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The plant debris beds of the Early Cretaceous (Barremian) Wessex Formation of theIsle of Wight, southern England: their genesis and palaeontological significance

Steven C. Sweetman a,⁎, Allan N. Insole b

a School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth, PO1 3QL, UKb Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, UK

a b s t r a c ta r t i c l e i n f o

Article history:Received 6 March 2009Received in revised form 25 March 2010Accepted 29 March 2010Available online 7 April 2010

Keywords:Debris flowDinosaursEarly CretaceousMicrovertebrateSouthern EnglandWealden

The Early Cretaceous (Barremian) Wessex Formation of the Isle of Wight primarily represents high sinuosityfluvial, but also lacustrine and terrestrial deposition on a low relief floodplain occupying half grabens, withactive normal faults immediately to the north. Plant debris beds form a very small proportion of thesuccession but are the main source of vertebrate fossils, including dinosaurs. These conspicuous, but thin,grey units are distributed randomly throughout the Wessex Formation and are of limited lateral extent. Thelower part of most plant debris beds comprises a poorly sorted, matrix-supported conglomerate. The upperpart of most consists of structureless mudstone, which usually grades up into colour-mottled butpredominantly red and purple floodplain mudstones. The plant debris beds contain an unusual mixture offreshwater molluscs, diverse aquatic and terrestrial vertebrates, and mainly terrestrial plants with a variablequality of preservation. We conclude that each plant debris bed represents a locally generated sheetflood,which was then transformed on the floodplain into a debris flow by the acquisition of surface material. Thedebris flows surged into, and were deposited in depressions on the alluvial plain, including abandoned chuteand cut-off channels, oxbows and ponds. These units are therefore intrabasinal and interfluvial, and thefossils that they contain are locally derived. Similar deposits of palaeontological significance have not beenreported elsewhere in the literature. In view of this, their origin and sedimentology, their palaeontologicalsignificance, and the taphonomy of the flora and fauna they contain are discussed.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The Lower Cretaceous Wealden Supergroup of southern England,comprising terrestrial, fluvial, lacustrine and lagoonal strata, wasdeposited in two sub-basins, the Weald sub-basin in south-easternEngland, and the smaller Wessex sub-basin in central-southernEngland (Fig. 1). The Wealden Group (Daley and Stewart, 1979) ofthe Isle of Wight crops out in two areas, a small area on the south-eastcoast within the core of the Sandown Anticline and a larger one on thesouth-west coast in the core of the Brighstone Anticline (Fig. 2). Inboth areas, the Wealden Group can be subdivided into two units: theWessex Formation (formerlyWealdenMarls) and the overlying VectisFormation (formerly Wealden Shales) (Daley and Stewart, 1979,Stewart, 1981a).

Plant debris beds (Oldham, 1976) make up only about 4% of theexposed Wessex Formation floodplain succession (Stewart, 1978)(Fig. 3), but they constitute the main source of macro- and microfossilmaterial. The Barremian marked a point of transition for terrestrialvertebrates from an essentially relict assemblage of Late Jurassic

aspect to the highly derived assemblages of the Late Cretaceous.Elsewhere in the world there are few deposits of similar age yieldingterrestrial vertebrates. This, together with the palaeogeographicallocation of the Isle of Wight during the Early Cretaceous (centralLaurasia, latitude about 30–35° N), render the vertebrate-bearingplant debris beds of the Wessex Formation of considerable impor-tance. Similar deposits, which we conclude represent locally gener-ated sheetfloods that were transformed on the floodplain into debrisflows by the acquisition of surface material, do not appear to havebeen described elsewhere in the literature. Their origin andsedimentology, their palaeontological significance, and the taphono-my of the flora and fauna they contain are discussed.

2. Palaeogeomorphic and geological setting

At the time of deposition of the Wessex Formation crustalextension was taking place (Chadwick, 1985; Underhill, 2002) andthis is characterized by the development of tilt-blocks and halfgrabens bounded by normal faults (Leeder and Gawthorpe, 1987). Inthe case of the Wessex Formation, these faults delineated thenorthern margin of an east–west orientated valley. Deposition withinthe river valley generally consisted of extrabasinal sediment laiddown in channel, point bar and floodplain environments by a

Palaeogeography, Palaeoclimatology, Palaeoecology 292 (2010) 409–424

⁎ Corresponding author. Tel.: +44 7860 499851.E-mail address: steven.sweetman@port.ac.uk (S.C. Sweetman).

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perennial, moderately sized, high sinuosity river (Insole and Hutt,1994). Palaeocurrent, petrographic and heavy mineral data indicatesediment derivation from the Breton-Cornubian Massif to the west(Stewart, 1978; Allen, 1998). The river carried a dominant suspensionload with reduced bed load leading to the development of muddypoint bars (Stewart, 1983). However, at times the bed load increased,probably in response to climatic events in the hinterland. During theseperiods of increased flow, sandy point bars were formed (Stewart,1981b). It appears that lakes and/or ponds occurred locally on thefloodplain, but they were commonly temporary features andfrequently dried out with pond and overbank mudstones oftenexhibiting oxidization and deep desiccation cracks. Some areas of thealluvial plain were sediment-starved for sufficiently long intervals oftime for pedogenesis and immature calcrete formation to occur(Fig. 4). Insole and Hutt (1994, pp. 199–202) recognized six majorfacies associations in the succession.

The maximum outcrop thickness of Isle of Wight Wealden Groupstrata is about 240 m and of this the exposed Wessex Formationcomprises about 180 m (Stewart, 1978). However, the Arreton 1borehole proved a total thickness of about 580 m for the WealdenGroup of the Isle of Wight (Falcon and Kent, 1960), althoughgeophysical evidence indicates that, north of the Isle of WightMonocline (the flexure in Fig. 2), the Wealden Group is either absentor very thin (Allen and Holloway, 1984; Chadwick, 1985).

Although correlation of the non-marine Wessex Formation of theIsle of Wight with the marine Lower Cretaceous of the North Sea areaand the Tethyan Realm remains uncertain (Rawson et al., 1978; Allenand Wimbledon, 1991), palynological, magnetostratigraphic andfossil wood carbon isotope data suggest that the whole of the exposedWessex Formation is Barremian in age (Kerth and Hailwood, 1988;Hughes and McDougall, 1990; Robinson and Hesselbo, 2004). It alsoindicates that the overlying Vectis Formation comprises deposits of

Fig. 1. Outline geological map of southern Britain showing the distribution of theWessex Formation and otherWealden Supergroup strata, and the location of theWeald andWessexsub-basins.

Fig. 2. A, outline geological map of the Isle of Wight; B, location map.

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late Barremian to early Aptian age and that the Hauterivian–Barremian boundary lies close to the level of the so-called “PineRaft” (White, 1921) exposed on the foreshore at Hanover Point (NGRSZ 379837) (Figs. 2 and 3).

3. Palaeoclimate

The Early Cretaceous climate in north-west Europe is considered tohave been warm and humid (Allen, 1975, 1989, 1998; Batten, 1975;Sladen, 1983; Sladen and Batten, 1984; Hallam et al., 1992; Haywoodet al., 2004). However, there is evidence of a phase of increasing

aridity during the late Barremian to early Aptian (Ruffell and Batten,1990). In the Wessex Basin, sedimentological evidence, includingpseudoanticlines, mudcracks and incipient calcretes, suggests that thearea experienced a warm, equable palaeoclimate (mean annualtemperature 20–25 °C) with a low seasonal rainfall (b500 mm a−1)(Insole and Hutt, 1994). Watson and Alvin (1996) and Allen (1998)showed that the Wessex Formation flora was both fire- and drought-resistant and suggested that it was adapted to a seasonal climate withperiods ofmarked aridity. Evidence for a wet season is provided by thefrequent occurrence of fungal decay in plant material from the plantdebris beds (Alvin et al., 1981).

Fig. 3. Schematic lithological logs of theWessex Formation exposed on the Isle ofWight to show the distribution of plant debris beds. South-west coast sections after Stewart (1978);south-east coast section after Radley (1994).

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Based on a numeric climate model, Haywood et al. (2004)concluded that cold month mean temperatures in the Wessex Basinwere as low as 4–8 °C, while warm month mean temperatures mayhave reached 36–40 °C. Their model also indicates that precipitationrates were significant throughout the year with little evidence forseasonal drought suggested by palaeontological and sedimentologicaldata. The average precipitation rate simulated for any one month was4–8 mm/day, whilst during parts of the winter (December throughFebruary) precipitation rates may have exceeded 16 mm/day.However, they also concluded that during the warm months (Junethrough August), moisture lost by evaporation would exceed thatobtained from precipitation causing drought conditions. At such timesvegetation would have been vulnerable to wildfires ignited bylightning strike. The presence of large quantities of pyrofusinite(Scott, 1989) in many of the plant debris beds serves to confirm thatwildfires commonly occurred and, as further discussed, this issignificant when considering the origin of these beds.

4. Plant debris bed lithology and sedimentology

In the account that follows bed numbers for beds exposed on thesouth-west coast of the Isle of Wight follow Stewart (1978). Numbersfor those exposed on the south-east coast follow Radley (1994)(Fig. 3).

The plant debris beds are randomly distributed throughout thesuccession and it appears that there is no direct association betweenthem and any other facies (Fig. 3). They occur as relatively thin unitsvarying in thickness from about 0.1 m to as much as 1.5 m. However,the majority are between 0.7 and 0.9 m thick, but the thickness ofmost varies laterally. Variations in thickness of some, e.g. L1 (Fig. 3), inpart reflects minor variations in the surface topography of theunderlying stratum. However, lateral variations in thickness alsoappear to reflect variations in sediment supply at the time ofdeposition.

The two dimensional nature of coastal exposures frustratesattempts to determine the three-dimensional form of the plant debrisbeds. However, coastal erosion is rapid and observations made overseveral decades suggest that all are lenticular.While some of them canbe seen to infill shallow depressions between 20 and 50 m wide (e.g.

bed CL3 in Compton Bay (Figs. 2 and 3), now almost entirely erodedaway), others appear to be laterally continuous within the availableoutcrop (e.g. L9 north-west of Grange Chine, Fig. 3). Some units, e.g.bed L1 between Chilton Chine and Sudmoor Point (Figs. 2 and 3), arelaterally continuous but are very variable laterally in both thicknessand lithology. In some places bed L1 is nearly 1 m thick with abundantplant material and allochthonous siderite nodules, but elsewhere isdiminished to a thin band of grey siltstone with scattered, fine plantmaterial.

In a small number of examples, an otherwise laterally extensiveunit can be seen to abruptly terminate against the apparent margin ofa depression (e.g. bed L6 exposed south-east of Chilton Chine, Fig. 5).It appears, therefore, that the three-dimensional form of each bed isunique. Furthermore, recent excavations for dinosaur fossils haveconfirmed the limited lateral extent of some of these units (Hutt inWright et al., 2000).

Most plant debris beds occur as single, well-defined beds, but at afew horizons several plant debris beds occur stacked on top of eachother, forming compound units, e.g. beds 26–30 at Yaverland (Figs. 2and 3). However, each bed appears to represent a discrete depositionalevent unrelated to that of the overlying and/or underlying bed(s).

Other than in the case of stacked plant debris beds, the base of eachrests with a sharp, sometimes irregular, contact on floodplainmudstones, siltstones and sandstones (Fig. 6). Where significant

Fig. 4. Calcrete horizons in siltstones and mudstones of the Wessex Formation exposednorth-west of Chilton Chine (Fig. 2) on the south-west coast of the Isle of Wight.Hammer shaft, 300 mm.

Fig. 5. Top, photograph of part of plant debris bed L6 exposed near Chilton Chine (Figs. 2and 3) in August 2004. To the right of this photograph the bed is well developed andlaterally continuous with abundant, poorly sorted plant material, including amber,supported in a grey siltstone matrix. The trampled area (right arrow below) evidentlyrepresents a topographic high around which the debris flow was deposited. Below,diagrammatic representation to define: A, finely laminated mudstones and siltstones;B, colour-mottled, overbank mudstone with occasional desiccation cracks; left arrow,eroded and, right arrow, trampled surface of B; C, area of secondary reduction affectingA and B; D, plant debris bed, conglomeratic at base rapidly fining upwards into; E, lightcoloured siltstone with occasional plant material towards the base, fining upward withno well-defined boundary into F; and F, colour-mottled overbank mudstone withabundant, deep, desiccation cracks.

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amounts of plant material are present in a plant debris bed, there maybe a variably developed zone of secondary iron reduction in theunderlying unit (Fig. 5). An underlying seat-earth with rootlets isnever present and the units are therefore allochthonous. Some units(e.g. bed 38 at Yaverland) appear to have an erosional base but others(e.g. bed L6 near Chilton Chine, Fig. 5) appear to have been depositedon a previously eroded surface. However, in many examples (e.g. L11,the “Grange Chine Black Band,” Fig. 3), there is no clear indication thaterosion took place. In many cases, the undulatory nature of the base ofa unit is due to the topographic irregularity of the depositional surfaceat the onset of plant debris bed deposition and appears unrelated toerosion. That this topographic irregularity is in many cases almostcertainly related to trampling by dinosaurs is clearly illustrated inCompton Bay (Fig. 2). Here, crevasse splay sandstones overlie suchsurfaces. The sandstones are well-cemented producing a durable castof the underlying surfaces including the footprints imprintedupon them (see Martill and Naish, 2001, plate 13A and chapter 10,pp. 310–319).

The basal part of most plant debris beds generally comprises a verypoorly sorted, matrix-supported conglomerate, with clasts of clays-tone, siltstone, carbonate pebbles, plant debris, including rare piecesof amber (Nicholas et al., 1993) and vertebrate material, of all sizes, ina generally uniform siltstone or fine sandstone matrix (Figs. 6 and 7).About 50% of the plant material occurs in the form of fusain. However,

the amount of fusain is extremely variable both between beds and atdifferent exposures of individual beds. The larger pieces of fusain areusually, but not always, rounded (Alvin et al., 1981). However, in viewof the softness of this material transport distances need only havebeen short to produce the observed rounding. The large particle sizeand very high reflectivity of the fusain is characteristic of pyrofusiniteor fossil charcoal (Stach et al., 1982). Since pyrofusinite is inert, resistsdecomposition and floats on water, it is likely to be over-representedrelative to non-fusainized material (Alvin et al., 1981; Stach et al.,1982). The composition of the matrix, as of beds as a whole, isextremely variable and each bed is lithologically unique. The grainsize of the clasts is generally in the 5 to 50 mm range, but large logs upto 1 m in diameter and 3 m long occur in some plant debris beds. Largepartly articulated dinosaur skeletons are found occasionally as well aslarge isolated bones and fragmentary skeletal material. The carbonatepebbles are of two types. In some beds the most abundant typecomprises generally small calcrete pebbles (≤5 mm diameter)derived from immature calcrete horizons within the local alluvialplain (Insole and Hutt, 1994) (Fig. 4). In others, generally larger,irregular to subrounded sideritic clasts (5–150 mm maximumdimension) predominate (Fig. 7). These often contain angularclaystone intraclasts, bivalves, bone fragments and plant material.They typically weather to a brownish colour and one example hasbeen shown to comprise mainly manganoan siderite with minor

Fig. 6. Plant debris bed CL2 exposed in Compton Bay (Figs. 2 and 3) to show: A, sharpcontact with the somewhat undulatory upper surface the underlying crevasse splaysandstone; B, basal conglomerate including very poorly sorted plant material,allochthonous siderite clasts and autochthonous pyrite; C, finely and irregularlylaminated zone with fine comminuted plant material and P, matted, comminuted plantmaterial; D, very fine, unlaminated siltstone; E, zone of transition between D and F; and F,massive, colour-mottled overbank mudstone. Hammer shaft, 300 mm. Insert: petro-graphic thin section viewed under crossed polars of the poorly sorted, subangular, fine,predominantly quartz sandstone comprising zone C and incorporating plant remains withlong axes predominantly parallel to bedding, but note quartz grain Q with long axisperpendicular to bedding. Opaque grain Pt is pyrite.

Fig. 7. Bed L5 exposed north-east of Chilton Chine (Figs. 2 and 3) to show rapidtransition from basal conglomerate to the overlying siltstone and gradual transitionfrom this into the overlying mudstone facies. Arrows labelled S point to allochthonous,subrounded siderite clasts of variable size. Hammer shaft, 300 mm. Insert: petrographicthin section viewed under crossed polars of matrix supporting clasts in the basalconglomerate. Arrow points to approximate level in the bed fromwhich the sample wascollected. Matrix consists of very fine silt with very poorly sorted subangular quartz,rare heavy minerals and plagioclase feldspar (not shown in this view), and smallintraformational clasts of siltstone (Sst) and claystone (Cst).

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amounts of quartz and ?kaolinite (Prior, 1993). These sideritic clastsare clearly allochthonous. Their source is uncertain but they may beearly diagenetic concretions of the type believed to be formed byprecipitation from slightly reducing groundwater in permanentlysaturated soils (Wilson, 1965; Retallack, 1976, 1977). They may thushave been derived from alluvial plain marsh deposits. In addition toallochthonous siderite, autochthonous siderite occurring as concre-tionary masses of variable size, some large enough to partly enclosedinosaur skeletons, is abundant in some plant debris beds (Hutt et al.,2001). These also often enclose intraclasts, plant, invertebrate andsmall vertebrate remains (Fig. 8). In many plant debris beds sideritealso occurs as small (about 1 mm maximum dimension), botryoidalcrystalline particles seen in residues obtained from bulk screening.

Where a plant debris bed can be demonstrated to have beendeposited in a shallow depression, (e.g. bed 12 at Yaverland, Figs. 2and 3, intermittently exposed on the foreshore), the lithology isnoticeably coarser at the base of the lens and fines both upwards andlaterally. Above the basal conglomerate, the lowest part of a plantdebris bed typically comprises an apparently structureless orindistinctly laminated siltstone or fine sandstone. However, in anumber of plant debris beds, distinct laminations delineated by theconcentration of fine carbonaceous material can be seen, e.g. beds CL2and L5 (Figs. 6 and 9). In a few cases, the unit commences withirregular lenses of small-scale cross-bedded, fine sand separated bylaminated silt implying a low flow regime. The laminations in suchcases exhibit numerous small-scale erosion surfaces, suggesting

repeated scouring or higher energy events. Some laminations,including those occurring above the basal conglomerate in someunits, exhibit small-scale deformation caused bymicro-slumping and/or post-depositional compaction and some are packed with matted,comminuted plant debris (Fig. 6). Compressed carbonised logs andstems are usually confined to the basal zone, the largest logs beinglocated at the base of the unit. In this zone, logs and stems arerandomly oriented, some being sub-vertical. Higher in the unit,elongated plant fragments are parallel to bedding and an approxi-mately N–S or NNE–SSW sub-parallel alignment is apparent in someof the few outcrops where long-axis orientation can be observed (e.g.Fig. 10A) (Francis and Harland, 2006). However, measurements ofplant fragment orientation in bed 33 at Yaverland (Fig. 10B) showed amean orientation of 301º–121º. It seems probable that plant fragmentorientation as measured in these exposures reflects flow patternscaused by the topography at the depositional site rather than thegeneral dispersal pattern on the alluvial plain.

Above the basal zone of the bed, there is an upward decrease ingrain size and plant debris also becomes both smaller and sparser. Theupper part of many plant debris beds commonly appears structure-less. The top of plant debris beds is variable. In some units, there is agradation upward into massive, colour-mottled floodplain mudstonesvia a zone of reticulate mottling (Wright et al., 2000) (Fig. 6).Alternatively, the bed is capped by an irregular, thin organic-richzone, in which there is an upward increase in brown goethite patches.Alvin et al. (1981) suggest that this goethite-rich zone is a palaeosol,but Wright et al. (2000) suggest that goethite records oxidation of thetop of the unit during periods of lowered water table. Very thincarbonised rootlet systems occasionally occur in the upper parts ofplant debris beds but are uncommon. In one example, bed CL7 in

Fig. 8. A, autochthonous siderite concretion, containing lignified wood, bivalves andbone fragments, from bed L1 near Sudmoor Point (Figs. 2 and 3). B, C, petrographic thinsections of an autochthonous siderite concretion from Yaverland on the south-eastcoast viewed under crossed polars to show: B, pyrofusinite with secondary calcite fillingpore spaces and cracks, in a matrix comprising small carbonate clasts in amorphousmuddy siderite; C, another thin section from the same sample showing quartz grainsand a plant spore (S) enclosed by a small calcrete nodule.

Fig. 9. Irregular laminations including fine plant material occurring above the basalconglomerate in an exposure of bed L5 north-west of Chilton Chine (Figs. 2 and 3). Thisexposure was situated about 25 m south-east of that shown in Fig. 7 and demonstrateslateral variability in this bed, which is also typical of others. Width of hammer head,175 mm. Insert: petrographic thin section viewed in plain polarized light to show plantremains from one lamina in a very fine, predominantly quartz sand matrix.

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Compton Bay (Figs. 2 and 3, 11), there is an abrupt transition frompoorly sorted plant material in a muddy, very poorly sorted sandmatrix to colour-mottled overbank mudstone.

The upper part of some plant debris beds, e.g. beds 27 and 28 atYaverland, is considerably thicker than the lower conglomeratic part.In good exposures of these, fine parallel laminations are apparent,delineated by differences in the amount of organic matter theycontain (Fig. 12). This part of a bed appears to represent verticalaccretion of sediment in dysaerobic standing water (Radley, 1994).

5. Plant debris bed sedimentation

5.1. Conventional explanations

Stewart (1978, 1981a) interpreted the plant debris beds asdeposits of violent flooding events which carried debris from thechannel system out onto the alluvial flood plain, where depositiontook place in ephemeral lakes. However, while some plant debris bedslie above crevasse splay sandstones, there is no direct faciesassociation between them as would be expected if they were theresult of overbank flooding from an adjacent channel (Wright et al.,2000). Furthermore, the Wessex Formation plant debris beds shownone of the characteristics of sediments deposited in large lakes(Gersib and McCabe, 1981; Talbot and Allen, 1996) and in a few

examples, they can be seen to occupy shallow depressions 20 to 50 mwide.

Within the Wessex Formation, plant material, apart from rootlettraces, is almost entirely confined to the plant debris beds. Thisindicates that wood and other plant debris was usually rapidlydestroyed either on or below the floodplain surface, presumably bydecomposition as a result of insect, bacterial and fungal attack andoxidation. In the case of the plant debris beds, it is clear that thisnormal process was halted. While rootlet traces are fairly common,

Fig. 10. Long-axis orientation of plant material in plant debris beds exposed atYaverland (Figs. 2 and 3): A, bed 22; B, bed 33. Total number of measurements for bed22=117, mean orientation=261°–81° True. Total number of measurements for bed33=81, mean orientation=301°–121° True.

Fig. 11. Plant debris bed CL7 exposed in Compton Bay (Figs. 2 and 3) showing undulatorycontact with the underlying mudstone and abrupt contact with the overlying mudstone.Hammer shaft, 300 mm.

Fig. 12. Fine laminations in organic-rich mudstone occurring above bed 27 at Yaverland(Figs. 2 and 3).

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large tree-like root systems are rare. This suggests that the vegetationcover on the floodplain was for the most part herbaceous. Many of thelarge logs found within the plant debris beds must therefore havebeen transported from outside the immediate vicinity of thedepositional site. Their source is likely to have been some drierinterfluvial area or alternatively a nearby upland region. Both Mantell(1846) and Stewart (1978) suggested that the large logs were derivedfrom sites far upstream, perhaps as far as the eastern fringe of theBreton–Cornubian Massif some 75 to 100 km to the west and south-west (Allen, 1998, Fig. 1, p. 198), but there is no need to hypothesizethat transport distances were large. On the contrary, the poor degreeof sorting of the plant material and the preservation of delicate plantand animal remains supports the conclusion that transport distanceswere short (Alvin et al., 1981; Watson and Alvin, 1996), and that theywere derived from areas close to the sites of deposition and burial.Furthermore, Alvin et al. (1981) noted that some of the plant debrisresembles forest floor litter and that similar concentrations have beenrecorded in modern flood deposits when extensive forest floorerosion occurs. Long distance transport in, and deposition from, themain river system appears therefore unlikely.

All the evidence indicates that each plant debris bed was the resultof a single, relatively rapid depositional event. The fact that plantdebris beds form only a small part of the Wessex Formationsuccession also indicates that these were rare rather than annual,seasonal events. Furthermore, the distribution of plant debris bedswithin the succession shows that the time difference betweendeposition of individual beds was extremely variable and that thereis no cyclicity in their pattern of deposition.

5.2. Debris flow deposition

While the base of some plant debris beds was clearly depositedfrom flowing water, the fabric of basal conglomerates (poor sorting,dispersed megaclasts in a fine-grained matrix) is characteristic ofdeposition from debris flow rather than streamflow (Nemec and Steel,1984). This origin was first suggested by Insole (1982), although nosupporting evidence was presented.

Shultz (1984) suggested that four debris flow regimes can bedistinguished on the basis of yield strength, flow turbulence andintergranular interaction support mechanisms. The generally lowabundance of clasts, the relatively large size of clasts and the lack ofgrading would indicate that the lower part of a typical plant debrisbedwas deposited as a plastic debris flow (type I flow of Shultz, 1984).Furthermore, the concentration of the largest clasts including logs andother plant material at the base of plant debris beds, which wouldnormally be buoyant in fluid flood flow, is consistent with itsincorporation into the front of a moving debris flow wave in a waysimilar to that proposed by Vallance and Scott (1997) for large clastsoccurring in the boulder-rich Osceola mud flow deposits of MountRainier, Washington, USA. In their model, large clasts incorporated inthe front of a debris flowwave are deposited first and then overlain byfiner sediments as the wave advances.

In the case of many of the plant debris beds, the upper part of thebasal conglomerate can be identified as the deposit of a pseudoplasticdebris flow (type III flow of Shultz (1984)) on the basis of the smallersize and greater dispersion of the clasts. This would imply thatdilution of the debris flow caused a reduction in the flow's yieldstrength, and hence viscosity, as deposition took place. Shultz (1984)suggested that dilution of a debris flow might be due to interactionbetween a flow and a sheetflood or streamfloods or alternatively thatthe top of a flowmight be remobilized by a subsequent flood. It seemsreasonable to propose that dilution would also occur when a debrisflow entered a standing body of water, although no modern exampleappears to have been reported in the literature. Whatever the cause ofthe dilution, the result would have been a change from a high-strength type I to a low-strength type III flow. At this point, sediment

is deposited or possibly moved as bed load, so that elongate clasts,such as plant material, become oriented parallel to the flow direction(Bluck and Kelling, 1963; Bull, 1972). The same process also accountsfor the absence of well-defined, steep-sided margins, which aretypical of modern debris flow deposits (Nilsen, 1982). In the finalphase of deposition, the slurry thixotropically froze, thus maintainingthe dispersion of clasts within the matrix (Shultz, 1984). The vaguelaminations seen in some plant debris beds may reflect the surgingmotion recorded in debris flows by Sharp and Nobles (1953) andPierson (1981) which can result in repeated partial remobilization ofdeposited material.

The top of the conglomeratic part of plant debris beds is commonlymarked by a transition involving the sudden reduction in the quantityof large clasts, including plant debris, but no great change in matrixgrain size (e.g. Fig. 6). This phase of deposition represents suspensionfallout in still water. Finally, the surface water was lost, either byinfiltration, drainage, evaporation or a combination of these. Thisresulted in the gradual resumption of normal depositional conditionson the floodplain with a fluctuating water table and the onset of soil-forming processes in an oxidizing environment on and within theoverbank (Insole and Hutt, 1994; Wright et al., 2000).

Typical debris flow deposits have smooth, rather flat uppersurfaces through which large clasts may project (Collinson, 1996),unless the top is reworked by rain-wash (Bull, 1972). However, withthe exception of bed CL7 (see above) the plant debris beds usuallyshow a gradational contact with overlying floodplain depositsindicative of a final phase of deposition involving suspension fall outand desiccation as described above.

While large quantities of pyrofusinite are present in many of theplant debris beds, the majority also contain abundant unburned plantmaterial (cf. Lovelace, 2006). This suggests that the wildfiresresponsible for pyrofusinite production were not particularly intense.It also suggests that they may have been restricted to “crown fires” inwhich smaller branches, twigs and pteridophyte groundcover wereburnt, but the trunks and larger woody structures escaped largelyunscathed. The extreme rarity of burnt logs in the Wessex Formationsupports this hypothesis. The most significant effect of wildfires maytherefore have been to denude the forest floor and floodplain, andopen forest canopies (Swanson, 1981), thereby significantly increas-ing runoff and erosion during rainstorms.

With the exception of a small number of beds, e.g. beds 27 and 28at Yaverland (Figs. 2 and 3) where the upper part comprises themajority of the bed and represents an extended period of depositionin standing water (Fig. 12), the sedimentological evidence indicatesthat most of a typical plant debris bed is a debris flow deposit. Debrisflows are generated in areas where there is seasonal or irregularrainfall, vegetation cover is sparse, bedrock is muddy or weathers toproduce muddy debris and slopes are steep enough to generate rapidrunoff and erosion. Most debris flows originate as slumps of saturatedsediment on steep slopes (Shultz, 1984; Lovelace, 2006). The obvioussite of origin for debris flows in the Wessex Formation wouldtherefore appear to be the elevated footwall bounding the northernmargin of the river valley. However, the lithology of the non-biogenicclasts indicates that the source of the sediment was the surroundingalluvial plain and not the footwall which comprised strata of LateJurassic age (Radley et al., 1998).

The slopes required to generate debris flows could have beenproduced by one or a combination of two processes. The presence ofcalcretes within interfluvial areas indicates temporarily stabilizedgeomorphic surfaces, perhaps slightly elevated above the adjacentriver valleys, as postulated for Old Red Sandstone alluvial plains(Allen, 1974; Allen and Williams, 1979). Heavy rainfall on elevatedpedified areas including the elevated footwall, where soils were alsostabilized by tree roots (Roering et al., 2003) could produce therequired runoff volumes, while calcrete geomorphic terraces on thefloodplain could have provided the necessary slopes and sediment

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sources to generate debris flows. Alternatively, the required slopescould have been produced by localized subsidence due to differentialcompaction within the underlying floodplain, and/or tectonic activity.The former is a function of the sand/mud ratio beneath the floodplain(Bridge and Mackey, 1993; North, 1996) and common fluid escape(Fig. 13) and slump structures provide evidence for the latter(J. Hendry and D. Martill, pers. comm. to S.C. Sweetman, 2008).

In extensional basins, tectonic slopes are produced by a combina-tion of footwall uplift and hanging wall subsidence and in most casesthe footwall scarp is steep in comparison to the hanging wall dip slope(Brun and Choukroune, 1983). In these settings the footwall area isthe main sediment source for the adjacent basin. However, in theWessex Formation the lack of sediment fans incorporating Jurassicclasts and derived fossils (Radley, 2005; Sweetman and Underwood,2006) suggest that movement on the faults were of low magnitudeand progressive rather than sporadic, and that the higher groundcomprising the footwall to the north was of low relief. In addition torestricting sediment supply from the footwall, low magnitude,progressive movement on the faults would also have affected channelmigration which would also have been progressive rather thancomprising periodic and abrupt avulsions as seen in more activetectonic settings, e.g. the Rio Grande rift of Colorado and NewMexico.That movement on the faults was of low magnitude is supported bythe sedimentology of Wessex Formation fluvial deposits which, inavailable outcrops, show no evidence of avulsions, and by the averagesediment accumulation rate. This was in the order of 40 mm/1000 years, the precise value being uncertain due to difficultiesassociated with dating the base and top of the exposed succession.

Bull (1972) suggested that a water flow could ingest sufficientmaterial from the surface over which it is flowing to produceirreversible entrainment and that the flow would then transforminto a plastic flow, but Shultz (1984) disagreed. He considered that adebris flow is unlikely to be produced by this mechanism because thecritical sediment concentration necessary to produce plastic behav-iour would not be reached. Nevertheless, while the currently availableevidence does not permit unambiguous recognition of the origins oftheWessex Formation plant debris beds, it is considered probable thatthey were generated on the floodplain by the contamination of sheetfloodwaters by collected surface material. In view of the proximity ofthe footwall to plant debris bed exposures (between about 0.5 and2 km) on both the south-east and south-west coasts, it seems likely

that some of the floodwater responsible for the subsequentproduction of a debris flow would have had its origin on the footwall(Fig. 14). However, sheet floods could also have been generated on thefloodplain and flowed towards or away from the footwall from slopesgenerated as described above. In either case, the occurrence of water-lain deposits at the base of some units would support the conclusionthat the debris flows formed in response to contamination of sheetfloodwaters.

Once generated, debris flows can move rapidly, even over verygentle slopes, and for great distances. Sharp and Nobles (1953) recordpeak velocities of 4.4 m s−1 on slopes of 0.014. Debris flows aresufficiently dense and viscous to be able to transport clasts weighingmany tonnes (Nilsen, 1982). A debris flow would thus have been ableto transport whole or partly disarticulated dinosaur carcasses, evenwithout decompositional gases, and/or large logs with ease.

6. Palaeontological significance of the plant debris beds

Until recently, study of the fauna and flora contained in the plantdebris beds was hampered by the relative scarcity of well-preservedfossils. It was also made more difficult by the lack of modern researchon those fossils which have been recovered, many of the species beingknown only from very fragmentary material and many being in needof taxonomic revision. However, research in the last few decades onthe flora (Oldham, 1976; Watson and Sincock, 1992; Watson andAlvin, 1996; Collinson et al., 1999), and fauna (Butler and Ford, 1975;Buffetaut and Ford, 1979; Buffetaut and Hutt, 1980; Barker et al.,1997; Radley and Barker, 1998, 2000; Martill and Naish, 2001 andreferences therein; Naish, 2002; Naish and Martill, 2002; Evans et al.,2004; Naish et al., 2004; Sweetman, 2004, 2006a,b, 2007, 2008, 2009;Steel et al., 2005; Sweetman and Underwood, 2006; Mannion, 2009;Witton et al., 2009; Sweetman and Martill, in press) has significantlychanged the situation. There remains a lack of quantitatively usefuldata, largely reflecting the possibly unique taphonomic history ofplant and animal remains in the plant debris beds. However, thediversity of the vertebrate assemblage as a whole sheds valuable lighton vertebrate evolution during a period of time otherwise poorlyrepresented in the fossil record.

6.1. Plants

The flora includes few recognisable macrofossils, other than twigs,logs and occasional cones, but spores (Fig. 8C) and dispersed cuticles areabundant. It contains charophytes, pteridophytes, caytoniales, cycado-phytes, gingkophytes, coniferophytes and angiosperms (Batten, 1975;Hughes, 1975;Oldham, 1976;Alvinet al., 1981;Alvin, 1983; Collinsonetal., 1999). Amber has been recorded from a number of plant debris beds(Stewart, 1978; Nicholas et al., 1993; Selden, 2002; Bray and Anderson,2008; Jarzembowski et al., 2008) and infrared spectroscopy supports aconiferous origin for the original resin (Nicholas et al., 1993; Bray andAnderson, 2008). The conifer Pseudofrenelopsis parceramosa, appears tohave been a dominant element in the flora, with perhaps just a fewassociated species (Oldham, l976; Watson and Alvin, 1996) but seedsand spores recently recoveredwhile bulk screening formicrovertebrateremains indicate that the flora was more diverse than previouslyrecognized (Sweetman, 2007). The recorded flora suggests that thefloodplain was covered by savannah or chaparral-like vegetationconsisting of a ground cover primarily of pteridophytes with scattered,open conifer stands. Wildfires would have tended to prevent thedevelopment of dense growth of gymnosperms (Harris, 1981). At somehorizons, large numbers of megaspores occur. Some of these probablyrepresent aquatic plants and this suggests that some of the pools ofstanding water were colonised by such plants (Batten et al., 1996;Batten, 1998).

Fig. 13. A fluid escape structure in a sandstone exposed about 20 m south-east ofChilton Chine (Fig. 2). Hammer shaft, 300 mm.

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6.2. Molluscs

Macroinvertebrates are relatively uncommon in the plant debrisbeds. Nippononaia (Subnippononiaia) fordi, Margaritifera (Pseudunio)valdensis, Unio elongata and other unionids are abundant in someunits (Radley, 1994; Barker et al., 1997; Radley and Barker, 1998,2000). However, the unionid assemblage as a whole, which appears tobe diverse, awaits detailed study. Comparison with the living relativesof these bivalves indicates that they were semi-infaunal, low-levelsuspension feeders inhabiting shallow, perennial, well-oxygenatedpools and lakes of low turbidity (Good, 2004).

Grazing gastropods are uncommon, although Viviparus and Physahave been recorded (Radley, 1994; Radley and Barker, 2000) andplanorbids are common in one of the plant debris beds (L2, Fig. 3)exposed at Sudmoor Point (Fig. 2) (Sweetman, 2007). Viviparidgastropods are epifaunal, ciliary feeders, which typically live instanding or slowly flowing freshwater environments where thesubstrate is fine-grained (Gray, 1988). Physids are air-breathinggastropods, which normally inhabit rooted vegetation in the marginalzones of ponds and lakes (Good, 1987; Gray, 1988). Modernplanorbids inhabit slow moving to still, well-vegetated ponds andlakes (Kerney, 1999). The comparative rarity of gastropods in theplant debris beds may indicate that ponds on the floodplain wereephemeral but probably, at least in part, reflects a preservation bias.Gastropod eggs adhering to bone and plant material are abundant atexposures of bed L1 where it approaches the beach north-west ofChilton Chine on the south-west coast of the island (Martill, 2001; S.C.Sweetman, pers. obs.) (Figs. 2 and 3).

6.3. Arthropods

Arthropod remains (with the exception of carbonaceous coprolitesattributable to termites, discussed below) are uncommon in theWessex Formation. However, detailed examination of the low densityresidue fractions obtained from bulk screening, which has not yet

taken place, may alter this perception. Cursory review of two samplesindicates the presence of ?beetle elytra, but whether or not thesebelong to fossil taxa has not yet been determined (S.C. Sweetman,pers. obs.). Recent work on amber has however revealed the presenceinsects and a spider.

Amber has been reported from several horizons but to date only thatobtained from a beach-level exposure of bed L6 south-east of ChiltonChine (Figs. 2 and 3) has been found to contain fossiliferous inclusions.Nicholas et al. (1993) were the first to describe amber from this localityand horizon, which is a typical plant debris bed but which wasinterpreted by them to be afluviatile channel lag deposit. Althoughmostof the amber from this source is dark brown in colour and brittle, it hasyielded chironomids and other Diptera, a wasp (Hymenoptera), acockroach (Blattodea), a beetle (Coleoptera) and a spider (Araneae)(Jarzembowski, 1995; Selden, 2002; Jarzembowski et al., 2008). Thesewere interpreted by Jarzembowski et al. (2008) to have inhabitedaraucarian riparian woodland with seasonal resin production.

Indeterminate ostracods have been reported from one thin, shelly,calcareous sandstone horizon occurring at Yaverland (Radley, 1994)but, until bulk screening for microvertebrates, were otherwiseunknown. Considerable numbers of well-preserved specimens havenow been obtained from a small number of plant debris beds(Sweetman, 2007). They may provide valuable palaeoenvironmentalinformation and aid in correlation between sections on the Isle ofWight and ostracod assemblages recorded from the WealdenSupergroup of mainland Britain, (e.g. Nye et al., 2008), but at presentthis material also awaits study. A number of morphotypes are pres-ent and some of these are cyprids. However, in view of theconsiderable intraspecific variation in valve ornamentation amongcyprids (Anderson, 1967), including species of Cypridia, which cursoryexamination indicates is present in the assemblage, it is not yetpossible to determine how many taxa are present. Conchostracansalso occur in abundance in a thin sandstone occasionally exposed onthe foreshore below the car park at Yaverland but remain to bedescribed (Martill and Naish, 2001).

Fig. 14. Simplified schematic representation of one of the geomorphic and geological settings in which plant debris beds were deposited. See text for discussion of other settings.

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6.4. Vertebrates

Recent research has shown that the vertebrate assemblage of theplant debris beds is highly diverse. Furthermore, the co-occurrence ofremains of large and small forms from both terrestrial and aquaticenvironments appears to be unique to them. The assemblage includeschondrichthyan and osteichthyan fishes, amongwhich are hybodonts,the earliest record of a neoselachian from a freshwater deposit(Sweetman and Underwood, 2006), amiiforms, seminotiforms andpycnodontiforms (Woodward, 1889; Patterson, 1966). Fishremains are often abundant in residues obtained from bulk screening(Sweetman and Underwood, 2006; Sweetman, 2007) but most of thismaterial, which probably includes new and/or previously unrecordedspecies, awaits detailed study. The tetrapod assemblage (Fig. 15)comprises: lissamphibians, including several frogs and salamanders,and an albanerpetontid; perhaps the most diverse lizard assemblageyet recovered from deposits of Early Cretaceous age, includingscincomorphs and anguimorphs; rare turtle remains, representingone genus of Pleurosternidae and one of Plesiochelydae; a diverseassemblage of crocodilians, including members of at least fourneosuchian families, Goniopholididae, Pholidosauridae, Atoposaur-idae and Bernissartiidae, and one eusuchian family incertae sedis;Europe's most speciose dinosaur assemblage including ornithischians,among which are basal forms, a polacanthid ankylosaur, andornithopods, and saurischians including sauropods and theropods,among which are a number of rare, apparently avian forms;uncommon pterosaur remains representing istiodactylids, ornithoch-cheirids, a ctenochasmatid, an azhdarchoid and at least one othercurrently indeterminate taxon and; mammals, including a gobicono-dontid and at least one other eutriconodontan, a spalacolestinespalacotheriid, one or possibly two multituberculates, at least onedryolestid, a stem boreosphenidan possibly a member of Aegialo-dontidae, and perhaps more than one other taxon currentlyrepresented by indeterminate anterior teeth (Benton and Spencer,1995; Martill and Naish, 2001 and references therein; Naish, 2002;Naish and Martill, 2002; Naish et al., 2004; Sweetman, 2004, 2006a,b,2007, 2008, 2009; Steel et al., 2005; Mannion, 2009; Witton et al.,2009; Sweetman and Martill, in press).

In sharp contrast to vertebrate remains obtained from other facies,those obtained from the plant debris beds demonstrate that aconsiderable diversity of trophic resources were being exploited by awide range of vertebrates in the Wessex Formation ecosystem. This isindicative of similar habitat diversity and high vegetational productivityon the floodplain, at least seasonally (contra Insole and Hutt, 1994).

6.5. Palaeoichnology

Burrows are uncommon within the plant debris beds. Identifiableforms include Skolithos, Cylindricum and Planolites. These trace fossilsare typical of the low diversity Scoyenia ichnofacies, which isconcluded to indicate deposition in the shore zone of ephemerallakes and in sluggish streams (Ekdale et al., 1984 pp. 160–162). Thelow diversity and density of trace fossils within the plant debris bedssuggests only temporary colonisation by a pioneer community priorto final burial.

Large (about 100 mm maximum dimension) coprolites areoccasionally encountered while surface prospecting plant debrisbeds for fossils but are relatively uncommon. Irregular to ovoidlumps of phosphatic material probably represent faecal materialproduced by dinosaurs and crocodilians and of these, those containingbone fragments and/or fish scales may be attributable to theropodsand not crocodilians as might be supposed (Fisher, 1981; Goldringet al., 2005). Those with spiralling and grooving are probablyattributable to the larger hybodont sharks recorded from the plantdebris beds (Duffin, 1979; Goldring et al., 2005). In contrast to thelarge coprolites, small coprolites are often abundant in residuesobtained from bulk screening. They also display a variety of shapesand inclusions reflecting the diet and morphology of the alimentarytract of the animals that produced them. Many of those without spiralornament are probably attributable to bony fishes. All residues alsocontain often abundant, small (generally b1 mm long axis), cylindri-cal, carbonaceous coprolites. These are sub-hexagonal in cross-sectionindicating that they were produced by termites (Francis and Harland,2006). Their abundance suggests that termites were an importantelement of the Wessex Formation ecosystem.

Quartz, occurring as beekite ‘discs,’ is also found in many of theplant debris beds. This forms as a replacement of calcite in bivalveshells (Carson, 1991) and either represents the residue of shellmaterial lost to dissolution after plant debris bed deposition or grainsreworked from floodplain sediments prior to final deposition in theplant debris beds. They suggest that bivalves were ubiquitous infreshwater environments on the floodplain.

Occasional extrabasinal clasts are also found in the plant debrisbeds. These are extremely variable in size (2–3 mm to about 10 cmmaximum dimension. S.C. Sweetman, pers. obs.) and lithology, andthe majority are well rounded, and polished. Most are considered tobe gastroliths (Martill and Barker, 2000; Radley, 2005) but the largeLower Jurassic clast reported by Radley (1993) is considered by him tobe a drop stone transported in tree roots.

Fig. 15. Pie charts depicting: A, the relative abundance of tetrapod genera known from theWessex Formation prior to studies involving bulk screening techniques. Data from Bentonand Spencer, 1995; Martill and Naish, 2001; Steel et al., 2005; Paul, 2008; Witton et al., 2009; and Naish, pers. comm. to S.C Sweetman, 2007; B, the relative abundance of tetrapodgenera present in the Wessex Formation combining data from A and that obtained following bulk screening for vertebrate remains (Sweetman, 2007 and pers. obs., 2010). Thenumber of genera in B for each group either represents the known number of genera present or a conservative estimate of the minimum number.

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7. Taphonomy

7.1. Plants

Of necessity, some aspects relating to the taphonomy of plants hasbeen discussed above in connection with the genesis and sedimen-tology of the plant debris beds. However, it is worth reemphasizingthat plant material preserved in them ranges in size from cuticles ofstems and leaves, to logs and that both burned and unburnedmaterialis present in varying amounts. Most of the burned material, occurringas pyrofusinite, represents leaves, twigs and small branches ratherthan logs. While much of the plant material is pyritized, occasionallyother minerals, including calcite, siderite, silica and barite, are alsopresent (Alvin et al., 1981). The preservational quality of the cuticularmaterial is often extremely good, with little evidence of attrition(Oldham, 1976). However, a significant proportion of the plant debris,including some of the fusain, shows evidence of pre-burial decay. Inthe case of fusain much of this decay probably occurred beforecharring took place (Alvin et al., 1981). The abundance of the smalland fragile leaves of the conifer Pseudofrenelopsis parceramosa inseveral plant debris beds indicates that transport distances were short(Watson and Alvin, 1996). Much of the larger unburned plantmaterialis crushed, lignitic, and devoid of primary organic structures.

The permineralised coniferous wood (Alvin et al., 1981) occurringin the “Pine Raft” at Hanover Point is not a constituent of a plant debrisbed. It occurs immediately below plant debris bed CL1 (Fig. 3) in asandstone matrix and probably represents a collection of logsstranded on a point bar.

7.2. Bivalves

Bivalves are preserved as broken to complete and articulatedspecimens. Shell orientation is variable. They are normally articulatedand closed, but occasionally are gaping or disarticulated indicatingreworking of individuals that had recently died (Radley and Barker,2000). Only rarely are bivalves found in inferred life position and thenonly in the fine-grained upper parts of some plant debris beds. Thesmaller unionids sometimes occur as rolled and corroded sideriticclay–ironstone internal moulds (Radley and Barker, 2000) but theoriginal aragonitic shell material is preserved in many occurring inplant debris beds exposed at Yaverland. Shells of unionid species withpermanent openings for the siphons and foot are usually filled withsediment identical to the surrounding matrix. However, some specieslack such permanent openings and in these fine-grained geopetal fillsalso occur as do mineral infills comprising pyrite, siderite and calcite(Radley and Barker, 2000). The degree of articulation and lack ofabrasion indicates that the specimens preserved in the plant debrisbeds were subject to little transport prior to burial and that manywere probably alive when deposition occurred. However, the lack ofescape structures at the top of plant debris beds and the lack ofburrows within them also suggest that the viscosity of the materialenclosing them was too high to allow burrowing and escape. The lackof shell damage also indicates that molluscs were not subjected to thein situ abrasion described by Newell et al. (2007).

7.3. Vertebrates

Althoughmost of the vertebrate material recovered from the plantdebris beds is fragmentary comprising isolated bones and teeth orfragments thereof, these elements mostly show little or no sign ofabrasion. Articulated skeletons do occur, but are uncommon suggest-ing that most of the vertebrate material present in the plant debrisbeds represents carcases that had decomposed prior to incorporationin the beds fromwhich they are derived. The variation in preservationof dinosaur and other bones present in the plant debris beds reflectsthe degree of taphonomic processing to which individual bones were

subject. Modern observations indicate that natural breakage of bonein continental settings is usually the result of subaerial weatheringrather than fluvial transport, while abrasion is normally due tointeraction with water-borne moving sediment during transport(Behrensmeyer, 1991). It appears therefore that during deposition ofthe Wessex Formation, the majority of dinosaur and other carcassestypically decomposed in open areas on the floodplain or in standingbodies of water before being transported relatively short distancesprior to burial. Those finds, which are partly or completely articulated,were evidently the few carcasses which were buried beforesubstantial decomposition and dissociation due to scavenging andother processes, such as weathering and trampling, could take place.Examples include remains of the theropod Eotyrannus, Hutt et al.,2001 and the dryosaurid Valdosaurus found together in bed L11, andnumerous examples of the iguanodontid ‘Iguanodon’ now the subjectof taxonomic review (Paul, 2008).

Although substantially less common, dinosaur remains have alsobeen found in other facies including overbank mudstones, pond/lakemudstones and some of the sandstones and conglomerates. Verte-brate remains in the latter are for the most part very fragmentary andoften substantially abraded and/or polished but occasionally deter-minate material is preserved (Sweetman, 2004). Skeletal materialrecovered from overbank and pond/lake deposits (e.g. that represent-ing the so-called “Barnes High sauropod” obtained from bed SS44(Fig. 3), often shows a greater degree of articulation than thatobtained from the plant debris beds. This suggests that transportdistances for these skeletonswereminimal. In contrast to themajorityof bones recovered from plant debris beds, the quality of bonepreservation in other facies is usually poor with bone exhibitingsubstantial demineralization prior to and possibly after final burial. Itappears therefore that it is low preservation potential for organicmaterial in facies other than the plant debris beds that accounts forthe lack of fossils from these horizons and there is no reason toassume that plants and animals were absent from the environmentsthey represent. This concept is supported by the occurrence ofcomplete skeletons of Hypsilophodon in a red bed (SS45, Fig. 3) at thetop of the succession. Here, as yet poorly understood mechanismsresulted in a mass death event and rapid burial of the resultingcarcases. Although the sediment comprising bed SS45 was oxidizedafter deposition, it appears that rapid burial of the Hypsilophodoncarcases prevented demineralization and disarticulation of skeletalmaterial. Other microvertebrate material is also preserved in this bedbut is otherwise extremely uncommon in facies other than the plantdebris beds (Sweetman, 2007). However, in the plant debris bedsmicrovertebrate remains, which co-occur with macro-remains, areoften exquisitely preserved (Fig. 16). This very gracile albanerpetontidhumerus was found in a sample which also yielded a substantiallyabraded ornithischian caudal vertebral centrumwith a circumferenceof about 80 mm and weighing some 650 g. Despite their often goodstate of preservation, microvertebrate remains also display the samevariations in style of preservation observed in macro-remains (seeMartill, 2001). Whether or not any of these remains were associatedor articulated in the matrix from which they were obtained isuncertain due to the bulk screening techniques (modified fromWard,1981) employed in their recovery.

Fig. 16. Albanerpetontid humerus from bed 38 at Yaverland (Figs. 2 and 3).

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Shipman (1975) reported that, on the East African savannah,during severe drought edible vegetation within a critical distance ofwater is soon eaten, so that many animals die of starvation (and notthirst as might be supposed) near to major water sources. In thesecircumstances, whole carcasses may be readily available for transportover short distances to depositional areas. Decomposition gasestrapped in the body cavity increase buoyancy and therefore aid thetransport of a carcass. It is possible that this occasionally occurred onthe Wessex Formation alluvial plain.

The high taxonomic diversity of the vertebrate assemblage, themixture of forms from terrestrial and aquatic environments, and thevariability of skeletal preservation indicate that the plant debris bedassemblages are not catastrophic death assemblages. Thus, whilewildfire and flood events were involved in the formation of theseunits and may have caused the death of some of the vertebratesrecovered from them, these were not the main cause of death.

7.4. General observations

Pyrite is very common within the plant debris beds, occurring asdiscrete nodules, overgrowths on lignitic plant material and bones,and as void fill within both plants and bones. In the absence of anyevidence for raised salinities during deposition of the plant debrisbeds and indeed theWessex Formation as awhole, the primary sourceof the sulphur appears to be organic material trapped in the deposits.However, the amount of pyrite and therefore sulphur present appearsto be too great to have been derived from the organic material aloneand further research is required to ascertain its origin. Nevertheless,the abundance of pyrite indicates early anoxic diagenesis within thesediment (Canfield and Raiswell, 1991). Clarke and Barker (1993)noted the lithified remains of bacterial mucilages (preserved askutnohorite) within some lguanodon bones. Clarke (1994), followingthe work of Jørgensen (1982), suggested that bacterial films couldhave coated organic material and locally promoted anoxic conditionsby insulating it from the surrounding oxic environment. Whilelocalized anoxic environments may have been formed in this way,this mechanism does not account for the distinctive grey coloration ofthe plant debris beds. The volume of plantmaterial and the abundanceof pyrite all indicate general dysaerobic conditions resulting from thedecay of organic matter immediately after deposition and probablyafter final burial.

The abundance and diversity of vertebrate fossils within a plantdebris bed cannot be predicted from lithology. The exception to thisgeneral rule is that microvertebrate fossils are very rare or absent inthose parts of a plant debris bed occurring above the basalconglomerate. Each plant debris bed is not only unique in terms ofits lithology but also its palaeontology. As well as lateral and verticalvariation in lithology within individual beds, there is also variation inthe amount and taxonomic diversity of their palaeontological content.In many plant debris beds (e.g. bed L14 at Barnes High and bed 38 atYaverland, Figs. 2 and 3), samples taken from as little as 0.5 m apart atthe same level within the bed and of apparently identical lithologyyielded very different microvertebrate assemblages in terms of bothdiversity and abundance (Sweetman, 2007). Also, in almost all cases,microvertebrate remains were obtained in very close specialproximity to the macro-remains of both plants and animals. Theapparent randommixing and uneven distribution of fossils containedin the plant debris beds is consistent with their incorporation in debrisflows. There are, however, some examples where a more predictablesorting of vertebrate remains appears to have occurred. Both Butlerand Ford (1975, multituberculate teeth) and Buffetaut and Ford(1979, crocodile teeth) reported sorting by flowing water in bed CL3in Compton Bay. Similar sorting has also been recorded in other plantdebris beds including bed L9 (S. Hutt, pers. comm. to S.C. Sweetman,2002). In all these cases, currents flowing over stranded logsdeposited silt on the upstream side of the log and downstream

scoured depressions in which vertebrate remains were concentrated.This may relate to local reworking of debris flow deposits prior to finalburial (S.C. Sweetman, pers. obs.).

Previous studies (e.g. Sankey and Baszio, 2008) indicate that themajority of microvertebrate remains are obtained from well sortedfluvial, lagoonal and marginal marine deposits. The degree of sortingobserved in these deposits permits statistical analysis of smallvertebrate remains and often detailed palaeoenvironmental andpalaeogeographical reconstructions, but the remains obtained fromthese settings are often affected by considerable time and/or spaceaveraging. In contrast, the mode of deposition of the plant debris bedshas resulted in minimal sorting of vertebrate remains and similarstatistical analyses cannot be undertaken. However, in view of thelimited lateral extent of each plant debris bed and, therefore, thesource area for both their palaeontological and non-biogenic content,the flora and fauna obtained from them has been subject to minimalspace averaging. The model proposed for the generation of the plantdebris beds involving the contamination of sheetfloods by surfacematerial and erosion of floodplain surface sediments also suggestsminimal time averaging. Their palaeontological content thereforerepresents a ‘snapshot’ of the biota present at the time of deposition ofeach. Differences between them reflect the area of origin of thefloodwaters responsible for their genesis, and the palaeoenviron-ments and the biota present in them sampled by each flood event.

7.5. Comparison with other fossiliferous debris flows

Other deposits exhibiting minimal time and space averaging, andsorting of vertebrate remains are those representing mass mortalityevents, e.g. China's Lower Cretaceous Jehol biota. However, thesedeposits do not generally incorporate debris flows and fossiliferousdebris flows are otherwise rarely reported from the geological record.Those that have been described bear no resemblance in detail to theplant debris beds of the Wessex Formation (Shultz, 1984; Turnbulland Martill, 1988; Andrews and Ersoy, 1990; Bestland, 1990;Andrews, 1995; Bell and Padian, 1995; Fastovsky et al., 1995; Loopeet al., 1999; Rogers, 2005; Eberth et al., 2006; Lovelace, 2006; Kelley,2008; Lauters et al., 2008; Scherzer, 2008). However, the lowerPermian pond bone beds occurring in a red bed dominated, floodplainsequence in central Archer County, Texas, USA, described by Sander(1989), have been considered to be comparable to the plant debrisbeds of the Wessex Formation (Radley, 1994). These organic-richdeposits accumulated in oxbow lakes or swamps and contain bothterrestrial and aquatic vertebrate remains. The plant material alsoincludes fusain and in these respects the Permian deposits doresemble the plant debris beds. However, Sander (1989) states thatbiological and physical processes led to the gradual accumulation ofall of the organic debris. The mode of deposition of these deposits istherefore entirely different to that of the plant debris beds and thelatter appear to be unique in the geological record.

8. Conclusions

Diverse evidence suggests that the plant debris beds are productsof an exceptional combination of otherwise common events, i.e.wildfires and floods. The genesis of each plant debris bed beganwith aforest/brush fire caused by a lightning strike, probably at the end of adry period (Komarek, 1972; Cope and Chaloner, 1980; Haywood et al.,2004). These fires removed ground cover vegetation that normallytraps and holds water, and opened forest canopies. In some areas,calcrete and tree roots would have impeded the infiltration ofrainwater into the soil and thus further increased runoff.

Floodwaters produced by exceptionally heavy rainfall occurringbefore regeneration of the forest canopies and ground covervegetation then caused erosion of the margins of geomorphic terraceson the floodplain, flowed towards low-lying areas, picking up

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floodplain sediments and surface debris en route and transformedinto debris flows. Potential debris included charcoal, leaf litter, logs,dinosaur and other vertebrate carcasses in various states of decom-position and disintegration, invertebrates, and a relatively smallnumber of live vertebrates, including dinosaurs, that were unable toescape the flow.

The debris flows ran into depressions on the alluvial plain, some ofwhich may have already been partly occupied by standing bodies ofwater. The final phase of deposition usually involved the verticalaggradation of fine silts and/or organic-rich muds. Anaerobic condi-tions became established at or before this stage, resulting in thepreservation of organic matter and the production of pyrite.

The plant debris beds are therefore intrabasinal and interfluvialdeposits and the fossils they contain are locally derived. They arelenticular in nature and the majority appear to be of very limitedlateral extent. Close inspection confirms that each is sedimentolog-ically and palaeontologically unique. Differences between themreflect the area of origin of the floodwaters and the palaeoenviron-ments and biota sampled by each flood event. They also reflect thecomposition of floodplain sediments incorporated into each and theenvironment and topography at the site of deposition. The highlydiverse vertebrate assemblages they contain, which include co-occurring taxa representing some of the largest and smallestterrestrial vertebrates to have inhabited the Earth, and which alsoinclude forms from both aquatic and terrestrial environments, areunique in the palaeontological record. The biota as a whole shedsvaluable light on the evolution of terrestrial and freshwater organismsduring the Early Cretaceous, a period of time otherwise very poorlyrepresented in the fossil record.

Acknowledgements

S.C. Sweetman thanks the University of Portsmouth, UK for itssupport and Dave Martill, Jim Hendry and Andy Gale, University ofPortsmouth, for helpful discussions. Also, Geoff Long and Bob Loveridge,University of Portsmouth, for the preparation of thin sections andphotographs respectively. Don Brinkman, Dave Eberth and JimGardner,Royal TyrrellMuseumof Palaeontology, Alberta, Canada, are thanked foradvice concerning the accumulation ofmicrovertebrate remains and theoccurrence of these in debris flows. Financial support by the RTMPCooperating Society is gratefully acknowledged. A.N. Insole thanksMikeBenton and Brian Williams for helpful discussion on the plant debrisbeds over many years. We join in thanking Finn Surlyk for editorialadvice, and John Collinson and Dave Eberth for their comments whichgreatly improved the manuscript.

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