Melchin et al.—Llandovery retiolitine graptolites from Arctic Canada We also document an unnamed species questionably
assigned to Paraplectograptus. The fact that this taxon occurs in the S. guerichi Zone, and possibly the L. convolutus Zone, is important because it may represent both a temporal and phylogenetic link between typical Aeronian forms and the more derived taxa that have been commonly regarded as “plectograptines” in later Telychian and younger strata.
Phylogenetic analysis
Much of the history of our understanding of the phylogeny and systematics of retiolitines was summarized by Lenz and Melchin (1997) and Bates et al. (2005). Those two papers both presented phylogenetic analyses of retiolitines based on different sets of taxa and character state codings. In particular, the Lenz and Melchin (1997) study included data from some new Aeronian species described in this study, but Bates et al. (2005) focused primarily on Wenlock and Ludlow taxa and did not include all of the taxa analyzed by Lenz and Melchin (1997). The results of those two cladistics analyses suggested very different patterns of emergence of those taxa traditionally assigned to the Retiolitinae (e.g., Pseudoplegmatograptus, Stomatograptus and Retiolites) and the Plectograptinae (e.g., Paraplectograptus, Sokolovograptus and Plectograptus), as well as conflicting interpretations about the relationship of Rotaretiolites to these two groups. One of the objectives of this study was to use all of new data available from the study of these newly described Aeronian and early Telychian faunas, together with a new character coding set incorporating information from all of the previous studies, to test these different hypotheses of the relationships among Llandovery retiolitines.
Selection of taxa and characters.—Our phylogenetic analysis focuses on the Aeronian and early Telychian retiolitines that were recovered from the Cape Phillips Formation and are systematically describedinthispaper: Pseudoretiolites perlatus, P. decurtatus, P. hyrichus, Pseudoretiolites?sp., Pseudoplegmatograptus obesus, Rotaretiolites exutus, Rotaretiolites cf. exutus, Eorograptus spirifer, Aeroretiolites cancellatus, Aeroretiolites? sp., and Paraplectograptus? sp. In addition, to understand the phylogenetic relationships between our relatively early retiolitine forms and some later Telychian taxa that have been included in previous phylogenetic studies, we included the following later Llandovery taxa: Retiolites geinitzianus Barrande, 1850, Stomatograptus canadensis Lenz, 1988, Paraplectograptus eiseli (Manck, 1917), and Sokolovograptus textor (Bouček and Münch, 1952).We have also included Pseudorthograptus obuti (Rickards and Koren’, 1974), P. inopinatus (Bouček, 1944) (Fig. 4), and Hercograptus introversus Melchin, 1999, as the most closely related representatives of the Petalolithinae (Melchin et al., 2011) for comparison and testing the hypothesis of themonophyly of the Retiolitinae. Separate analyses were run with each of these three taxa as the outgroup. We have not included Pseudoretiolites cf. tianbaensis,
described in this paper, in our analysis because too little is known of its internal structure and proximal development for reliable coding. In addition, since Pseudoretiolites decurtatus and P. perlatus differ only in minor quantitative parameters, they showed no differences that were captured by our
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Figure 4. Scanning electron microscope images of Pseudorthograptus inopinatus (Bouček, 1944), GSC137621, early growth stage specimen showing well-preserved ancora umbrella: (1) profile view of complete specimen; (2) oblique proximal end view showing broken, but otherwise well- preserved spiral lists on the shallow ancora umbrella, unknown locality, Cornwallis Island, C. curtus or lower L. convolutus Zone.
modified after Bates et al. (2005) to accommodate the features specifically seen among our study taxa. We have simplified the coding of ancora types to reflect the lesser degree of variation seen among our earlier study species. As a result, our coding of ancora types more closely reflects that of Lenz and Melchin (1997). The list of characters and their states is presented in Appendix 2, which also provides the sources of the morphological information for taxa not described in this study. The matrix showing the taxa and their character states is shown in Table 1. We conducted one analysis in which all characters were
coding system. Of the two species, Pseudoretiolites perlatus was included in the analysis because the structure of its proximal end is better preserved in our material. The two taxa can be regarded as sister species, more closely related to each other than any other taxa in our analysis. The characters and their states that we have used are
unordered and another in which the following characters were ordered: 4—prosicula preservation; 5—metasicula preservation. The analysis using all unordered characters produced a better-resolved tree and used no a priori assumptions about character directionality, so we present here only the results of the analysis using all unordered characters.
Cladistic analysis methods.—Preparation of the data matrix and analysis of the trees resulting from the parsimony searches were conducted using MacClade 4.06 (Maddison and Maddison, 2003). The branch-and-bound parsimony searches and bootstrap analyses were conducted using PAUP*4.10b (Swofford, 2002). Initial analyses used all equally weighted characters. Based on these analyses the character weights were recalculated using the rescaled consistency index (RCI). The rationale for this approach was extensively outlined by Donoghue et al. (2008 and references therein), and this methodology was also employed by Melchin et al. (2011). After a second analysis, the character weights were again recalculated using the RCI, and run again. After this point, further re-analyses produced no additional change in RCIs
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