1999a–c; David et al. 2000; deuterostomes. This is especially important in the light of Lefebvre 2000a) or as chordates (the so−called “calcichor− contributory new information from such diverse fields as de− dates”; Jefferies 1981, 1986, 1991, 1997, 2001; Jefferies and velopmental biology and palaeontology (e.g., Bromham and Lewis 1978; Cripps 1988, 1989a, b, 1990, 1991; Daley 1992; Degnan 1999; Donoghue et al. 2000; Peterson and Eernisse Jefferies et al. 1996; Ruta 1997a, b; Ruta and Theron 1997; 2001). A stunning series of recent fossil discoveries from the Jefferies and Jacobson 1998; Sutcliffe et al. 2000; Domin− Lower Cambrian of China have improved our understanding guez et al. 2002) have led to a flurry of research targeted at of basal chordate evolution. A multitude of spectacular and evaluating the various biological and systematic implications challenging new fossils have been proposed as stem gnatho− as well as evolutionary scenarios associated with each of the stomes, stem myopterygians (sensu Janvier 1981, i.e., the competing hypotheses (Peterson 1995; Bergström et al. clade consisting of lampreys plus gnathostomes), or even 1998; Lefebvre et al. 1998; Lefebvre and Vizcaino 1999; stem craniates (e.g., Smith et al. 2001; Shu et al. 2003, and Ruta 1999b, c; Lefebvre 2000a, b, 2001, 2003; Martí Mus references therein). Together with recent advances in devel− 2002). At the same time, a large amount of new information opmental biology, these new finds are slowly forcing a re− and character revision have improved considerably the broad thinking of long−established views about the nature and dis− picture of stylophoran intrinsic relationships (Cripps 1991; tribution of jaws, respiratory structures, sense organs, miner− Daley 1992; Parsley 1997, 1998; Ruta 1997a, 1999b; Ruta alized parts and paired appendages in basal chordates and and Theron 1997; Lefebvre and Vizcaino 1999; Ruta and Jell deuterostomes (Janvier 1996a, b, 2001; Coates and Cohn Acta Palaeontol. Pol. 48 (4): 559–568, 2003 http://app.pan.pl/acta48/app48−559.pdf 560 ACTA PALAEONTOLOGICA POLONICA 48 (4), 2003 1998; Donoghue et al. 2000; Smith and Coates 2000; Dono− in tree search are often a major hurdle to jump and represent ghue and Aldridge 2001; Smith et al. 2001; Dominguez et al. common problems with large data sets. The construction of 2002; Donoghue 2003). On casual judgement, attempts to in− all−encompassing matrixes is often impractical. For example, tegrate this new information with data on the enigmatic different data sources (morphology; molecules; phisiology; stylophorans seem unrealistic, given the highly controversial behaviour) might be available for different clades within the nature of the latter. However, comprehensive investigations same group of organisms. Therefore, it might be difficult to of deuterostome and, more specifically, chordate and echino− combine in the same matrix partially overlapping taxon sets derm evolution call for critical scrutiny of all lines of evi− that have been coded for rather heterogenous character types. dence (including problematic extinct groups), and for evalu− Systematists encounter similar problems in analyses of ation of additional, less frequently used data sources (e.g., combined fossil and living representatives of a group. degree of congruence between stratigraphical order of ap− In many cases, however, it is desirable to produce detailed pearance of key taxa and sequence of phylogenetic bran− phylogenies, e.g., to infer large−scale macroevolutionary pat− ching events; see comments in Jefferies 1986; Smith et al. terns or to analyze profiles of diversity and disparity through 2001; and Dominguez et al. 2002). time (Bininda−Emonds et al. 1998, 1999, 2002). In these cases, In this paper, I address stylophoran intrinsic relationships supertrees provide a powerful means of collating information from the perspective of supertree building (Baum 1992; from smaller−scale, character−derived trees. Supertree con− Ragan 1992), and consider all the species that have been in− struction overcomes several of the problems associated with cluded in cladistic analyses published over the last 15 years. very large taxon/character sets and has been applied to several I point out that the nature of the present work is chiefly meth− organisms, both living (e.g., some orders of birds, mammals odological and does not aim to replace primary, charac− and plants) and fossils (e.g., dinosaurs) (reviews in Bininda− ter−based trees. It does, however, address the merits of a rela− Emonds et al. 2002). The method offers several advantages. tively new and still largely unexplored approach to phylog− First, it allows multiple trees, consisting of partially or totally eny reconstruction (see Bininda−Emonds et al. 2002, Golo− overlapping taxon sets, to be combined in a single, large−scale boff and Pol 2002 and Pisani et al. 2002 for detailed reviews). phylogeny. Second, it is unique in its ability to synthesize It also provides a framework for future character analyses of competing hypotheses of relationship for exceedingly numer− stylophorans through the erection of a novel hypothesis that ous operational taxonomic units. Third, supertrees provide in is independent of any assumption of skeletal plate homology, many cases partial or full resolution for groups that are poorly as well as of any anatomical orientation and systematic resolved in one or more of the source trees. Fourth, conflicting position of the group. positions of some groups or single species in the source trees often tend to cancel out in favour of a single, unambiguous placement in the supertree. A final aspect of supertree con− Supertrees struction relates to their ability to generate, in some cases, groups that do not appear in any of the contributory clado− Terminology.—Component trees (i.e., primary, character− grams (but see comments below). In these cases, supertrees based cladograms) are also known as source trees (Pisani et call for a critical scrutiny of the evidence (characters) in sup− al. 2002). The analyses from which they derive are original port of such novel groups. Finally, supertrees might be used as or primary analyses. The topology of each source tree is simple tests of the ability of various nodes in the source trees to translated into its matrix representation, or