The Age of the Isle of Wight Dinosaurs
Figure 2. Ages of the Wealden Group on the Isle of Wight
Almost all of the dinosaurs from the Isle of Wight have been found in the Wealden Group (Insole 1980, 1982. Hutt et al. 1996). Isolated bones found in the overlying strata < #150; usually the Lower Greensand < #150; may have been derived from the Wealden Group, but there are records of associated, and therefore contemporaneous, skeletal elements. Because the Wealden Group was deposited in non-marine, alluvial and coastal lagoonal environments, it largely lacks fossils useful for reliable biostratigraphic correlation, and consequently these deposits have been difficult to correlate directly with rocks elsewhere in the UK. Lack of volcanic ash deposits within the sequence means that radiometric age determination has not been possible. Thus the Wealden Group is not very precisely dated, although the top is we11 constrained as it is overlain by marine strata. Attempts to use the microscopic remains of fossil pollen and spores have proved partially successful, and some approximate dates have been determined suggesting that the exposed Wessex Formation is entirely Barremian (Hughes and McDougall 1990). However, good conelations can be made between the first marine strata overlying the Wealden Group on the Isle of Wight (Rawson et al. 1978), and as a result the top of the Wealden Group can be dated at approximately 114 million years b.p. (Harland et al. 1989<). As none of the Wealden Group is considered to be older than Barremian, an approximate maximum age for the Isle of Wight dinosaur assemblage is approximateley 130 million years b.p.
Magnetostratigraphic age determinations suggest that the top of the Vectis Formation may lie within the early Aptian (Kerth and Hailwood 1988). The base of the Wealden Supergroup is not seen on the Island, but in the Weald of Kent and Sussex it is dated at about 144 million years. The base of the Wealden Group is dated at around 131 million years on the mainland, but it is not clear if the base of the Wealden Group on the mainland is equivalent to the base of the Wealden Group on the Isle of Wight as detected in boreholes. Nevertheless, the time span of the Wealden Group on the Isle of Wight can be estimated at an incredible 13 million years. The oldest part of the Wealden Group on the Isle of Wight is the lowest part of the exposed Wessex Formation in the cliffs and foreshore between Hanover Point and Sudmoor Point. Here the rocks are probably around 118 million years old. The time span for the entire Wealden Group of the Wessex sub- basin encompasses three stages of the Lower Cretaceous, in ascending order: the Valanginian, Hauterivian and Barremian. On the Isle of Wight only the upper part of the Hauterivian is seen, where, at Hanover Point the so-called Pine raft, an accumulation of fossil tree trunks in a sandstone channel is placed at the Hauterivian-Barremian boundary by Hughes and McDougall (1990). The remaining part of the Wealden Group is therefore largely of Barremian age. The very top of the Vectis Formation is probably of lowest Aptian age (Kerth and Hailwood 1988, Hughes and McDougall 1990, Allen and Wimbledon 1991). Thus the dinosaur-bearing strata on the Isle of Wight are somewhat younger than strata yielding dinosaurs in Kent, Sussex and Surrey (Blows 1987, Norman 1987). In short, there are some interesting differences as well as some similarities between the dinosaur faunas of the Wealden of the Isle of Wight and the Wealden of the mainland. For example, the ornithopod Iguanodon is abundant in both the Wealden Group of the Isle of Wight and in the Weald. By contrast, the hypsilophodontid Hypsilophodon foxi, which is so common in the Isle of Wight, has yet to be recorded with certainty in the Weald. The ankylosaur Hylaeosaurus known from the Weald has not definitely been recorded from the Isle of Wight, while many of the smaller theropods known from the Island have never been recorded from the Weald. Norman (1987a) has suggested a biostratigraphic scheme for the Weald based on Iguanodon species. Similarly, Pereda-Suberbiola (1993) attempted to utilise the ankylosaurs in a stratigraphic context. The scarcity of dinosaurs in the English Wealderf Group is, however, a major stumbling block to their utility as biostratigraphic indicators. Present perceived differences between the Isle of Wight dinosaur assemblage and that of the Weald may have no statistical significance, instead being merely a consequence of a very small and imprecisely known data set.
