Palaeoenvironment and Taphonomy of Dinosaur Tracks in the Vectis Formation (Lower Cretaceous) of the Wessex Sub-Basin, Southern England

Home , Chine, Dinosaur Beds, Greensand, Southern England, Vectis Formation, Wealden Group

Cretaceous Research (1998) 19, 471±487 Article No. cr970107

Palaeoenvironment and taphonomy of dinosaur tracks in the Vectis Formation (Lower Cretaceous) of the Wessex Sub-basin, southern England

*Jonathan D. Radley1, *Michael J. Barker and {Ian C. Harding * Department of Geology, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth, PO1 3QL, UK; 1current address: Postgraduate Research Institute for Sedimentology, The University of Reading, PO Box 227, Whiteknights, Reading RG6 6AB, UK { Department of Geology, University of Southampton, Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH, UK

Revised manuscript accepted 20 June 1997

Reptilian ichnofossils are documented from three levels within the coastal lagoonal Vectis Formation (Wealden Group, Lower Cretaceous) of the Wessex Sub-basin, southern England (coastal exposures of the Isle of Wight). Footprints attributable to Iguanodon occur in arenaceous, strongly trampled, marginal lagoonal deposits at the base of the formation, indicating relatively intense ornithopod activity. These were rapidly buried by in¯uxes of terrestrial and lagoonal sediment. Poorly-preserved footcasts within the upper part of the Barnes High Sandstone Member are tentatively interpreted as undertracks. In the stratigraphically higher Shepherd's Chine Member, footcasts of a small to medium-sized theropod and a small ornithopod originally constituted two or more trackways and are preserved beneath a distinctive, laterally persistent bioclastic limestone bed, characterised by hypichnial Diplocraterion. These suggest relatively low rates of dinosaurian activity on a low salinity, periodically wetted mud¯at. Trackway preservation in this case is due to storm-induced shoreward water move- ments which generated in¯uxes of distinctive bioclastic lithologies from marginal and offshore lagoonal settings. The rapidly-deposited footprint-®lls occasionally contain fully articulated shallow burrowing bivalves. # 1998 Academic Press

KEY WORDS: Vectis Formation; Lower Cretaceous; southern England; dinosaur tracks; palaeoenvironments; taphonomy.

1. Introduction and geological setting Dinosaur tracks were ®rst recognised in the Lower Cretaceous Wealden strata of southern England in the mid nineteenth century (Delair, 1989). These generally occur in alluvial plain facies, both in the Wessex (or `Channel') Sub-basin (Wes- sex Formation of the Isle of Wight, Barremian; Allen & Wimbledon, 1991) and the Weald Sub-basin (principally within the Hastings Beds of Sussex, Berriasian- Valanginian; Allen & Wimbledon, 1991). Most records refer to medium to large robust three-toed tracks, usually attributed to the pes (hind feet) of Iguanodon, the common large Wealden ornithopod (Tylor, 1862; Ticehurst, 1928; Delair, 1983). More recently, theropod footprints and footcasts as well as `four-toed' ankylosaur or sauropod pes footcasts have been recognised (Woodhams & Hines, 1989; Parkes, 1993; Radley, 1994a). The Vectis Formation (Barremian to possibly early Aptian in age: Allen, 1989; Allen & Wimbledon, 1991) of the Isle of Wight (Figure 1) overlies the Wessex Formation, recording a phase of warm-temperate or subtropical coastal lagoonal deposition in the Wessex Sub-basin, prior to the Lower Greensand (Aptian) mar-

0195±6671/98/030471 + 17 $30.00/0 # 1998 Academic Press 472 J. D. Radley et al.

Figure 1. Outline map of the Isle of Wight, southern England, showing outcrop of Wealden (Lower Cretaceous) strata (shaded) and localities mentioned in the text. ine transgression (Allen, 1981, 1989; Stewart, 1981; Stewart et al., 1991). It comprises two essentially argillacous units (the Cowleaze Chine and Shepherd's Chine Members) enclosing the arenaceous Barnes High Sandstone Member (Figure 2). Sections occur at Sandown Bay on the southeast coast [National Grid Reference (NGR) SZ 615 852-621 853] and between Barnes High and Ather®eld Point (SZ 438 805-452 791; Ather®eld section of this account), Shippards Chine (SZ 376 843-375 845) and at Compton Bay (SZ 372 848-370 850) on the south- west coast (Figure 1). The Vectis Formation reaches a thickness of nearly 70 m (White, 1921). Sedimentological studies (Stewart, 1981; Stewart et al., 1991) indicate that muddy horizons are of low-energy subaqueous and mud¯at origin whilst the Barnes High Sandstone and thinner sand bodies represent deltaic in¯uxes into the lagoon. Thin (generally less than 10 cm), laterally persistent beds of shelly arenaceous limestone (`coquinas') occur within the Shepherd's Chine Member (Figure 2; Table 1) and represent storm deposits. Subsurface data and provenance studies (Stewart et al., 1991; Radley et al., in press) indicate that the faulted northern margin of the Wessex Sub-basin occurred immediately to the north of the present-day Sandown and Compton Bay outcrops. Faunas are dominated by bivalves, gastropods, ostracods, and ®sh remains. Palaeoecological analyses indicate ¯uctuating oligohaline-low brachyhaline salinities throughout deposition (Stewart, 1981; Stewart et al., 1991). The overall increase in salinity and storm events recorded upwards through the Shepherd's Chine Member (Stewart et al., 1991) indicates increasing marine in¯uence prior to the Aptian transgression. Research into reptilian ichnofossils has traditionally been of a palaeobiological nature, providing information on the morphology and behaviour of trackway pro- ducers. In recent years however, palaeoenvironmental studies have realised the potential of vertebrate ichnofossils in the understanding of palaeogeographical, palaeohydrological and sedimentological problems (Lockley, 1986). Dinosaur tracks in southern England 473

Figure 2. Generalised stratigraphic column for the Vectis Formation (Wealden Group) at Ather- ®eld, Isle of Wight, showing distribution of dinosaur tracks and limestone marker bands (lime- stones not to scale).

Table 1. Characterisation of limestone marker bands in the Shepherd's Chine Member, Vectis For- mation, Ather®eld, Isle of White.

Shelly limestone (bed number) 1 2 3 4

Thickness 1±2 cm, up to 10 1±2 cm, possibly 1±5 cm 2±5 cm cm in footcasts thickening to 5 cm at Shepherd's Chine Dominant taxa abundant Filosina abundant Filosina abundant abundant Filosina, and Viviparus Praeexogyra, rarer Viviparus Procerithium, Paraglauconia Other features strongly pyritic, abundant ®sh locally pinches out, locally encased in Diplocraterion sp. on debris, red abundant grey- ®brous calcite, base, trackways colouration brown muddy shells frequently locally present on matrix preserved in white base, dark calcite colouration 474 J. D. Radley et al.

The examples documented and discussed below comprise an in situ footprint horizon and occurrences of ex situ footcasts from two higher levels. The terminol- ogy used for reptilian ichnofossils throughout this account is essentially that of Lockley (1991, appendix B).

2. History of ®nds in the Vectis Formation The White Rock The basal metre of the Cowleaze Chine Member within the Ather®eld section is a sandstone unit known as the White Rock (Figure 2). Over the last ®fteen years this has yielded occasional large iguanodontid pes footcasts, found as loose blocks on the shore in the vicinity of Cowleaze Chine (SZ 444 801). In situ footprints have been recently detected (see below and Figure 3a, b).

The Barnes High Sandstone Member Loose blocks of cross-laminated, bioturbated and shelly sandstone on the shore between Shepherd's Chine and Cowleaze Chine (SZ 445 798, Figures 1, 2) are derived from the highest 2 m of the Barnes High Sandstone Member. These have recently revealed large, poorly-preserved and incomplete iguanodontid pes footcasts (see below and Figure 3c) and a small theropod footcast (Dr D. Martill, pers. comm., 1998).

The Shepherd's Chine Member Mantell (1846, p. 94) recorded ``....markings....supposed to be the imprints of the feet either of birds or reptiles...'' from shelly limestone slabs, weathered from the Shepherd's Chine Member in Sandown Bay. Examples of these have not been traced, either in the ®eld or in available collections. Consequently their authen- ticity cannot be con®rmed. In 1984 a schoolboy collector, James Crouch, presented a well-preserved theropod pes footcast to the Museum of Isle of Wight Geology (MIWG 5768; Figure 3d), preserved on the underside of a slab of shelly limestone. It was discovered on the foreshore near Cowleaze Chine, and has been cited by Ruffell (1988), Woodhams & Hines (1989), Stewart et al. (1991), brie¯y described by Radley (1992) and ®gured by Radley (1994a, b). Since the initial discovery, at least 50 variably preserved theropod pes footcasts have been further recovered as ex situ limestone slabs, between Cowleaze Chine and Ather®eld Point, and in the mouth of Shepherd's Chine (SZ 447 798). Many of these are in private or university collections; however to date (1996), 27 specimens are held in the collections of the Museum of Isle of Wight Geology. This study is based largely on these specimens. These are preserved in an identical lithology (see below). Undistorted examples are all of closely comparable morphology and size to the initial ®nd (Figure 3d, e), indicating derivation from a single track-bearing layer and generation by one or more theropods of the same species. In recent years, the majority of recovered specimens have exhibited considerable wear, suggesting that they were mostly eroded from their parent bed at approximately the same time as the initial ®nd. Palaeoecological and sedimentological investigation of Vectis Formation lime- stones has now successfully located the track-bearing layer (Figure 2), and culminated in the discovery of a recently eroded footcast-bearing slab (Figure 3e). In situ material may still exist at this important site, but excavation is presently Dinosaur tracks in southern England 475

Figure 3. Dinosaur tracks from the Vectis Formation. a, Iguanodon footprint (pes) in loose block derived from lower part of White Rock, Cowleaze Chine foreshore (NGR SZ 444 801). Note the conglomeratic in®ll, 0.07. b, Iguanodon footprint (pes), middle part of White Rock, Cowleaze Chine foreshore (NGRÂ SZ 444 801). Print is impressed into pale silt, subsequently ®lled with darker, carbonaceous sediment, 0.065. c, Iguanodon footcast (pes), upper part of Barnes High Sandstone Member between CowleazeÂ Chine and Shepherd's Chine (NGR SZ 445 798), adapted from ®eld sketch, 0.05. d, theropod footcast (pes; MIWG 5768), limestone bed 1 (Diplocraterion limestone), Shepherd'sÂ Chine Member, found loose on Cowleaze Chine foreshore (NGR SZ 444 801), 0.12. e, incomplete theropod footcast (pes; IWCMS: 1995.856), limestone bed 1 (Diplocraterion Âlimestone), Shepherd's Chine Member, recovered from slope to north-west of Shepherd's Chine (NGR SZ 4456 7989), 0.18. f, small ornithopod footcast (pes; M. Green collection), limestone bed 1 (Diplocraterion limestone),Â Shepherd's Chine Member, base of cliff between Shepherd's Chine and Ather®eld Point (NGR SZ 448 795), 0.26. Â 476 J. D. Radley et al. impractical (see below). More recently (1996), Mr M. Green of Ryde, Isle of Wight, has discovered a small ornithopod footcast, weathered from an outcrop of the same layer south-east of Shepherd's Chine (SZ 448 795; Figure 3f and below).

3. Footprints in the White Rock Lithology The White Rock rests on oxidised, bioturbated and mudcracked lenticular- bedded alluvial mudstones and sandstones of the Wessex Formation (the Hypsilo- phodon Bed; Reid & Strahan, 1889). The contact is locally gradational but commonly irregular or channelised. In the vicinity of Cowleaze Chine, the lower part of the White Rock comprises laminated, cross-laminated or burrow-mottled grey sandstone, intercalated with lenses of fusain-rich carbonaceous sandstone, organic-rich silty mudstone and poorly-sorted granule-pebble conglomerate. The mudstones yield poorly preserved (?acid-etched) corbiculid bivalves (Filosina cf. gregaria Casey) and sporadic ostracods [Theriosynoecum ®ttoni (Mantell)]. The conglomerates comprise varying proportions of calcrete and fusain clasts, worn reptilian bone fragments and disarticulated or more rarely articulated unionoid bivalves, as well as disarticulated Filosina. These variable layers display much sedimentary disturbance (contortion, upturning and pinching of laminae), and grade up into muddy carbonaceous grey sandstones, penetrated by Beaconites burrows (Goldring & Pollard, 1995) and rootlet traces. In an unpublished thesis, D. J. Stewart recognised a similar suite of lithologies constituting the White Rock in cliff sections running for 700 m to the north-west (see also Stewart, 1981).

Dinosaur tracks Iguanodontid footcasts found on the shore at Cowleaze Chine are preserved in pale laminated sandstone, indicating derivation from adjacent exposures of the White Rock. One of these is preserved in the collections of the Museum of Isle of Wight Geology (MIWG 5311). In the spring of 1996, foreshore exposures immediately west of Cowleaze Chine exposed hummocky surfaces at several levels in the lower, relatively coarse- grained and lithologically variable part of the White Rock. These preserve shallow, broadly rounded depressions, sometimes with surrounding rims of deformed sediment, and correspond to contortions seen in cross-section in the adjacent cliffs. Laminated sands and fusain-rich sediments beneath these depressions displayed compression and some distortion. Several of these structures were recognisable as large, robust, variably-oriented tridactyl footprints (Figure 3a, b), and match tracks generally attributed to Iguanodon pes. Examples have been described by Woodhams & Hines (1989). Footprint in®lls include both coarse and ®ne-grained lithologies. The former include coarse pebbly sandstone containing small (up to 5 mm) sub-rounded calcrete clasts, fusain fragments and disarticulated bivalve shells (Figure 3a). The highest observed footprint was in the middle of the bed and was in®lled by grey carbonaceous silt (Figure 3b).

Track taphonomy Taphonomic studies of ancient and Recent vertebrate tracks and trackways potentially provide important sedimentological data, and enhance our understanding of Dinosaur tracks in southern England 477 preservational mechanisms (Lockley, 1986; Cohen et al., 1991). Sediment grain- size and substrate water content are important factors affecting the formation of prints. Subsequent entry into the sedimentary record depends on relative burial rates and substrate trampling (Laporte & Behrensmeyer, 1980). The strongly trampled nature of the lower part of the White Rock (dinoturba- tion; Dodson et al., 1980) indicates repeated disturbance of marginal lagoonal substrates and consequential time-averaging of several trackway generations over an unknown, but probably considerable, time period. The occasional preservation of sharply-de®ned iguanodontid footprints within these surfaces indicates periodic rapid burial of freshly-trampled, moist, or slightly damp substrates (Laporte & Behrensmeyer, 1980), within this essentially destructive regime. We suggest that coarser, conglomeratic or sandstone ®lls were introduced by seasonal ¯ood-generated alluvial in¯uxes, and raised rims around some footprints (see above) may have enhanced their ef®ciency as sediment sinks (Laporte & Beh- rensmeyer, 1980). The highest preserved footprint (Figure 3b) is in®lled with ®ner sediment, indicating rapid depositional in®ll during lagoonal transgression. Consequently, track preservation in the White Rock was primarily climatically- controlled, and enhanced by alluvial in¯ux in the lower part and by lagoonal transgression in the upper part.

Deposition The White Rock was interpreted by Stewart (1981) as a low-salinity marginal lagoonal sand¯at deposit, dissected by narrow ¯uvial channels. Accordingly, the coarser, cleaner sands in the lower part of the unit at Cowleaze Chine are envi- saged as low-energy marginal lagoonal sediments, deposited primarily by runoff from alluvial environments to the west (Stewart et al., 1991), but subject to shoreline reworking. Given the strongly seasonal Wealden climate (Allen, 1981), the associated poorly-sorted lenticular intraformational conglomerates are interpreted as rapidly deposited, channelised ¯ood sediments, largely comprising admixtures of lithologi- cal and biogenic ¯oodplain debris. Articulated, closed, unionoid bivalves [includ- ing Margaritifera (Pseudunio) valdensis (Mantell)] indicate input from alluvial environments (Tevesz & Carter, 1980; Taylor, 1988). Conversely, Filosina typi®es subaqueous lagoonal muds in the Vectis Formation and is interpreted as a shallow infaunal suspension feeder, characteristic of oligohaline-mesohaline salinities (0.5± 18 parts per thousand). The disarticulated nature of these shells in the conglomeratic lenses suggests some local input from marginal lagoonal shell pavements. Fusain-rich layers re¯ect dry-season hinterland wild®re events (Batten, 1974; Sellwood & Price, 1993) and subsequent wet-season ¯ushing of charcoal-rich debris into the lagoon. This material may have been periodically deposited on marginal sand¯ats by seiches. Lenses of ®ner, fossiliferous grey-coloured mudstone indicate bodies of standing water on the ¯ats, similarly fed by lagoonal washover. The ostracod Theriosynoecum ®ttoni was considered by Horne (1995) to be a probable indicator of freshwater conditions. Exposures of the lower part of the White Rock in sections 200 m, 300 m and 800 m north-west of Cowleaze Chine reveal ®ner-grained and more uniform ligni- tic clays containing dispersed, well-preserved Filosina. Additionally, micropalaeon- tological preparation of sediment from the latter site (below Barnes High; SZ 438 807) has revealed abundant Theriosynoecum ®ttoni, suggesting an onshore-offshore gradient in this direction. 478 J. D. Radley et al.

At Cowleaze Chine, the ®ner-grained and bioturbated nature of the higher part of the bed similarly indicates an increase in water depth, linked to lagoonal transgression. This culminated in deposition of the fossiliferous dysaerobic muds of the Cowleaze Chine Member (Stewart, 1981). Rootlets penetrating the top of the White Rock indicate temporary plant colonisation during this transgressive phase.

4. Footcasts in the Barnes High Sandstone Member Lithology and track preservation The Barnes High Sandstone at Ather®eld (approximately 6 m thick) is a coarsen- ing-up succession, comprising lenticular and wavy-bedded mudstone/sandstone units passing up into cross-bedded shelly and bioturbated calcite-cemented and ferruginous sandstones (Daley & Stewart, 1979). Fallen slabs of sandstone containing abundant disarticulated and fully articulated Filosina sp. and Unio cf. elon- gata Cornuel occur on the shore between Shepherd's Chine and Cowleaze Chine (Figure 1) and occasionally display distorted or otherwise poorly-preserved robust tridactyl hypichnial sandstone footcasts (Figure 3c). These are approximately 60 cm long and match Iguanodon pes tracks (Woodhams & Hines, 1989 and above). Recently, Dr D. Martill (pers. comm., 1998) has additionally recorded a small theropod footcast. The fossil content indicates that the slabs are derived from a level within the highest two metres of the member.

Deposition The Barnes High Sandstone is interpreted as a shallowing-upward river mouth bar succession and the higher cross-bedded units may have been deposited in a bar crest setting (Stewart, 1981). The distorted nature of many of the footcasts suggests that they may be undertracks (Lockley, 1989), thus possibly relating to dinosaur activity at a higher level within the top of the Barnes High Sandstone or basal Shepherd's Chine Member.

5. Footcasts in the Shepherd's Chine Member Description of theropod footcasts Medium-sized footcasts, tridactyl, digitigrade, deepening distally and terminated by three, slender, clawed digits (ii, iii, iv). Angles between digits are approximately 35. Mean length is approximately 265 mm, mean width is approximately 245 mm. Overall length of digit iii and distal claw is approximately 190 mm. Casts of hallux and digit v are absent. Possible phalangeal pad casts are occasionally preserved.

Discussion The footcasts are attributed to the pes of a small to medium-sized theropod dinosaur on account of their gracile `bird's foot' appearance and clear traces of sharp elongate claws (Thulborn & Wade, 1984; Pittman, 1989; Woodhams & Hines, 1989, Figure 3d). English Wealden theropods are known to include coelurosaurs, carnosaurians and the enigmatic Baryonyx walkeri (Benton & Spencer, 1995); however, the traces cannot con®dently be attributed to a known taxon, given the relatively generalised nature of theropod pedal morphology. Using the morpho- metric ratios of Thulborn (1989), the biometric data suggest that hip height of the track producer was approximately 125 cms. Data from other bipedal dinosaur Dinosaur tracks in southern England 479 trackways and estimates of stride (Alonso, 1989; Thulborn, 1989) suggest that stride length of the trackway producer may have been in the order of 1.5±1.7 m. Similar footprints or casts have frequently been attributed to the ichnogenus Gral- lator (Lull, 1904; Irby, 1995).

Description of ornithopod footcast Small footcast, tridactyl, digitigrade, deepening distally and terminated by three blunt digits (ii, iii, iv). Angles between digits are approximately 50. Length is 170 mm, width is 150 mm.

Discussion The footcast is attributed to the pes of a small ornithopod on account of the presence of relatively robust digit casts, which do not bear claws (Thulborn, 1990; Figure 3f). The biometric data indicate a hip height of approximately 80 cm (Thulborn, 1989).

Preservation The theropod and small ornithopod footcasts described above occur as hypichnial casts on tabular grey limestone slabs. The lithologies of the casts and the overlying tabular layer differ, although mixing often occurs. Sharply de®ned theropod footcasts are common (Figure 3d, e), but range into distorted examples. Examples of shallow, `ghost' casts have also been found. Maximum footcast depth is preserved distally, and ranges from approximately 20 mm in undistorted examples to around 75 mm in distorted specimens. The high-®delity of preservation and nature of the cast medium (see below) indicate that these are primary casts, rather than remnants of an undertrack (Lockley, 1989). Descriptions of the preserving lithology are outlined below.

Footcast in®ll This comprises sand-grade bioclastic sediment with a ®ne-grained sparite cement. It is packed with gastropod shells (Viviparus infracretacicus Huckriede), scattered quartz grains, clay and much comminuted phosphatic ®sh debris (Figure 4a). This lithology is referred to herein as Viviparus coquina (see below and Figure 5). Low angle lamination is occasionally discernible, and traces of elongate shallow burrow casts (up to 17 mm) occur sporadically on the footcasts, especially in the vicinity of the shallow heel region. Fish debris is sometimes concentrated as a basal lag. The gastropods are small (up to 8 mm long), and are ®lled with sparry bioclast sand or rarely, lithi®ed mud. Both disarticulated and articulated Filosina gregaria occur, but are relatively rare. Articulated examples are in®lled by bioclastic sand, sparry calcite, or rarely by laminated mudstone. Algal borings sometimes occur, preserved as pyritic in®lls around shell peripheries. Slabs without footcasts occasionally preserve the Viviparus coquina as a thin (up to 15 mm) planar veneer. The slabs sometimes retain patches of pale to dark grey silty mudstone in inter- stices adjacent to toe casts, as intraclasts, or as lenses within the skeletal accumulation. The shells have been neomorphically replaced by pseudopleochroic calcite, or pyritised.

Overlying tabular layer This is referred to herein as the burrow-cast coquina (see below and Figure 5). The base constitutes a slightly irregular to planar pavement of dominantly con- 480 J. D. Radley et al.

Figure 4. Features of theropod trackway horizon (limestone bed 1: Diplocraterion limestone), Shep- herd's Chine Member, Vectis Formation. a, sediment in®lling theropod track, representing the ®rst depositional phase (IWCMS: 1995.856, `Viviparus coquina', see text). Note the well-preserved, relatively uncompacted Viviparus shells, 1.3. b, Diplocraterion preserved on the underside of the `burrow-cast cquina', which representsÂ the second depositional phase (MIWG 6813), 0.26. c, articulated, closed, spar-®lled Filosina preserved in worn, fragmentary theropod footcast (MIWGÂ 7037, `Viviparus coquina', ®rst depositional phase), 0.25. Â vex-up Filosina shells (up to 25 mm long), interspersed with Viviparus and ®sh debris in a matrix of bioclastic-quartz sand. Rarely, articulated Filosina occur. Hypichnial Diplocraterion frequently occur below the basal shell pavement (Figures 4b, 5), preserved as a lithi®ed admixture of clastic grains and comminu-

Figure 5. Diagrammatic representation of the main biostratinomic divisions of the theropod trackway horizon (limestone bed 1: Diplocraterion limestone, Shepherd's Chine Member, Vectis For- mation). Dinosaur tracks in southern England 481 ted vertebrate debris. They are randomly-oriented, vertical or subvertical, and up to about 10 mm deep and 15 mm long. Individual burrows sometimes intersect or merge. Smaller sinuous burrow casts also occur and are tentatively attributed to a small variety of the ichnogenus Planolites. The main skeletal accumulation comprises 15±30 mm of disarticulated Filosina shells in a matrix of Viviparus-rich bioclastic sand and mud with a ®ne sparite cement. The clay mineral content of the rock comprises illite and kaolinite, with illite-smectite mixed layer minerals and some smectite (Dr I. M. West, pers. comm., 1996). Extensive pyritisation has occurred in higher parts of the layer. Filosina shells occur in convex-up, convex-down and rarely, oblique con®gur- ations and are sometimes stacked (Kidwell et al., 1986). The top surface is a pavement of disarticulated, dominantly convex-up Filosina, interspersed with Viviparus.

Provenance The footcast-bearing slabs match the lowest of four shelly limestone beds which are traceable throughout the Ather®eld section (Figure 2; Table 1). This is the U burrow-bearing layer noted by Wach & Ruffell (1991). Stewart et al. (1991) ®rst assigned the burrows to the ichnogenus Diplocraterion (composite shell layer of their account). The bed occurs approximately 17 m above the base of the Shep- herd's Chine Member (Figure 2; limestone bed 1). It is intermittently exposed from the cliff top south-east of Cowleaze Chine (SZ 4444 8000) to a low bank south-east of Shepherd's Chine (SZ 4491 7953) and in the sides of Shepherd's Chine (SZ 4470 7979-4485 7985). Diplocraterion is abundant on the base of the bed throughout these sections (Figure 4b; Table 1), along with occasional straight to curved groove casts. We propose the name Diplocraterion limestone for this distinctive unit, which is easily distinguishable from the thicker and ®ner grained Diplocraterion ironstone higher in the sequence (Daley & Stewart, 1979). Signi®cantly, subaerially-weathered fragments of the Diplocraterion limestone on shale slopes around SZ 4456 7989 have yielded a single, fresh, theropod footcast- bearing slab (Museum of Isle of Wight Geology specimen IWCMS: 1995.856). Although incomplete (Figure 3e), it matches the beach ®nds in terms of size and morphology. Consequently this indicates that part of a trackway probably remains in situ. Small, disturbed sections of the Diplocraterion limestone occur amongst the extensive slips near the site of the recent ®nd and may well yield track-bearing material. Ornithopod footcasts do not appear to have been recorded from the track-bearing layer prior to the recent ®nd (see above and Figure 3f). This indicates the existence of a largely unrevealed trackway near the place of discovery.

Depositional environment The Diplocraterion limestone (limestone bed 1) occurs within ®ne grey blocky mudstones and paper shales containing thin ostracod shell beds. The ostracods are mainly Cypridea cf. valdensis (J. de C. Sowerby) and Theriosynoecum ®ttoni (Mantell) which are low salinity (possibly freshwater) taxa (Horne, 1995). Taxa indicative of higher salinities occur occasionally at this level (Jones, unpublished thesis; cited by Stewart et al., 1991) implying ¯uctuating, but nevertheless essentially low salinities. Ex situ theropod footcasts, a sample of the Diplocraterion limestone and enclosing shale yield abundant low salinity dino¯agellate cysts (also see Batten & Lister, 1988; Harding & Allen, 1995). Assemblages are dominated by 482 J. D. Radley et al.

Vesperopsis fragilis (Harding), with rarer occurrences of taxa such as Loboniella hir- suta Batten & Lister, Corculodinium uniconicum Batten & Lister, and Valensiella (ex Cassiculosphaeridia) parvulum (Batten & Lister). Chlorococcalean algae are represented by rare Botryococcus sp. and Tetraedron paraincus Batten & Lister. Additionally, palynological preparations from ex situ footcasts yielded abundant equidimensional brown wood (co-dominant with Vesperopsis fragilis), whilst the in situ limestone sample was dominated by sheet-like plant tissue material. This indicates a degree of variability in palynological signature, but overall a low salinity. Thin layers of Viviparus infracretacicus occur in the mudstone immediately above the limestone, and the limestone itself largely comprises Viviparus infracretacicus and Filosina gregaria (see above). Recent viviparid gastropods typify muddy substrates in standing or slow moving freshwater environments (Gray, 1988). Lower Cretaceous viviparids are generally taken to indicate freshwater to lower oligohaline salinities (0±5 parts per thousand, FuÈrsich & Kauffman, 1984; Morter, 1984). The common co-occurrence of Viviparus and the probably oligohaline-mesohaline Filosina (see above) in parautochthonous assemblages (Kidwell et al., 1986) throughout the Vectis Formation indicates comparable tolerances, and in all probability a degree of euryhaline tol- erance within the oligohaline-mesohaline range. The ®nely laminated nature and dark grey colouration of the underlying mudstones indicate deposition under at least episodically dysaerobic conditions. The presence of thin ostracod shell beds implies periodic wave-induced subaqueous winnowing. Shallow (a few mm deep) ostracod-®lled vertical polygonal cracks were noted 30 cm below the coquina layer in Shepherd's Chine (SZ 4485 7985), implying periodic emergence and desiccation of muddy substrates. The high ®delity preservation of the footcasts indicates that at the time of trackway formation, the underlying mud was moist and moderately to ®rmly cohesive (Laporte & Beh- rensmeyer, 1980; Lockley, 1991) and therefore submerged to a shallow depth or temporarily emergent. Shallow, poorly-preserved examples suggest variation in sediment water content across the mud¯at (Lockley, 1986), as does a partial cast of a probable polygonal desiccation crack on the underside of one limestone slab.

Biostratinomy of Viviparus coquina As a subdiscipline of taphonomy, biostratinomic studies chart the post-mortem, pre- burial histories of skeletal particles (Seilacher, 1973) and potentially provide important sedimentological data. The high grade preservation of the footcasts indicates a minimal time lapse between trackway emplacement and deposition of the footprint ®ll. The clean-washed bioclastic in®ll within most gastropods from the footprint ®lls suggests derivation from an agitated shoreface or strandline setting. The sharp-based nature of the ®ll and the occasional preservation of low-angle lamination implies subaqueous deposition as a migrating sheet of skeletal sand. The occurrence of a silty lens (see above) indicates that this ¯ood event com- prised at least two depositional pulses with brief intervening quiescence. High resolution footcast preservation also implies rapid deposition of the ®ll, with only localised erosion of the underlying substrate (Laporte & Behrensmeyer, 1980). Supporting evidence for rapid deposition comes from the presence of articulated bivalves within the skeletal concentration. Within the Vectis Formation, Filo- sina occurs commonly in background mud facies as disarticulated shells in parautochthonous winnowed pavements. Closed, articulated examples are relatively rare at most levels, and con®rm the susceptibility of shallow infauna to Dinosaur tracks in southern England 483 reworking (Tanabe & Arimura, 1987; Bromley, 1990). Local concentrations of articulated Filosina in footprint in®lls (Figure 4c) therefore indicate exhumation of live bivalves from subaqueous muds and onshore entrainment within the gastropod-rich sediment. Rapid sedimentation of this material resulted in near-instan- taneous burial in an unfamiliar sedimentary medium. Some of the bivalves occur in the inferred normal life position, with the commissural plane oriented vertically or subvertically, and posterior margin directed upwards (Stanley, 1970; FuÈrsich, 1980) whereas others are disoriented. All orientations indicate failed escape responses, due either to siphonal clogging or toxic effects (Kranz, 1974; Brett & Seilacher, 1991). Sparry calcite sometimes partially or completely ®lls the remain- ing voids between valves (Figure 4c), indicating aerobic decay of soft tissues (Brett & Baird, 1986; Lockley, 1986). In®lls of mud and geopetal bioclast sand in some examples suggest post-depositional sedimentary in®ltration of perforated or slightly gaping bivalves. Biostratinomic features of the overlying burrow-cast coquina (see below) imply that the ¯ood event described above was due to a barometrically and wind induced shoreward water swell, during the onset of an onshore-directed storm (Aigner, 1985; Brett & Seilacher, 1991). Occasional groove casts indicate tool marks created by onshore movement of small objects (probably biogenic debris) across the mud¯ats during this phase.

Biostratinomy of burrow-cast coquina The basal layer of convex-up Filosina (Figure 5) indicates renewed bioclastic input, from an offshore subaqueous Filosina-rich substrate, followed by wave or current reworking (Futterer, 1978; Kidwell & Bosence, 1991). This material accumulated as a uniform sheet, well beyond the vicinity of the trackways. The presence of obruted Filosina unsuccessfully escaping from the underlying Vivi- parus coquina into this higher layer indicates at best only a short intermission between the two events. At least some of the gastropod shells present in the higher layer (Figure 5) must be reworked products of pre-existing skeletal accumulations, deposited on mud- ¯ats around the trackways during the earlier phase. The abundance of disarticulated and variably-oriented, sometimes stacked Filosina shells in the main part of the burrow-cast coquina indicates continued bioclast input from Filosina pavements and rapid subaqueous storm deposition of this material (Kidwell & Bosence, 1991; Banerjee & Kidwell, 1991). The resulting storm sheet sealed the deeper footprint in®lls and armoured the surrounding mud¯ats, but locally reworked Viviparus shells from shallower prints. The Diplocraterion burrows are interpreted as the dwelling structures of soft- bodied suspension or deposit feeders. They are absent from the deeper, distal parts of the footcasts, but may be sporadically developed on the shallow heel regions and attain normal background abundance between digits. This suggests that the burrowers colonised the freshly ¯ooded mud¯ats in the brief interval between the in¯uxes of Viviparus and burrow-cast coquinas. Such widespread colonisation must have been rapid and hindered only by local accumulations of poorly-penetrable Viviparus sand such as that ®lling the ¯ooded theropod tracks. Bioturbation was curtailed by the ensuing in¯ux of Filosina-rich sediment comprising the burrow-cast coquina. The apparent absence of endichnial Diplocrater- ion within the burrow-cast coquina suggests that the burrowers did not survive this latter depositional event. 484 J. D. Radley et al.

The shell pavement capping the deposit re¯ects decreased sediment supply and winnowing of the recently deposited coquina as the storm abated (Seilacher & Aigner, 1991). Thin, parautochthonous Viviparus and ostracod shell beds in shales capping the coquina re¯ect recolonisation of submerged substrates by opportunistic feeders and periodic winnowing during and after terminal mud fallout. Pyrite within the burrow-cast coquina re¯ects early diagenetic reduction of rapidly buried organic material.

6. Discussion and conclusions These occurrences of vertebrate ichnofossils provide further evidence for optimum preservational conditions in wetted areas, marginal to long-standing water bodies (Laporte & Behrensmeyer, 1980; Scrivner & Bottjer, 1986; Cohen et al., 1991). Lithologically variable sediments comprising the lower part of the White Rock were deposited in a marginal lagoonal environment by an interplay of climatically- controlled terrestrial runoff and occasional lagoonal in¯uxes. Iguanodon footprints and extensive dinosaur-induced trampling indicate at least periodic subaerial exposure of damp substrates. Prints are in®lled by both conglomeratic and ®ner- grained ®lls, and high resolution preservation indicates that they escaped erosion or further trampling, by rapid burial. Iguanodon and theropod footcasts on fallen blocks of the Barnes High Sandstone may possibly be transmitted undertracks. Molluscs have inhabited non-marine environments since the mid-Palaeozoic period (Gray, 1988) but it was only in post-Triassic times that they became important sedimentary constituents in low salinity regimes and potentially signi®- cant contributors to trackway preservation. Theropod and ornithopod trackways in the Shepherd's Chine Member owe their preservation to molluscan productivity within the Vectis lagoon, and a combination of background and event biostratinomic processes. In contrast to the underlying alluvial facies (Wessex For- mation), dinosaur remains are rare in the Vectis Formation, and generally restricted to partial Iguanodon skeletons (Hooley, 1925; Clarke & Barker, 1993; Benton & Spencer, 1995). Consequently the chance preservation of these trackways provides unique evidence of potentially widespread theropod and ornithopod activity on low salinity marginal lagoonal mud¯ats. Live reptilian prey, stranded or drowned vertebrate carcasses or live aquatic prey may have attracted theropods; additionally, open shoreline corridors may have been advantageous in providing unobstructed all-round views. The preservation of footcasts in the Shepherd's Chine Member re¯ects the dynamics of a powerful storm. Initial surges inundated a marginal lagoonal mud- ¯at and had the dual effect of introducing Viviparus-rich shell sand from a relatively high-energy area and exhuming shallow-burrowing bivalves (Filosina) from ®ner substrates. The fresh trackways acted as a series of taphonomic `traps' for this material (Laporte & Behrensmeyer, 1980) entombing the bivalves. The small size and relatively compact, unornamented morphology of the gastropod shells (Figure 4a) enhanced the high resolution footcast preservation. These and associated burrow casts were subsequently armoured by a sheet of Filosina- rich sediment (the burrow-cast coquina), which also effectively sealed the fate of rapidly-buried Filosina and opportunistic soft-bodied burrowers. Mud fallout resumed in standing water as storm conditions waned, burying and preserving the shelly accumulation. Dinosaur tracks in southern England 485

Figure 6. Model relating track preservation in the White Rock and Shepherd's Chine Member to rates of trampling and deposition. Preservation of primary trackways in the White Rock and Shepherd's Chine Member was due to rapid burial soon after formation. The co-occurrence of well- preserved Iguanodon footprints and indeterminate trampling features in sand¯at facies at the base of the formation (White Rock) indicates relatively high rates of trampling (Figure 6) in an area supporting large dinosaur populations. By contrast, the higher, spatially separated occurrences of theropod and ornithopod trackways in storm-in¯uenced mud¯at facies (Shepherd's Chine Member) indicates a low productivity environment, occasionally visited by opportunistic preda- tors or scavengers (Figure 6). To our knowledge these are the youngest occurrences of dinosaur ichnofossils in the Cretaceous deposits of Great Britain.

Acknowledgements We thank Roland Goldring (University of Reading), for constructive comments on an early draft of this paper. Ian West (University of Southampton) provided details of clay mineralogy and Owen Sutcliffe (University of Bristol) provided use- ful advice on invertebrate ichnology. JDR thanks Steve Hutt (Museum of Isle of Wight Geology) and Mick Green (Ryde, Isle of Wight) for assistance in the ®eld and allowing access to collections in their care. Dave Martill (University of Ports- mouth) provided unpublished data concerning a theropod track.

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