The Floristic Relationship Between The
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Cleal and Cascales-Miñana Journal of Palaeogeography (2019) 8:14 https://doi.org/10.1186/s42501-019-0029-3 Journal of Palaeogeography ORIGINALARTICLE Open Access The floristic relationship between the upland and lowland Carboniferous wetlands of Variscan Euramerica — Evidence from some medullosalean pteridosperm fronds Christopher J. Cleal1* and Borja Cascales-Miñana2 Abstract Alethopteris grandinii represents remains of fronds of a medullosalean pteridosperm (probably a small tree) that rapidly migrated across the lowland wetland habitats of Variscan Euramerica in middle Asturian (late Moscovian) times. This was probably caused by changing drainage patterns within the lowland coal swamps, in response to climate and landscape changes. However, these medullosaleans had first appeared rather earlier, in early Bolsovian (early Moscovian) times, in upland wetland habitats. These upland habitats may have pre-adapted these plants to the changed condition in the lowland coal swamps. Keywords: Carboniferous, Coal swamps, Medullosales, Upland, Migration 1 Introduction 2010a; Fig. 1). We now believe that these floristic differ- During Pennsylvanian (late Carboniferous) times, large ences provide important insights into how the coal swamp parts of palaeotropical Euramerica were covered by wet- vegetation evolved during Pennsylvanian times, especially lands, often referred to as coal swamps because of the in response to the landscape and climate changes occurring extensive coal deposits that were formed from their at this time (Gastaldo et al. 1996;ClealandThomas1999, peats. These heavily vegetated wetlands consisted of a 2005;Opluštil and Cleal 2007; Cleal et al. 2010a, 2011). complex of clastic and peat substrates habitats formed In this paper we examine evidence for the migration in a fluvio-lacustrine setting. During most of Westpha- of vegetation from upland to lowland wetland habitats lian (late Bashkirian to late Moscovian) times, the vege- during late Moscovian times, with particular reference tation tended to be dominated by arborescent lycopsids, to the Saar-Lorraine Basin that straddles the border with subsidiary ferns, sphenophytes, and pteridosper- between France and Germany (Schindler and Heidtke mous and cordaitean gymnosperms; later in Westphalian 2007). This basin formed in a half-graben between the and Stephanian (latest Moscovian to Gzhelian) times, Rheno-Hercynian and Saxo-Thuringian zones of the marattialean ferns and alethopterid medullosaleans Variscan Orogen (Kneuper 1964, 1966; Korsch and became the dominant plants. Schäfer 1995; Schäfer and Korsch 1998) and is one of Although at any one time the general character of the the best documented of the Westphalian age intramon- coal swamp vegetation was broadly similar across Eura- tane basins, with an Upper Paleozoic sedimentary fill of merica, some palaeobiogeographical variation has been c. 10 km thickness. We will be focussing in particular on recognised, especially between the paralic lowland areas a group of medullosalean pteridosperms that bore fronds and the intra-montane basins (Gothan 1915, 1925, 1951, of the fossil-genus Alethopteris von Sternberg, which in 1954; Thomas 2007; Cleal 2008a, 2008b; Cleal et al. late Moscovian and Kasimovian times was a major com- ponent of the Euramerican wetland vegetation; in the low- * Correspondence: [email protected] land paralic basins, its appearance was part of a major 1Department of Natural Sciences, National Museum Wales, Cardiff CF10 3NP, vegetational change represented by the base of the Crenu- UK lopteris acadica Zone, formerly known as the Lobatopteris Full list of author information is available at the end of the article © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Cleal and Cascales-Miñana Journal of Palaeogeography (2019) 8:14 Page 2 of 12 Fig. 1 Palaeogeographical map of Variscan Euramerica during middle Westphalian times, showing large area of paralic coal swamps and the two main intramontane basins, Saar-Lorraine and West and Central Bohemia. Adapted with permission from Cleal et al. (2010a, their Fig. 1) vestita Zone (Wagner 1984;Cleal1991). However, we upland floras were not upland in any meaningful sense; will show that there is evidence that these plants the term was instead mainly being used to refer to vege- were present rather earlier in the upland wetlands tation of dryland or better drained lowland habitats. such as the Saar-Lorraine Basin. There is nevertheless evidence of some truly upland We will limit our discussion to the evidence available Westphalian vegetation in Variscan Euramerica, in the in the area of palaeotropical coal swamps between the intra-montane basins of the orogen. The elevation of Acadian Mountains (Pfefferkorn and Gillespie 1980)in these basins relative to the paralic areas has been the sub- the west, which approximates in position to the ject of debate. Based on the distinctive nature of the vege- modern-day Appalachian Mountains, and the Palaeo- tation, Holub et al. (1977) and Tenchov (1976, 1977) tethys Ocean in the east. This area has been named estimated elevations of 1000–2000 m above sea level, Variscan Euramerica (Cleal et al. 2010a) and at least whilst Becq-Giraudon and Van Den Driessche (1994)and during early Bashkirian to middle Moscovian time repre- Becq-Giraudon et al. (1996) interpreted deposits in the sented the largest area of coal swamps in Euramerica Autunian basins of Central France as periglacial, from (Cleal and Thomas 2005). There are extensive adpression which they suggested elevations of 4000–5000 m above floras known from coal swamp habitats west of the Aca- sea-level based. More recently, by modelling drainage pat- dian Mountains (e.g. Pfefferkorn and Gillespie 1980;Blake terns within the West and Central Bohemia Basin and et al. 2002) but their taxonomy and biostratigraphy have comparing its palaeogeographical position relative to the not been investigated in detail in recent years, making a foreland basins to the north, Opluštil (2005) estimated it comparison with the data from the more easterly basins formed at an elevation of about 1000 m. difficult. We will hereafter use the Heerlen Regional Chronostratigraphy, as reviewed by (Wagner 1974)and 3 Stratigraphical correlation Wagner and Winkler Prins (2016) as this still provides a A difficulty with trying to compare the vegetation dy- far better temporal resolution of the terrestrial namics of upland intramontane and lowland paralic ba- coal-bearing sequences of Variscan Euramerica; for a sins is stratigraphical correlation. In the paralic basins, correlation between this scheme and the IUGS Global marine bands resulting from eustatic flooding have Chronostratigraphy based mainly on marine stratotypes, provided what are in effect time-lines that allow accurate see Fig. 2. This will include the use of the name Asturian correlation (e.g. Bless and Winkler Prins 1972; Bless et for what was traditionally referred to as the Westphalian al. 1972, 1977) but these are absent from the D Substage (Wagner et al. 2002). For biostratigraphy, we intra-montane basins. Until relatively recently, macro- will use the scheme developed by Wagner (1984)and floral and palynological biostratigraphy have been the modified by Cleal (1991) and Cleal and Thomas (1994); main alternative means of correlation, but using these see also Fig. 2. correlations to compare the vegetation dynamics in the two types of area clearly has the potential to introduce 2 What were the upland wetlands? circular arguments. Correlations independent of biostra- Thomas and Cleal (2017) recently discussed what tigraphy have been mainly based around ash bands, palaeobotanists have meant in the past when using the often termed tonsteins. Initially this was based around term upland floras. It became clear that most so-called the simple lithostratigraphical correlation of the tonsteins Cleal and Cascales-Miñana Journal of Palaeogeography (2019) 8:14 Page 3 of 12 Fig. 2 Summary of the currently accepted correlations between the IUGS Global Chronostratigraphy and the Heerlen Regional Chronostratigraphy, and the macrofloral biozones referred to in this paper. Adapted from Cleal (2018, his Fig. 3) between different basins and this was used to establish a lycopsid- to fern-dominated vegetation that occurred in detailed correlation between the intra-montane Saar- the peat-substrate habitats of Variscan Euramerica at this Lorraine and paralic Nord-Pas-de-Calais sequences time (Gastaldo et al. 1996; Cleal and Thomas 1999, (Bouroz 1967). More recently, high resolution radiometric 2005;Opluštil and Cleal 2007; Cleal et al. 2010a, 2011). dating has further improved the correlations with the However, there was also a marked change in the clastic West and Central Bohemia Basin (Opluštil et al. 2016)but substrate vegetation reflected in the base of the Crenu- similar work has yet to be done in Saar-Lorraine Basin. lopteris acadica Zone (formerly the Lobatopteris vestita Zone) in the Wagner (1984) and Cleal (1991) biostrati- 4 Major changes in coal swamp floras of Variscan graphical scheme. As with the peat substrate vegeta- Euramerica tion, there is a marked increase in abundance