Abstract
AbstractQuantitative analysis of the distribution of dispersed spores and pollen (sporomorphs) has been used to assess temporal floral variation through the Middle Jurassic Ravenscar Group (Aalenian–Bathonian), North Yorkshire, UK. Aalenian, Bajocian and Bathonian strata possess relatively distinct sporomorph and palynofacies assemblages, which potentially reflect a dynamic history regarding the nature of parent vegetation. Specifically, Aalenian palynofloras are composed of a heterogeneous mixture of conifers, ferns, simple monosulcate pollen producers, sphenophytes and Caytoniales; Bajocian palynofloras are codominated by conifers and ferns; and Bathonian palynofloras are highly rich and contain assemblages of abundant ferns, conifers, lycophytes, pteridosperms/conifers and Caytoniales. Individual‐ and sample‐based rarefaction demonstrates that Bathonian samples are richer than Aalenian and Bajocian samples. Temporal variations in assemblages are a result of long‐term depositional and possible climatic fluctuations through the Middle Jurassic. Ordinations of sporomorph data using non‐metric multidimensional scaling (NMDS) demonstrate that short‐term variations between samples are largely governed by taphonomic biases as a result of slight changes in depositional processes, which give rise to highly variable catchment areas that supply deposits with sporomorphs. Long‐term compositional changes are apparent in sporomorph assemblages regardless of lithology/local depositional environments, suggesting that long‐term variations are more substantial than short‐term variations and potentially include genuine regional temporal changes in parent vegetation. Relating sporomorph assemblages with their respective depositional environments and relative catchment area sizes using lithological and palynofacies information suggests that the basin interior was occupied by mostly low‐standing species and extrabasinal vegetation was dominated by coniferous taxa. Comparisons of the dispersed sporomorph and plant megafossil records indicate that both fossil assemblages reflect different aspects of the palaeoflora due to a multitude of taphonomic and ecological biases. Such biases include variation in sporomorph production levels, depositional environment and differential sporomorph and parent plant durability.
Highlights
T H E Middle Jurassic deposits of Yorkshire, UK, represent a rare example of an extensive development of predominantly non-marine deposits of this age and provide important insight into understanding terrestrial ecosystems from this time period
Following extensive investigations over the past half century into the rare occurrences where spores/pollen are preserved in situ (e.g. Van Konijnenburg-Van Cittert 1968, 1971, 1978, 1981, 1989, 1993, 2000, 2010; Pedersen et al 1989; Hill 1990; Osborn and Taylor 1993; Balme 1995; Friis and Pedersen 1996; Yang et al 2008) the majority of Middle Jurassic sporomorphs can be assigned at least to family-level plant classification
This study focused on the dominantly non-marine sequences of the Middle Jurassic Ravenscar Group (Aalenian to Bathonian) from the Cleveland Basin (Fig. 2)
Summary
T H E Middle Jurassic deposits of Yorkshire, UK, represent a rare example of an extensive development of predominantly non-marine deposits of this age and provide important insight into understanding terrestrial ecosystems from this time period. The sequences contain abundant and often exceptionally preserved plant megafossil remains and have been studied in detail by palaeobotanists over the past century (see Van Konijnenburg-Van Cittert and Morgans 1999) Palaeofloristic reconstructions from these deposits rely primarily on plant megafossil assemblages (e.g. Spicer and Hill 1979), and several dispersed sporomorph investigations have been carried out (Couper 1958; Chaloner 1968; Chaloner and Muir 1968; Riding and Wright 1989; Boulter and Windle 1993; Hubbard and Boulter 1997; Srivastava 2011; Slater and Wellman 2015), much of this work has focused on taxonomy and biostratigraphy (Couper 1958; Riding and Wright 1989; Srivastava 2011). The Ravenscar Group represents predominantly paralic, fluvial and lacustrine deposits, with three marine units that are present as a result of marine incursions from the south and east: the Eller Beck Formation, the Lebberston Member and the Scarborough Formation (Knox 1973; Hemingway and Knox 1973; Livera and Leeder 1981; Alexander 1989, 1992; Powell 2010)
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