Melchin et al.—Llandovery retiolitine graptolites from Arctic Canada
Arctic Canada (Modified after Melchin,1989)
SJF Devon Island Cy. insectus
Cy. sakmaricus O. spiralis
Mcl. crenulata/ Mcl. griestoniensis
Str. crispus 76°
Dundas Island
Sp. turriculatus Sp. guerichi
Bathurst Island
CM RC SC 75°
Cornwallis Island
Resolute 0
98° 96° 94°
Figure 1. Map of the central Canadian Arctic Islands, showing the collecting localities: CM=Cape Manning, SC=Snowblind Creek, RC=Rookery Creek, SJF=Cape Sir John Franklin, and Dundas Island.
Melchin (1989), although Melchin and Holmden (2006) noted the discovery of rare graptolites characteristic of this zone at Cape Manning, Cornwallis Island. Recently acquired data (Dawson, 2007; Melchin, 2013) also show that an interval marked by the first appearance of Demirastrites triangulatus occurs below the first occurrence of Demirastrites pectinatus at Cape Manning. As a result, a combined D. triangulatus-D. pectinatus Subzone and a S. sedgwickii Zone can now be recognized at that section (Figs. 2, 3), which can be readily correlated with the well-known biozonal schemes of Avalonia (Zalasiewicz et al., 2009) and peri-Gondwana (Loydell, 2012). Here we review the previously known biostratigraphic ranges of the each of the retiolitine genera known from the Aeronian and early Telychian and describe the new information provided by these Canadian Arctic faunas. The biostratigraphic occurrences at Cape Manning, the most completely sampled locality in this study, are shown in Figure 3. All other occurrence data are presented in Appendix 1.
Sti. sedgwickii Lit. convolutus R. orbitus
D. pectinatus/ D. triangulatus
Cor. cyphus Hut. acinaces
Britain
Zalasiewicz et al., 2009
Cy. insectus Cy. lapworthi O. spiralis
Mcl. crenulata Mcl. griestoniensis Str. sartorius Str. crispus Sp. turriculatus Sp. guerichi
Sti. halli
Sti. sedgwickii Lit. convolutus
Pr. leptotheca Neo. magnus M. triangulatus
M. revolutus Hut. acinaces
Cor.=Coronograptus,
117
Loydell, 2012 Peri-Gondwana
Cy. sakmaricus Cy. insectus
Mcl. griestoniensis Tor. tullbergi O. spiralis
Sp. turriculatus Str. crispus
Sti. sedgwickii R. linnaei
Lit. convolutus
Pr. leptotheca M. simulans
D. pectinatus/ D. triangulatus
Cor. cyphus Cyst. vesiculosus
Figure 2. Llandovery (mid-Rhuddanian – upper Telychian) biostratigraphic scheme for Arctic Islands, and comparison with those of the Great Britain and Peri-Gondwana (in particular, Czech Republic). Abbreviations of generic names:
Hut.=Huttagraptus, Lit.=Lituigraptus, M.=Monograptus, Mcl.=Monoclimacis, O.=Oktavites,
Pr.=Pristiograptus,
Cy.=Cyrtograptus, D.=Demirastrites, R.=Rastrites,
Sp.=Spirograptus, Sti.=Stimulograptus, Str.=Streptograptus, Tor.=Torquigraptus.
25 50 km
Pseudoretiolites.—Historically, the oldest known records of retiolitine graptolites globally are those of species of Pseudoretiolites Bouček and Münch, 1944, particularly P. perlatus (Nicholson, 1868), from the upper part of the Coronograptus gregarius Zone (lower Aeronian—Elles and Wood, 1908; Bouček and Münch, 1944), and in strata of the upper part of their lower Aeronian Demirastrites triangulatus Zone in association with Demirastrites pectinatus (Obut and Sobolevskaya,1968). However, more recent compilations of graptolite biostratigraphic occurrences in the United Kingdom (Rickards, 1976; Zalasiewicz et al., 2009) showed the oldest retiolitine taxon, P. perlatus, occurring no lower than the Lituigraptus convolutus Zone. A detailed study of the mid-Aeronian strata in the Czech sequence by Štorch (1998) showed that P. perlatus commonly occurs in the L. convolutus Zone, but that it also occurs rarely in the underlying Pribylograptus leptotheca and Demirastrites simulans zones (= Rastrites orbitus Subzone of Arctic Canada). Other species of Pseudoretiolites, such as P. decurtatus Bouček and Münch, 1944, and P. tianbaensis Ge, 1990 from China, first occur in most studies no lower than the L. convolutus Zone (e.g., Obut and Sobolevskaya, 1967; Štorch, 1998; Ge, 1990), or in the lowest Telychian Spirograptus guerichi and Spirograptus turriculatus zones (e.g., Bouček and Münch, 1944; Münch, 1952; Chen, 1984), although Lenz and Melchin (1987a) and Melchin (1989) recorded P. cf. decurtatus in the upper Campograptus curtus Zone in Arctic Canada. Although some summary range figures show Pseudoretiolites appearing at the base of the Aeronian (e.g., Lenz and Melchin, 1997, fig. 4;
Rhud.
Aeronian C. curtus
Telychian
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