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The age, fauna and palaeoenvironment of the Late Triassic fissure deposits of Tytherington, South Gloucestershire, UK

D. I. WHITESIDE^*,* and J. E. A. MARSHALL

^* 14 Crown Road, Chelsfield, Orpington, Kent BR6 6JN, UK
School of Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Waterfront Campus, European Way, Southampton, SO14 3ZH, UK

View larger version (27K): [in this window] [in a new window]   Figure 1. Map showing the principal Upper Triassic/Lower Jurassic tetrapod-bearing fissure deposits of the English West Country around Bristol and in South Wales. The three important groups of fissure localities are Durdham Down (in Bristol), Tytherington and Cromhall, north of Bristol; Holwell, Emborough, Windsor Hill and Batscombe in the Mendip Hills; and a west of Cardiff group including Pant-y-ffynon and Ruthin, together with those on St Bride’s palaeo-island (e.g. Pant, Duchy, Ewenny and Pontalun). The map shows the inferred latest Triassic palaeo-highs that would have formed an archipelago of small limestone islands lying in a Rhaetian sea. A UK location map is shown on Figure 2. The map also shows the locations of complete Penarth Group palynological profiles and the more limited spot sample palynomorph records. Compiled from Ordnance Survey Digimap © and the BGS 1:250 000 solid geology Bristol Channel sheet with input from Audley-Charles (1970), Tucker (1977), Kellaway & Welch (1993) and Robinson (1957a). Sources for the palynological data points are The Cross & Lavernock Point (Orbell, 1973); Brent Knoll Borehole (Warrington, 1981); Watchet and St Audries Bay (Warrington & Whittaker, 1984); Selworthy (Warrington in Edwards, 1999; Warrington et al. 1995); High Ham (Warrington, Whittaker & Scrivener, 1986); Chilcompton and Vallis Vale (Warrington, 1984); Dundry Borehole (Warrington in Kellaway & Welch, 1993); Aust (Manor Farm Quarry, pers. obs.) and Hampstead Farm Quarry, Chipping Sodbury (this paper).

View larger version (75K): [in this window] [in a new window]   Figure 2. (a) Map of Tytherington Quarry (derived from map supplied by Amey Roadstone Corporation, 1981) showing location of the fissure localities referred to in the text. (b) Map of the UK showing the location of Tytherington and the area of Figure 1. (c) Photograph of Tytherington Quarry, looking to the east, taken in May, 1982. Only fissures 12 (not shown), 11, 13, 14, 15 and 16 (circled) in the top quarried level are still present. The other fissures have been destroyed by quarrying operations. Compared to the map of the quarry (a) there is now a fourth quarried level. No vertebrate fossils or significant fissures were found in the fourth level.

View larger version (35K): [in this window] [in a new window]   Figure 3. Cross-sections of the 16 Triassic fissure localities described in the text. Except for the top quarried level that retained the original limestone surface, the vertical extents of the fissures are generally unknown due to earlier quarrying operations or unexcavated base levels. The upper and lower parts of some fissure fills were also obscured by quarry scree. The uppermost parts of fissures 1, 5, 8, 11, 12, 14, 15 and 16 were observed through binoculars and details described from those observations, telephoto slides and hand collection of fallen rocks. The remaining sections were completed from in situ rock collections. Note the diversity of morphology and fill lithologies. The locations of the reptile bones and the productive palynological samples are shown.

View larger version (42K): [in this window] [in a new window]   Figure 4. Simplified large-scale geological map and geological succession of the area around Tytherington Quarry. The limestone was quarried from the Black Rock Group. Also note the outcrop of bedded Penarth Group at a height of 96 m, about 600 m south of the quarry. From BGS map sheet ST 68 NE, co-ordinates used are British National Grid. Geological column from Kellaway & Welch (1993).

View larger version (145K): [in this window] [in a new window]   Figure 5. Vertebrates from the Tytherington fissures. All vertebrate material was prepared in 10 % acetic acid following the method of Rixon (1976). (a) Severnichthys. BRSUG specimen 23730. Blackened cusp of tooth from palynomorph-bearing rock; fissure 1, x15, scale bar 500 µm. (b) Fossil charcoal. BRSUG specimen 23880. Fissure 1, x20, scale bar 250 µm. (c, d) Clevosaurus. BRSUG specimen 23731. A worn tooth, (c) occlusal and (d) view of tooth base. Found in the same palynomorph-bearing rock specimen as BRSUG specimen 23730. Fissure 1, x20, scale bar 250 µm. (e) Gyrolepis. BRSUG specimen 23735. Tooth, fissure 2, x15, scale bar 500 µm. (f, g) Unknown chondrichthyan. BRSUG specimen 23729. Unidentified fossil, possibly a shark (hybodont?) denticle, fissure 2, x15, scale bar 500 µm. (h) ‘Palaeosaurus.’ BRSUG specimen 23601. Tooth type, fissure 2, x5, scale bar 1 mm. (i) Thecodontosaurus. BRSUG specimen 23600. Tooth, fissure 2, x5, scale bar 1 mm. (j) Diphydontosaurus. BRSUG specimen 23725. Worn bone, showing corrosion, fissure 2, x10, scale bar 500 µm. (k) Glauconitic clay-mineral BRSUG 23726. Fissure 2, x20, scale bar 250 µm. (l) Ooids in aggregated cluster, BRSUG 23727. Fissure 2, x10, scale bar 500 µm. (m) Section through aggregated ooid showing concentric calcite rings (arrowed) around a haematitic core, BRSUG 23736. Fissure 2, x15, scale bar 500 µm. (n) Diphydontosaurus avonis. BRSUG specimen 23987. Part of left dentary, medial view, showing pleurodont teeth, fissure 2, x10, scale bar 500 µm. (o) Diphydontosaurus avonis. BRSUG specimen 23988, Part of left dentary in lateral view, showing flaking of outer bone (arrowed) caused by sub-aerial exposure, fissure 2, x10, scale bar 500 µm. (p) Diphydontosaurus avonis, BRSUG specimen 23989. Anterior part of right dentary in medial view, showing wear due to post mortem transport, fissure 2, x10, scale bar 500 µm. (q) Planocephalosaurus robinsonae. BRSUG 23755. Part of right maxilla, lateral view, from palynomorph-bearing deposit, fissure 2, x5, scale bar 1 mm. (r) Steinkern of ‘Rhaetic’ gastropod. BRSUG 23728. Fissure 2, x20, scale bar 250 µm. (s) Diphydontosaurus avonis. BRSUG specimen 23990. Part of left maxilla, lateral view showing wear due to post-mortem transport, fissure 2, x15, scale bar 500 µm. (t, u) Clevosaurus. BRSUG specimen 23700. Left dentary, (t) lateral and (u) medial view, fissure 2, x2.5, scale bar 2 mm. (v) Unidentified Lepidosaur. BRSUG specimen 23737. Limb bone with metallic sulphide coating (arrowed), fissure 2, x15, scale bar 500 µm. (w, x) Pholidophorus. BRSUG specimen 23738. White-coloured scale found in the rock that produced the holotype of Diphydontosaurus avonis, (w) internal and (x) external view, fissure 2, x15, scale bar 500 µm. (y) Euestheria minuta. BRSUG specimen 23640. Fissure 4, x5, scale bar 1 mm. (z) Collection of bones in a typical assemblage of white, yellow and brown bones (and bone fragments) from the rock that produced the type specimens of D. avonis, WB, white bones and YB, yellow bones of Diphydontosaurus. BRSUG specimen 23739. Fissure 2, x5, scale bar 1 mm. (aa, bb) Lepidosaur ‘B’ of Whiteside (D. I. Whiteside, unpub. Ph.D. thesis, Univ. Bristol, 1984). BRSUG specimen 23670. Anterior of blackened left maxilla of sphenodontian related to Diphydontosaurus and Gephyrosaurus in (aa) lateral and (bb) medial view, fissure 12, x10, scale bar 500 µm. (cc) Euestheria minuta var. brodeiana. BRSUG specimen 23641. Two individuals, fissure 12, x5, scale bar 1 mm. (dd) Pelecymala sp. BRSUG specimen 23897. Part of left dentary in lateral view. The tooth-wear facets are caused by orthal shear rather than propalinal movement found in the similar Clevosaurus latidens Fraser, 1993, fissure 14, x3.5, scale bar 1 mm. (ee) Clevosaurus. BRSUG specimen 23890. Part of left dentary of a juvenile in lateral view, fissure 14, x10, scale bar 500 µm. (ff) Planocephalosaurus. BRSUG specimen 23891. Scanning electron micrograph; the posterior view of the right premaxilla of a juvenile showing (?replacement) pits at the base of the teeth, fissure 14, x25, scale bar 200 µm. (gg) Clevosaurus. BRSUG specimen 23691. Lateral view of toothed part of blackened right dentary, fissure 13, x10, scale bar 500 µm. (hh) Pholidophorus. BRSUG specimen 23692. External view of brown coloured scale showing growth rings (arrowed), fissure 13, x10, scale bar 500 µm. (ii, jj) Pholidophorus. BRSUG specimen 23690. (ii) External and (jj) internal view of brown coloured scale, fissure 13, x15, scale bar 500 µm. (kk) Conodont. BRSUG 23732. Scanning electron micrograph of reworked Carboniferous conodont, probably Bisphathodus, fissure 14, x25, scale bar 200 µm. (ll) Gyrolepis. BRSUG 23733. Scanning electron micrograph of white tooth found in the same rock with Planocephalosaurus, Tricuspisaurus and Pelecymala preserved in the same manner, fissure 14, x25, scale bar 200 µm. (mm, nn) Diphydontosaurus. BRSUG specimen 23892. Part of blackened right dentary showing alternating sized-teeth, (mm) medial and (nn) lateral view, fissure 16, x15, scale bar 500 µm. (oo, pp) Tricuspisaurus. BRSUG specimen 23893. Teeth, coloured grey, in (oo) lateral and (pp) occlusal view, fissure 14, x10, scale bar 500 µm. (qq) Planocephalosaurus. BRSUG specimen 23894. Part of right maxilla brown coloured in lateral view, fissure 14, x10, scale bar 500 µm. (rr) Planocephalosaurus. BRSUG specimen 23895. Left grey coloured dentary in lateral view, fissure 14, x5, scale bar 1 mm. (ss) Part of rock hand specimen BRSUG 23896 showing possible associated whitish/grey-coloured Clevosaurus bones including a maxilla ‘M’ and a dentary ‘D’, fissure 16, x4, scale bar 2 mm.

View larger version (151K): [in this window] [in a new window]   Figure 6. Palynomorphs recorded from the Tytherington fissures. Processing involved successive HCl and HF treatments followed by sieving at 20 µm and a further treatment in hot HCl to remove neoformed fluorides. No oxidation was required. The palynomorphs were mounted in Elvacite 2044 and the slides are curated in the collections of Bristol University Geology Museum. Each palynomorph name is followed by a palynological sample number (e.g. FP2.1), a BRSUG specimen number and England Finder co-ordinates (e.g. M29–3). In addition, all specimens are ringed. All x600 unless indicated; appropriate scale bars are given. (a) Calamospora tener, FP1.1, BRSUG 23860, N45–1/3. (b) Concavisporites toralis, FP2.1, BRSUG 23861, N23. (c) Deltoidospora auritora, FP2.1, BRSUG 23861, H26–1. (d) Granulatisporites subgranulosus, FP2.1, BRSUG 23862, F41–1. (e) Carnisporites megaspiniger, FP2.1, BRSUG 23863, K22–1. (f) Carnisporites spiniger, FP2.1, BRSUG 23864, L26. (g) Microreticulatisporites fuscus, FP2.1, BRSUG 23865, U40. (h) Uvaesporites reissingerii, FP1.1, BRSUG 23860, M39–4. (i) Deltoidospora minor, FP2.1, BRSUG 23866, L43–4. (j) Taurocusporites sp A, FP2.1, BRSUG 23867, M29–3. (k) Acanthotriletes varius, FP2.1, BRSUG 23868, S36–3. (l) Cycadopites, FP2.1, BRSUG 23861, P15. (m) Cingutriletes infrapunctatus, FP2.1, BRSUG 23865, W36–1. (n) Cingulizonates rhaeticus, FP2.1, BRSUG 23862, T44. (o) Carnisporites leviornatus, FP13.2, BRSUG 23869, J36–4, x400. (p) Limbosporites lundbladii, FP2.1, BRSUG 23864, L26–4. (q) Rhaetipollis germanicus, FP2.1, BRSUG 23861, N22–1. (r) Triancoraesporites ancorae, FP2.1, BRSUG 23868, Q43–3. (s) Krauselisporites reissingerii, FP2.1, BRSUG 23864, Q42–4. (t) Ricciisporites tuberculatus, FP2.1, BRSUG 23870, Q36, x400. (u) Protohaploxypinus hercynicus, FP2.1, BRSUG 23868, M31–3. (v) Lunatisporites rhaeticus, FP2.1, BRSUG 23865, D36–3. (w) Tsugaepollenites ?pseudomassulae, FP2.1, BRSUG 23861, F26. (x) Ovalipollis pseudoalatus, FP2.1, BRSUG 23865, P38–4. (y) Vesicaspora fuscus, FP2.1, BRSUG 23865, Q36–4. (z) Alisporites thomasii, FP2.1, BRSUG 23861, P26. (aa) Alisporites dunrobinensis, FP2.1, BRSUG 23865, Q38–1. (bb) Granuloperculatipollis rudis, FP2.1, BRSUG 23868, J42. (cc) Quadraeculina anellaeformis, FP2.1, BRSUG 23871, K36. (dd) Classopollis classoides, FP2.1, BRSUG 23865, S34. (ee) Vitreisporites pallidus, FP2.1, BRSUG 23861, G14. (ff) Gliscopollis meyeriana, FP2.1, BRSUG 23872, O31–2. (gg) Suessia schwabiana, BRSUG 23873, N19. (hh) Rhaetogonyaulax rhaetica, FP2.1, BRSUG 23861, E29–3. (ii) Rhaetogonyaulax rhaetica, FP2.1, BRSUG 23866, L36. (jj) Micrhystridium sp, FP2.1, BRSUG 23861, Q18–1. (kk) Celyphus stenlillensis, FP6.1, BRSUG 23874, E42–4. (ll) Dapcodinium priscum, FP2.1, BRSUG 23871, K36–1. (mm) Pterospermella australis, FP2.1, BRSUG 23861, R23. (nn) Tytthodiscus, FP9.2, BRSUG 23875, N37, x400. (oo) Cymatiosphaera polypartita, FP2.1, BRSUG 23870, L35–1, x400. (pp) Botryococcus, FP2.1, BRSUG 23865, G39–1, x400. (qq) foraminiferal test lining, FP2.1, BRSUG 23876, V45–3. (rr) foraminiferal test lining, FP2.1, BRSUG 23865, Q39.

View larger version (152K): [in this window] [in a new window]   Figure 7. (a) Fissure 1 showing convoluted dark-grey palynomorph-bearing (FP1.1) sandy mudstone of typical Westbury-type lithology ‘WL’ with fractured calcite band. The Carboniferous Limestone block ‘CLB’ has slipped and caused the disruption of the bed. (b) Fissure 1; the lower quarry scree has been removed (Fig. 7a is outlined) and a more complete exposure below the slipped Carboniferous Limestone block of (a) is shown. There is dark-grey convoluted sandy mudstone to the left of the fractured calcite band (‘fc’ in Fig. 7c) and a near-horizontal layered alternation of mudstone and conglomerate sequence on the right. The slumped sequence will have been related to earth movements and was formed after deposition, which is clearly demonstrated by the broken calcite and mineral vein. Such seismic movements have been identified by Mayall (1983) at the beginning of the Cotham Member, and Simms (2006) regards the cause as bolide impact. The palynomorph assemblage FP1.2 (Table 2) was taken from the right-hand side and the sample point is shown in close-up in Figure 7c. The scale-figure (DIW) is 1.7 m tall. (c) Fissure 1. A close-up of the sequence described in (b); there are repeating conglomeratic bands that fine upwards (graded bedding) and have fine laminated mudstones between them. Note location of FP1.2 and ‘fc’ the fractured calcite vein. (d) Fissure 1. A close-up of (c) that shows two conglomeratic bands (a clast is circled). (e) Thecodontosaurus. BRSUG specimen 23613. Lateral view of left ilium showing characteristic short anterior process ‘AP’. The bone is whitish coloured which is typical of many of the Thecodontosaurus bones of the original discovery at Tytherington. Fissure 2, x0.75, scale bar 2 cm. (f) Thecodontosaurus. BRSUG specimen 23613. Medial view of same left ilium as (e) with dark-grey silty sandstone ‘GSS’ matrix attached. This matrix is lithologically the same as the rock abutting the Thecodontosaurus bones that produced palynomorph assemblage FP2.1. Fissure 2, x0.75, scale bar 2 cm. (g) Thecodontosaurus. BRSUG specimen 23642. A postero-medial view of part of a white and yellow coloured right femur showing well-developed trochanter ‘tr’ with muscle scar ‘ms’. Fissure 2, x0.75, scale bar 2 cm. (h) Acid-etched rock showing brown- and white-coloured bones of Thecodontosaurus ‘Th’ in a breccia above finely laminated cross-bedded calcareous silty sandstone, including a layer of the grey lithology that elsewhere, and below the Thecodontosaurus breccia, yielded the palynomorph assemblage FP2.1. Fissure 2, x0.6, scale bar 2 cm. (i) Opposite side of (h) showing angular white dolomitized Carboniferous limestone ‘DCL’ breccia with clasts above, and cutting into, finely laminated grey silty sandstone. Fissure 2, x0.6, scale bar 2 cm.

View larger version (177K): [in this window] [in a new window]   Figure 8. (a) Fissure 2, first exposure (mid-November, 1975) showing the locality of the original find of the breccia and conglomerates (outlined as ‘b’) that yielded Thecodontosaurus, Diphydontosaurus and other reptiles. Palynomorph-bearing siltstone laminae occur below this original find with a similar but non-laminated lithology in the same horizon as the conglomerate. (b) Fissure 2, closeup of the Thecodontosaurus breccia ‘Th’ and conglomerate band in situ. The location of the limestone that yielded the holotype of Diphydontosaurus is marked ‘D’, as is the palynomorph-bearing rock FP2.1, the original assemblage described by Marshall & Whiteside (1980). (c) Later exposure of fissure 2 (mid-December, 1975), about 17 m to the southeast of the first exposure, showing fissure formation on a vertical joint ‘VJ’, with an upper fissure fill ‘UFF’ lying above ‘keyhole’ fissure fill. The two sedimentary fills were undoubtedly continuous and the intermediate section was removed as a result of quarrying. (d) Two blocks of Thecodontosaurus breccia and conglomerate from fissure 2 showing bones (circled) and yellow dolomitized limestone. (e) Rock specimen from fissure 2 showing bands of red, that is, a ‘Keuper’ type lithology that yielded Planocephalosaurus ‘Pl’ with bands of green calcareous sediment, plus thin laminae of black silt that contained the Rhaetic palynomorph assemblage FP2.2, x1.1, scale bar 2 cm. (f, g) Thin-section of rock that yielded palynomorph assemblage FP2.1; (g) shows crystals of baroque dolomite ‘d’ that are diagenetically replaced calcite. This dolomite implies dolomitization in Penarth Group times which may have produced dolomitized Carboniferous Limestone that later was deposited as clasts in the breccia layer that lay directly above (see Fig. 8b, d). Organic matter ‘om’ is present as thin orange-coloured bands which contain AOM and represent a short-lived algal bloom that formed as a discrete event. The fine silty layers are characterized by larger phytoclasts and represent terrestrial input. The palynomorphs are discrete entities and not introduced as clasts by reworking or slumps. The R. tuberculatus specimen shown in (g) is circled, x50, scale bar 300 µm. (f) Close up of thin-section (g) showing in situ specimen of Ricciisporites tuberculatus as circled in Figure 8b; (compare Fig. 12t), x150, scale bar 50 µm.

View larger version (55K): [in this window] [in a new window]   Figure 9. Fissure 2 showing the morphological evolution of fissure 2 as it was quarried from NW to SE. Overall there is a change from large clast breccias and conglomerates at 0 m to smaller clast reddish conglomerates in the middle section and to grey-black conglomerates and siltstones in the last sections at 23–26 m. The main fissure is accompanied by a brown sand and mud-filled satellite fissure development which can be followed from 0 m to the section at 11 m. The first (discovery) exposure of the fissure at 0 m is a complex fill with thick laminated red sandy marls succeeded by grey laminated Westbury-type sediments that yielded palynomorph assemblage FP2.1. This sequence continued upwards with fining-up breccia and conglomeratic sediments that included Westbury-type grey sandy mudstones; this is the rock that yielded the original find of Thecodontosaurus at Tytherington. At 5 m, the exposure shows a similar fill (where exposed) with the upper part of the fill separated into two. This separation is lost in the next exposure (8 m), which is a repeating sequence of conglomerates and finer reddish sediments. Some of the conglomerates, which are identical to ‘dolomitic conglomerate’, have large rounded clasts of sandstone and limestone including dolomitized limestone. At 11 m the lower part of the fissure has become simply a vertical joint but the upper part is much wider and a continuation of the section at 8 m. The upper infilling is also similar to that at 8 m with alternating beds of large and small clast conglomerates. The large clast conglomerates then disappear by 14 m where the fill is solely composed of red small-clast conglomerates. At 14 m and 17 m there is the complex development of a short phreatic tube. The section at 17 m has been interpreted by some (e.g. Simms, 1990) as a phreatic tube with a vadose notch cut into the base. It is more likely to have been formed by tectonic and solutional activity along a vertical joint entirely below the water table. The very localized circular structure would have been in direct receipt of surface waters that swirled around at the freshwater lens/saline water boundary to create the feature (see Fig. 16 and Whiteside & Robinson, 1983). A comparison with the previous cross-section at 14 m demonstrates that the right hand side of the ‘tube’ at 17 m was produced later than the left hand side and is strong evidence for void formation during the Penarth Group. The void was then filled by ‘glauconitic clay’-rich conglomerates on the right hand side of the ‘tube’ at 17 m. The fissure rapidly closes to an enlarged joint by 20 m and remains so until the final section at 26 m. In parallel with this closure, the satellite fissure has opened. The satellite fissure seen at 23 m is likely to be an extension of fissure 1 and implies that there was an inter-connectivity of the two systems. The locations of the three fissure 2 samples are shown. FP2.2 can only be positioned approximately between 17 m to 18 m. These series of sections were compiled from in situ collection and measurements, photos and field notes made between successive quarry blasts. The top of the section is hung on the top of the third quarried level which is shown as the ‘horizontal’ lines on the second exposure at 5 m (29/11/75). The base of the fissure remains unknown, but as these fissures were not located during the quarrying of the fourth level it may lie in the upper part of this deepest level (at approximately 60 m OD; about 44 m below the Carboniferous Limestone surface).

View larger version (165K): [in this window] [in a new window]   Figure 10. (a) Fissure 3, first exposure showing strongly developed ochreous mineralization with fine-grained limestone ‘FGL’ to bottom right. Although barren of palynomorphs this limestone is similar to the palyniferous fine-grained limestone of fissure 16 that produced FP16.1. (b) Fissure 3, second exposure about 5 m behind (a) with a vertical notch ‘VN’ cut into its base and with ochreous layers filling the cavity. (c) Fissure 11 labelled to show the three exposures of this fissure. Scale figure is 1.75 m tall. The extent of the red sandy breccia ‘RSB’ that contained the fossils is arrowed. (d) Fissure 14 showing the extent of the funnel shape opening ‘o’ of a doline. The vertical extent of the main fossil-bearing ‘FB’ green calcareous sandy rock is arrowed. (e) Fissure 14, this is a close-up of the lower section (marked in Fig. 10d) and is filled with red cross-bedded sandy material that contained a few black lepidosaur bones. There are green reduction spots in this deposit and these are frequently associated with the bones. Note the smooth limestone wall on the left of the red sediments that is due to solution. (f ) Fissure 12, the layers included the cross-bedded sandstone with water drop marks ‘WDM’ and thin red sandstones. Some white bone fragments were found in situ in these rocks. However, the main fossil-bearing rock is above this horizon, and although the fossils collected were not found in situ, their lithologies were recognized using binoculars and therefore their position can be accurately located. Sample BLB (position estimated from observation through binoculars) contained black lepidosaur bones; WLB, white lepidosaur bones and Eu, Euestheria (estimated position where the lithology is uniquely identical to the Euestheria-bearing rock collected from the scree below).

View larger version (159K): [in this window] [in a new window]   Figure 11. (a) Fissure 13 looking towards the SW. The top ‘entrance’ to the fissure is indicated ‘o’. The fissure widens from this opening. At the extreme left is a keyhole-shaped pocket ‘KSP’ and at the centre are horizontal solution grooves with cusps (marked ‘b’). At the lower right is the main fossiliferous deposit ‘d’ that formed part of a cavernous structure. The whole structure is interpreted as a flank margin cave. (b) The main group of solution cusps. (c) Rock specimen from fissure 13 that shows cavity filled with ‘caymanite’; note the characteristic layering; x 0.85, scale bar 2 cm. (d) Main fossiliferous deposit with Richard Bennett collecting in situ reptile bones. Pholidophorus scales and bones were found in the same horizon. The bones collected at this position were very corroded in a halo of green sediment within the calcareous yellow limonite clay sandstone. Note thin layers of hard sandy limestone above his head. To the right of the deposit, the lower limestone bands thicken and sag ‘S’ downwards. (e) Main fossiliferous deposit, looking SE with DIW (1.70 m tall) indicating the site of palynomorph-bearing limestone (FP13.1). Note the harder, thin sandy-limestone bands ‘SL’ just above the centre of the picture.

View larger version (25K): [in this window] [in a new window]   Figure 12. A representative stratigraphical section of the Penarth Group with selected palynological markers (inceptions and extinctions). Section based on Bristol district (Kellaway & Welch, 1993) with palynological markers after Hounslow, Posen & Warrington (2004); Warrington & Whittaker (1984) and Orbell (1973). Note that the characteristic Penarth Group palynomorphs have inceptions in the very uppermost part of the Blue Anchor Formation.

View larger version (7K): [in this window] [in a new window]   Figure 13. Cluster analysis of palynological assemblages from Tytherington plus the Clifton Suspension Bridge ‘FPC’ sample. Palynological data were recalculated to include only the major elements (Ricciisporites tuberculatus, Ovalipollis pseudoalatus, Rhaetipollis germanicus, the Circumpolles pollen, Rhaetogonyaulax rhaetica and Cymatiosphaera polypartita). A number of different methods were used to make the similarity matrix and perform the cluster analysis. These generally gave robust results.The illustrated dendrogram was clustered using UPGMA and a Cosine similarity measure using the MVSP© program (Kovach Computer Services). The analysis shows three clear assemblage groups in the fissure deposits. The two palynological samples that were in a limestone lithology are marked L, that is, FP13.1L and FP16.1L.

View larger version (17K): [in this window] [in a new window]   Figure 14. Quantitative palynological profile of the main terrestrial and marine palynomorphs. from the Penarth Group at Hampstead Farm Quarry. Quantitative numbers were determined using a Lycopodium spike as per Thomas et al. 1993 and are expressed in thousands of palynomorph g–1 rock. Three numbers (114 000, 123 000 and 390 000) are off the scales used. Also shown is the Total Organic Carbon content (TOC %), calcite % (both measured using a CE-1108 elemental analyser) and the relative abundance of AOM. Bed numbers are from Donovan, Curtis & Curtis (1989). Note the very characteristic double peak of Rhaetogonyaulax rhaetica separated by an acme of Cymatiosphaera polypartita. The abundances of all the palynomorphs declines markedly in the Cotham Member with R. rhaetica declining the least and hence becoming relatively abundant.

View larger version (15K): [in this window] [in a new window]   Figure 15. Ternary plots showing comparative assemblages based on the recalculated relative percentages of Rhaetogonyaulax rhaetica, Ovalipollis pseudoalatus and Ricciisporites tuberculatus from Hampstead Farm Quarry and the palynomorph-bearing fissure deposits at Tytherington and the Clifton Suspension Bridge ‘FPC’.

View larger version (29K): [in this window] [in a new window]   Figure 16. Palaeoenvironmental model of the Tytherington reptile-bearing fissures during their Rhaetian infilling. The cross-section runs N–W to S–E and includes fissures 2, 13 and 14. The occurrence of seasonal rainfall is predicted from the presence of Euestheria minuta, the palaeolatitude, the dolomitization of the calcite layers in the Rhaetian laminated sediments and general climatic indicators in the region at the time. The vegetation cover (not shown) would be expected to be fairly dense, especially around the entrances to the fissures, as the freshwater lens that was supported by the saline waters of the Rhaetian sea would maintain fresh water very near the limestone surface. Modern islands below about 10 ha can lack a permanent freshwater lens (Whittaker, 1998), but the field relationship of local Rhaetic deposits suggest that the Tytherington paleo-island would have been much larger than this, perhaps about 700 ha.

View larger version (20K): [in this window] [in a new window]   Figure 17. Map of the fissures showing the relative abundance of the three common sphenodontids, Clevosaurus, Diphydontosaurus and Planocephalosaurus, based on skeletal elements that can be attributed with certainty (all are skull or lower jaw bones). There is a clear change in the relative abundance of these sphenodontids from west to east across the quarry. The western part of the quarry is bordered by the Lower Limestone Shale Group, and the easterly fissures are furthest away from that stratum and the Old Red Sandstone.

View larger version (16K): [in this window] [in a new window]   Figure 18. Stratigraphical comparisons between reptiles found in the Tytherington Triassic fissures and the same or closely related genera from normal bedded sequences worldwide. Data from Shubin & Sues (1991), Wu (1994), Shubin, Olsen & Sues (1994), Luo & Wu (1994), Sues, Clark & Jenkins (1994), Benton (1994), Sues & Reisz (1995), Sues, Shubin & Olsen (1994), Renesto (1995), Duffin (1995), Lucas (1998, 1999), Irmis (2004) and Säila (2005). Ch is China, Eu is Europe, NAm is North America and SAfr South Africa. The Tytherington coelurosaur is most similar to Megapnosaurus which Lucas & Tanner (2006) regard as first appearing in, and defining the beginning of, the Dawan LVF (equivalent to the Sinemurian), but which Bristowe & Raath (2004) regard as congeneric with Coelophysis. In addition to the data displayed, Bonaparte & Sues (2006) have recently described Clevosaurus from the Caturitta Formation (?Carnian–Norian) of Rio do Sul, Brazil.