Sedimentology and vertebrate taphonomy of bone-bearing facies fromthe Clockhouse Rock Store (Weald Clay, late Hauterivian),

Capel, Surrey, UK

Elizabeth Cook

COOK, E. 1997 Sedimentology and vertebrate taphonomy of bone-bearing facies from theClockhouse Rock Store (Weald Clay, late Hauterivian), Capel, Surrey, UK. Proceedings of theGeologists' Association, 108, 49-56. The Clockhouse Rock Store [TQ 174384] has for the past twentyyears been a major source of Wealden insect fossils, but the presence of vertebrate remains has only beenbriefly documented. Nine distinct vertebrate-bearing facies are described here. Sedimentary structures inthe sandstones indicate fluvial deposition. Taxa recorded include hybodont sharks, holostean-grade andteleost fish and goniopholidid crocodiles. The vertebrate fossils show taphonomic features confirmingdeposition by rivers.

Department ofGeology, Wills Memorial Building, University ofBristol, Queens Road, BristolBS81RJ.

1. INTRODUCTION

The Wealden Supergroup sediments of southern Englandrange in age from late Berriasian to Barremian or earliestAptian (Allen & Wimbledon, 1991). The sediments containa variety of different bone accumulations, ranging fromscattered bones preserved in unconsolidated clays toconsolidated vertebrate-rich bone-beds. The WealdenSupergroup has two subdivisions: the Hastings Group andthe overlying Weald Clay Group. The Hastings Group isdominated by arenaceous fluvial sandstones and overbanksilts and clays (Allen, 1981, 1990). The Weald Clay wasdeposited under predominantly non-marine conditions, andthe palaeoenvironmental reconstruction is of a lagoonal orlacustrine mudplain complex forming in the subsidingWeald Basin (Allen, 1981, 1990).

The Rock Store is a collection of approximately 100tonnes of siltstone and sandstone slabs recovered from theold clay pit prior to infill. During September 1991 theNature Conservancy Council (now English Nature), SurreyCounty Council, Clockhouse Brickworks and theCurry Fund of the Geologists'Association organized arescue operation to remove the slabs to an enclosure at thetop of the pit. This area and the clay pits have beendesignated a Site of Special Scientific Interest (Ross, 1991,1992).

The Clockhouse Rock Store is significant as it con­tains nine different sedimentary contexts for bonepreservation. The vertebrate material is often heavilyabraded and fragmented. Taxa represented includehybodont sharks, bony fish (for example holostean-gradeand teleost remains) and goniopholidid crocodiles. Thepurpose of this paper is to describe the sedimentology of thesite, and to outline the diverse facies of vertebratepreservation.

Proceedings of the Geologists' Association, 108,49-56.

2. LOCATION AND STRATIGRAPHY

The Clockhouse Brick Works (Fig. 1), near Capel, Surrey[National Grid Reference TQ 174384] exposes a sequenceof clays with lenses and scour fills of silt and sandstones.Approximately 35 m of sediments were exposed here,belonging to the Lower Weald Clay (Fig. 2); of lateHauterivian age (Jarzembowski, 1991a; Ross & Cook,1995). The strata lie stratigraphically below BritishGeological Survey Bed 3a and dip slightly (1.5°) to thenorth (Jarzembowski, 1991a).

3. SEDIMENTOLOGY

A section exposed in the old pit, as measured by Kirkaldy &Bull (1948) consisted of clay with some siltstone andsandstone beds.

Although most of the sediments were probably depositedunder low-energy freshwater conditions, some of the beds,located at the base of the section, were laid down in abrackish-marine environment (Kilenyi & Allen, 1968;Jarzembowski, 199Ia). The coarser-grained, frequentlycross-bedded facies, for example the bone-beds, representshort-lived episodes of fluvial deposition. The presence ofbasin casts and asymetrical ripples suggests that thesechannelized flows were sufficiently fast-flowing to erodethe clays, and that the current velocity decreasedsignificantly prior to deposition. Similar structures havebeen recorded from the Warnham Brickworks, Horsham(Prentice, 1962).

4. PALAEONTOLOGY

A wide variety of taxa are known from the Clockhouse claypits, including various plants, invertebrates, vertebrates and

0016-7878/97 $10·00 © 1997 Geologists' Association

50 E.COOK

10 km

Fig. I. South-eastern England showing the outcrop of the Weald Clay (stippled). The Clockhouse Rock Store is located by an asterisk.

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Fig. 2. Simplified stratigraphy of the Wealden Group, showing theposition of the Clock house Brickworks. Abrev iation : Apt. ,Aptian.(A. J. Ross, pers . cornm., 1994)

trace fossils. Insect fossils are especially abundant and arefound in fine-grained siltstone scour-fills. Common groupsare beetles, cockroaches, true flies and bugs. Rarer formsare dragonflies, scorpionflies, caddisflies, lacewings, grass­hoppers/crickets, termites and wasps (Jarzernbowski , 1991a& b). Other invertebrates include isopods, conchostracans,

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molluscs (for example the bivalves Unio and Filosina) ,ostracods and foraminiferans. Plant fossils include frag­ments of wood, Weichselia fronds and pinnules, and thehorsetail Equisetum (Jarzembowski, 199 Ia).

Vertebrate material is preserved in the clays and con­solidated silty sandstones. The commonest fossils are fishbones, scales and teeth. Taxa include holostean-grade fish(for example Lepidotesi , and the sharks Hybodu s andLissodus, and rare goniopholidid crocodi le teeth. A fewfossils found weathering out of the clay include Hybodusteeth and spines, holostean-grade teeth and scales (forexample Lepidotes), bones of crocodile, pterosaur, turtleand the ornithopod dinosaur Iguanodon (Jarzcmbowski,199Ia) .

S. VERTEBRATE TAPHO NOMY

As the sedimentary section is inaccessible, the bone-bedslabs were not seen in situ. All the material was sampledfrom the rock store. It proved impossible to match the bone­bearing lithologies with the sediments described inWorssam's (1978) section, although it is probable that thebone-bearing slabs came from between the ClockhouseSandstone and the Cassiope Band (A. J. Ross, pers. comm.,1995). The vertical distribution of the bone beds is notknown. Where possible, the slabs were treated with diluteacetic acid to free the vertebrate debris from the matrix.Data from the non-calcareous sediments were collectedfrom the surfaces of the slabs.

The vertebrate material preserved in each bone accumu­lation was described according to several taphonomic para­meters. The size distribution, based on maximum length, ofthe fossils was noted. The nature of any fractures wasrecorded and, where possi ble, their relative timingidentified. The abrasion characteristics of the assemblageswere described following the methodology of Fiorillo

B ONE - B E A RI N G FACIES, C L OCK HOUS E R O C K S T O R E, UK 51

Table I. Taphonomic characteristics of abraded bone material(modified from Fiorillo, 1988; Cook, 1995)

Stage 0 Very angular: the bone (or tooth) is fresh andunabraded. Processes and bone edges well definedand sharp. (Little or no abrasion.)

Stage 1 Subangular: the bone edges and processes are slightlyabraded and polished.

Stage 2 Subrounded: bone edges well rounded, processesrecognizable. (Moderate abrasion.)

Stage 3 Rounded: edges show a high degree of rounding,processes generally remnant.

Stage 4 Extremely rounded. (Extreme abrasion.)

(1988) and Cook (1995). Abrasion, generally producedduring fluvial transportation , causes rounding of the bonesand teeth (Table I). Evidence for in situ weathering, charac­terized by cracks in the surface of the bones (Behrensmeyer,1978; Fiorillo, 1988), and predator or scavenger activitywere also recorded . Although the taphonomic charac­teristics of the bone beds are similar, the sedimentarycontexts are sufficiently different to justify describing themseparately.

Clockhouse Type I Bone Bed

All the material preserved in this sedimentary context (Fig.3a) is heavily fragmented and abraded and individualelements cannot be identified. The hand specimens are ofthree lithofacies: silty clay, silty sandstone and fine- tomedium-grained sandstone. The sandstone is composedprimarily of quartz, with some opaque minerals con­centrated along laminae. The sediment is generally poorlysorted. The silty clay and sandstone do not show anysedimentary structures. The silty sandstone has well­developed fine-scale laminations . The vertebrate materialsare found disseminated throughout the sandstone.

Clockhouse Type II Bone Bed

This facies (Fig. 3b) contains a wide range of sedimentgrades and structures. The lower surfaces of the slabs showtool marks, and the upper surfaces are rippled. Thevertebrate material is found in the coarse-gra ined,structureless conglomerate and associated sandstones. Thesandstones show well-developed cross-laminations.

The vertebrate assemblage is composed entirely of fishremains. Most are not identifiable to generic level, broadlycompri sed of bony fish remains, for example scales,vertebrae, skull bones, indeterminate bones and teeth.Recognizable fossils include holostean-grade fish bones andHybodus teeth. Vertebrate fragments arc up to 13.4 mmlong, although most fall in the 1.0-6.4 mm range. All thematerial is disarticulated and incomplete. Breakage surfaces

show the characteristics of post-mineralization fracturing;the damage probably occurred during collection.

Abrasion stages range from I to 2 (slight to moderaterounding) to 4 (extremely well rounded). Most of the speci­mens fall between stages 2 and 3 (moderately to wellrounded ).

Most of the material shows no evidence of in situweathering, although a few specimens show surfacemodification features characteristic of weathering stage I ;that is, parallel cracks developed in the outer layers of thetissue.

No evidence was found for predator or scavenger activity,for example etching by stomach acids or bite marks.However, this may reflect the small grain size, taxa presentor the environment of deposition. Small bones are morelikely to be swallowed whole, which limits the possibility oftheir being marked by teeth. Moreover, transport tends toremove surface modification feature s result ing fromdigestion .

The sedimentary structures and taphonomic profile ofthis facies suggest deposition by high-energy currents. Thesmall amount of matrix in the conglomerate suggests somedegree of fluvial winnowing, reworking having producedthe generally high levels of abrasion. The scarcity of signsof in situ weathering is consistent with death and depositionin water.

Clockhouse Type III Bone Bed

Weathered surfaces of the sediment (Fig. 3c) are grey,speckled with orange and black; unweathered surfaces aregrey with little iron staining. Sediment grain size rangesfrom medium sand to fine gravel (0.5 mm to 5.0 mm indiameter). The sediment is very poorly sorted. Clasts aresubrounded to rounded and comprise quartz with smallquantities of opaques, vertebrate material (pale brown witha fibrous texture) and Iithics. Most of the grains are matrix­supported and cemented by calcite which is unevenlydistributed. Where the rock is finer-grained there are moregrain-to-grain contacts, less cement and less vertebratedebris. Some poorly developed graded bedding is present.The upper and lower surfaces of the beds are irregular, thelower surface representing scour infills, the upper, ripples.Vertebrate material is disseminated throughout thesediment.

The poor preservation state of the vertebrate material isindicated by high proportions of indeterminate bone andteeth . Identifi able fossils include teeth of the sharksHybodus and Lissodus, with smaller numbers of holostean­grade fish remains. Most of the bone fragments are between1.0 mm and 3.9 mm long; the maximum recorded lengthwas 9.9 mm. Most of the material extracted in this facies isincomplete and bone fractures are characteristic of post­mineralisation damage. Pre-mineralization damage, charac­terised by uneven fracture surfaces, is less common.Abrasion states range from stage I (slightly rounded) tostage 4 (extremely well rounded). Bone fragments aregenerally more heavily abraded than the teeth or scales.

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B ONE - BEARING F A CfE S , C LOCK HOUSE ROCK ST O RE, U K 53

Approximately 12% show features characteristic of stage Iweathering (cracking of the outer layers of bone tissue). Noevidence of predator or scavenger activity was seen, forexample etching of the outer layers of tissue by stomachacids.

The coarse -grained sediment, and the scours and ripplessuggest a high-energ y regime. The presen ce of fine-grainedmatrix indic ates that the sediment was not heavilywinnowed. The high degree of rounding confirms transportin a high-energy environment and some reworking. Theabsence of semi-aquatic and terrestrial vertebrates, such ascrocod iles, turtles and dinosaurs, suggests that the fossilassemblage accum ulated in a fluvi al-lacustrineenvironment.

Clockhouse Type IV Bone Bed

This facies is pale grey in colour, the vertebrate debrisvisible as dark specks. The quartz grains are poorly sorted,ranging from medium to coarse sand. This supports largerfragments of vertebrate fossils, which range from sand-sizeto 16 mm long. Most are approximately 2.0 mm long;pieces over 5.4 rnrn long are rare. Much of the material istoo heavily fragmented and abraded to be identified, butHybodus, holostean-grade fish and indeterminate fish arerecognizable. All of the skeletal remains were found fullydisarticulated. It was impossible (because of poor preser­vation) to quantify the relative abundan ce of pre- and post­mineralization fractures.

The abrasion profile ranges from stages 1- 2 (slightabrasion) to stage 4 (extreme abrasion). Most of the materialis moderately to highly abraded (stages 2, 3, and 4); teethare less heavily abraded. Only 4% of the fragments showeviden ce of ill situ weathering; the remainder areunweathered. No signs of tooth marks or acid etching wereseen, but this does not rule out the possibility of predator orscavenger activity.

The coarse grain size of the sediment indicates transportand deposition under high-energy conditions. The poorsorting suggests rapid deposition, with little winnowing.The abrasion profile of the vertebrate material is charac­teristic of an assemblage that has been subjected to fluvialtransport and reworking. The low incidence of ill situweathering is to be expected for an aquatic assemblagewhich was not exposed for long periods.

Clockhouse Type V Bone Bed

This facies association (Figs 3d & e) consists of a series oflaminated and massive sandstones . The lower surfaces ofthe slabs are marked by sub-parallel ridges approximately30 mm long which are interpreted as tool marks. Thedominant clasts are quartz, with subordinate vertebratedebris. Generally, the sediments are poorly sorted andinclude both matrix and grain supported fabrics. Thevertebrate material is concentrated in the medium- tocoarse-grained sandstone, and is generally found scatteredacross the lower bedding surfaces. Smaller fragments of

bone highlight the cross-lamination surfaces in the finer­grained sandstones.

All the identifiable vertebrate material is attributed tofish; most of the fossils are indeterminate bone fragments.Hyb odus dominates the identifi able remains; holostean­grade fish and Lissodus are present. Vertebrate fragmenrsare generally between 1.0 mm and 4.4 mm long; all arefully disarticulated and none is complete. It was impossibleto work OuI the relative timing of the breaks on the bones.All the fracture surfaces showed the characteristic smoothtexture of post-mineralization breakage. The abrasionranges from stage I (slight abra sion) to stage 4 (extremeabrasion). Stage I is rare, and no fragments are unabraded.Teeth are less heavily abraded than bone. Most of the fossils(96%) show no evidence of subaerial weathering. Theremainder have well-developed parallel cracks in the outerlayers of tissue, characteri stic of weathering stage 1. Thereis no sign of predator or scavenger activity, punctures,grooves and acid etching being absent. The sandstones weredeposited by high-energy fluvial activity. Currents areindicated by tool marks and cross laminations, and thegenerally poor sorting suggests rapid deposition with littleor no winnowing.

The moderate to extreme rounding of the vertebratefragments reflect significant transport and reworking. Thelow incidence of in situ weathering is consi stent with thepreservation of an aquat ic faunal assemblage in fluvialsediments.

Clockhouse Type VI Bone Bed

On weathered surfaces this bone bed (Fig. 3f) is paleyellowish-grey in colour; fresh surfaces are pale grey. Thematrix ranges from silt to medium -grained sand andsupports clasts which range from 0.75 mm to 6.0 mm indiameter (average diameter of 3.0 mm). Bioclastic materialis generally larger, with a maximum length of 17.0 mm;overall the sorting is poor. whereas the matrix is well sorted.The clasts are predominantly subrounded to well-roundedquartz grains with some vertebrate debris and phosphaticnodules.

No evidence of planar or cross-bedding was seen,although there are some indications of graded bedding .Scattered sole structures on the bedding plane probablyrepresent tool marks or bioturbation. Most of the fossilmaterial is concentrated on the bedding planes and showsno preferred orientation. The cement is calcite.

The vertebrate fossils include teeth, scales, vertebrae,elongate bones (probably ribs and fin spines) and abundantindetermin ate skeletal fragments. All the identified verte­brate material is attributed to bony fish or sharks. Taxainclude holostean-grade fish (teeth and scales), Hybodus(teeth) and indeterminate bony fish fragments. Bony fishremains form the bulk of the sample. None of the materialis complete. In most cases the fossils were broken aftermineralization, although pre-mineralization fracture isfairly common. Probably most of the damage occurredduring re-exposure when the pit was being worked, during

54 E.COOK

assembly of the rock store, or during preparation in thelaboratory.

Abrasion ranges from stage 1- 2 (slight abrasion) to stage4 (extremely well rounded). Bones show the widest rangeof abrasion stages; dermal material and teeth are allmoderately abraded. Differen ces in these profiles isaccounted for by the nature of the tissue: teeth and osteo­derms are more resistant to abrasion than bone because ofthe more compact nature of their tissue. Most of thevertebrate material (95.9%) shows no evidence of in situweathering . The remainder bear surface modificationfeatures consistent with weathering stage I. None of thematerial shows any evidence of damage due to carnivores.

The sedimentary structures characteristic of this bone bedsuggest deposition under high-energy conditions. The widerange of abrasion indicates that the vertebrate assemblage isattritional in origin.

Clockhouse Type VII Bone Bed

Fresh surfaces of this lithology (Fig. 3g) are mid-grey incolour and weather slightly darker grey with some ironstaining. The sediment is a poorly-sorted silty sandstonesupporting a few sand-sized grains and vertebrate debris. Itis dominated by angular to subangular quartz clasts withsome larger white nodules and vertebrate debris. The rock iswell cemented by calcite.

The bedding surfaces are planar and sub-parallel. There isno evidence of other sedimentary structures. The vertebratefossils are concentrated on, and orientated parallel to, thebeddin g planes. Vertebrate material is also scatteredthroughout the sediment, becoming more common towardsthe bedding surfaces. Identifiabl e remains, which arescarce, include indeterminate bony fish debris, holostean­grade fish, Hybodus and rare crocodile teeth . Theunidentified material consists of bone fragments and teeth.

Vertebrate fragments with long-axes greater than 6.4 romare rare. All the material is disarticulated and broken. Poorpreservation made it impossible to describe with any degreeof certainty the relative timing of the fragmentation of thebones. The range of abrasion states is wide, from 1-2 (slightabrasion) to stage 4 (extreme abrasion). Most of the materialis moderately to heavily abraded. Bones are generally moreheavily abraded than teeth or osteoderms. Only 1.7% ofbones and teeth showed signs of in situ weathering ; thesedisplay parallel cracks in the outer layer s of tissueattributable to weathering stage I. No evidence of predatoror scavenger activity was observed, for example puncturesand grooves produced by teeth, or pitting produced bystomach acids.

Deposition evidently took place in a moderately energeticenvironment. There are no cross-beds or cross-laminationsto suggest a high-energy regime, and this is confirmed bythe poor sorting which indicates little winnowing. Never­theless the abrasion profile of the vertebrate materialsugg~sts fl.uvial transport and reworking. The weatheringstate IS typical for a dominantly aquatic assemblage.

Clockhouse Type VIII Bone Bed

This facies (Fig. 3h) comprises a bedded sequence of fine­to medium-grained sandstones with small lenses of coarserclastics. Vertebrate debris is scattered throughout the latteras well as scattered randomly throughout the finer-grainedsediments. The finer-grained sediments are moderately wellsorted. The sandstones are composed of quartz and arecemented by calcite. Lower surfaces of the blocks aremarked by straight sub-parallel linear crests interpreted astool marks.

Most of the vertebrate debris is indeterminate bonefragments and teeth. A substantial proportion can beidentified only as bony fish fragments, although Hybodusand holostean-grade fish remains are recognizable. Sizesgenerally lie between 1.0 and 2.0 mm long, althoughfragments up to 9.4 mm long were noted. All the material isdisarticulated and broken. Poor preservati on precludeddetermination of the nature of the fractures .

All the material described is bone which shows moderateto high abrasion. The range of abrasion states is from stage2-3 to stage 4. The majority of the vertebrate debris ismoderately rounded; most (91.7%) is unweathered, the restshowing parallel cracks in the outer layers of tissue typicalof weathering stage I. No signs of tooth punctures, groovesor acid etching were found. The high level of disarticu­lation, fragmentation, weathering and rounding indicate asuccession of high-energy fluvial events.

Clockhousc Type IX Bone Bed

This facies (Fig. 3i) consists of a fine-grained sandstonesandwiched between two layers of finely-laminated silt­stone. The sandstone is composed primarily of quartz withvertebrate material (brown with a fibrous texture) and smallcubic crystals of an opaque mineral (iron pyrites). Fromtheir shape it is assumed that the crystals grew in situ,perhaps suggesting some degree of marine influence duringdeposition of this bed (R. Allen, pers. comm., 1994;Canfield & Raiswell, 1991). The vertebrate material occursas floating grains in the sandstone matrix. The bioclasticdebris shows no preferred orientation and is concentratedtowards the base of the bed. Most of the sample (62%) isindeterminate bone fragments, and the remainder is fishbone and scale debris. All of the sample is highlyfragmented and abraded.

The preserv ation indicates depo sition in a fluvialenvironment , perhaps coastal or estuarine, if the presence ofiron pyrites reflects a marine or brackish influence.

6. CONCLUSIONS

The sediments at Clockhouse record deposition underpredominantly low-energy, freshwater conditions, with atleast once a slight brackish-marine influence. The LowerWeald Clay is thought to represent an area of wetlanddeposit ion (Allen, 1981, 1990), composed of manyinterconnecting lakes, lagoons and rivers supporting a

B ONE -BE A RING FACIES , C L OC K HOUSE ROCK STORE, U K 55

Tab le 2. Summary of the taphonomic and sedimentologica l charac teristics of the Clockhouse bone accumulations

Sedimentology Fossil alignment Fossil preservation Vertebrate fauna(decreasing order ofabundance)

Type I Laminated silty- Disseminated Heavily fragmented lndet. bone material.sandstone sandwiched throughout the (sand-sizedbetween fine- to medium- sandstone. fragments) and abraded.grained sandstone.

Type II Variety of sediment grain Found in grits and Material generally Bony fish remains.sizes in discrete layers associated sandstones. moderately- to well- indet. bones,separated by irregular rounded. Fragments up Hybodus teeth,boundaries. Lower surfaces to 13 mm long. Lepidotes teeth.show parallel, linear toolmarks. Upper surfacerippled.

Type III Grain size ranges from Disseminated Heavily fragmented, up Bony fish remains,medium-sand (0.5 mm) to throughout the to 9.9 mm long. indet. bones,5.0 mm diameter. Poorly sandstone . Moderately abraded. Hyb odus teeth,developed graded bedding. Lep idotes teeth.Lower surface representsscour infill.

Type IV Poorly sorted (medium- to Disseminated Heavily fragmented, up Indet. bonecoarse-sa nd. Structureless. throughout the to 5.4 mm long. fragments, indet.

sandstone. Moderately abraded. fish, Hybodus teeth,Lepidotes teeth.

Type V Poorly sorted (medium- to Concentrated in Heavily fragmented, up lnde t. bonecoarse-sand. Cross-laminated medium-coarse sands. to I 1.4 mm long. fragments, indet.and massive sandstone. Small bone fragments Moderately abraded. fish , Hybodus teeth,Lower surfaces show tool highlight cross- Lep idotes teeth andmarks. lamination surfaces. scales, Lissodus teeth.

Type VI Poorly sorted (silt to Generally concentrated Heavily fragmented. up Bony fish remains.medium-sand) with scattered on bedding planes. but to 18.0 mm long. indet. bones,larger clasts (0.75--6.0 mm present throughout the Moderate to heavy Lepidotes teeth anddiameter). Poorly defined sediment. abrasion. scales, Hybodus teeth.graded bedding.

Type VII Poorly sorted (silty- Scattered throughout Heavily fragmented, up lndet. bonesandstone) supporting few the sediment, to 6.4 mm long. fragments, indet.coarser grains. Bedding concentrated on the Moderate to heavy fish, Hyb odu s teeth,planes are planar and bedding plane . abrasion. Lepidotes teeth andsub-parallel. scales, crocodi le teeth.

Type VIII Discrete layers of fine- and Scattered throughout Heavily fragmented, up lndet. bone fragments,medium-coarse sandstone. the sediment. to 3.0 mm long. indet. bony fish,Scours infilled with coarse r Moderate to heavy Hybodus teeth, Lepidotessediments. Lower surfaces abrasion. teeth and scales.show tool marks.

Type IX Fine-grained sandstone Concentrated towards Heavily abraded and lndet. bone. indet. fishsandwiched between layers of base of the sandstone fragmented. scales and bones.laminated siltstone. Cubes layer.of pyrites common in bone-bearing sediment.

56 E.COOK

diverse and rich flora and fauna. The vertebrate fossilsfound at Clockhouse, predominantly shark and bony fish,support this view.

The taphonomic characteristics (high fragment ation,heavy abrasion and little in situ weathering) of the bonebeds arc all typical of assemblages deposited by rivers(Table 2). The sedimentary characteristics allow a three-foldclassification to be constructed: highly concentrated win­nowed lag deposits, scatters of vertebrate material pre­served on the bedding planes, and low concentrat ions ofbone fragm ents within the sediment. Winnowed lagdeposits arc represented by Type III, and parts of Types I, II,V and VIII. Such accumulations are characterized bycoarse-grained clasts and little matrix (Allen, 1965).Scatters of vertebrate material supported in fine-grained

sands and silts include Types V, VI, VII and VIII. Thesewere probably not winnowed. Material held within the bedsis seen in bone-bed types VI, VII and IX.

ACKNOWLEDGEMENTS

Thanks to Andrew Ross and Mike Benton for their helpcollecting the material described in this paper. I am gratefulto Andrew Ross, Mike Benton, Perce Allen and theanonymous referees for comments on early drafts of themanuscript. Thanks also to Rachel Allen for valuablediscussions about pyrite. This work was funded by theNational Geographic Society (fieldwork grant number4705-92) and the University of Bristol Alumni Association.

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Received 8 March 1996; revised typescript accepted I I November 1996.