a quantitative comparison of dispersed spore/pollen and...

A quantitative comparison of dispersed spore/pollen and plantmegafossil assemblages from a Middle Jurassic plant bed fromYorkshire, UK

Sam M. Slater and Charles H. Wellman

Abstract.Detailed quantitative data has previously been collected from plant megafossil assemblagesfrom a Middle Jurassic (Aalenian) plant bed from Hasty Bank, North Yorkshire, UK. We conducted asimilar analysis of palynological dispersed sporomorph (spore and pollen) assemblages collected fromthe same section using the same sampling regime: 67 sporomorph taxa were recorded from 50 samplestaken at 10 cm intervals through the plant bed. Basic palynofacies analysis was also undertaken on eachsample. Both dispersed sporomorph and plant megafossil assemblages display consistent changes incomposition, diversity (richness), and abundance through time. However, the dispersed sporomorphand plant megafossil records provide conflicting evidence for the nature of parent vegetation.Specifically, conifers and ferns are underrepresented in plant megafossil assemblages, bryophytesand lycopsids are represented only in sporomorph assemblages, and sphenophytes, pteridosperms,Caytoniales, Cycadales, Ginkgoales and Bennettitales are comparatively underrepresented insporomorph assemblages. Combined multivariate analysis (correspondence analysis and nonmetricmultidimensional scaling) of sporomorph occurrence/abundance data demonstrates that temporalvariation in sporomorph assemblages is the result of depositional change through the plant bed. Thereproductive strategies of parent plants are considered to be a principal factor in shaping many ofthe major abundance and diversity irregularities between dispersed sporomorph and plant megafossildata sets that seemingly reflects different parent vegetation. Preferential occurrence/preservation ofsporomorphs and equivalent parent plants is a consequence of a complex array of biological, ecological,geographical, taphonomic, and depositional factors that act inconsistently between and within fossilassemblages, which results in notable discrepancies between data sets.

Sam M. Slater and Charles H. Wellman. Department of Animal and Plant Sciences, University of Sheffield,Alfred Denny Building, Western Bank, Sheffield, S10 2TN, United Kingdom. E-mail: [email protected]

Accepted: 3 June 2015Published online: 14 October 2015Supplemental materials deposited at Dryad: doi:10.5061/dryad.tg469

Introduction

Understanding the causes of temporal varia-tion in paleofloras is a fundamental objective ofpaleobotany. Extracting these causes is, how-ever, frequently problematic as it is oftendifficult to determine the dominant controlson the constituents of fossil assemblages. Suchcontrols include ecological, climatic, deposi-tional, and preservational factors. Establishingthe causes of paleofloristic temporal variationis clearly enhanced when a multidisciplinaryapproach is used, as the overreliance onsingular lines of evidence can often lead toover interpretation. Comparing data from theplant megafossil and terrestrial palynologicalrecords can provide important insight intoecological and preservational biases that can

shape diversity (richness) and abundancepatterns of these fossil assemblages. Thus,such comparisons can help to determine thereliability of paleofloristic interpretationsbased on the spore and pollen (sporomorph)and plant megafossil records in isolation.

The sequences of North Yorkshire, UK offer arare example of extensive Middle Jurassicterrestrial deposits. The sedimentary successionshave previously been studied in detail for theirpaleontological significance (e.g., Romano andWhyte 2003) and particularly for the famousplant beds that are scattered throughout thesesequences (e.g., Black 1929; van Konijnenburg-van Cittert 1968, 1975, 1996, 2008; Crane andHerendeen 2009; Spicer and Hill 1979; vanKonijnenburg-van Cittert and Morgans 1999).Although plant megafossil studies from these

Paleobiology, 41(4), 2015, pp. 640660DOI: 10.1017/pab.2015.27

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deposits are common in the literature (e.g.,Harris 1941, 1944, 1952, 1953, 1961a,b, 1964,1969, 1978, 1979; van Konijnenburg-van Cittert1972, 1975, 1978, 1981, 1989, 1996, 2008; Harriset al. 1974; Spicer and Hill 1979; Hill 1990;Morgans 1999; van Konijnenburg-van Cittertand Morgans 1999), published dispersed sporeand pollen investigations remain comparativelysparse (e.g., Couper 1958; Chaloner 1968;Chaloner and Muir 1968; Riding 1984; Ridingand Wright 1989; Gowland and Riding 1991;Boulter and Windle 1993; Hubbard and Boulter1997; Butler et al. 2005; Srivastava 2011).This study provides a detailed palynological

assessment of a plant bed from Hasty Bank,North Yorkshire, UK that was previouslyanalyzed for its plant megafossil contents.Spicer and Hill (1979) carried out a compre-hensive quantitative study of this plant bed inwhich they sampled contiguous plant mega-fossil census counts through a through a 5moutcrop section. Plant megafossil counts werecarried out on rock blocks of either 50 or 25 cm2

parallel to bedding and 10 or 20 cm in depth,perpendicular to bedding. Counts were thenmultiplied accordingly so that all abundancescorrespond to a 50 50 20 cm3 block of sedi-ment. The histogram of their results is pro-vided in Supplementary Figure 1. Sporomorphquantitative data was analyzed here from thesame section discussed in Spicer and Hill(1979) in order to compare this with thequantitative plant megafossil data. Palynofa-cies analysis was also carried out in order to aidenvironmental reconstructions (Tyson 1995).Paleofloristic comparisons of sporomorph andplant megafossil data are possible due tocomprehensive in situ spore/pollen (e.g., vanKonijnenburg-van Cittert 1968, 1971, 1978,1981, 1989, 1993, 2000; Pedersen et al. 1989;Hill 1990; Osborn and Taylor 1993; Balme 1995;Friis and Pedersen 1996; Yang et al. 2008) andultrastructural transmission electron micro-scope (TEM) studies (e.g., Batten and Dutta1997) which means that the majority of MiddleJurassic sporomorphs can now be assigned atleast to family level plant classification.By comparing dispersed spore/pollen

assemblages with plant megafossil data inassociation with palynofacies analysis it wasanticipated that a more realistic paleofloristic

and paleoenvironmental interpretation wouldbe possible and potential discrepancies in datasets would help to explain preservationalbiases between sporomorph and plant mega-fossil records. Since sporomorph and plantmegafossil assemblages undergo differenttransportation and depositional processes, itwas expected that the respective fossil assem-blages would be notably dissimilar in compo-sition (e.g., Bercovici et al. 2008, 2009), with theanticipation that our analyses may shed lighton the reasons behind these differences.

Geological Setting

The Mesozoic sequences of the ClevelandBasin, northeast England (Fig. 1) have beenintensively studied since the early nineteenthcentury (e.g., Young and Bird 1822) and offerimportant insight into both terrestrial andmarine environments of this time. MiddleJurassic sediments of the Cleveland Basin aredominated by the chiefly terrestrial sequencesof the Ravenscar Group (Fig. 2). Regional upliftand associated relative sea level fall led to thedeposition of extensive fluviodeltaic sequencesderived from upland areas surrounding theCleveland Basin. Marine beds occur sporadi-cally throughout the Ravenscar Group as aresult of marine incursions from the south andeast (Hemingway and Knox 1973; Knox 1973;Hemingway 1974;Nami andLeeder 1978; Leederand Nami 1979; Hancock and Fisher 1981;

FIGURE 1. Location and geological setting of Hasty Bank,northeast England. Modified from Milsom and Rawson(1989); Mjs and Prestholm (1993); Cox and Sumbler(2002); Palliani and Riding (2000); Slater et al. (2015).

PALYNOLOGY AND MEGAFLORAL COMPARISON 641



Livera and Leeder 1981; Fisher and Hancock1985; Kantorowicz 1985; Alexander 1989, 1992;Riding and Wright 1989; Gowland and Riding1991; Rawson and Wright 2000; Powell 2010).The Ravenscar Group provides an exceptionalexample of extensiveMiddle Jurassic terrestrialsequences and the plethora of plant fossils

(e.g., van Konijnenburg-van Cittert andMorgans1999) and dinosaur footprints (Whyte andRomano 1993, 2001a,b; Romano et al. 1999;Romano and Whyte 2003; Whyte et al. 2006,2007, 2010) make the Cleveland Basin animportant region for paleontology.

The plant bed under investigation is locatedon the northern slope of Hasty Bank (NZ 567037), situated within the northwest region ofthe North YorkMoors National Park, northeastEngland. The plant bed occurs at the base of theAalenian Saltwick Formation, stratigraphicallylocated at the base of the Ravenscar Group(Fig. 2) and lies unconformably above themarine Dogger Formation. The bed is approxi-mately 7m thick and has previously yielded avaried flora of 90 species (Hill and vanKonijnenburg-van Cittert 1973; Hill 1974; Spi-cer and Hill 1979). Two lithologies dominatethe plant bed (Fig. 3), a claystone that forms thelower part of the section and a siltstone thatoccupies the majority of the upper part of thesection. An erosional surface is presentbetween the claystone and the siltstone. A thinlens of dark gray clay is also present at the topof the section (Hill and van Konijnenburg-vanCittert 1973; Hill 1974).

The claystone is uniform dark gray in color.Grain size is homogeneous through the unit and

FIGURE 2. Subdivision of Middle Jurassic sequences ofthe North Yorkshire Coast. Marine units shaded. Thearrow indicates the stratigraphic position of the plant bed.Modified from Rawson and Wright (2000); Slater et al.(2015).

FIGURE 3. Cross section of the geology of the main plant bed at Hasty Bank. Vertical and horizontal scales provided(vertical scale exaggerated four times). The section discussed is shown by the rectangle. Adapted from Hill and vanKonijnenburg-van Cittert (1973); Spicer and Hill (1979); van Konijnenburg-van Cittert and Morgans (1999).

642 SAM M. SLATER AND CHARLES H. WELLMAN



thin (13mm scale) horizontal laminae areabundant. The rock is relatively soft and breaksapart easily along laminae, which often revealhighly abundant fragmentary plantmegafossils.The siltstone is a homogeneous medium

gray color. Grain size is uniform through theunit and larger, more prominent horizontallaminae (510mm scale) are present. The rockis harder than the claystone and fragmentaryplant megafossils are abundant, although lessso than in the claystone. Horizontal roots occurin low abundance within the basal ~1m of thesiltstone, these are typically less than 2 cm inlength and ~2mm in width.The gray clay at the top of the section is a

homogeneous very dark gray color. Grain sizeis uniform through the unit and no sedimen-tary structures are visible. The rock is very softand not fully lithified. Plant megafossils areless common in the gray clay than in theclaystone and siltstone.

Previous Interpretations of the DepositionalEnvironments at Hasty Bank.The Hasty Bankplant bed was first recognized as an importantfossil locality by Black (1929). Subsequentpaleobotanical and paleoecological studies havecommented on possible environments ofdeposition for the plant bed, most notably byHarris (1964), Hill and van Konijnenburg-vanCittert (1973), Hill (1974), and vanKonijnenburg-van Cittert and Morgans (1999).

Harris (1964) postulated that the claystone atthe base of the plant bed (Fig. 3) was depositedin a coastal environment periodically floodedby seawater. These interpretations werebased on the occurrence of the pteridospermPachypteris papillosa in association with raremarine microfossils thought to be derived frommarine flooding events. Harris (1983) recon-structed P. papillosa as a large shrub that formedmangrove-like thickets along tidal rivers. Spicerand Hill (1979) showed that P. papillosa ismarkedly more abundant within the claystonedeposit compared to the rest of the section.

The siltstone is interpreted as the peripheralfringes of a large channel sandstone depositimmediately adjacent to the southeast of theplant bed (Fig. 3). Hill and van Konijnenburg-van Cittert (1973) concluded that the siltstonewas deposited in the slower flowing region of

the channel. It is possible that the siltstonecould however represent a levee or floodplaindeposit peripheral to the sandstone. The chan-nel has cut into the underlying sedimentsforming an erosional surface between theclaystone and the siltstone and thus there is atime gap between these deposits.

Previous depositional environmental inter-pretations for the gray clay are lacking. How-ever, sedimentological, sporomorph andpalynofacies evidence from this study suggeststhat this deposit represents a swamp or anabandoned channel.

Materials and Methods

Collection.A total of 50 samples (HB1HB50,numbered in reverse stratigraphic order, i.e.,HB1 is at the top of the section) were collected at10 cm vertical intervals from the main HastyBank plant bed (NZ 567 037) for palynologicalprocessing. Samples were taken from theidentical section of that discussed by Spicer andHill (1979), shown in Figure 3. Christopher R.Hill (of Spicer and Hill [1979]) was presentduring collection of samples to ensure the exactposition of the section was located. Samplingrequired the excavation of approximately 50 cmof modern deposits to access the outcrop.The exterior of the outcrop was weatheredbetween 5 and 20 cm deep into the rock. Thesection was therefore excavated a further ~30 cminto the outcrop to ensure fresh exposure.Samples HB1HB3 are from the gray clay atthe top of the section; HB4HB28 are from thesiltstone unit; and HB29HB50 are from theclaystone unit.

Processing.Dry rock samples were weighedat 20 g before being dissolved in 40%hydrochloric acid for at least 24 hours toremove carbonates, followed by two weekmaceration in 40% hydrofluoric acid to removesilicates. Samples were agitated every two daysto ensure full break down of rock material.Samples were then decanted and fresh wateradded, repeating the process until neutral. Oneday was left between decants to ensure minimalloss of palynomorphs. Two Lycopodium tablets(produced by the University of Lund, Sweden;batch 1031) were added before sieving at 10 m.Centrifuging residues in zinc chloride was then




undertaken to remove heavyminerals. Residueswere then sieved again at 10 m to remove theheavy liquid and final residues were spreadacross cover slips and gently heated on a hotplate to remove excess water. Cover slips werethenmounted onto slides using epoxy resin on ahot plate. Allmaterials (rock samples and slides)are housed in the collections of the Centre forPalynology at the University of Sheffield.

Counting.Slides were examined under aMeiji Techno (MA151/35/50) light microscope.A minimum of 200 indigenous Jurassicsporomorphs were counted from each sample inaddition to any Lycopodium spores from tablets inorder to assess the relative organic richnessof samples. The Lycopodium tablets contain aknown quantity of spores (20,8481546 sporesper tablet). This allows the palynomorphproductivity of each sample to be assessed whencounting sporomorphs, as numbers ofLycopodiumspores can be compared with numbers ofindigenous Jurassic sporomorphs to assess thepalynomorph richness of samples. In this study,increased numbers of Lycopodium correspond toa decrease in palynomorph productivity perunit of sediment. Counts were carried out insystematic traverses across slides to ensure nograins were missed. For presence/absence data,the remainder of the slide was then examined inthe same fashion to identify species that werenot present in the count data. The complete rawdata set is provided in Supplementary Table 1.For sporomorph images that refer closely to thetaxonomic identifications used in this study,see Couper (1958), Boulter and Windle (1993)and Srivastava (2011).

Palynofacies Analysis.The term palynofaciestypically refers to all of the visible organicparticles (usually 2250 m in size) that occurwithin palynological maceration residues(Traverse 2007). Palynofacies analysis iscommonly used to assess depositionalenvironments (e.g., Parry et al. 1981; Boulterand Riddick 1986; Van der Zwan 1990; Brugmanet al. 1994; Oboh-Ikuenobe and Yepes 1997;Oboh-Ikuenobe et al. 2005; Carvalho et al. 2006).In this study, palynofacies analysis attempts toprovide a more in depth interpretation of thedepositional environments at Hasty Bank. Asimilar palynofacies classification scheme toTyson (1995) and Batten and Stead (2005) was

used to categorize organic matter. Categoriesfor palynofacies debris are: spores; pollen;algae; dinoflagellate cysts; acritarchs; humicdebris; amorphous organic matter (AOM);Botryococcus; structured vitrinite; unstructuredvitrinite; cuticle; and inertinite. Counts of 200palynodebris were carried out on all samples,the complete raw data set is provided inSupplementary Table 2.

Statistical Analysis.Correspondence analysis(CA) and nonmetric multidimensional scaling(NMDS) were performed on sporomorph datasets to further understand the causes of floralvariation through the Hasty Bank plant bed.Correspondence analysis and NMDS areordination methods that plot complexmultivariate data onto a minimal number ofaxes (e.g., Jardine et al. 2012). Correspondenceanalysis is an eigenvector method of ordinationthat produces a graphical representation of acontingency table (Spicer and Hill 1979).Nonmetric multidimensional scaling is anonparametric ordination technique that usesranked distances between samples to assessthe degree of similarity between samples(Chatfield and Collins 1980; ter Braak 1995;Legendre and Legendre 2012; Hammer andHarper 2006; Jardine et al. 2012), henceclustering of samples in ordination spaceindicates high compositional similarity betweenthose samples. For comprehensive descriptionsof CA and NMDS see Greenacre (2007) and Coxand Cox (2001), respectively. Correspondenceanalysis and NMDS are becoming increasinglyused in palynological analysis of quantitativepaleoecological studies (e.g., Kovach 1989, 1993;Wing and Harrington 2001; Hammer andHarper 2006; Bonis and Krschner 2012;Jardine et al. 2012; Stukins et al. 2013) as suchmethods allow the user to extract information onthe major causes of variation from complex datasets. Correspondence analysis was chosen overdetrended correspondence analysis (DCA) asthis method was employed by Spicer and Hill(1979) on megafossil data, thus to allowcomparison of ordinations we used the sametechnique here. Furthermore, CA ordinations donot indicate the need for DCA. Both CA andNMDS are used here to assess abundance andpresence/absence data. For NMDS ordinations,the Bray-Curtis dissimilarity metric was used




to generate distances between samples asthis method is considered to perform well inecological analyses (e.g., Minchin 1987;Harrington 2008; Bowman et al. 2014). Repeatedruns were carried out for two dimensions until aconvergent solution was established. Principalcomponents rotation and centering was thenapplied to the final ordination. Nonmetricmultidimensional scaling ordinations wereperformed using R, version 3.1.2 (R Core Team2014), within the package vegan, version2.2-1 (Oksanen et al. 2015). Sporomorphrelative abundances have been transformedlogarithmically for CA and NMDS. Thisprocedure condenses the differences in scoresbetween abundant and rare species betweensamples, thus reducing the impact of highlyabundant taxa on the data set and also reducingstatistical noise. Spicer andHill (1979) suggestedthat the most effective way to assess megafossildata in ordinations was to logarithmicallytransform abundances. Species that are presentin samples but not in counts have been excludedfrom logarithmically transformed relativeabundance ordinations. For taxonomic CA thesame data has been used with the exclusion ofspecies that contribute less than 1% of the totalcount to eliminate statistical noise. Presence/absence CA and NMDS were performedto assess co-occurrence and compositionalvariation between samples. For presence/absence analyses all species are included;species that are present are scored as 1, speciesthat absent are scored as 0. Spiked Lycopodiumdata has been excluded from all ordinations.The statistical program PAST (Hammer et al.2001) was used to create CA plots.

Results

Sporomorph Diversity (Richness) and AbundanceVariation.A total of 67 sporomorph taxawere recognized from 50 samples; the entiretaxonomic list with associations betweensporomorphs and parent plant groups isprovided in Supplementary Table 3. Thecommonly used Chao2 species richnessestimator (Colwell and Coddington 1994) gavea species estimate of 69.94 taxa (standarddeviation = 3.36) for the entire data set, whichsuggests the data set was not severely

undersampled. Diversities (richness) usingpresence/absence data (Fig. 4B) are highestwithin the claystone; diversity (richness) isslightly lower within the siltstone assemblageand lower again in gray clay samples. Thisindicates that the claystone was deposited ata time of increased floral diversity and/orclaystone samples are capturing moresporomorph taxa due to preservational biases.

Relative abundances of all taxa are displayedgraphically in Supplementary Figure 2. Relativeabundances of the ten most abundant spor-omorph taxa are provided in Figure 4A. Spor-omorphs have been grouped into their botanicalaffinities in Figure 5A to extract information onhigher taxonomic level temporal floral varia-tions. Spiked Lycopodium spores exhibit the mostprominent abundance variation through thesequence. Abundant spiked Lycopodium in grayclay (HB1HB3) and siltstone (HB4HB28)samples indicates that claystone samples(HB29HB50) preserve far greater numbers ofindigenous Jurassic sporomorphs.

Bryophytes (Fig. 5A) are represented by twospecies and are in low abundance throughoutthe section. Lycopsids are highly diverse(16 taxa) and abundances are low throughoutthe plant bed, but slightly higher in the silt-stone compared to the claystone and gray clay.Sphenophytes are represented by Calamosporamesozoica and abundances are low andrelatively constant throughout the section.Ferns are the most diverse group (23 taxa) andare highly abundant through the plant bed.Abundances increase from the claystone intothe siltstone and increase again into the grayclay. Pteridosperms are represented by twospecies ofAlisporites; diversity and abundancesare low throughout the section, but slightlyhigher in the siltstone compared to theclaystone and gray clay. Caytoniales arerepresented by the single species Vitreisporitespallidus; similarly to pteridosperms, abun-dances are low throughout the section, butslightly higher in the siltstone compared to theclaystone and gray clay. Conifers constitute themost abundant group and are highly diverse(14 taxa). Abundances decrease markedlyfrom the claystone into the siltstone anddecrease again within the gray clay. Relativeabundance variation within the conifers is




most apparent within the three most abundantspecies: Classopollis torosus, Perinopolleniteselatoides, and Araucariacites australis (Fig. 4A).Classopollis torosus and P. elatoides are con-siderably more abundant within claystonesamples, whereas A. australis contrasts theoverall abundance pattern of the conifers and ismore abundant within siltstone samples.Cycadopsida/Ginkgopsida pollen is moder-ately diverse (four taxa) and displays littlechange in abundance (Fig. 5A) through theplant bed. Chasmatosporites hians exhibits themost pronounced abundance variation withinthe Cycadopsida/Ginkgopsida group; C. hiansis relatively common in the claystone, butbecomes increasingly rare within siltstonesamples and is absent above sample HB19.Bennettitales are low in abundance anddiversity throughout the plant bed (two taxa).The largest relative abundance variationwithin the Bennettitales is displayed byCycadopites carpentieri, which occurs frequentlyin the basal and upper samples of the

section, but is absent from samples HB26 toHB16.

Sporomorph Sample Ordination.CA andNMDS (Fig. 6) reveal samples from claystone,siltstone, and gray clay units are well separatedin ordination space, with only minimaloverlap. This illustrates that lithology andapparent floristic composition are correlatedand perhaps both are responding to the sameprincipal factors of deposition and/orenvironment. Logarithmically transformedrelative abundance (Figs. 6A,C) and presence/absence sample multivariate plots (Figs. 6B,D)demonstrate little clustering of stratigra-phically adjacent samples within lithologicalpoint clouds, suggesting little temporal floralchange and habitat partitioning within eachof the three depositional settings. Forlogarithmically transformed relative abundanceCA (Fig. 6A) the variance is 14% for axis oneand 9% for axis two. For presence/absenceCA (Fig. 6B) the variance is 11% for axis oneand 8% for axis two. Gray clay samples display

FIGURE 4. A, Relative abundance chart of the ten most abundant sporomorph taxa in addition to spiked Lycopodium.Relative abundances of Jurassic taxa are calculated from counts of at least 200 grains. Spiked Lycopodium count data isshown as a percentage of the indigenous Jurassic taxa plus the spiked Lycopodium itself. B, Sporomorph diversity(richness) for all samples.




some volatility between relative abundanceand presence/absence CA (Figs. 6A,B), whichpotentially reflects the comparatively lowabundance of conifers and low overall diversitieswithin the gray clay. However, in all ordinations(Fig. 6) gray clay samples are consistently moresimilar to siltstone samples than claystonesamples. In NMDS ordinations (Figs. 6C,D)samples of different lithologies are separatedpredominantly on axis one, suggesting thiscorresponds to depositional setting. Data forFigure 6 is provided in Supplementary Tables 47.Taxon Correspondence Analysis.The scatter

plot of the first two CA axes (43% cumulativevariation) of logarithmically transformedrelative abundances of taxa reveals fourgroupings, labeled AD (Fig. 7A). Eigenvaluesand percentage variance data for Figure 7 isprovided in Supplementary Table 8. Relativeabundances of taxa from clusters AD havebeen grouped and plotted in Figure 7B.

Cluster A (Fig. 7A) is occupied by P. elatoides(Taxodiaceae [van Konijnenburg-van Cittert andvan der Burgh 1989; Boulter and Windle 1993;Balme 1995; Couper 1958; Mander et al. 2010;Mander 2011]), Callialasporites turbatus (Arau-cariaceae [Boulter and Windle 1993]), C. torosus(Cheirolepidiaceae [Harris 1979; Boulter andWindle 1993; Balme 1995; Couper 1958; Ziaja2006; Mander et al. 2010; Mander 2011]),Spheripollenites subgranulatus (Taxodiaceae[Couper 1958; Boulter and Windle 1993]), andSpheripollenites scabratus (Taxodiaceae [Couper1958; Boulter and Windle 1993]). Taxa fromcluster A are considerably more abundantwithin claystone samples (Fig. 7B). Cluster B(Fig. 7A) is occupied by C. mesozoica (Spheno-phyta [Harris 1978; Boulter and Windle 1993;Balme 1995]), Dictyophyllidites harrisii (Dipter-idaceae/Matoniaceae [Couper 1958; Boulterand Windle 1993; van Konijnenburg-vanCittert 1993; Guignard et al. 2009]), and

FIGURE 5. Relative abundance charts of (A) sporomorphs and (B) plant megafossils (Spicer and Hill 1979) grouped intotheir respective botanical affinities. Relative abundances of Jurassic sporomorph taxa are calculated from counts of atleast 200 grains. Spiked Lycopodium count data is shown as a percentage of the indigenous Jurassic taxa plus the spikedLycopodium itself.




Marattisporites scabratus (Marattiaceae [Couper1958; Filatoff 1975; Boulter and Windle 1993;Balme 1995]). Taxa from cluster B display littlechange in relative abundances through the sec-tion (Fig. 7B). Cluster C (Fig. 7A) is occupied byA. australis (Araucariaceae [Couper 1958; Boulterand Windle 1993; Balme 1995; Mander et al.2010; Mander 2011; Bonis and Krschner 2012]),Pityosporites microalatus (Pinaceae [Couper 1958;Boulter and Windle 1993]), Cerebropollenitesmesozoicus (Coniferales [van Konijnenburg-vanCittert and van der Burgh 1989; Boulter andWindle 1993; Balme 1995]), Cycadopites minimus(Cycadales/Ginkgoales [Boulter and Windle1993; Balme 1995; Mander et al. 2010]), andDeltoidospora minor (Cyatheaceae/Dicksonia-ceae/Dipteridaceae/Matoniaceae [Couper 1958;Boulter andWindle 1993; van Konijnenburg-van

Cittert 1993; Guignard et al. 2009]). Taxa fromcluster C are considerably more abundantwithin siltstone and gray clay samples (Fig. 7B).Cluster D is occupied by Chasmatosporitesapertus (?Cycadales/?Ginkgoales/?Gnetales[Boulter and Windle 1993; Balme 1995]) andDictyophyllidites equiexinus (Dipteridaceae/Matoniaceae [Boulter and Windle 1993; vanKonijnenburg-van Cittert 1993]). Taxa fromcluster D are more abundant within siltstonesamples compared to claystone and grayclay samples (Fig. 7B). Cycadopites carpentieri(Bennettitales [Boulter and Windle 1993]) plotsseparately in ordination space from clustersAD. Figure 7B demonstrates that C. carpentieriis considerably more abundant within claystonesamples. Relative abundances of clusters(Fig. 7B) reveal each point cloud (Fig. 7A) has a

FIGURE 6. Combined ordination plots of samples (spiked Lycopodium excluded). A, B, correspondence analysis; C, D,nonmetric multidimensional scaling. A, C, logarithmically transformed relative abundance data; B, D, presence/absence data.White circles represent claystone samples; gray circles represent siltstone samples; black circles represent gray clay samples.




distinct abundance pattern between differentlithological/depositional units that correlatewith variation on axis one (29% variation),signifying axis one corresponds to depositionalsetting.

The conifer dominated clusters A and Crepresent possible upland communities, asconifers are typical of upland and well-drainedsettings (e.g., Stukins et al. 2013). Megafloralstudies have hypothesized that Mesozoic fernswere typical of areas of high disturbanceand water supply (Harris 1961b; vanKonijnenburg-van Cittert and van der Burgh1989; Stukins et al. 2013). The co-occurrence ofSphenophyta and fern spores in cluster B issuggestive of such a community. Ascribing aspecific control for axis two (14% variation)with confidence is problematic as this could beone of a myriad of possible factors related tothe principal drivingmechanisms of vegetationdynamics; differential site availability, speciesavailability and species performance (Picketand Cadenasso 2005; Stukins et al. 2013).Palynofacies Analysis.Two palynofacies are

recognized that correspond directly withlithological variation, herein referred to aspalynofacies 1 and 2. Palynofacies 1 representsthe gray clay and siltstone assemblage(samples HB1HB28) and palynofacies 2

represents the claystone assemblage (samplesHB29HB50). Relative abundances of organicdebris are provided in Figure 8. Palynofacies 1is dominated by inertinite (~43%) andunstructured vitrinite (~31%). Palynofacies 2comprises a more heterogeneous mixture oforganic debris and is co-dominated by AOM(~23%), pollen (~23%), inertinite (~21%) andunstructured vitrinite (~17%).

Comparison of Sporomorph and Plant MegafossilAssemblages.A direct comparison ofsporomorph and plant megafossil abundancevariation through the plant bed is provided inFigure 5. A comparison of the sporomorph andplant megafossil diversity (richness) data isprovided in Figure 9. Sporomorph assemblagesare more diverse (67 taxa) than megafossilassemblages (49 taxa in Spicer and Hill 1979).

Bryophytes are represented by two speciesin sporomorph assemblages and are verylow in abundance through the section(Figs. 5, 9). Bryophytes are absent from mega-fossil assemblages (Spicer and Hill 1979).

Lycopsids are highly diverse (16 taxa)within the sporomorph assemblage, howeverthey are absent from megafossil counts (Fig. 9)(Spicer and Hill 1979). Abundances are lowthrough the section within sporomorphassemblages (Fig. 5).

FIGURE 7. A, Correspondence analysis of taxa using logarithmically transformed sporomorph relative abundance data(excluding spiked Lycopodium). Squares represent sphenophytes; white circles represent ferns; gray circles representconifers; crosses represent Cycadopsida/Ginkgopsida; triangles represent Bennettitales. Ellipses AD show clusteringof plots. B, Relative abundances of grouped taxa from clusters AD from Figure 7A.




Sphenophytes are markedly more abundantwithin the megafossil assemblage (Fig. 5B) anddiversity is the same (one species) in bothsporomorph and megafossil assemblages(Fig. 9). Abundances are relatively constantthrough the section within sporomorphassemblages, however, megafossil assem-blages record an increase in abundance withinthe siltstone compared to the claystone (Fig. 5).

Ferns are the most diverse plant groupwithin sporomorph assemblages atHasty Bank.Abundances (Fig. 5) and diversities (Fig. 9) areconsiderably higher within sporomorphassemblages (23 taxa) compared to megafossilassemblages (nine taxa). Abundances increasethrough the section in both sporomorph andplant megafossil assemblages (Fig. 5).

Pteridosperms are considerably moreabundant (Fig. 5) and diverse (Fig. 9) within

FIGURE 8. Relative abundance chart of palynofacies categories.

FIGURE 9. Comparative diversities (richness) of thedominant plant groups within sporomorph and plantmegafossil assemblages.




megafossil assemblages (11 taxa) compared tosporomorph assemblages (two taxa). Interest-ingly, sporomorph and megafossil assemblagesdisplay conflicting records for pteridospermtemporal abundance variation. Specifically,megafossil assemblages record a large decreasein pteridospermabundances across the claystonesiltstone boundary, however in sporomorphsamples, pteridosperm abundances are slightlyhigher within the siltstone unit.

Caytoniales are more abundant (Fig. 5) anddiverse (Fig. 9) in megafossil assemblages(two taxa) compared to sporomorph assem-blages (one taxa). Both sporomorph andmegafossil assemblages document a slightincrease in Caytoniales abundance within thesiltstone unit (Fig. 5).

Coniferales are the most abundant plantgroup within sporomorph assemblages (Fig. 5A)and diversity is high (14 taxa) (Fig. 9). Abun-dance (Fig. 5B) and diversity (eight taxa) (Fig. 9)in megafossil assemblages is considerably lower.Abundances decrease through the section in bothsporomorph andmegafossil assemblages (Fig. 5).

Cycadopsida/Ginkgopsida are considerablymore abundant (Fig. 5) and diverse (Fig. 9)within megafossil assemblages (10 taxa), com-pared to sporomorph assemblages (four taxa).Abundances of Cycadopsida/Ginkgopsidadisplay little change through the section withinsporomorph assemblages (Fig. 5A). Conversely,abundances of Cycadales and Ginkgoalesdecrease through the section within megafossilassemblages (Fig. 5B).

Bennettitales are notably more abundant(Fig. 5) and diverse (Fig. 9) in megafossilassemblages (eight taxa), compared to spor-omorph assemblages (two taxa). Abundancesare slightly higher in the siltstone unit withinboth sporomorph and plant megafossilassemblages (Fig. 5).

Discussion

Depositional Environments.The sedimen-tology and palynofacies of the claystoneindicates a low energy, low oxygen and highnutrient depositional environment. Theoccurrence of Tasmanites spp. and Crassosphaeraspp. from samples HB50 to HB4 reveals a

marine component through the plant bed andsuggests periodic flooding by seawater, asdescribed by Harris (1964). However,Tasmanites spp. and Crassosphaera spp. arevery rare (typically less than one specimenper 1000 palynomorphs) and the presence ofBotryococcus and absence of dinoflagellates inpalynofacies counts demonstrates dominantlyfresh water conditions (Gray 1960; Tyson1995). The possibility that the rare marinepalynomorphs could be reworked, potentiallyfrom the underlying marine Dogger Formation,cannot be discounted. Abundant AOM inclaystone samples indicates relatively lowoxygen and high nutrient levels within theoriginal water during deposition (Tyson 1995;Roncaglia 2004; Traverse 2007; Pacton et al.2011). It is difficult to state with certaintya definitive depositional setting for theclaystone, however the results of this studyagree with Harris (1964) interpretations andare suggestive of a coastal plain periodicallyflooded by seawater, occupied by mangrove-like vegetation.

Interpretations of the depositional environ-ment for the siltstone are consistent with pre-vious studies that indicate this unit representsthe slow moving part of a fluvial channel (Hilland van Konijnenburg-van Cittert 1973; vanKonijnenburg-van Cittert and Morgans 1999).Decreased abundances of AOM signify lowernutrient levels than the claystone (Tyson 1995).

The absence of marine palynomorphs in thegray clay demonstrates marine influence isnegligible to absent. Sample ordination (Fig. 6)reveals that the gray clay is compositionallymore similar to the siltstone than the claystone.Sporomorph assemblages of the gray claycontain higher abundances of ferns and lowerabundances of wind blown taxa, suggesting asmaller catchment area and reduced sporomorphtransportation distances compared to theclaystone and siltstone (Chaloner and Muir1968). These combined factors are suggestive ofa swamp or an abandoned channel environmentfor the gray clay.

Taphonomical models of time-averagingverses catchment area for multiple deposi-tional environments demonstrate that thecomponents of fossil assemblages can beused to indicate their temporal and spatial




representations (Behrensmeyer and Kidwell1985; Behrensmeyer et al. 2000). Fossil assem-blages that contain transported plant mega-fossils are typical of floodplain, pond and lakedeposits. Such deposits generally representtime periods of ~10010,000 years and sourceareas of ~100,000m2. Sporomorphs are gen-erally representative of larger source areas,frequently in excess of 1,000,000 m2 (Behrens-meyer et al. 2000). Estimates of sourceareas and time-averaging for floodplain andchannel environments are highly variable(Behrensmeyer et al. 2000). Channels generallyrepresent increased time-averaging and sourcearea sizes compared to floodplain deposits,although there is significant overlap in sourcearea sizes and the degree of time-averagingbetween channels and floodplains. Interest-ingly, the claystone unit (coastal plain) at HastyBank is interpreted to represent a longer timeinterval and a larger source area than the silt-stone (fluvial channel) due to slower sedi-mentation rates and the probability thatnumerous rivers potentially flowed into thecoastal plain environment.

Explanations for Temporal SporomorphVariation.Variation in sporomorph and plantmegafossil assemblages through the sectioncan be correlated strongly with depositionalchange. The depositional environment isa primary control on parent vegetation, thus achange in depositional setting typically resultsin a change in sporomorph and megafossilassemblages. The erosional surface between theclaystone and siltstone separates two distinctdepositional environments by a period ofunknown duration; therefore a change in floralcomposition between the claystone and theupper part of the section (siltstone and grayclay) is not particularly surprising. Althoughthe claystone and gray clay are more similar toeach other in terms of lithology than they are tothe siltstone, the reason for their differingsporomorph assemblages is attributed to theirdifferent depositional environments andassociated variable catchment areas.

The higher number of sporomorphs withinthe claystone unit is a result of a combinationof ecological and non-ecological variables.Non-ecological variables include lithologicalfactors, sedimentation rates and the depositional

environment. Although governed by thedepositional environment, the lithology itselfcan impact on the preservation of spor-omorphs and therefore result in apparenttemporal floral variation. Spores and pollencan be considered as sedimentary particlesduring transportation and depositional pro-cesses; hence certain taxa are preferentiallypreserved based on factors such as particlesize, particle shape and durability (Traverse2007). Thus, changes in transportation anddepositional processes between claystone,siltstone and gray clay units would havepresumably resulted in the preferentialpreservation of particular taxa based on thesephysical factors. The preservation potential ofsporomorphs is generally increased whensediment grain size is reduced and sedimenta-tion rates are slower (Traverse 2007).Therefore, the high number of sporomorphsand high diversities within the claystone areprobably a result of the relatively small grainsize and slow sedimentation rates of this unit.Conversely, the siltstone is considered tohave been deposited more rapidly than theclaystone, thus the lower abundances anddiversities of coniferous pollen within thesiltstone are probably a result of faster sedi-mentation rates, as there would have been lesstime for such pollen rain to accumulate. Thelower diversities within the siltstone comparedto the claystone are also a consequence of thesmaller catchment area supplying this depositwith sporomorphs. Specifically, the channelenvironment of the siltstone would have pre-sumably had fewer tributaries feeding thisdeposit compared to the numerous rivers thatwould have potentially flowed into the coastalplain setting of the claystone.

Spicer and Hill (1979) postulated thatdifferential rates of compaction could haveaffected floral compositions between lithologies.This could have had some influence on abun-dances; however, there is no conclusivesedimentological evidence that suggestscompaction was vastly different betweenlithologies. The differential sedimentation ratesand sediment grain sizes between depositionalenvironments are considered to be far largercontributors to such abundance differences.For example, diversity is likely to be lower




within the fluvial siltstone (compared tothe same unit of claystone), because it wasdeposited more rapidly.Ecological Causes for Variation between

Sporomorph and Plant Megafossil Assemblages.Table 1 shows the generalized sporomorphdispersal methods of the dominant plantgroups through the Hasty Bank plant bed.The majority of plant groups in Table 1 rely onwind to disperse sporomorphs. The life habitsand reproductive methods of wind dispersedtaxa result in abundance and diversitydiscrepancies between the sporomorph andmegafossil assemblages. Pollen and sporeproduction in wind dispersed species istypically very high as the efficiency of windpollination increases as the concentrationof airborne pollen increases (Regal 1982;Whitehead 1983; Allison 1990; Friedman andBarrett 2009). The sporomorph and megafossilrecords at Hasty Bank reflect this bias; conifersand ferns that produce vast numbers of pollenand spores are considerably more abundant

(Fig. 5) and diverse (Fig. 9) within sporomorphassemblages.

The nature of wind pollination means thatsporomorph assemblages capture spores andpollen from a significantly larger geographicarea compared to plant megafossil assem-blages, which are more representative of thelocal paleoflora (e.g., Prentice 1985). Many ofthe coniferous species within the sporomorphassemblage are therefore potentially notrepresentative of the flora close to the site ofdeposition and are possibly more indicative ofupland communities.

The physical size of parent plants alsoimpacts on the composition of sporomorphassemblages. Pollen released at elevated heightsincreases dispersal distances as: (1) wind speedsare greater, (2) pollen remains within theairstream longer, and (3) there is usually lessintervening vegetation to intercept pollen(Levin andKerster 1974; Okubo and Levin 1989;Friedman and Barrett 2009). Many coniferoussporomorph taxa from the Hasty Bank plant

TABLE 1. Sporomorph dispersal methods, sporomorph production levels, and relative parent plant heights of modernequivalents of the major plant groups in the Hasty Bank plant bed.

Plant groupTypical sporomorphdispersal method

Typical sporomorphproduction level

Typical modern equivalentparent plant heights References

Bryophyta Mostly wind Relatively high Very low (Some epiphytic) Pohjamo et al. 2006Lycopsida Mostly wind High Low (Some epiphytic) Brack-Hanes 1981;

Traverse 2007Sphenophyta Wind High Relatively small van Konijnenburg-van

Cittert and Morgans1999

Ferns Mostly wind Very High Variable, mostly low(Some epiphytic)

Durand and Goldstein2001

Pteridosperms Mostly wind, somepotentially insect

Labandeira et al. 2007

Caytoniales Wind and ?insect Harris 1933, 1945;Schwendemann et al.2007; Ren et al. 2009;Labandeira 2010

Coniferales Wind Very high Variable, mostly very tall Critchfield 1985; vanKonijnenburg-vanCittert and Morgans1999

Cycadales Wind and insect Variable (Relatively highin wind pollinatedtaxa, low in wind andinsect pollinated taxa)

Variable, low tomoderately tall

Norstog 1987

Ginkgoales Wind and ?insect High Very tall Del Tredici 1989; vanKonijnenburg-vanCittert 2010; Crane2013; Bhowmik andParveen 2014

Bennettitales Wind and ?insect Crepet et al. 1991




bed originate from large trees; this is aprincipal factor contributing to their highabundances within sporomorph assemblages.

Harris (1964) described the pollen organPteroma thomasi from Hasty Bank and asso-ciated this with the pteridospermP. papillosa based on their similar cuticles andco-occurrence. The pollen of P. thomasi ismost similar to the wind dispersed pollen ofAlisporites thomasii (Harris 1964; Ziaja 2006).Spicer and Hill (1979) demonstrated that theparent plant, P. papillosa is markedly moreabundant within the claystone, however thedispersed pollen, A. thomasii does not recordthis abundance change and is low in abundancethroughout the section. Temporal changes inlocal vegetation are generally less well recordedamong wind dispersed taxa in the sporomorphrecord as such taxa are captured fromlarger geographic areas than correspondingmegafossils.

Animal-plant interactions could also be acause of inconsistencies between sporomorphand plant megafossil assemblages. Potentialinsect assisted pollination in Caytoniales(Harris 1945; Labandeira 2010) could be acause of the underrepresentation of such pollenin sporomorph assemblages (Figs. 5 and 9) aspollen production in insect pollinated plants istypically very low compared to wind dis-persed taxa (e.g., Norstog 1987).

Cycads and Bennettitales display markedlylower abundances (Fig. 5) and diversities(Fig. 9) within the sporomorph record. Thereproductive methods of these plants differconsiderably from the exclusively wind dis-persed conifers and ferns. Some moderncycads rely on a combination of wind andinsect pollination (Niklas and Norstog 1984;Clark and Clark 1987; Tang 1987; Norstog andFawcett 1989; Ornduff 1990; Pellmyr et al. 1991;Wilson 2002; Kono and Tobe 2007; Terry et al.2007), and both fossil cycads and Bennettitalesdisplay early evidence of possible insect polli-nation (Crepet et al. 1991; Klavins et al. 2005;Labandeira et al. 2007). Pollen productionvaries greatly among modern cycads, depend-ing on whether wind or insect pollination isdominant (Norstog 1987). Kono and Tobe(2007) demonstrated that the pollen of themodern cycad, Cycas revoluta occurs only in

abundance within very close proximity (~2m)to the cones fromwhich it is released. If Jurassiccycads share such a characteristic, cycad pollenwould almost certainly be underrepresented inthe sporomorph assemblage.Nilssonia kendalliaeis the most common species within megafossilcounts, constituting ~29% of the total assem-blage (Spicer and Hill 1979). Harris (1964)presumed N. kendalliae to be of cycad orpteridosperm affinity. In situ pollen studieshave associated the pollen Androstrobus withN. kendalliae (van Konijnenburg-van Cittert1968) and more generally Nilssoniaceae(Hill 1990). In situ Androstrobus pollen isconsidered to be equivalent to dispersedChasmatosporites pollen (Balme 1995). As agenus Chasmatosporites constitutes only ~2.9%of the total sporomorph assemblage. Thisabundance discrepancy is interpreted to be dueto low pollen production and small dispersalranges of cycads compared to many of thewholly wind pollinated plants. Hence, the spe-cialized reproductive nature of cycads, Bennet-titales and potentially Caytoniales (Delevoryas1963; Harris 1974; Labandeira 2010; Manderet al. 2010) is probably the principal factor con-tributing to their underrepresentation in spor-omorph assemblages.

The underrepresentation of cycads, Bennet-titales and ginkgos in the sporomorphdiversity record is potentially compounded byrecognition biases. The leaves of these groupstypically possess distinctive morphologicalfeatures that enable easy differentiation togeneric and species level (Lidgard and Crane1990), thus the diversity of such taxa in themegafossil assemblage is high. However,the pollen of these groups is often simple andmonosulcate, with little morphological andsculptural variation visible under light micro-scopy (Frederiksen 1980). Sporomorph speciestherefore potentially represent numerous parentplant species, thus diversity in the dispersedsporomorph record is underrepresented.

The high diversities of lycopsids, ferns andbryophytes in sporomorph assemblages com-pared to megafossil assemblages (Fig. 9) couldbe related to epiphytic communities. Epiphytesare generally poorly represented in themegafloral record as the burial and subsequentfossilization of such species is unlikely




compared to most other plants (Schneiderand Kenrick 2001; Frahm and Newton 2005;Tstutsumi and Kato 2006; Schuettpelz andPryer 2007, 2009; Dubuisson et al. 2009;Penika and Oplutil 2013). In contrast,sporomorphs released from epiphytes donot experience this bias, thus diversities ofepiphytic groups are comparatively unaffectedin the sporomorph record.

The absence of lycopsids and low diversityof ferns in megafossil assemblages couldalso be linked to the relatively low preser-vation potential of many non-arborescentspecies. Scheihing (1980) demonstrated thatnon-arborescent taxa are frequently under-represented in the megafloral record as a resultof: (1) the increased biomass of arborescentspecies, (2) difficulty in recognition of non-arborescent plant parts, and (3) shielding of thenon-arborescent understory by the arborescentcanopy during high energy transport anddepositional processes.Variation in Plant and Sporomorph Durability

CausingMegafossilSporomorph Inconsistencies.Many of the inconsistencies between the plantmegafossil and sporomorph assemblages areinterpreted to be a result of differences indurability between parent plants andassociated sporomorphs.

Equisetum columnare is the second mostabundant plant species in megafossil assem-blages, constituting ~19% of the megaflora(Spicer and Hill 1979). However, its corre-sponding microspore, C. mesozoica representsonly ~2.5% of the sporomorph assemblage.This discrepancy is probably due to the highlydurable nature of Equisetum, which means thatthis genus is overrepresented compared toother megafloral taxa. Conversely, C. mesozoicahas a low preservation potential due to its thinwall and low sporopollenin content (Traverse2007; Grauvogel-Stamm and Lugardon 2009),thus this species is underrepresented in thesporomorph assemblage. These combined fac-tors give rise to a notable differential pre-servation potential between the parent plantand sporomorph.

The high diversity of lycopsids (16 taxa) insporomorph assemblages and their absencefrom megafossil assemblages suggests anextremely low preservation potential for

lycopsid remains within this deposit. The samepattern is also present within TriassicJurassicplant beds from East Greenland (Mander et al.2010, 2013) and Lower Jurassic deposits ofOdrow, central Poland (Ziaja 2006). Lycop-sids are notable both for their diversity in themegaspore record of the Middle Jurassicdeposits of Yorkshire (reviewed in Slater et al.2015) and for their lack of megafossils withinthese deposits. Harris (1961b) summarizesstudies on the lycopsid megafossil Selaginellitesfalcatus. With the exception of this species thereare no other convincing reports of lycopsidmegafossils from the Middle Jurassic of York-shire (Lindley and Hutton 1833; Hill et al. 1985;Schweitzer et al. 1997). The absence of lycopsidmegafossils may also be exacerbated by thelack of recognition of delicate lycopsid remains(Skog and Hill 1992).

Are Sporomorphs or Plant Megafossils MoreInformative regarding Paleofloristic Recon-structions?Comparison of palynologicaland plant megafossil records demonstratesthat respective data sets reflect differentaspects of the paleoflora as they preferentiallypreserve certain taxa based on a multitude ofecological and non-ecological variables. Suchvariables include spore/pollen and plantdurability, absolute abundances of plantspecies in life, proximity of parent plants todepositional location, spore/pollen dispersalmethods, spore/pollen dispersal distances,absolute numbers of spores/pollen releasedfrom parent plants, transportation distances,transportation processes, climatic variations,and the depositional environment. This studyillustrates that sporomorphs preserve someaspects of the paleoflora more completely(mostly wind dispersed taxa) than megafossilassemblages. However, megafossil assemblagesequally preserve other aspects of the paleoflora(mostly reproductively specialized taxa) morecompletely than sporomorph assemblages.

Direct quantitative comparative studies ofdispersed sporomorph and plant megafossilassemblages from pre-angiosperm Mesozoicfloras are relatively uncommon in the literaturedue to the rarity of such fossil sites (e.g.,Pedersen and Lund 1980; Ziaja 2006; Manderet al. 2010). Most previous studies thatincorporate dispersed sporomorphs and plant




megafossils are confined to Paleozoic (e.g., LooyandHotton 2014), Cretaceous (e.g., Lidgard andCrane 1990; Bercovici et al. 2008, 2009) andCenozoic (e.g., Tinner et al. 1996; Wingand Harrington 2001) floras. These floras arefundamentally different to pre-angiospermMesozoic communities, thus comparison ofsuch floras with those at Hasty Bank is highlyproblematic. Rare examples where combinedsporomorph and megafossil data have beenused in vegetation reconstructions frompre-angiosperm Mesozoic floras demonstrateconsistency with findings from Hasty Bank.Specifically, conifers and ferns are typically wellrepresented in sporomorph assemblages (Janaand Hilton 2007), cycads, Bennettitales andpteridosperms are generally well represented inmegafossil assemblages (Pedersen and Lund1980; Gtz et al. 2011), and bryophytes andlycopsids are often confined to sporomorphassemblages (Ziaja 2006; Mander et al. 2010,2013). Such large discrepancies between parentplant and dispersed sporomorph assemblagesquestions the reliability of local vegetationreconstructions based on megafossil or spor-omorph evidence in isolation and suggests thatwhere possible a combined approach is con-siderably more informative.

Conclusions

Variation in sporomorph assemblagesthrough the Hasty Bank plant bed is the resultof a change in depositional setting between thethree lithological units. Changes in the deposi-tional environments consequently influencethe vegetation, catchment areas, and preserva-tion potential of sporomorphs and plantmegafossils; hence the fossil assemblages varynotably between lithologies. Discrepanciesbetween sporomorph and plant megafossilassemblages are primarily a result of thedifferent life habits and reproductive strategiesemployed by parent plants. Such differencesoften cause large variation in sporomorphproduction and dispersal distances. Differen-tial preservation potentials between parentplants and associated spores/pollen also hasa substantial impact on generating inconsis-tencies between sporomorph and plant mega-fossil data sets. This is particularly apparent

regarding the absence of lycopsids and theelevated abundances of E. columnare in plantmegafossil assemblages (Spicer and Hill 1979).Based on the results at Hasty Bank and similarstudies (e.g., Ziaja 2006; Mander et al. 2010;2013), explaining discrepancies between spor-omorph and plant megafossil assemblagesrequires considerable analysis and there is nobest method of reconstructing paleofloras.Assemblage compositions are the product of acomplex array of biological, geographical,and depositional factors that act inconsistentlybetween and within sporomorph andmegafossil assemblages, resulting in notabledisparities between respective data sets.Refining parent plant affinities with spore andpollen in situ studies will aid in futurereconstructions of paleofloras using dispersedsporomorphs.

Acknowledgments

We thank J. L. Clark and C. R. Hill for theirhelp with collection of samples. C. R. Hill alsoprovided useful comments throughout thisresearch. B. J. Slater and T. M. Young alsohelpedwith fieldwork. S. Ellin provided labora-tory support. D. Cameron provided advice onstatisticalmethods. This research forms part of aPh.D. studentship to SMS at the University ofSheffield funded by a Natural EnvironmentResearch Council CASE Award with RoyalDutch Shell, jointly supervised by C. H.Wellman and I. Prince. Finally, we thankM. Kowalewski, A. Bercovici and three anon-ymous reviewers for their comments, whichsignificantly improved this paper.

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