Upper Chicksgrove Quarry, Wiltshire

[ST 962 296]

Introduction

Upper Chicksgrove Quarry yielded a diverse fauna of fossil reptiles and mammals during an excavation in the early 1980s (Figure 2.16). The site was first noted by a remarkable pioneering female geologist, Etheldred Bennett, who made one of the very first bed-by-bed stratigraphical descriptions in the literature. That section, in the east of the Chicksgrove Quarry complex, can still be accurately proved today. The site has been noted several times in the literature (Fitton, 1836; Arkell, 1933; Wimbledon, 1976, 1980) and is one of the most significant for stratigraphical studies in the Late Jurassic strata, because of its rich ammonite faunas. The site had never figured in the vertebrate palaeontological literature, and its only claim to vertebrate palaeontology had been as the source of fragmentary fish material from the basal Purbeck Limestone.

In the late 1970s and early 1980s, however, quarrying revealed new stratigraphical units that yielded common vertebrate material. Discoveries (Anon., 1983) included fishes, dinosaurs and crocodilians, in a unit lying within the otherwise marine Portland Group (Wimbledon, 1980).

Description

The site shows a thick succession of Portland Limestone and basal Purbeck Limestone strata above, totalling around 27 m. The actively quarried locality was monitored by W.A. Wimbledon since the 1970s. The plant and reptile bed of the Wardour Portland Limestone was discovered in the early 1980s. Stratigraphically, the unit is the basal bed of the Wockley Member (Wimbledon, 1976), traditionally termed the Itagstone beds'. The bed is a fine quartz sand with many carbonate fragments, mostly shell fragments: it is laminated and shows abundant remains of current-aligned flattened carbonized plant material. In places, the sediment is silicified (Astin, 1987), as are some of the biotic contents, notably seeds, wood and gastropods. The unit was deposited in a depression in the top of the glauconitic building stone of the Tisbury Member, and is overlain by a discontinuous gastropod-rich micrite. The sand deposit lenses out rapidly eastwards, and the basal unit towards the east is a thin clay with serpulid debris. This lensing unit rests on a highly irregular erosion surface cut down into the topmost bed of the Tisbury Member (and embedded in that surface are crushed okusensis zone ammonites). The deposit, up to 0.60 m thick, is of limited extent within the depression, and it was excavated in the 1980s and removed for screening for microvertebrate and other fossils. Nineteen tonnes were screened and washed on site. Sorting of the washings and acid residues is still not finished. The bare surface left by removal of the plant bed shows the extent (15 x 7 m) and shape of the trough.

The fauna of the plant–reptile bed is diverse (Anon, 1983). In addition to abundant carbonized plant remains, large reptilian bone fragments, teeth, silicified molluscs and wood also occur. The bed contains the bivalve Myrene, ostracods, around 20 species of gastropods, and bones and teeth of fishes, crocodilians, pterosaurs, ornithischian and saurischian dinosaurs, lepidosaurs and mammal material, as well as megaspores and microspores, conifer seeds and wood.

Vertebrate material is abundant and varied, from microscopic teeth to large limb bones. Lepidosaurian and mammal specimens are the smallest, together with many minute crocodilian and pterosaur teeth. Groups present, on the basis of teeth, include one diplodocid sauropod, a camarasaurid sauropod, small and large theropods, presumed coelurosaurs, a fabrosaurid and Iguanodon. In addition, armoured forms (nodosaur or stegosaur) also occur. Crocodilians are represented by three types: abundant goniopholids, theriosuchians and Bernissartia. There are two identifiable species of pterosaur: assigned to Pterodactylus and Gnathosaurus. Mammal remains consist of several dozen complete teeth and fragments. No precise specific assignments have yet been attempted. However, members of three orders of mammals have been positively identified (W A. Wimbledon, pers. comm.): Multituberculata, Triconodonta and Tupantotheria'.

Interpretation

The plant bed is an anomalous unit; lithologically it is like no other in the Vale of Wardour Late Jurassic succession. Its facies is atypical in what is a predominantly open-water sequence with a shelled marine molluscan fauna: the norm for the Portland Group. The bed is remarkable also in that it contains a fossil fauna and flora that came from adjacent non-marine environments, aqueous and terrestrial. Non-marine environments are typical in the Purbeck Limestone, but not in the Portland Group. Wimbledon (1976), after a revision of Portlandian stratigraphy, placed the plant-reptile bed in the Galbanites kerberus biozone, well down in the Portland Group. No other non-marine intercalations or biotas have been found in the British Portlandian Stage, apart from rare dinosaur teeth.

Comparison with other localities

The Late Jurassic Tithonian Stage has been split locally into two substages: the Bolonian below (spanning the upper Kimmeridge Clay) and the Portlandian above (covering the Portland and basal Purbeck Limestone Groups). This vertebrate assemblage is the only one known of its age. No land vertebrate assemblage has been recorded from the Portlandian substage, only rare isolated dinosaur teeth. The non-marine Morrison Formation faunas of dinosaurs and mammals from mid-western North America have always been vaguely described as late Jurassic in age (Clemens et al., 1979). Most of the animals are from the upper Brushy Basin member (Engelmann and Callison, 1998), dated as ranging in age from late Kimmeridgian to early Tithonian (Steiner, 1998), thus slightly older than the Upper Chicksgrove fauna. The Guimarota deposit of is approximately coeval with the Morrison Formation, as is Tendaguru in Tanzania (Heinrich, 1999). The Purbeck Limestone, whose mammals are from Durlston Bay, Swanage and other localities in the Isle of Purbeck, associated with crocodilians and limited dinosaurian remains, are considerably younger, being of early to mid-Berriasian age (Allen and Wimbledon, 1991).

Thus the Upper Chicksgrove Quarry deposit and contained mammals fall approximately midway between these two clusters, of latest Kimmeridgian to early Bolonian and Berriasian age, a span of about 3 Ma. The Morrison Formation mammals fall near the Kimmeridgian–Bolonian boundary. Upper Chicksgrove Quarry is unequivocally mid-Portlandian in age, and Durlston Bay is earliest Cretaceous in age.

Conclusions

The fossil mammals from Upper Chicksgrove Quarry are apparently diverse, though rare.

They have not been studied yet, so their full import still has to be determined. However, their age lends them considerable international importance: they fall midway in age between the famous Morrison Formation mammals of North America and the Purbeck-Wealden mammals of Britain and other countries.

References