Introduction
Understanding the causes of temporal variation in paleofloras is a fundamental objective of paleobotany. Extracting these causes is, however, frequently problematic as it is often difficult to determine the dominant controls on the constituents of fossil assemblages. Such controls include ecological, climatic, depositional, and preservational factors. Establishing the causes of paleofloristic temporal variation is clearly enhanced when a multidisciplinary approach is used, as the overreliance on singular lines of evidence can often lead to over interpretation. Comparing data from the plant megafossil and terrestrial palynological records can provide important insight into ecological and preservational biases that can shape diversity (richness) and abundance patterns of these fossil assemblages. Thus, such comparisons can help to determine the reliability of paleofloristic interpretations based on the spore and pollen (sporomorph) and plant megafossil records in isolation.
The sequences of North Yorkshire, UK offer a rare example of extensive Middle Jurassic terrestrial deposits. The sedimentary successions have previously been studied in detail for their paleontological significance (e.g., Romano and Whyte Reference Romano and Whyte2003) and particularly for the famous plant beds that are scattered throughout these sequences (e.g., Black Reference Black1929; van Konijnenburg-van Cittert Reference van Konijnenburg-van Cittert1968, Reference van Konijnenburg-van Cittert1975, Reference van Konijnenburg-van Cittert1996, Reference van Konijnenburg-van Cittert2008; Crane and Herendeen Reference Crane and Herendeen2009; Spicer and Hill Reference Spicer and Hill1979; van Konijnenburg-van Cittert and Morgans Reference van Konijnenburg-van Cittert and Morgans1999). Although plant megafossil studies from these deposits are common in the literature (e.g., Harris Reference Harris1941, Reference Harris1944, Reference Harris1952, Reference Harris1953, Reference Harris1961a,Reference Harrisb, Reference Harris1964, Reference Harris1969, Reference Harris1978, Reference Harris1979; van Konijnenburg-van Cittert Reference van Konijnenburg-van Cittert1972, Reference van Konijnenburg-van Cittert1975, Reference van Konijnenburg-van Cittert1978, Reference van Konijnenburg-van Cittert1981, Reference van Konijnenburg-van Cittert1989, Reference van Konijnenburg-van Cittert1996, Reference van Konijnenburg-van Cittert2008; Harris et al. Reference Harris, Millington and Miller1974; Spicer and Hill 1979; Hill Reference Hill1990; Morgans Reference Morgans1999; van Konijnenburg-van Cittert and Morgans Reference van Konijnenburg-van Cittert and Morgans1999), published dispersed spore and pollen investigations remain comparatively sparse (e.g., Couper Reference Couper1958; Chaloner Reference Chaloner1968; Chaloner and Muir Reference Chaloner and Muir1968; Riding Reference Riding1984; Riding and Wright 1989; Gowland and Riding 1991; Boulter and Windle Reference Boulter and Windle1993; Hubbard and Boulter Reference Hubbard and Boulter1997; Butler et al. Reference Butler, Charnock, Hager and Watkins2005; Srivastava Reference Srivastava2011).
This study provides a detailed palynological assessment of a plant bed from Hasty Bank, North Yorkshire, UK that was previously analyzed for its plant megafossil contents. Spicer and Hill (Reference Spicer and Hill1979) carried out a comprehensive quantitative study of this plant bed in which they sampled contiguous plant megafossil census counts through a through a 5 m outcrop section. Plant megafossil counts were carried 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 then multiplied accordingly so that all abundances correspond to a 50×50×20 cm3 block of sediment. The histogram of their results is provided in Supplementary Figure 1. Sporomorph quantitative data was analyzed here from the same section discussed in Spicer and Hill (Reference Spicer and Hill1979) in order to compare this with the quantitative plant megafossil data. Palynofacies analysis was also carried out in order to aid environmental reconstructions (Tyson Reference Tyson1995). Paleofloristic comparisons of sporomorph and plant megafossil data are possible due to comprehensive in situ spore/pollen (e.g., van Konijnenburg-van Cittert Reference van Konijnenburg-van Cittert1968, Reference van Konijnenburg-van Cittert1971, Reference van Konijnenburg-van Cittert1978, Reference van Konijnenburg-van Cittert1981, Reference van Konijnenburg-van Cittert1989, Reference van Konijnenburg-van Cittert1993, Reference van Konijnenburg-van Cittert2000; Pedersen et al. Reference Pedersen, Crane and Friis1989; Hill Reference Hill1990; Osborn and Taylor Reference Osborn and Taylor1993; Balme Reference Balme1995; Friis and Pedersen Reference Friis and Pedersen1996; Yang et al. Reference Yang, Friis and Zhou2008) and ultrastructural transmission electron microscope (TEM) studies (e.g., Batten and Dutta Reference Batten and Dutta1997) which means that the majority of Middle Jurassic sporomorphs can now be assigned at least to family level plant classification.
By comparing dispersed spore/pollen assemblages with plant megafossil data in association with palynofacies analysis it was anticipated that a more realistic paleofloristic and paleoenvironmental interpretation would be possible and potential discrepancies in data sets would help to explain preservational biases between sporomorph and plant megafossil records. Since sporomorph and plant megafossil assemblages undergo different transportation and depositional processes, it was expected that the respective fossil assemblages would be notably dissimilar in composition (e.g., Bercovici et al. Reference Bercovici, Nichols and Pearson2008, Reference Bercovici, Pearson, Nichols and Wood2009), with the anticipation that our analyses may shed light on the reasons behind these differences.
Geological Setting
The Mesozoic sequences of the Cleveland Basin, northeast England (Fig. 1) have been intensively studied since the early nineteenth century (e.g., Young and Bird Reference Young and Bird1822) and offer important insight into both terrestrial and marine environments of this time. Middle Jurassic sediments of the Cleveland Basin are dominated by the chiefly terrestrial sequences of the Ravenscar Group (Fig. 2). Regional uplift and associated relative sea level fall led to the deposition of extensive fluviodeltaic sequences derived from upland areas surrounding the Cleveland Basin. Marine beds occur sporadically throughout the Ravenscar Group as a result of marine incursions from the south and east (Hemingway and Knox Reference Hemingway and Knox1973; Knox Reference Knox1973; Hemingway Reference Hemingway1974; Nami and Leeder Reference Nami and Leeder1978; Leeder and Nami Reference Leeder and Nami1979; Hancock and Fisher Reference Hancock and Fisher1981; Livera and Leeder Reference Livera and Leeder1981; Fisher and Hancock Reference Fisher and Hancock1985; Kantorowicz Reference Kantorowicz1985; Alexander Reference Alexander1989, Reference Alexander1992; Riding and Wright Reference Riding and Wright1989; Gowland and Riding Reference Gowland and Riding1991; Rawson and Wright Reference Rawson and Wright2000; Powell Reference Powell2010). The Ravenscar Group provides an exceptional example of extensive Middle Jurassic terrestrial sequences and the plethora of plant fossils (e.g., van Konijnenburg-van Cittert and Morgans Reference van Konijnenburg-van Cittert and Morgans1999) and dinosaur footprints (Whyte and Romano Reference Whyte and Romano1993, Reference Whyte and Romano2001a,Reference Whyte and Romanob; Romano et al. Reference Romano, Whyte and Manning1999; Romano and Whyte Reference Romano and Whyte2003; Whyte et al. Reference Whyte, Romano, Hudson and Watts2006, Reference Whyte, Romano and Elvidge2007, Reference Whyte, Romano and Watts2010) make the Cleveland Basin an important region for paleontology.
Figure 1 Location and geological setting of Hasty Bank, northeast England. Modified from Milsom and Rawson (Reference Milsom and Rawson1989); Mjøs and Prestholm (Reference Mjøs and Prestholm1993); Cox and Sumbler (Reference Cox and Sumbler2002); Palliani and Riding (Reference Palliani and Riding2000); Slater et al. (Reference Slater, Taylor, Batten, Hill and Wellman2015).
Figure 2 Subdivision of Middle Jurassic sequences of the North Yorkshire Coast. Marine units shaded. The arrow indicates the stratigraphic position of the plant bed. Modified from Rawson and Wright (Reference Rawson and Wright2000); Slater et al. (Reference Slater, Taylor, Batten, Hill and Wellman2015).
The plant bed under investigation is located on the northern slope of Hasty Bank (NZ 567 037), situated within the northwest region of the North York Moors National Park, northeast England. The plant bed occurs at the base of the Aalenian Saltwick Formation, stratigraphically located at the base of the Ravenscar Group (Fig. 2) and lies unconformably above the marine Dogger Formation. The bed is approximately 7 m thick and has previously yielded a varied flora of 90 species (Hill and van Konijnenburg-van Cittert Reference Hill and van Konijnenburg-van Cittert1973; Hill Reference Hill1974; Spicer and Hill Reference Spicer and Hill1979). Two lithologies dominate the plant bed (Fig. 3), a claystone that forms the lower part of the section and a siltstone that occupies the majority of the upper part of the section. An erosional surface is present between the claystone and the siltstone. A thin lens of dark gray clay is also present at the top of the section (Hill and van Konijnenburg-van Cittert Reference Hill and van Konijnenburg-van Cittert1973; Hill Reference Hill1974).
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 van Konijnenburg-van Cittert (Reference Hill and van Konijnenburg-van Cittert1973); Spicer and Hill (Reference Spicer and Hill1979); van Konijnenburg-van Cittert and Morgans (Reference van Konijnenburg-van Cittert and Morgans1999).
The claystone is uniform dark gray in color. Grain size is homogeneous through the unit and thin (1–3 mm scale) horizontal laminae are abundant. The rock is relatively soft and breaks apart easily along laminae, which often reveal highly abundant fragmentary plant megafossils.
The siltstone is a homogeneous medium gray color. Grain size is uniform through the unit and larger, more prominent horizontal laminae (5–10 mm scale) are present. The rock is harder than the claystone and fragmentary plant megafossils are abundant, although less so than in the claystone. Horizontal roots occur in low abundance within the basal ~1 m of the siltstone, these are typically less than 2 cm in length and ~2 mm in width.
The gray clay at the top of the section is a homogeneous very dark gray color. Grain size is uniform through the unit and no sedimentary structures are visible. The rock is very soft and not fully lithified. Plant megafossils are less common in the gray clay than in the claystone and siltstone.
Previous Interpretations of the Depositional Environments at Hasty Bank
The Hasty Bank plant bed was first recognized as an important fossil locality by Black (Reference Black1929). Subsequent paleobotanical and paleoecological studies have commented on possible environments of deposition for the plant bed, most notably by Harris (Reference Harris1964), Hill and van Konijnenburg-van Cittert (Reference Hill and van Konijnenburg-van Cittert1973), Hill (Reference Hill1974), and van Konijnenburg-van Cittert and Morgans (Reference van Konijnenburg-van Cittert and Morgans1999).
Harris (Reference Harris1964) postulated that the claystone at the base of the plant bed (Fig. 3) was deposited in a coastal environment periodically flooded by seawater. These interpretations were based on the occurrence of the pteridosperm Pachypteris papillosa in association with rare marine microfossils thought to be derived from marine flooding events. Harris (Reference Harris1983) reconstructed P. papillosa as a large shrub that formed mangrove-like thickets along tidal rivers. Spicer and Hill (Reference Spicer and Hill1979) showed that P. papillosa is markedly more abundant within the claystone deposit compared to the rest of the section.
The siltstone is interpreted as the peripheral fringes of a large channel sandstone deposit immediately adjacent to the southeast of the plant bed (Fig. 3). Hill and van Konijnenburg-van Cittert (Reference Hill and van Konijnenburg-van Cittert1973) concluded that the siltstone was deposited in the slower flowing region of the channel. It is possible that the siltstone could however represent a levee or floodplain deposit peripheral to the sandstone. The channel has cut into the underlying sediments forming an erosional surface between the claystone and the siltstone and thus there is a time gap between these deposits.
Previous depositional environmental interpretations for the gray clay are lacking. However, sedimentological, sporomorph and palynofacies evidence from this study suggests that this deposit represents a swamp or an abandoned channel.
Materials and Methods
Collection
A total of 50 samples (HB1–HB50, numbered in reverse stratigraphic order, i.e., HB1 is at the top of the section) were collected at 10 cm vertical intervals from the main Hasty Bank plant bed (NZ 567 037) for palynological processing. Samples were taken from the identical section of that discussed by Spicer and Hill (Reference Spicer and Hill1979), shown in Figure 3. Christopher R. Hill (of Spicer and Hill [Reference Spicer and Hill1979]) was present during collection of samples to ensure the exact position of the section was located. Sampling required the excavation of approximately 50 cm of modern deposits to access the outcrop. The exterior of the outcrop was weathered between 5 and 20 cm deep into the rock. The section was therefore excavated a further ~30 cm into the outcrop to ensure fresh exposure. Samples HB1–HB3 are from the gray clay at the top of the section; HB4–HB28 are from the siltstone unit; and HB29–HB50 are from the claystone unit.
Processing
Dry rock samples were weighed at 20 g before being dissolved in 40% hydrochloric acid for at least 24 hours to remove carbonates, followed by two week maceration in 40% hydrofluoric acid to remove silicates. Samples were agitated every two days to ensure full break down of rock material. Samples were then decanted and fresh water added, repeating the process until neutral. One day was left between decants to ensure minimal loss of palynomorphs. Two Lycopodium tablets (produced by the University of Lund, Sweden; batch 1031) were added before sieving at 10 µm. Centrifuging residues in zinc chloride was then undertaken to remove heavy minerals. Residues were then sieved again at 10 µm to remove the heavy liquid and final residues were spread across cover slips and gently heated on a hot plate to remove excess water. Cover slips were then mounted onto slides using epoxy resin on a hot plate. All materials (rock samples and slides) are housed in the collections of the Centre for Palynology at the University of Sheffield.
Counting
Slides were examined under a Meiji Techno (MA151/35/50) light microscope. A minimum of 200 indigenous Jurassic sporomorphs were counted from each sample in addition to any Lycopodium spores from tablets in order to assess the relative organic richness of samples. The Lycopodium tablets contain a known quantity of spores (20,848±1546 spores per tablet). This allows the palynomorph productivity of each sample to be assessed when counting sporomorphs, as numbers of Lycopodium spores can be compared with numbers of indigenous Jurassic sporomorphs to assess the palynomorph richness of samples. In this study, increased numbers of Lycopodium correspond to a decrease in palynomorph productivity per unit of sediment. Counts were carried out in systematic traverses across slides to ensure no grains were missed. For presence/absence data, the remainder of the slide was then examined in the same fashion to identify species that were not present in the count data. The complete raw data set is provided in Supplementary Table 1. For sporomorph images that refer closely to the taxonomic identifications used in this study, see Couper (Reference Couper1958), Boulter and Windle (Reference Boulter and Windle1993) and Srivastava (Reference Srivastava2011).
Palynofacies Analysis
The term palynofacies typically refers to all of the visible organic particles (usually 2–250 µm in size) that occur within palynological maceration residues (Traverse Reference Traverse2007). Palynofacies analysis is commonly used to assess depositional environments (e.g., Parry et al. Reference Parry, Whitley and Simpson1981; Boulter and Riddick Reference Boulter and Riddick1986; Van der Zwan Reference Van der Zwan1990; Brugman et al. Reference Brugman, Van Bergen and Kerp1994; Oboh-Ikuenobe and Yepes Reference Oboh-Ikuenobe and Yepes1997; Oboh-Ikuenobe et al. Reference Oboh-Ikuenobe, Obi and Jaramillo2005; Carvalho et al. Reference Carvalho, Filho and Menezes2006). In this study, palynofacies analysis attempts to provide a more in depth interpretation of the depositional environments at Hasty Bank. A similar palynofacies classification scheme to Tyson (Reference Tyson1995) and Batten and Stead (Reference Batten and Stead2005) was used to categorize organic matter. Categories for palynofacies debris are: spores; pollen; algae; dinoflagellate cysts; acritarchs; humic debris; amorphous organic matter (AOM); Botryococcus; structured vitrinite; unstructured vitrinite; cuticle; and inertinite. Counts of 200 palynodebris were carried out on all samples, the complete raw data set is provided in Supplementary Table 2.
Statistical Analysis
Correspondence analysis (CA) and nonmetric multidimensional scaling (NMDS) were performed on sporomorph data sets to further understand the causes of floral variation through the Hasty Bank plant bed. Correspondence analysis and NMDS are ordination methods that plot complex multivariate data onto a minimal number of axes (e.g., Jardine et al. Reference Jardine, Harrington and Stidham2012). Correspondence analysis is an eigenvector method of ordination that produces a graphical representation of a contingency table (Spicer and Hill Reference Spicer and Hill1979). Nonmetric multidimensional scaling is a nonparametric ordination technique that uses ranked distances between samples to assess the degree of similarity between samples (Chatfield and Collins Reference Chatfield and Collins1980; ter Braak Reference ter Braak1995; Legendre and Legendre Reference Legendre and Legendre2012; Hammer and Harper Reference Hammer and Harper2006; Jardine et al. Reference Jardine, Harrington and Stidham2012), hence clustering of samples in ordination space indicates high compositional similarity between those samples. For comprehensive descriptions of CA and NMDS see Greenacre (Reference Greenacre2007) and Cox and Cox (Reference Cox and Cox2001), respectively. Correspondence analysis and NMDS are becoming increasingly used in palynological analysis of quantitative paleoecological studies (e.g., Kovach Reference Kovach1989, Reference Kovach1993; Wing and Harrington Reference Wing and Harrington2001; Hammer and Harper Reference Hammer and Harper2006; Bonis and Kürschner Reference Bonis and Kürschner2012; Jardine et al. Reference Jardine, Harrington and Stidham2012; Stukins et al. Reference Stukins, Jolley, McIlroy and Hartley2013) as such methods allow the user to extract information on the major causes of variation from complex data sets. Correspondence analysis was chosen over detrended correspondence analysis (DCA) as this method was employed by Spicer and Hill (Reference Spicer and Hill1979) on megafossil data, thus to allow comparison of ordinations we used the same technique here. Furthermore, CA ordinations do not indicate the need for DCA. Both CA and NMDS are used here to assess abundance and presence/absence data. For NMDS ordinations, the Bray-Curtis dissimilarity metric was used to generate distances between samples as this method is considered to perform well in ecological analyses (e.g., Minchin Reference Minchin1987; Harrington Reference Harrington2008; Bowman et al. Reference Bowman, Francis, Askin, Riding and Swindles2014). Repeated runs were carried out for two dimensions until a convergent solution was established. Principal components rotation and centering was then applied to the final ordination. Nonmetric multidimensional scaling ordinations were performed using R, version 3.1.2 (R Core Team 2014), within the package “vegan”, version 2.2-1 (Oksanen et al. Reference Oksanen, Blanchet, Kindt, Legendre, Minchin, O’Hara, Simpson, Solymos, Stevens and Wagner2015). Sporomorph relative abundances have been transformed logarithmically for CA and NMDS. This procedure condenses the differences in scores between abundant and rare species between samples, thus reducing the impact of highly abundant taxa on the data set and also reducing statistical noise. Spicer and Hill (Reference Spicer and Hill1979) suggested that the most effective way to assess megafossil data in ordinations was to logarithmically transform abundances. Species that are present in samples but not in counts have been excluded from logarithmically transformed relative abundance ordinations. For taxonomic CA the same data has been used with the exclusion of species that contribute less than 1% of the total count to eliminate statistical noise. Presence/absence CA and NMDS were performed to assess co-occurrence and compositional variation between samples. For presence/absence analyses all species are included; species that are present are scored as 1, species that absent are scored as 0. Spiked Lycopodium data has been excluded from all ordinations. The statistical program PAST (Hammer et al. Reference Hammer, Harper and Ryan2001) was used to create CA plots.
Results
Sporomorph Diversity (Richness) and Abundance Variation
A total of 67 sporomorph taxa were recognized from 50 samples; the entire taxonomic list with associations between sporomorphs and parent plant groups is provided in Supplementary Table 3. The commonly used Chao2 species richness estimator (Colwell and Coddington Reference Colwell and Coddington1994) gave a species estimate of 69.94 taxa (standard deviation = 3.36) for the entire data set, which suggests the data set was not severely undersampled. Diversities (richness) using presence/absence data (Fig. 4B) are highest within the claystone; diversity (richness) is slightly lower within the siltstone assemblage and lower again in gray clay samples. This indicates that the claystone was deposited at a time of increased floral diversity and/or claystone samples are capturing more sporomorph taxa due to preservational biases.
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 is shown as a percentage of the indigenous Jurassic taxa plus the spiked Lycopodium itself. B, Sporomorph diversity (richness) for all samples.
Relative abundances of all taxa are displayed graphically in Supplementary Figure 2. Relative abundances of the ten most abundant sporomorph taxa are provided in Figure 4A. Sporomorphs have been grouped into their botanical affinities in Figure 5A to extract information on higher taxonomic level temporal floral variations. Spiked Lycopodium spores exhibit the most prominent abundance variation through the sequence. Abundant spiked Lycopodium in gray clay (HB1–HB3) and siltstone (HB4–HB28) samples indicates that claystone samples (HB29–HB50) preserve far greater numbers of indigenous Jurassic sporomorphs.
Figure 5 Relative abundance charts of (A) sporomorphs and (B) plant megafossils (Spicer and Hill Reference Spicer and Hill1979) grouped into their respective botanical affinities. Relative abundances of Jurassic sporomorph taxa are calculated from counts of at least 200 grains. Spiked Lycopodium count data is shown as a percentage of the indigenous Jurassic taxa plus the spiked Lycopodium itself.
Bryophytes (Fig. 5A) are represented by two species and are in low abundance throughout the section. Lycopsids are highly diverse (16 taxa) and abundances are low throughout the plant bed, but slightly higher in the siltstone compared to the claystone and gray clay. Sphenophytes are represented by Calamospora mesozoica and abundances are low and relatively constant throughout the section. Ferns are the most diverse group (23 taxa) and are highly abundant through the plant bed. Abundances increase from the claystone into the siltstone and increase again into the gray clay. Pteridosperms are represented by two species of Alisporites; diversity and abundances are low throughout the section, but slightly higher in the siltstone compared to the claystone and gray clay. Caytoniales are represented by the single species Vitreisporites pallidus; similarly to pteridosperms, abundances are low throughout the section, but slightly higher in the siltstone compared to the claystone and gray clay. Conifers constitute the most abundant group and are highly diverse (14 taxa). Abundances decrease markedly from the claystone into the siltstone and decrease again within the gray clay. Relative abundance variation within the conifers is most apparent within the three most abundant species: Classopollis torosus, Perinopollenites elatoides, and Araucariacites australis (Fig. 4A). Classopollis torosus and P. elatoides are considerably more abundant within claystone samples, whereas A. australis contrasts the overall abundance pattern of the conifers and is more abundant within siltstone samples. Cycadopsida/Ginkgopsida pollen is moderately diverse (four taxa) and displays little change in abundance (Fig. 5A) through the plant bed. Chasmatosporites hians exhibits the most pronounced abundance variation within the Cycadopsida/Ginkgopsida group; C. hians is relatively common in the claystone, but becomes increasingly rare within siltstone samples and is absent above sample HB19. Bennettitales are low in abundance and diversity throughout the plant bed (two taxa). The largest relative abundance variation within the Bennettitales is displayed by Cycadopites carpentieri, which occurs frequently in the basal and upper samples of the section, but is absent from samples HB26 to HB16.
Sporomorph Sample Ordination
CA and NMDS (Fig. 6) reveal samples from claystone, siltstone, and gray clay units are well separated in ordination space, with only minimal overlap. This illustrates that lithology and apparent floristic composition are correlated and perhaps both are responding to the same principal factors of deposition and/or environment. Logarithmically transformed relative abundance (Figs. 6A,C) and presence/absence sample multivariate plots (Figs. 6B,D) demonstrate little clustering of stratigraphically adjacent samples within lithological point clouds, suggesting little temporal floral change and habitat partitioning within each of the three depositional settings. For logarithmically transformed relative abundance CA (Fig. 6A) the variance is 14% for axis one and 9% for axis two. For presence/absence CA (Fig. 6B) the variance is 11% for axis one and 8% for axis two. Gray clay samples display some volatility between relative abundance and presence/absence CA (Figs. 6A,B), which potentially reflects the comparatively low abundance of conifers and low overall diversities within the gray clay. However, in all ordinations (Fig. 6) gray clay samples are consistently more similar to siltstone samples than claystone samples. In NMDS ordinations (Figs. 6C,D) samples of different lithologies are separated predominantly on axis one, suggesting this corresponds to depositional setting. Data for Figure 6 is provided in Supplementary Tables 4–7.
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.
Taxon Correspondence Analysis
The scatter plot of the first two CA axes (43% cumulative variation) of logarithmically transformed relative abundances of taxa reveals four groupings, labeled A–D (Fig. 7A). Eigenvalues and percentage variance data for Figure 7 is provided in Supplementary Table 8. Relative abundances of taxa from clusters A–D have been grouped and plotted in Figure 7B.
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 represent conifers; crosses represent Cycadopsida/Ginkgopsida; triangles represent Bennettitales. Ellipses A–D show clustering of plots. B, Relative abundances of grouped taxa from clusters A–D from Figure 7A.
Cluster A (Fig. 7A) is occupied by P. elatoides (Taxodiaceae [van Konijnenburg-van Cittert and van der Burgh Reference van Konijnenburg-van Cittert and van der Burgh1989; Boulter and Windle Reference Boulter and Windle1993; Balme Reference Balme1995; Couper Reference Couper1958; Mander et al. Reference Mander, Kürschner and McElwain2010; Mander Reference Mander2011]), Callialasporites turbatus (Araucariaceae [Boulter and Windle Reference Boulter and Windle1993]), C. torosus (Cheirolepidiaceae [Harris Reference Harris1979; Boulter and Windle Reference Boulter and Windle1993; Balme Reference Balme1995; Couper Reference Couper1958; Ziaja Reference Ziaja2006; Mander et al. Reference Mander, Kürschner and McElwain2010; Mander Reference Mander2011]), Spheripollenites subgranulatus (Taxodiaceae [Couper Reference Couper1958; Boulter and Windle Reference Boulter and Windle1993]), and Spheripollenites scabratus (Taxodiaceae [Couper Reference Couper1958; Boulter and Windle Reference Boulter and Windle1993]). Taxa from cluster A are considerably more abundant within claystone samples (Fig. 7B). Cluster B (Fig. 7A) is occupied by C. mesozoica (Sphenophyta [Harris Reference Harris1978; Boulter and Windle Reference Boulter and Windle1993; Balme Reference Balme1995]), Dictyophyllidites harrisii (Dipteridaceae/Matoniaceae [Couper Reference Couper1958; Boulter and Windle Reference Boulter and Windle1993; van Konijnenburg-van Cittert Reference van Konijnenburg-van Cittert1993; Guignard et al. Reference Guignard, Wang, Ni, Tian and Jiang2009]), and Marattisporites scabratus (Marattiaceae [Couper Reference Couper1958; Filatoff Reference Filatoff1975; Boulter and Windle Reference Boulter and Windle1993; Balme Reference Balme1995]). Taxa from cluster B display little change in relative abundances through the section (Fig. 7B). Cluster C (Fig. 7A) is occupied by A. australis (Araucariaceae [Couper Reference Couper1958; Boulter and Windle Reference Boulter and Windle1993; Balme Reference Balme1995; Mander et al. Reference Mander, Kürschner and McElwain2010; Mander Reference Mander2011; Bonis and Kürschner Reference Bonis and Kürschner2012]), Pityosporites microalatus (Pinaceae [Couper Reference Couper1958; Boulter and Windle Reference Boulter and Windle1993]), Cerebropollenites mesozoicus (Coniferales [van Konijnenburg-van Cittert and van der Burgh Reference van Konijnenburg-van Cittert and van der Burgh1989; Boulter and Windle Reference Boulter and Windle1993; Balme Reference Balme1995]), Cycadopites minimus (Cycadales/Ginkgoales [Boulter and Windle Reference Boulter and Windle1993; Balme Reference Balme1995; Mander et al. Reference Mander, Kürschner and McElwain2010]), and Deltoidospora minor (Cyatheaceae/Dicksoniaceae/Dipteridaceae/Matoniaceae [Couper Reference Couper1958; Boulter and Windle Reference Boulter and Windle1993; van Konijnenburg-van Cittert Reference van Konijnenburg-van Cittert1993; Guignard et al. Reference Guignard, Wang, Ni, Tian and Jiang2009]). Taxa from cluster C are considerably more abundant within siltstone and gray clay samples (Fig. 7B). Cluster D is occupied by Chasmatosporites apertus (?Cycadales/?Ginkgoales/?Gnetales [Boulter and Windle Reference Boulter and Windle1993; Balme Reference Balme1995]) and Dictyophyllidites equiexinus (Dipteridaceae/Matoniaceae [Boulter and Windle Reference Boulter and Windle1993; van Konijnenburg-van Cittert Reference van Konijnenburg-van Cittert1993]). Taxa from cluster D are more abundant within siltstone samples compared to claystone and gray clay samples (Fig. 7B). Cycadopites carpentieri (Bennettitales [Boulter and Windle Reference Boulter and Windle1993]) plots separately in ordination space from clusters A–D. Figure 7B demonstrates that C. carpentieri is considerably more abundant within claystone samples. Relative abundances of clusters (Fig. 7B) reveal each point cloud (Fig. 7A) has a distinct abundance pattern between different lithological/depositional units that correlate with variation on axis one (29% variation), signifying axis one corresponds to depositional setting.
The conifer dominated clusters A and C represent possible upland communities, as conifers are typical of upland and well-drained settings (e.g., Stukins et al. Reference Stukins, Jolley, McIlroy and Hartley2013). Megafloral studies have hypothesized that Mesozoic ferns were typical of areas of high disturbance and water supply (Harris Reference Harris1961b; van Konijnenburg-van Cittert and van der Burgh Reference van Konijnenburg-van Cittert and van der Burgh1989; Stukins et al. Reference Stukins, Jolley, McIlroy and Hartley2013). The co-occurrence of Sphenophyta and fern spores in cluster B is suggestive of such a community. Ascribing a specific control for axis two (14% variation) with confidence is problematic as this could be one of a myriad of possible factors related to the principal driving mechanisms of vegetation dynamics; differential site availability, species availability and species performance (Picket and Cadenasso Reference Picket and Cadenasso2005; Stukins et al. Reference Stukins, Jolley, McIlroy and Hartley2013).
Palynofacies Analysis
Two palynofacies are recognized that correspond directly with lithological variation, herein referred to as palynofacies 1 and 2. Palynofacies 1 represents the gray clay and siltstone assemblage (samples HB1–HB28) and palynofacies 2 represents the claystone assemblage (samples HB29–HB50). Relative abundances of organic debris are provided in Figure 8. Palynofacies 1 is dominated by inertinite (~43%) and unstructured vitrinite (~31%). Palynofacies 2 comprises a more heterogeneous mixture of organic debris and is co-dominated by AOM (~23%), pollen (~23%), inertinite (~21%) and unstructured vitrinite (~17%).
Figure 8 Relative abundance chart of palynofacies categories.
Comparison of Sporomorph and Plant Megafossil Assemblages
A direct comparison of sporomorph and plant megafossil abundance variation through the plant bed is provided in Figure 5. A comparison of the sporomorph and plant megafossil diversity (richness) data is provided in Figure 9. Sporomorph assemblages are more diverse (67 taxa) than megafossil assemblages (49 taxa in Spicer and Hill Reference Spicer and Hill1979).
Figure 9 Comparative diversities (richness) of the dominant plant groups within sporomorph and plant megafossil assemblages.
Bryophytes are represented by two species in sporomorph assemblages and are very low in abundance through the section (Figs. 5, 9). Bryophytes are absent from megafossil assemblages (Spicer and Hill Reference Spicer and Hill1979).
Lycopsids are highly diverse (16 taxa) within the sporomorph assemblage, however they are absent from megafossil counts (Fig. 9) (Spicer and Hill Reference Spicer and Hill1979). Abundances are low through the section within sporomorph assemblages (Fig. 5).
Sphenophytes are markedly more abundant within the megafossil assemblage (Fig. 5B) and diversity is the same (one species) in both sporomorph and megafossil assemblages (Fig. 9). Abundances are relatively constant through the section within sporomorph assemblages, however, megafossil assemblages record an increase in abundance within the siltstone compared to the claystone (Fig. 5).
Ferns are the most diverse plant group within sporomorph assemblages at Hasty Bank. Abundances (Fig. 5) and diversities (Fig. 9) are considerably higher within sporomorph assemblages (23 taxa) compared to megafossil assemblages (nine taxa). Abundances increase through the section in both sporomorph and plant megafossil assemblages (Fig. 5).
Pteridosperms are considerably more abundant (Fig. 5) and diverse (Fig. 9) within megafossil assemblages (11 taxa) compared to sporomorph assemblages (two taxa). Interestingly, sporomorph and megafossil assemblages display conflicting records for pteridosperm temporal abundance variation. Specifically, megafossil assemblages record a large decrease in pteridosperm abundances across the claystone–siltstone boundary, however in sporomorph samples, pteridosperm abundances are slightly higher within the siltstone unit.
Caytoniales are more abundant (Fig. 5) and diverse (Fig. 9) in megafossil assemblages (two taxa) compared to sporomorph assemblages (one taxa). Both sporomorph and megafossil assemblages document a slight increase in Caytoniales abundance within the siltstone unit (Fig. 5).
Coniferales are the most abundant plant group within sporomorph assemblages (Fig. 5A) and diversity is high (14 taxa) (Fig. 9). Abundance (Fig. 5B) and diversity (eight taxa) (Fig. 9) in megafossil assemblages is considerably lower. Abundances decrease through the section in both sporomorph and megafossil assemblages (Fig. 5).
Cycadopsida/Ginkgopsida are considerably more abundant (Fig. 5) and diverse (Fig. 9) within megafossil assemblages (10 taxa), compared to sporomorph assemblages (four taxa). Abundances of Cycadopsida/Ginkgopsida display little change through the section within sporomorph assemblages (Fig. 5A). Conversely, abundances of Cycadales and Ginkgoales decrease through the section within megafossil assemblages (Fig. 5B).
Bennettitales are notably more abundant (Fig. 5) and diverse (Fig. 9) in megafossil assemblages (eight taxa), compared to sporomorph assemblages (two taxa). Abundances are slightly higher in the siltstone unit within both sporomorph and plant megafossil assemblages (Fig. 5).
Discussion
Depositional Environments
The sedimentology and palynofacies of the claystone indicates a low energy, low oxygen and high nutrient depositional environment. The occurrence of Tasmanites spp. and Crassosphaera spp. from samples HB50 to HB4 reveals a marine component through the plant bed and suggests periodic flooding by seawater, as described by Harris (Reference Harris1964). However, Tasmanites spp. and Crassosphaera spp. are very rare (typically less than one specimen per 1000 palynomorphs) and the presence of Botryococcus and absence of dinoflagellates in palynofacies counts demonstrates dominantly fresh water conditions (Gray Reference Gray1960; Tyson Reference Tyson1995). The possibility that the rare marine palynomorphs could be reworked, potentially from the underlying marine Dogger Formation, cannot be discounted. Abundant AOM in claystone samples indicates relatively low oxygen and high nutrient levels within the original water during deposition (Tyson Reference Tyson1995; Roncaglia Reference Roncaglia2004; Traverse Reference Traverse2007; Pacton et al. Reference Pacton, Gorin and Vasconcelos2011). It is difficult to state with certainty a definitive depositional setting for the claystone, however the results of this study agree with Harris’ (Reference Harris1964) interpretations and are suggestive of a coastal plain periodically flooded by seawater, occupied by mangrove-like vegetation.
Interpretations of the depositional environment for the siltstone are consistent with previous studies that indicate this unit represents the slow moving part of a fluvial channel (Hill and van Konijnenburg-van Cittert Reference Hill and van Konijnenburg-van Cittert1973; van Konijnenburg-van Cittert and Morgans Reference van Konijnenburg-van Cittert and Morgans1999). Decreased abundances of AOM signify lower nutrient levels than the claystone (Tyson Reference Tyson1995).
The absence of marine palynomorphs in the gray clay demonstrates marine influence is negligible to absent. Sample ordination (Fig. 6) reveals that the gray clay is compositionally more similar to the siltstone than the claystone. Sporomorph assemblages of the gray clay contain higher abundances of ferns and lower abundances of wind blown taxa, suggesting a smaller catchment area and reduced sporomorph transportation distances compared to the claystone and siltstone (Chaloner and Muir Reference Chaloner and Muir1968). These combined factors are suggestive of a swamp or an abandoned channel environment for the gray clay.
Taphonomical models of time-averaging verses catchment area for multiple depositional environments demonstrate that the components of fossil assemblages can be used to indicate their temporal and spatial representations (Behrensmeyer and Kidwell Reference Behrensmeyer and Kidwell1985; Behrensmeyer et al. Reference Behrensmeyer, Kidwell and Gastaldo2000). Fossil assemblages that contain transported plant megafossils are typical of floodplain, pond and lake deposits. Such deposits generally represent time periods of ~100–10,000 years and source areas of ~100,000 m2. Sporomorphs are generally representative of larger source areas, frequently in excess of 1,000,000 m2 (Behrensmeyer et al. Reference Behrensmeyer, Kidwell and Gastaldo2000). Estimates of source areas and time-averaging for floodplain and channel environments are highly variable (Behrensmeyer et al. Reference Behrensmeyer, Kidwell and Gastaldo2000). Channels generally represent increased time-averaging and source area sizes compared to floodplain deposits, although there is significant overlap in source area sizes and the degree of time-averaging between channels and floodplains. Interestingly, the claystone unit (coastal plain) at Hasty Bank is interpreted to represent a longer time interval and a larger source area than the siltstone (fluvial channel) due to slower sedimentation rates and the probability that numerous rivers potentially flowed into the coastal plain environment.
Explanations for Temporal Sporomorph Variation
Variation in sporomorph and plant megafossil assemblages through the section can be correlated strongly with depositional change. The depositional environment is a primary control on parent vegetation, thus a change in depositional setting typically results in a change in sporomorph and megafossil assemblages. The erosional surface between the claystone and siltstone separates two distinct depositional environments by a period of unknown duration; therefore a change in floral composition between the claystone and the upper part of the section (siltstone and gray clay) is not particularly surprising. Although the claystone and gray clay are more similar to each other in terms of lithology than they are to the siltstone, the reason for their differing sporomorph assemblages is attributed to their different depositional environments and associated variable catchment areas.
The higher number of sporomorphs within the claystone unit is a result of a combination of ecological and non-ecological variables. Non-ecological variables include lithological factors, sedimentation rates and the depositional environment. Although governed by the depositional environment, the lithology itself can impact on the preservation of sporomorphs and therefore result in apparent temporal floral variation. Spores and pollen can be considered as sedimentary particles during transportation and depositional processes; hence certain taxa are preferentially preserved based on factors such as particle size, particle shape and durability (Traverse Reference Traverse2007). Thus, changes in transportation and depositional processes between claystone, siltstone and gray clay units would have presumably resulted in the preferential preservation of particular taxa based on these physical factors. The preservation potential of sporomorphs is generally increased when sediment grain size is reduced and sedimentation rates are slower (Traverse Reference Traverse2007). Therefore, the high number of sporomorphs and high diversities within the claystone are probably a result of the relatively small grain size and slow sedimentation rates of this unit. Conversely, the siltstone is considered to have been deposited more rapidly than the claystone, thus the lower abundances and diversities of coniferous pollen within the siltstone are probably a result of faster sedimentation rates, as there would have been less time for such pollen “rain” to accumulate. The lower diversities within the siltstone compared to the claystone are also a consequence of the smaller catchment area supplying this deposit with sporomorphs. Specifically, the channel environment of the siltstone would have presumably had fewer tributaries feeding this deposit compared to the numerous rivers that would have potentially flowed into the coastal plain setting of the claystone.
Spicer and Hill (Reference Spicer and Hill1979) postulated that differential rates of compaction could have affected floral compositions between lithologies. This could have had some influence on abundances; however, there is no conclusive sedimentological evidence that suggests compaction was vastly different between lithologies. The differential sedimentation rates and sediment grain sizes between depositional environments are considered to be far larger contributors to such abundance differences. For example, diversity is likely to be lower within the fluvial siltstone (compared to the same unit of claystone), because it was deposited more rapidly.
Ecological Causes for Variation between Sporomorph and Plant Megafossil Assemblages
Table 1 shows the generalized sporomorph dispersal methods of the dominant plant groups through the Hasty Bank plant bed. The majority of plant groups in Table 1 rely on wind to disperse sporomorphs. The life habits and reproductive methods of wind dispersed taxa result in abundance and diversity discrepancies between the sporomorph and megafossil assemblages. Pollen and spore production in wind dispersed species is typically very high as the efficiency of wind pollination increases as the concentration of airborne pollen increases (Regal Reference Regal1982; Whitehead Reference Whitehead1983; Allison Reference Allison1990; Friedman and Barrett Reference Friedman and Barrett2009). The sporomorph and megafossil records at Hasty Bank reflect this bias; conifers and ferns that produce vast numbers of pollen and spores are considerably more abundant (Fig. 5) and diverse (Fig. 9) within sporomorph assemblages.
Table 1 Sporomorph dispersal methods, sporomorph production levels, and relative parent plant heights of modern equivalents of the major plant groups in the Hasty Bank plant bed.
The nature of wind pollination means that sporomorph assemblages capture spores and pollen from a significantly larger geographic area compared to plant megafossil assemblages, which are more representative of the local paleoflora (e.g., Prentice Reference Prentice1985). Many of the coniferous species within the sporomorph assemblage are therefore potentially not representative of the flora close to the site of deposition and are possibly more indicative of upland communities.
The physical size of parent plants also impacts on the composition of sporomorph assemblages. Pollen released at elevated heights increases dispersal distances as: (1) wind speeds are greater, (2) pollen remains within the airstream longer, and (3) there is usually less intervening vegetation to intercept pollen (Levin and Kerster Reference Levin and Kerster1974; Okubo and Levin Reference Okubo and Levin1989; Friedman and Barrett Reference Friedman and Barrett2009). Many coniferous sporomorph taxa from the Hasty Bank plant bed originate from large trees; this is a principal factor contributing to their high abundances within sporomorph assemblages.
Harris (Reference Harris1964) described the pollen organ Pteroma thomasi from Hasty Bank and associated this with the pteridosperm P. papillosa based on their similar cuticles and co-occurrence. The pollen of P. thomasi is most similar to the wind dispersed pollen of Alisporites thomasii (Harris Reference Harris1964; Ziaja Reference Ziaja2006). Spicer and Hill (Reference Spicer and Hill1979) demonstrated that the parent plant, P. papillosa is markedly more abundant within the claystone, however the dispersed pollen, A. thomasii does not record this abundance change and is low in abundance throughout the section. Temporal changes in local vegetation are generally less well recorded among wind dispersed taxa in the sporomorph record as such taxa are captured from larger geographic areas than corresponding megafossils.
Animal-plant interactions could also be a cause of inconsistencies between sporomorph and plant megafossil assemblages. Potential insect assisted pollination in Caytoniales (Harris Reference Harris1945; Labandeira Reference Labandeira2010) could be a cause of the underrepresentation of such pollen in sporomorph assemblages (Figs. 5 and 9) as pollen production in insect pollinated plants is typically very low compared to wind dispersed taxa (e.g., Norstog Reference Norstog1987).
Cycads and Bennettitales display markedly lower abundances (Fig. 5) and diversities (Fig. 9) within the sporomorph record. The reproductive methods of these plants differ considerably from the exclusively wind dispersed conifers and ferns. Some modern cycads rely on a combination of wind and insect pollination (Niklas and Norstog Reference Niklas and Norstog1984; Clark and Clark Reference Clark and Clark1987; Tang Reference Tang1987; Norstog and Fawcett Reference Norstog and Fawcett1989; Ornduff Reference Ornduff1990; Pellmyr et al. 1991; Wilson Reference Wilson2002; Kono and Tobe Reference Kono and Tobe2007; Terry et al. Reference Terry, Walter, Moore, Roemer and Hull2007), and both fossil cycads and Bennettitales display early evidence of possible insect pollination (Crepet et al. Reference Crepet, Friis and Nixon1991; Klavins et al. Reference Klavins, Kellogg, Krings, Taylor and Taylor2005; Labandeira et al. Reference Labandeira, Kvaček and Mostovski2007). Pollen production varies greatly among modern cycads, depending on whether wind or insect pollination is dominant (Norstog Reference Norstog1987). Kono and Tobe (Reference Kono and Tobe2007) demonstrated that the pollen of the modern cycad, Cycas revoluta occurs only in abundance within very close proximity (~2 m) to the cones from which it is released. If Jurassic cycads share such a characteristic, cycad pollen would almost certainly be underrepresented in the sporomorph assemblage. Nilssonia kendalliae is the most common species within megafossil counts, constituting ~29% of the total assemblage (Spicer and Hill Reference Spicer and Hill1979). Harris (Reference Harris1964) presumed N. kendalliae to be of cycad or pteridosperm affinity. In situ pollen studies have associated the pollen Androstrobus with N. kendalliae (van Konijnenburg-van Cittert Reference van Konijnenburg-van Cittert1968) and more generally Nilssoniaceae (Hill Reference Hill1990). In situ Androstrobus pollen is considered to be equivalent to dispersed Chasmatosporites pollen (Balme Reference Balme1995). As a genus Chasmatosporites constitutes only ~2.9% of the total sporomorph assemblage. This abundance discrepancy is interpreted to be due to low pollen production and small dispersal ranges of cycads compared to many of the wholly wind pollinated plants. Hence, the specialized reproductive nature of cycads, Bennettitales and potentially Caytoniales (Delevoryas Reference Delevoryas1963; Harris Reference Harris1974; Labandeira Reference Labandeira2010; Mander et al. Reference Mander, Kürschner and McElwain2010) is probably the principal factor contributing to their underrepresentation in sporomorph assemblages.
The underrepresentation of cycads, Bennettitales and ginkgos in the sporomorph diversity record is potentially compounded by recognition biases. The leaves of these groups typically possess distinctive morphological features that enable easy differentiation to generic and species level (Lidgard and Crane Reference Lidgard and Crane1990), thus the diversity of such taxa in the megafossil assemblage is high. However, the pollen of these groups is often simple and monosulcate, with little morphological and sculptural variation visible under light microscopy (Frederiksen Reference Frederiksen1980). Sporomorph species therefore potentially represent numerous parent plant species, thus diversity in the dispersed sporomorph record is underrepresented.
The high diversities of lycopsids, ferns and bryophytes in sporomorph assemblages compared to megafossil assemblages (Fig. 9) could be related to epiphytic communities. Epiphytes are generally poorly represented in the megafloral record as the burial and subsequent fossilization of such species is unlikely compared to most other plants (Schneider and Kenrick Reference Schneider and Kenrick2001; Frahm and Newton Reference Frahm and Newton2005; Tstutsumi and Kato Reference Tstutsumi and Kato2006; Schuettpelz and Pryer Reference Schuettpelz and Pryer2007, Reference Schuettpelz and Pryer2009; Dubuisson et al. Reference Dubuisson, Schneider and Hennequin2009; Pšenička and Opluštil Reference Pšenička and Opluštil2013). In contrast, sporomorphs released from epiphytes do not experience this bias, thus diversities of epiphytic groups are comparatively unaffected in the sporomorph record.
The absence of lycopsids and low diversity of ferns in megafossil assemblages could also be linked to the relatively low preservation potential of many non-arborescent species. Scheihing (Reference Scheihing1980) demonstrated that non-arborescent taxa are frequently underrepresented in the megafloral record as a result of: (1) the increased biomass of arborescent species, (2) difficulty in recognition of non-arborescent plant parts, and (3) shielding of the non-arborescent understory by the arborescent canopy during high energy transport and depositional processes.
Variation in Plant and Sporomorph Durability Causing Megafossil–Sporomorph Inconsistencies
Many of the inconsistencies between the plant megafossil and sporomorph assemblages are interpreted to be a result of differences in durability between parent plants and associated sporomorphs.
Equisetum columnare is the second most abundant plant species in megafossil assemblages, constituting ~19% of the megaflora (Spicer and Hill Reference Spicer and Hill1979). However, its corresponding microspore, C. mesozoica represents only ~2.5% of the sporomorph assemblage. This discrepancy is probably due to the highly durable nature of Equisetum, which means that this genus is overrepresented compared to other megafloral taxa. Conversely, C. mesozoica has a low preservation potential due to its thin wall and low sporopollenin content (Traverse Reference Traverse2007; Grauvogel-Stamm and Lugardon Reference Grauvogel-Stamm and Lugardon2009), thus this species is underrepresented in the sporomorph assemblage. These combined factors give rise to a notable differential preservation potential between the parent plant and sporomorph.
The high diversity of lycopsids (16 taxa) in sporomorph assemblages and their absence from megafossil assemblages suggests an extremely low preservation potential for lycopsid remains within this deposit. The same pattern is also present within Triassic–Jurassic plant beds from East Greenland (Mander et al. Reference Mander, Kürschner and McElwain2010, Reference Mander, Kürschner and McElwain2013) and Lower Jurassic deposits of Odrowąż, central Poland (Ziaja Reference Ziaja2006). Lycopsids are notable both for their diversity in the megaspore record of the Middle Jurassic deposits of Yorkshire (reviewed in Slater et al. Reference Slater, Taylor, Batten, Hill and Wellman2015) and for their lack of megafossils within these deposits. Harris (Reference Harris1961b) summarizes studies on the lycopsid megafossil Selaginellites falcatus. With the exception of this species there are no other convincing reports of lycopsid megafossils from the Middle Jurassic of Yorkshire (Lindley and Hutton Reference Lindley and Hutton1833; Hill et al. Reference Hill, Moore, Greensmith and Williams1985; Schweitzer et al. Reference Schweitzer, van Konijnenburg-van Cittert and van der Burgh1997). The absence of lycopsid megafossils may also be exacerbated by the lack of recognition of delicate lycopsid remains (Skog and Hill Reference Skog and Hill1992).
Are Sporomorphs or Plant Megafossils More Informative regarding Paleofloristic Reconstructions?
Comparison of palynological and plant megafossil records demonstrates that respective data sets reflect different aspects of the paleoflora as they preferentially preserve certain taxa based on a multitude of ecological and non-ecological variables. Such variables include spore/pollen and plant durability, absolute abundances of plant species in life, proximity of parent plants to depositional location, spore/pollen dispersal methods, spore/pollen dispersal distances, absolute numbers of spores/pollen released from parent plants, transportation distances, transportation processes, climatic variations, and the depositional environment. This study illustrates that sporomorphs preserve some aspects of the paleoflora more completely (mostly wind dispersed taxa) than megafossil assemblages. However, megafossil assemblages equally preserve other aspects of the paleoflora (mostly reproductively specialized taxa) more completely than sporomorph assemblages.
Direct quantitative comparative studies of dispersed sporomorph and plant megafossil assemblages from pre-angiosperm Mesozoic floras are relatively uncommon in the literature due to the rarity of such fossil sites (e.g., Pedersen and Lund Reference Pedersen and Lund1980; Ziaja Reference Ziaja2006; Mander et al. Reference Mander, Kürschner and McElwain2010). Most previous studies that incorporate dispersed sporomorphs and plant megafossils are confined to Paleozoic (e.g., Looy and Hotton Reference Looy and Hotton2014), Cretaceous (e.g., Lidgard and Crane Reference Lidgard and Crane1990; Bercovici et al. Reference Bercovici, Nichols and Pearson2008, Reference Bercovici, Pearson, Nichols and Wood2009) and Cenozoic (e.g., Tinner et al. Reference Tinner, Ammann and Germann1996; Wing and Harrington Reference Wing and Harrington2001) floras. These floras are fundamentally different to pre-angiosperm Mesozoic communities, thus comparison of such floras with those at Hasty Bank is highly problematic. Rare examples where combined sporomorph and megafossil data have been used in vegetation reconstructions from pre-angiosperm Mesozoic floras demonstrate consistency with findings from Hasty Bank. Specifically, conifers and ferns are typically well represented in sporomorph assemblages (Jana and Hilton Reference Jana and Hilton2007), cycads, Bennettitales and pteridosperms are generally well represented in megafossil assemblages (Pedersen and Lund Reference Pedersen and Lund1980; Götz et al. Reference Götz, Ruckwied and Barbacka2011), and bryophytes and lycopsids are often confined to sporomorph assemblages (Ziaja Reference Ziaja2006; Mander et al. Reference Mander, Kürschner and McElwain2010, Reference Mander, Kürschner and McElwain2013). Such large discrepancies between parent plant and dispersed sporomorph assemblages questions the reliability of local vegetation reconstructions based on megafossil or sporomorph evidence in isolation and suggests that where possible a combined approach is considerably more informative.
Conclusions
Variation in sporomorph assemblages through the Hasty Bank plant bed is the result of a change in depositional setting between the three lithological units. Changes in the depositional environments consequently influence the vegetation, catchment areas, and preservation potential of sporomorphs and plant megafossils; hence the fossil assemblages vary notably between lithologies. Discrepancies between sporomorph and plant megafossil assemblages are primarily a result of the different life habits and reproductive strategies employed by parent plants. Such differences often cause large variation in sporomorph production and dispersal distances. Differential preservation potentials between parent plants and associated spores/pollen also has a substantial impact on generating inconsistencies between sporomorph and plant megafossil data sets. This is particularly apparent regarding the absence of lycopsids and the elevated abundances of E. columnare in plant megafossil assemblages (Spicer and Hill Reference Spicer and Hill1979). Based on the results at Hasty Bank and similar studies (e.g., Ziaja Reference Ziaja2006; Mander et al. Reference Mander, Kürschner and McElwain2010; Reference Mander, Kürschner and McElwain2013), explaining discrepancies between sporomorph and plant megafossil assemblages requires considerable analysis and there is no “best” method of reconstructing paleofloras. Assemblage compositions are the product of a complex array of biological, geographical, and depositional factors that act inconsistently between and within sporomorph and megafossil assemblages, resulting in notable disparities between respective data sets. Refining parent plant affinities with spore and pollen in situ studies will aid in future reconstructions of paleofloras using dispersed sporomorphs.
Acknowledgments
We thank J. L. Clark and C. R. Hill for their help with collection of samples. C. R. Hill also provided useful comments throughout this research. B. J. Slater and T. M. Young also helped with fieldwork. S. Ellin provided laboratory support. D. Cameron provided advice on statistical methods. This research forms part of a Ph.D. studentship to SMS at the University of Sheffield funded by a Natural Environment Research Council CASE Award with Royal Dutch Shell, jointly supervised by C. H. Wellman and I. Prince. Finally, we thank M. Kowalewski, A. Bercovici and three anonymous reviewers for their comments, which significantly improved this paper.
Supplementary Material
Supplemental materials deposited at Dryad: doi:10.5061/dryad.tg469
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