[Palaeontology, Vol. 59, Part 2, 2016, pp. 305–328] MIDDLE JURASSIC VEGETATION DYNAMICS BASED ON QUANTITATIVE ANALYSIS OF SPORE/POLLEN ASSEMBLAGES FROM THE RAVENSCAR GROUP, NORTH YORKSHIRE, UK by SAM M. SLATER and CHARLES H. WELLMAN Department of Animal and Plant Sciences, University of Sheffield, Alfred Denny Building, Western Bank, Sheffield, S10 2TN, UK; e-mails: [email protected], c.wellman@sheffield.ac.uk Typescript received 1 July 2015; accepted in revised form 18 December 2015 Abstract: Quantitative analysis of the distribution of of slight changes in depositional processes, which give rise to dispersed spores and pollen (sporomorphs) has been used to highly variable catchment areas that supply deposits with assess temporal floral variation through the Middle Jurassic sporomorphs. Long-term compositional changes are apparent Ravenscar Group (Aalenian–Bathonian), North Yorkshire, in sporomorph assemblages regardless of lithology/local depo- UK. Aalenian, Bajocian and Bathonian strata possess relatively sitional environments, suggesting that long-term variations are distinct sporomorph and palynofacies assemblages, which more substantial than short-term variations and potentially potentially reflect a dynamic history regarding the nature of include genuine regional temporal changes in parent vegeta- parent vegetation. Specifically, Aalenian palynofloras are com- tion. Relating sporomorph assemblages with their respective posed of a heterogeneous mixture of conifers, ferns, simple depositional environments and relative catchment area sizes monosulcate pollen producers, sphenophytes and Caytoniales; using lithological and palynofacies information suggests that Bajocian palynofloras are codominated by conifers and ferns; the basin interior was occupied by mostly low-standing species and Bathonian palynofloras are highly rich and contain assem- and extrabasinal vegetation was dominated by coniferous taxa. blages of abundant ferns, conifers, lycophytes, pteridosperms/ Comparisons of the dispersed sporomorph and plant megafos- conifers and Caytoniales. Individual- and sample-based sil records indicate that both fossil assemblages reflect different rarefaction demonstrates that Bathonian samples are richer aspects of the palaeoflora due to a multitude of taphonomic than Aalenian and Bajocian samples. Temporal variations in and ecological biases. Such biases include variation in sporo- assemblages are a result of long-term depositional and possible morph production levels, depositional environment and dif- climatic fluctuations through the Middle Jurassic. Ordinations ferential sporomorph and parent plant durability. of sporomorph data using non-metric multidimensional scal- ing (NMDS) demonstrate that short-term variations between Key words: Palynology, sporomorphs, palaeobotany, paly- samples are largely governed by taphonomic biases as a result nofacies analysis, multivariate analysis, rarefaction analysis. T HE Middle Jurassic deposits of Yorkshire, UK, represent Boulter and Windle 1993; Hubbard and Boulter 1997; a rare example of an extensive development of predomi- Srivastava 2011; Slater and Wellman 2015), much of this nantly non-marine deposits of this age and therefore work has focused on taxonomy and biostratigraphy provide important insight into understanding terrestrial (Couper 1958; Riding and Wright 1989; Srivastava 2011). ecosystems from this time period. The sequences contain This study aimed to build upon previous investigations abundant and often exceptionally preserved plant into the palaeoflora of the Middle Jurassic using sporo- megafossil remains and have thus been studied in detail morphs (e.g. Boulter and Windle 1993) in an attempt to by palaeobotanists over the past century (see Van Konij- improve vegetation reconstructions and explain the causes nenburg-Van Cittert and Morgans 1999). Palaeofloristic of any potential variations in sporomorph assemblages reconstructions from these deposits rely primarily on through time. plant megafossil assemblages (e.g. Spicer and Hill 1979), Some plant species produce spores/pollen that appear and although several dispersed sporomorph investigations superficially indistinguishable, particularly when only have been carried out (Couper 1958; Chaloner 1968; using light microscopy (Frederiksen 1980; Mander and Chaloner and Muir 1968; Riding and Wright 1989; Punyasena 2014). Additionally, linking dispersed spores © 2016 The Authors. doi: 10.1111/pala.12229 305 Palaeontology published by John Wiley & Sons Ltd on behalf of The Palaeontological Association. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 306 PALAEONTOLOGY, VOLUME 59 and pollen to their parent plants has been hampered by a (Knox 1973; Hemingway and Knox 1973; Powell 2010). lack of data on the occurrences of spores and pollen The Cloughton Formation overlies the Eller Beck in situ within reproductive structures. These two factors Formation and represents the thickest formation of the represent barriers to reconstructing ancient vegetation Ravenscar Group (c. 85 m). This is divided into three using dispersed spores and pollen. However, following units: the Sycarham Member (non-marine), the extensive investigations over the past half century into the Lebberston Member (marine), and the Gristhorpe rare occurrences where spores/pollen are preserved in situ Member (non-marine). The non-marine units of the (e.g. Van Konijnenburg-Van Cittert 1968, 1971, 1978, Cloughton Formation are predominantly composed of 1981, 1989, 1993, 2000, 2010; Pedersen et al. 1989; Hill cross-bedded sandstones, siltstones and mudstones 1990; Osborn and Taylor 1993; Balme 1995; Friis and throughout which plant beds and thin coal measures are Pedersen 1996; Yang et al. 2008) the majority of Middle Jurassic sporomorphs can now be assigned at least to family-level plant classification. In addition, TEM studies 0 10km 20 30 of the spore/pollen wall ultrastructure have been used to Coastline refine parent plant affinities of dispersed taxa without the Redcar Jurassic 54.6° need for rare in situ preservation (e.g. Batten and Dutta sediments Jurassic 1997; Slater et al. 2015). Thus, dispersed spore/pollen boundary assemblages from the Jurassic of Yorkshire offer impor- Hasty Bank Fault tant insight into vegetation dynamics of this time period, Ravenscar Cleveland 54.4° and, when integrated with the plant megafossil record, Triassic Basin Jurassic these data sets can be used to improve local and regional- Triassic Scarborough N scale palaeoenvironmental reconstructions. 54.2° Jurassic Cretaceous GEOLOGICAL SETTING –1.5° –1° –0.5° The geology of the Mesozoic deposits of North Yorkshire FIG. 1. Generalized geological map of the North Yorkshire (Fig. 1) has been intensively studied since the early nine- coast. Modified from Milsom and Rawson (1989); Mjøs and teenth century (e.g. Young and Bird 1822) as these Prestholm (1993); Palliani and Riding (2000); Rawson and sequences offer well-exposed, extensive and often highly Wright (2000); Slater and Wellman (2015); Slater et al. (2015). complex sedimentary successions from this time (e.g. Ielpi and Ghinassi 2014). This study focused on the domi- nantly non-marine sequences of the Middle Jurassic Age Epoch Stage Lithostratigraphical Division Ravenscar Group (Aalenian to Bathonian) from the (Ma) Lower Cleveland Basin (Fig. 2). These successions were depos- Cornbrash Formation Callovian ited at a time of regional uplift and associated sea-level 166 Long Nab fall. The Ravenscar Group represents predominantly par- Bathonian Member alic, fluvial and lacustrine deposits, with three marine Scalby 168 units that are present as a result of marine incursions Upper Formation Moor Grit from the south and east: the Eller Beck Formation, the Bajocian Member ? Lebberston Member and the Scarborough Formation Scarborough Formation (Knox 1973; Hemingway and Knox 1973; Livera and Gristhorpe Leeder 1981; Alexander 1989, 1992; Powell 2010). Lower Member The non-marine Saltwick Formation (Aalenian), at the Bajocian Cloughton Lebberston base of the Ravenscar Group overlies the marine Dogger Formation Member Formation but often lies unconformably on the Lias Ravenscar Group Sycarham Member Group in areas where the Dogger is absent due to erosion Middle Jurassic 170 or non-deposition (Powell 2010). The Saltwick Formation Eller Beck Formation mostly consists of channel and crevasse-splay sandstones Aalenian Saltwick Formation and floodplain mudstones (Morgans 1999), in which Dogger Formation drifted and in situ plant megafossils are common (e.g. 174 Spicer and Hill 1979). The Eller Beck Formation repre- FIG. 2. Subdivisions of the Middle Jurassic of Yorkshire, mar- sents a thin (c. 4 m thick), marine incursion composed of ine units shaded. Modified from Rawson and Wright (2000); marine sandstone, ooidal ironstone and lime mudstone Slater and Wellman (2015); Slater et al. (2015). SLATER AND WELLMAN: JURASSIC VEGETATION RECONSTRUCTION 307 common (Van Konijnenburg-Van Cittert and Morgans fine-grained and reduced sediments likely to yield rich 1999). The Lebberston Member is laterally variable and is palynomorph assemblages; and (2) non-marine deposits subdivided into the Millepore Bed and Yons Nab Beds. In that are known for their plant megafossil contents (i.e. coastal exposures the Millepore Bed is composed of a plant beds) to allow comparison between plant megafossil sandy ooidal limestone and