The Cretaceous Period, which began 145.5 ma (Bui et al., 1995) was the final stage of the ‘’Age of Dinosaurs’’. The Cretaceous Period saw some major worldwide events, such as; increased sea floor spreading rates along mid-ocean ridges and the highest marine transgressions of the post- Pangean world (The Mesozoic Era II, 2010). Movements within the Earth’s mantle continued the break-up of the Pangea supercontinent (Fig.1) (Nudds & Selden, 2012). North America and Europe were divided, leading to the formation of the North Atlantic Ocean, while the southern and northern continents where further divided as the Tethys Ocean extended west (Nudds & Selden, 2012). This led to regional differences between the flora and fauna of the northern and southern continents. During this period modern mammals first appeared in the fossil record (Bui et al., 1995), and birds, which had first appeared in the Late Jurassic, began to diversify (Terrestrial Life, n.d.). Flowering plants (Angiosperms) also appeared approximately 135 million years ago, and by the end of the Cretaceous were the most diverse group of terrestrial plants, their rapid diversification coinciding with the diversification of insects, an example of co-evolution (Terrestrial Life, n.d.). During the early Cretaceous, dinosaurs were still the dominating terrestrial animals; pterosaurs ruled the sky, ichthyosaurs and plesiosaurs were the top marine predators (Nudds & Selden, 2012). While all these entities were devastated during the Cretaceous-Tertiary mass extinction, during the cretaceous they continued to adapt, with new groups of dinosaurs, such as ceratopsians, pachycephalosaurs and hadrosaurs emerging (Bui et al., 1995) (Terrestrial Life, n.d.). Top Predators were replaced in ecosystems, such as the replacement of allosaurs, icthysaurs and pterosaurs by coelurosaurs, mosasaurs and pterodactyloids respectively (Nudds & Selden, 2012).
Fig 1: Position of the continents during the Early Cretaceous, 130Ma (Source; (The Mesozoic Era II, 2010)).
HISTORY OF DISCOVERY OF THE WESSEX FORMATION
The Wessex formation, the Wealden Group, located on the Isle of Wight, has a significant importance for vertebrate palaeontology, with bones found in the Wessex Formation being at the forefront of dinosaur research (Hooker & Sweetman, 2009). In 1829 William Buckland first described the pedal phalanx, of the dinosaur iguanodon, which was first scientifically described and named by Gideon Mantall in 1825. The bone was found at Yaverland, located on the south-east coast of the Isle of Wight (Hooker & Sweetman, 2009).
STRATIGRAPHIC SETTING AND TAPHONOMY OF THE WESSEX FORMATION
The Lower Cretaceous Wealden Group outcrops in two sections in the Isle of Wight. These sections are coastal, with a small exposure in Sandown Bay, on the south-eastern coast of the Isle of Wight, and another exposure along the southwestern shore of the Isle of Wight, which is larger, and more laterally extensive (Evans et al., 2004). In this area the Wealden Beds have been given group status, and are divided into two formations, the Wessex Formation, and the Vectis Formation (formerly known as Wealden Marls and Wealden Shales respectively) (Insole & Hutt, 1994). The Wessex Formation is thought to be Hauterivian-Barremian in age, with the lower section of the formation potentially being Valanginian in age (Hutt et al., 2001) (Insole & Hutt, 1994). The beds were dated through the use of biostratigraphical correlations, which were based on palynological and ostracod data. The Wessex formation is a red-bed sequence of interbedded, varicoloured, but predominately red non-marine mudstones and sandstones, with the occasional crevasse splay deposit, and plant debris beds (Evans et al., 2004) (Insole & Hutt, 1994). The Isle of Wight Wealden Group strata reaches a maximum outcrop thickness of 240m (Insole & Sweetman, 2010), however the maximum depth of the Isle of Wight Wealden Group strata, as shown by the Arreton 1 borehole, created for hydrocarbon exploration (Hopson, 2011) (Insole & Sweetman, 2010) (Insole & Hutt, 1994), with the top 180m of sediments comprising of the exposed Wessex Formation (Insole & Sweetman, 2010) (Evans et al., 2004). There a six facies associated with the Wessex formation (Insole & Sweetman, 2010), the first consisting of fining-up conglomerates, siltstones, mudstones and sandstones with a distinctive arrangement of sedimentary structures (Insole & Hutt, 1994).The second facies consists of heterolithic bodies of sediment, ranging from predominantly mudstone and siltstone, to over 50% fine grained sandstone (Insole & Hutt, 1994). The third facies consists of sequences infilling recognizable channels; the fill consists of either red mudstone or sand-mud interbeds. Facies association four comprises of small-scale fine grained sandstone and red mudstone alternations (Insole & Hutt, 1994). The fifth facies, which makes up the bulk of the Wessex Formation sequence, is characterized by massive red mudstones and siltstones, mottled with purple, green, yellow, brown and grey, forming complex patterns. Polygonal mud cracks and rootlet traces are common in this facies (Insole & Hutt, 1994). The final facies association, the plant debris beds, are relatively thin beds, mostly less than 1mm in thickness, however with some units reaching a maximum thickness of 2m (Insole & Hutt, 1994) (Evans et al., 2004). These beds comprise of green-grey mudstones (Evans et al., 2004), as well as flood-dominated accumulations of poorly sorted fragments of lignite and fusain within the clay matrix (Hutt et al., 2001). These lignite and fusain fragments, which are representative of charcoal and wood fragments, are often pyritized (Evans et al., 2004). The plant debris beds are fossiliferous and representative of flash floods washing debris, such as flora and fauna remains into depressions within the Wealden alluvial floodplain (Evans et al., 2004). Large branches and stems caught in these floods may have acted as a net, catching and concentrating vertebrate material. This accounts for some of the plant debris bed’s abnormal features, such as the mixture of organisms from different habitats, the variability of preservation, and the discrepancy in grain size between the matrix and clasts (Insole & Hutt, 1994).
DESCRIPTION OF THE WESSEX FORMATION BIOTA
Flora: The flora contains few recognizable macrofossils, other than twigs, logs and the occasional cone (Insole & Sweetman, 2010) (Insole & Hutt, 1994). However there is an abundance of spores, and dispersed cuticles within the plant debris beds (Insole & Sweetman, 2010) (Insole & Hutt, 1994). Flora known within the plant debris beds includes pteridophytes, caytoniales, cycadophytes, gingkophytes, coniferophytes and angiosperms (Insole & Sweetman, 2010) (Insole & Hutt, 1994). Conifers and cycads are thought to have been major elements in the local vegetation, with a small proportion of these conifers being dominant, such as Pseudofrenelopsis parceramosa, which were large evergreen trees, potentially reaching heights of 10-15m (Insole & Hutt, 1994) (Insole & Sweetman, 2010). Infrared spectroscopy of amber, fossilized tree resin, found in the plant debris beds, supports a coniferous origin for the samples (Insole & Sweetman, 2010). It is thought that Wealden Group flood plains, or Early Cretaceous mid-latitude environments generally did not support coniferophyte-cycadophyte forests, instead they were similar to savannahs, covered in vegetation, with no closed canopy (Insole & Hutt, 1994). The abundance of fusain within the plant debris beds suggests that wildfires occurred, which may have prevented the development of dense gymnosperm growths (Insole & Sweetman, 2010) (Insole & Hutt, 1994). Megaspores are present in high numbers on some horizons, with some potentially representing aquatic plants, suggesting that several pools of standing water were colonised by these plants.
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