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Lower Aeronian Triangulate Monograptids of the Genus Demirastrites Eisel, 1912: Biostratigraphy, Palaeobiogeography, Anagenetic Changes and Speciation

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Lower Aeronian Triangulate Monograptids of the Genus Demirastrites Eisel, 1912: Biostratigraphy, Palaeobiogeography, Anagenetic Changes and Speciation Lower Aeronian triangulate monograptids of the genus Demirastrites Eisel, 1912: biostratigraphy, palaeobiogeography, anagenetic changes and speciation Petr Štorch & Michael J. Melchin Bed-by-bed sampling of the lower Aeronian black shale succession exposed at the Hlásná Třebaň section in the Prague Synform of the Czech Republic has provided a rich and continuous fossil record of biostratigraphically important “triangulate monograptid” graptolites referred to the genus Demirastrites Eisel, whose diagnosis is revised. Eight morphological forms assigned to four species, including the biozonal index taxa Demirastrites triangulatus (Harkness) and D. pectinatus (Richter), as well as D. major (Elles & Wood) and D. campograptoides sp. nov., are distinguished and described. Also documented are patterns of morphological change within species that we interpret to be anagenetic changes, as well as apparent patterns of morphological divergence that we interpret to represent speciation events. The proposed phylogeny suggests that the D. triangulatus lineage underwent significant anagenetic changes throughout its range before it split at the base of the pectinatus Biozone and gave rise to D. pectinatus, which then underwent further anagenetic changes between its early and late forms. At a slightly later stage, in the lower pectinatus Biozone, the D. triangulatus stem lineage underwent further significant changes, giving rise toD. major, which then saw further significant anagenetic changes before it became extinct in the middle pectinatus Biozone, well below the highest occurrence of D. pectinatus. D. campograptoides is a rare and apparently short-ranging species, thus far recorded only from Bohemia, derived from either early D. triangulatus or its direct ancestor. Reconsideration of the species’ records worldwide suggest that none of these lower Aeronian forms was truly cosmopolitan. • Key words: graptolite, Silurian, Aeronian, zonal index species, stratigraphy, taxonomy, evolution, intraspecific variability, Prague Synform, Czech Republic. ŠTORCH, P. & MELCHIN, M.J. 2018. Lower Aeronian triangulate monograptids of the genus Demirastrites Eisel, 1912: biostratigraphy, palaeobiogeography, anagenetic changes and speciation. Bulletin of Geosciences 93(4), 513–537 (12 figures, 1 table). Czech Geological Survey, Prague. ISSN 1214-1119. Manuscript received October 31, 2018; accepted in revised form December 3, 2018; published online December 20, 2018; issued December 20, 2018. Petr Štorch, Institute of Geology CAS, Rozvojová 269, Praha 6, 16502, Czech Republic; [email protected] • Michael J. Melchin, Department of Earth Sciences, St. Francis Xavier Univeristy, Antigonish, Nova Scotia, B2G 2W5, Canada Global correlation of Silurian rocks relies, to a large extent, relies critically on the correct taxonomic assignment of on planktonic graptolites, the most common and diversified the taxa involved. Modern taxonomic revisions are needed macrofossils to be found in anoxic black shales, which are not only for the purpose of high-resolution stratigraphy widespread in off-shore depositional settings, particularly and correlation, but also correct determination of the taxa in the lower part of the Silurian System. High-resolution is a primary prerequisite for any palaeobiogeographical correlation, particularly quantitative stratigraphical cor- analysis, particularly when closely similar taxa occur in relation based upon semi-automated (Shaw’s graphic a similar, but not always the same, stratigraphical interval correlation: Shaw 1964, Cooper & Lindholm 1990, Fan in different palaeobiogeographical provinces. et al. 2013) and automated systems (CONOP: Sadler et al. The present study focusses on the systematic revision 2003, 2011; Sadler 2004; Cooper et al. 2014 and Horizon of lower Aeronian monograptids with proximal rastritiform Annealing: Sheets et al. 2012, Melchin et al. 2016a) have to sub-rastritiform thecae and non-overlapping, sub- demanding requirements for primary data input. Densely rastritiform to high-triangular distal thecae with hooked sampled sections with abundant and highly diversified apertures that are transversely extended into a pair of lateral graptolite faunas are just one step on the way to maximum apertural processes. These graptolites are of particular resolution in stratigraphy and correlation, which also importance in lower Aeronian graptolite biostratigraphy DOI 10.3140/bull.geosci.1731 513 Bulletin of Geosciences • Vol. 93, 4, 2018 worldwide. They were often reported by earlier authors as an informal group of “triangulate monograptids” and assigned to the broadly based genus Monograptus (see e.g. Sudbury 1958, Rickards 1970, Hutt 1975, Rickards et al. 1977). New insights into graptolite thecal morphologies and recognition of several similar monograptid genera – Campograptus (Obut et al. 1968), Lituigraptus (Ni, 1978) and Torquigraptus (Loydell, 1993) – restricted some of the remaining “triangulate monograptids” to species with rhabdosomal and thecal forms that correspond with those of the genus Demirastrites (Eisel, 1912), as diagnosed by Přibyl & Münch (1942) and Štorch (2015). The first appearance datum of Demirastrites trian - g ulatus (Harkness, 1851), the type species of the genus and biozonal index fossil of the lowermost Aeronian graptolite biozone, has been widely used as a marker of the base of Figure 1. Location map of the Hlásná Třebaň section and other the Aeronian Stage (e.g. Štorch 1994, Melchin et al. 2012, localities within the Prague Synform. Abbreviations: HT – Hlásná Třebaň Štorch et al. 2018). Although some fragmentary specimens (49° 55´ 22.9˝ N, 14° 12´ 43.0˝ E); ZT – Zadní Třebaň (49° 55´ 10.5˝ N, 14° 11´ 5.8˝ E); K – Karlík (49° 56´ 30.0˝ N, 14° 16´ 8.6˝ E); with similar isolated thecae, assigned questionably C – Černošice (49° 57´ 27.1˝ N, 14° 18´ 23.0˝ E); V – Všeradice to Demirastrites brevis (Sudbury, 1958), have been (49° 52´ 36.3˝ N, 14° 6´ 12.9˝ E). See Štorch (2015) and Štorch et al. found in strata as low as the topmost Rhuddanian of the (2018) for locality details. Rheidol Gorge section (uppermost Coronograptus cyphus Biozone – Melchin et al. 2018), the first appearance and the interpretions of the proposed lineages among the abrupt proliferation of D. triangulatus marks lowermost taxa preserved at that section and brought substantial Aeronian strata and began a rapid Aeronian expansion progress in our understanding of the morphological details of monograptids with more-or-less isolated and hooked and the stratigraphical succession of the “triangulate metathecae (Demirastrites, Campograptus, Rastrites, monograptids”. In particular, Sudbury (1958, fig. 24) Lituigraptus, Torquigraptus and some species provisionally proposed a hypothesis for the evolution of members of retained in “Monograptus”). Although Demirastrites the D. triangulatus group based on data from the Rheidol triangulatus and its eponymous biozone have been reported Gorge section. It is important to note that Sudbury (1958) from Siberia (e.g. Obut et al. 1968) and China (Chen & Lin had named the various members of the triangulatus group 1978, Anhui Geological Survey 1982, Li 1995), specimens as subspecies of Monograptus separatus, but later corrected assigned to D. triangulatus in Siberia and China differ this, recognizing that the name Monograptus triangulatus from the European form in their lesser dorso-ventral width had priority over M. separatus (Sudbury 1959). Her study (DVW), less prominent thecal apertural processes and of the evolutionary relationships among the members of the some other morphological details. Demirastrites pectinatus Monograptus triangulatus group spanned the lower part of (Richter, 1853), likely a descendant of D. triangulatus, has the gregarius Biozone, an interval that is now recognized been employed as an index species of a graptolite biozone as the lower to upper part of the M. triangulatus Biozone overlying the D. triangulatus Biozone in peri-Gondwanan of Zalasiewicz et al. (2009), which correlates with the Europe (e.g. Bouček 1953, Štorch 1994, Loydell 2012) D. triangulatus Biozone and lower part of Demirastrites and as a subzonal index in Arctic Canada (Melchin 1989, pectinatus Biozone of continental Europe (see Melchin et Melchin & Holmden 2006). It is the senior synonym of al. 2018). Lower Aeronian graptolites of the Rheidol Gorge Monograptus fimbriatus (Nicholson, 1868) (e.g. Bjerreskov section, a locality proposed as one of the candidate sections 1975), commonly reported from the same stratigraphical for the new base Aeronian GSSP (Melchin et al. 2016b, interval in Britain, which was assigned to the upper 2018), are confined to a number of black-shale bands D. triangulatus Biozone by Zalasiewicz et al. (2009). interbedded with graptolite-barren mudstones (Jones 1909, A continuous, stratigraphically ordered morphological Sudbury 1958, Cullum & Loydell 2011, Melchin et al. series of “triangulate monograptids” assigned to 2018). As a result of the unfossiliferous intervals, the record Monograptus fimbriatus, M. raitzhainensis and M. trian- of graptolite evolution is not complete in that section. g ulatus was first recorded and interpreted by Challinor Rhabdosomes collected from some fossiliferous bands are (1945) from Rheidol Gorge, Wales. Based on more de- preserved in full relief but affected by varying degrees of tailed systematic study of larger, more densely sampled brittle cleavage deformation. Also, some apertural details collections of well-preserved, pyrite-infilled specimens are not
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