124


Journal of Paleontology 91(1):116–145


the most parsimonious solution is to followMelchin et al. (2011) and assign the entire retiolitid group (the clade above node 1) to the subfamily Retiolitinae. If further subdivision of this subfamily were to be proposed (at the level of tribe), we suggest that it should reflect the sister-group status of the two clades that include Pseudoplegmatograptus, Stomatograptus,and Retiolites on the one hand, and Aeroretiolites, Eorograptus, Rotaretiolites, Paraplectograptus, Sokolovograptus,and their “plectograptine” descendants on the other, as well as the paraphyletic, stem-group status of Pseudoretiolites (Fig. 5.1).


Retiolitine diversity and disparity in the Llandovery


Our data provide important new insights into the pattern of diversification of the Retiolitinae in Aeronian and early Telychian time (Figs. 3, 6). In particular, our data show that at least eight different retiolitine species belonging to four different genera occur within the L. convolutus Zone, whereas previously only two species of Pseudoretiolites had been identified with certainty from from this interval. We show that a significant disparity of retiolitine forms, ranging from large and complex species, such as Pseudoretiolites decurtatus, to the small and highly simplified form of Eorograptus spirifer, already existed by mid-Aeronian time. As noted above, there have been almost no graptolites


recovered from the S. sedgwickii Zone in Arctic Canada, despite considerable sampling effort. This appears to be the result of unfavourable preservational conditions in this interval (including a lack of concretions), which means that the lack of retiolitine species discovered in that interval in this study is clearly a tapho- nomic artefact. Our data, therefore, provide no new insights into the details of changing retiolitine faunas through the sedgwicki extinction event that characterizes the late Aeronian (e.g., Melchin et al., 1998; Štorch and Frýda, 2012). Our study does, however, provide data that allow us to reconstruct the phylogeny of retiolitine lineages through this time interval (Fig. 6). The morphology of many retiolitine species that occur in


later Llandovery-Wenlock strata have been described and illustrated by Lenz and Melchin (1987a), Lenz and Kozłowska- Dawidziuk (2001), Kozłowska-Dawidziuk and Lenz (2001), Lenz and Kozłowska (2007), and Lenz et al. (2012). Melchin (1989) documented the ranges of retiolitines through the Llandovery of Arctic Canada based on flattened material. These studies, together with our additional unpublished data, show that the following genera occur in mid-late Telychian strata in Arctic Canada: Retiolites Barrande, 1850, Stomatograptus Tullberg, 1883, Paraplectograptus Příbyl, 1948, Pseudoplectograptus Obut and Zaslavskaya, 1983, Sokolovograptus Obut and


Zaslavskaya, 1983, Pileograptus Lenz and Kozłowska, 2007, and Giganteograptus Lenz and Kozłowska, 2007. Detailed SEM studies on Retiolites and Stomatograptus from Arctic Canada have been carried out by Bates and Kirk (1997).


Locality and repository information


All of the material used in the present study has been chemically isolated from calcite concretions and studied using stereopair scanning electron microscopy (SEM) images. Much of the material was collected from from Cape Manning (Figs. 1, 3;CM 75o27’N, 94o21’W). Additional material came from Rookery Creek, Cornwallis Island (RC 75o22.5’N, 95o41’W), Snow- blind Creek, Cornwallis Island (SC 75o11’N, 93o56’W), Dundas Island (~76o05’N, 95o59’W), Cape Sir John Franklin, Devon Island (SJF 76o42.5’N, 96o53’W), and an unknown locality on Cornwallis Island. See Appendix 1 for further sample details. Illustrated specimens are given Geological Survey of


Canada (GSC) numbers, and are housed with The National Collection of Invertebrate and Plant Fossils of the Geological Survey of Canada, Ottawa, K1A 0E8. Collection localities and abbreviations are as follows: Cape


Manning (MCM; CM88), Snowblind Creek (LL1 and LL3; MSC; ML64–85; SB), and Rookery Creek (MRC), all three from Cornwallis Island (see Fig. 1). The other two localities are Cape Sir John Franklin (SJF) on westernmost Devon Island, and Dundas Island (DIS).


Systematic paleontology


Throughout the Systematics section, for species with numerous synonymies, only the original type publication and those con- sidered to be the most informative are listed. Morphological terminology follows that outlined by Bates et al. (2005) and the family-level systematics follows Melchin et al. (2011). 2TRD refers to two-thecae repeat distance (Howe, 1983). We introduce a new morphological term “pseudolip” to describe the lip-like, looping list structure underlying the apertural lip, attached to the lateral thecal apertural lists, which has been observed in our specimens of Pseudoretiolities cf. tianbaensis (Fig. 7.5, 7.7). The structure has an open space between it and the true apertural lip.


Superfamily Retiolitoidea Lapworth, 1873


Definition.—The most recent common ancestor of Metabolograptus ojsuensis and Retiolites geinitzianus and all of its descendants (Node 3, figs. 2 and 3 inMelchin et al., 2011).


Figure 8. Scanning electron microscopy images of Pseudoretiolites perlatus Nicholson, 1868: (1, 2) GSC137613, (1) beautifully preserved specimen with seven pairs of thecae, no visible sicula, (2) enlargement of two thecae showing zigzag thecal floor, part of thecal mid-ventral list (a), pleural list (b), and lateral apertural rod (c). Note zigzag distal part of thecal mid-ventral list, section MCM2-98, 59.1–59.2 m, L. convolutus Zone; (3) GSC137614, ancora umbrella with four spiral whorls, prosicula with fine longitudinal rods, and partial preservation of the metasicula (arrow), which is more complete than usual for this species, section DIS92-11, 17.5–18.0 m, L. convolutus Zone; (4) GSC78426, fairly mature specimen with at least six pairs of thecae, well-preserved prosicula, at least one stoma, ML64, upper, L. convolutus Zone, re-illustrated from Lenz and Melchin (1987a); (5, 8) GSC137615, specimen with two pairs of thecae, well-developed ancora umbrella with four spiral whorls, well-preserved prosicula, (5) oblique proximal view, (8) stereopair of profile view, section MRC02, L. convolutus Zone; (6) GSC 137616, stereopair of immature specimen, showing well-developed ancora umbrella with four complete spiral whorls, without preserved sicula, section MCM2-98, 59.1–59.2 m, L. convolutus Zone; (7) GSC 137617, stereopair of immature specimen with well developed ancora umbrella with five spiral whorls, base of theca11 mid-ventral list beginning within ancora umbrella (arrow), section MCM2-98, 54.1–59.2, L. convolutus Zone.


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