CORRESPONDENCE

Lower Greensand ammonites and ammonite zonation

Raymond Casey

Department ofPalaeontology, The Natural History Museum. Cromwell Road. London SW7 5BD.

1. INTRODUCTION

Dr Ruffell and his associates are to be congratulated inbringing to notice in the Proceedings important ammonitefinds in the Lower Greensand of Dorset and Kent (Ruffell& Batten, 1994; Ruffell & Owen, 1995). I am less enthusi­astic about the interpretations given some of the ammonitesand especially the far-reaching conclusions which seem tohave stemmed from the Kent discoveries. Perhaps thesecond paper was not the best place for Dr Owen to floathis ideas on the systematics of the relevant ammonites orto introduce radical changes in zonal nomenclature thateffectively rewrite much of the language of the LowerGreensand. Many of the assertions made in support of thesenew concepts are controversial, unsubstantiated or atvariance with published information and my own experi­ence. The zonal scheme for the Upper Aptian and LowerAlbian used in the paper by Ruffell & Owen is particularlyinappropriate in my opinion, as explained below.

2. PUNFIELD COVE, NEAR SWANAGE, DORSET

Two ammonites, identified by Dr H. G. Owen, requirecomment because of the historical or stratigraphical interestattached to them.

The first was obtained from above the Punfield MarineBand of the Punfield Cove area, from a level possiblyequivalent to the Upper Lobster Beds of the Isle ofWight, and was courageously identified as Deshayesitespunfieldensis Spath. This name is a misnomer, the speciesbeing unknown from Punfield and found to date only inthe Lower Lobster Beds tforbesi Zone, kiliani Subzone)of Atherfield, Isle of Wight (Casey, 1964, p. 338). Thespecimen to which this name has been applied (Ruffell &Batten, 1994, fig. 5.5) is a small shard, 14 mm long, lackingthe venter and the unfigured side and showing three pairs ofYshaped ribs. A number of species of Deshayesites andDufrenoyia could produce similar fragments. I regard thespecimen as indeterminable and without the historicalsignificance ascribed to it.

The second ammonite, from higher in the sequence, wasgiven the name Cheloniceras (Ch.) cf. gotteschei (vonKoenen) (sic, recte Kilian) (Ruffell & Batten, 1994, fig.5.3a, b) and taken to indicate the bowerbanki Zone ofthe Lower Aptian. The words attributed to me in respectof the range of this species refer properly to Ch. (Ch.)

Proceedings of the Geologists' Association. 107, 69-76.

cornuelianum (d'Orbigny) (Casey, 1961b, p. 202), to whichI understand (Ruffell, in litt.) the specimen was at firstassigned. The form described by me under the nameCh. (Ch.) cf. gotteschei (Kilian) was redescribed fromnew material in a later part of my monograph as Ch. (Ch.)rotundum Casey and firmly fixed in the bowerbanki Zone(Casey, 1980, p. 653). The matter is academic, however,because the Punfield Cove specimen clearly displays atits proximal end the ventral tubercles diagnostic of thesubgenus Epicheloniceras. It is an example of Ch. (E)eotypicum Casey and indicates not the Tropaeum bower­banki Zone of the Lower Aptian but a position low in theUpper Aptian.Zone of Ch. (E) martinioides.

3. M20 MOTORWAY CUTTING ATHEADCORN LANE BRIDGE,

NEAR ASHFORD, KENT

The most important ammonite discovery in the LowerGreensand for many years is reported by Ruffell & Owen(1995). The presence of a late jacobi Zone fauna near thetop of the Sandgate Formation in the M20 cutting atHeadcom Lane Bridge, near Ashford, Kent, was un­expected. Most of the specimens are too small or frag­mentary for positive specific determination; two (NHMC93984, C93987) are reminiscent of, though not identicalwith, forms of the rubricosus Subzone, the overall aspectbeing that of an early anglicus Subzone assemblage. Thisis further suggested by two examples (NHM C93983,C93985) of an ammonite recognized by Owen as belongingto Hypacanthoplites simmsi (Forbes). I would place thislevel at Headcom Lane Bridge provisionally at the baseof the anglicus Subzone of the jacobi Zone, which meansthat it corresponds to a small part only (not the whole) ofthe Folkestone Formation basement-bed at East Cliff,Folkestone, and is probably slightly earlier than that portionof the anglicus Subzone seen in the old Sandling Junctionpit about 8 km west of Folkestone (Casey, 1961a, p. 533).This indicates an even more remarkable westwardexpansion of the jacobi Zone than previously realized.

4. AMMONITE SYSTEMATICS, PHYLOGENYAND ZONATION

Differences in personal interpretation of the smallerammonite fragments from this new locality and in the

0016-7878/96 $07·00 © 1996 Geologists' Association

70 CORRESPONDENCE

precise correlation of the bed are relatively unimportant.Of greater concern are the confusing changes to establishedusages and nomenclature in ammonite systematics andzonation generated in the authors' paper. This is typifiedby the section on the systematics of the ammonite familyParahoplitidae, wherein Dr Owen attempts in a fewsentences to put aside decades of research by others onthis group and its complex phylogeny. He proposes tostreamline the taxonomy of the family by dropping theconcept of a subfamily Acanthohoplitinae and by elimin­ating those genera (Gargasiceras, Diadochoceras) thatcannot be fitted into his simplistic (and stratigraphicallyincongruous) picture of its evolution. Authoritativeaccounts of the Parahoplitidae (Parahoplitaceae ofSchindewolf and others), based mainly on well-developedsequences in Transcaspia and the Caucasus, are given byTovbina (1983) and Mikhailova (1979, 1983), the last withextensive bibliography. Briefly, the family/superfamily isnow believed by some (e.g. Kvantaliani & Sharikadze,1982; Tovbina, 1983) to have originated with the genusProcolombiceras, assigned to the Lower Aptian by itsauthor (Sharikadze, 1979), with the acanthohoplitinidGargasiceras straddling the Lower/Upper Aptian boundary.Other acanthohoplitinid branches may be followedthrough the Upper Aptian from Colombiceras, Acantha­hoplites (Protacanthoplites), Acanthohoplites s.s. toHypacanthoplites, and from Protacanthoplites toDiadochoceras (Tovbina, 1983). Parahoplites itself appearsat the base of the melchioris (= nutfieldiensis) Zone.Whereas Dr Owen thinks this genus gave rise toAcanthohoplites in the nolani Subzone, Mikhailova (1983),with knowledge of the stratigraphy, sees the reverse anddepicts Parahoplites as a dead-end offshoot from themain-stem Acanthohoplites.

I would take issue with most of what is said concerningthe stratigraphical ranges of various genera of theParahoplitidae (Ruffell & Owen, 1995, table 1). Thedepiction of Colombiceras as ranging through all threesubzones of the martinioides Zone is more speculative thaninformed, not a single specimen of Colombiceras havingbeen found as yet in any of these Lower Greensand sub­zones (debile, gracile and buxtorfl). In terms of the LowerGreensand sequence, it is difficult to place foreign occur­rences of the genus more precisely than martinioides andnutfieldiensis Zones, the latter being the probable horizonof the only known British examples (Casey, 1965, p. 421).Ironically, the authors' table 1 omits to show Colombicerasranging into the nutfleldiensis level, although there is acopious literature establishing it as a characteristicammonite at that level (melchioris Zone) in other countries.In the Caucasus and the Crimea, for example, Colombicerastobleri (Jacob) is used locally as a co-index fossil alongsideParahoplites melchioris and in some areas of Georgiareplaces it (Prozorovsky, 1989).

Especially in need of clarification in their table 1 is thestatement that the latest Parahoplites and the earliestHypacanthoplites occur together in the nolani Subzone with'the intermediate genus' Acanthohoplites. In fact,

Acanthohoplites is already present in strength in thenutfieldiensis (= melchioris) Zone, ranging into the no/aniSubzone and perhaps surviving into the ruhricosusSubzone. Parahoplites is found in association with forms ofthe nolani Subzone in condensed deposits (e.g. the famous'Clansayes' horizon of southeast France). The earliestHypacanthoplites are species of the rubricosus group,which, though frequently confused with forms of the nolaniSubzone, do not appear in uncondensed sequences untillater (ruhricosus Subzone). I do not know whereParahoplites and Hypacanthoplites can be found togetherin such sequences.

Specialists in this field can decide for themselves themerits and demerits of Dr Owen's novel ideas on theammonites, their ranges, systematics and evolution. Iquestion whether geologists in general should be asked totake on board his subjective views in support of a new zonalscheme for the Upper Aptian.

5. UPPER APTIAN, ZONE OFPARAHOPLITES NUTFIELDlENSlS

Of the three Upper Aptian zones recognized in the LowerGreensand (Casey, 1961a), Cheloniceras (Epicheloniceras)martinioides, Parahoplites nutfieldiensis and Hypa­canthoplites jacobi (Fig. 1), none escapes 'revision'. DrOwen first proposes to demote the nutfieldiensis Zone to asubzone and tag it on to the underlying martinioides Zone,thereby scaling down the Upper Aptian substage to twozones compared with four in the Lower Aptian. I considerthis wrong.

A zone characterized by Parahoplites nutfieldiensisor allied species is one of the key units of internationalcorrelation within the Aptian stage and is recognized overa large part of the northern hemisphere. This is Spath'sParahoplitan Age and together with the martinioides Zonecomprises the Middle Aptian substage of FSU (FormerSoviet Union) workers. England is one of the regions whereknowledge of the biostratigraphy is sufficiently advanced topermit subdivision of the zone. In Transcaspia, for example,the corresponding Zone of Parahoplites melchioris has anupper Subzone of Protacanthoplites monilis (Tovbina,1970). In the Northern Caucasus and the Crimea themelchioris Zone encompasses two foraminiferal zones,the Zone of Hedbergella trochoidea below and the Zoneof Planomalina cheniourensis above (Drushchits &Gorbachik, 1979). Stratigraphically and palaeontologieallythe junction of the martinioides and nutfleldiensis Zonesmakes a more significant boundary in England than doesthat of the bowerbanki and martinioides Zones (Lower/Upper Aptian), coinciding with the beginning of a longphase of impoverishment of the ammonite faunas.

The reasons for this break with established usage seemto be that Dr Owen considers my subdivision of thenutfieldiensis Zone into a lower Subzone of Tropaeum sub­arcticum and an upper Subzone of Parahoplites cunningtonito be illusory and that in some other region the genus

Substages Zones

CORRESPONDENCE

Subzones

71

Hypacanthop/ites ang/icusHypacanthop/ites rubricosusNo/aniceras no/ani

Parahoplites cunningtoniTropaeum subarcticum

Che/oniceras (Epiche/oniceras) buxtorfiChewn~eros(Ep~hewn~eros)grocile

Che/oniceras (Epiche/oniceras) debile

Lower Albian(part)

Upper Aptian

Leymeriella tardefurcata

Hypacanthop/ites jacobi

Parahop/ites nutfie/diensis

Che/oniceras(Epiche/oniceras) martinioides

{

Leymeriella regu/arisHypacanthoplites milletioidesFarnhamia farnhamensis

{{{

Fig. 1. Late Aptian-basal Albian zonal classification after Casey (l961a) (for discussion purposes only).

Parahoplites is accompanied throughout its vertical rangeby Che/oniceras (Epicheloniceras).

Dr Owen is entitled to his opinion (which I do not share)that P. cunningtoni and P. nutjieldiensis are conspecific,though I emphatically refute his statement that observeddifferences in specimens are due mostly to different modesof preservation. Also I disagree with his conjecture that thetype of P. cunningtoni came from sediments of the sameage as those included by me in the subarcticum Subzone.Not only does the cunningtoni Subzone contain a specialassemblage of Parahoplites, the distinctiveness of itsammonite fauna is emphasized by its failure to yield thelarge heteromorph Tropaeum, found in the subarcticumSubzone as a survivor from the martinioides Zone.

As part of the case for downgrading the nutjieldiensisZone, we are told that 'In the thick sedimentary sequencesof late Aptian age in the Vocontienne Trough in southernFrance and Switzerland (e.g. Delamette, 1988), Para­hoplites is associated with Cheloniceras (Epiche/oniceras)throughout its stratigraphical range' (Ruffell & Owen,1995, p. 8). The source of information on this hithertounsuspected association is not disclosed. The Delamettebook is irrelevant. Contrary to what is implied in thequotation above, it does not deal with thick sequences inthe Vocontian Basin but with condensed deposits of theHelvetian greensands, Here Delamette has recovered a fewphosphatized fragments of Epicheloniceras; he has nothingwhatsoever to say about Parahoplites and its range.Moreover, Dr Delamette tells me (in litt. 15.2.95) thatalthough he has collected hundreds of ammonites bed bybed through the 'Marnes Bleues ' Formation of theVocontian Basin, the genus Parahoplites is absent. Andwhere Parahop/ites does appear in uncondensed depositsin southern France, as in the Tave syncline (Gard)(Conte, 1985), it is isolated from the underlying bedswith Epicheloniceras.

However it is expressed in terms of ammonite chron­ology, the upper part of the nutfieldiensis Zone marks a new,transgressive sedimentary regime in southern England,

possibly linked to climatic change, as suggested by theincoming at this level of the rudist Toucasia, of Tethyanaffinity (Casey, 1961a, p. 578). The status of thenutfieldiensis Zone and the concept of two separatesubzones (subarcticum and cunningtoni) are thus endorsedby more than one line of enquiry.

6. UPPER APTIAN, ZONE OFHYPACANTHOPLITES JACOBI

Attention is next turned to the Zone of Hypacanthoplitesjacobi, which for the Lower Greensand I had divided intothree subzones, in ascending order: Nolaniceras nolani,Hypacanthoplites rubricosus and H. anglicus (Casey,196Ia). Dr Owen's theme is much the same as for thenutfieldiensis Zone: the rubricosus and anglicus Subzoneshave no separate existence; stratigraphically and palaeonto­logically they are the same and once again I was misled bydifferences in mode of preservation. He proposes to returnto a Zone of Diadochoceras nodosocostatum, with a lowerSubzone of Nolaniceras nolani and an upper Subzone ofH. jacobi, a classification harking back to the work of Jacoband Breistroffer in the first half of the century. This schemewas rejected by me 35 years ago as inappropriate to theEnglish succession because the genus Diadochoceras hasnot been found here and because two subzones alone do notreflect the extended sequence present in the LowerGreensand, and I reaffirm that position here.

The main basis for recognition of the rubricosus andanglicus Subzones of the jacobi Zone was provided by anextensive temporary exposure of the basement-bed of theFolkestone Formation at East Cliff, Folkestone. This bed isfar too important to allow misconceptions concerning it topass uncorrected. Here are the facts. Work in the late 1930sfor the foundations of a sea wall and promenade at EastCliff revealed two faunas of Hypacanthoplites in differentmodes of preservation in the basement-bed. The ammonitesoccurred either as steinkerns in black phosphorite, more

72 CORRESPONDENCE

or less rolled, or inside relatively large brown ferrugino­phosphatic concretions. When the nominal speciesH. rubricosus and H. anglicus were proposed (Casey,1950), the description of their stratigraphical relations wasunambiguous and explicit: there were two bands, withH. rubricosus below and H. anglicus above. Earlier, thequestion of whether these two occurrences occupieddifferent levels within the basement-bed, as described byPrice (1874), or formed a single band, as seen in the firstdiggings (Casey, 1939), had been answered when theexcavations were completed over a 200 m stretch of theshore. In the western part the large brown (ruhricosus)concretions were seen to lie at the very bottom of the bed,just as Price had said, with the rolled black (anglicus)nodules crowded into a seam above. Followed eastwards thetwo bands first converged into one, and then, in the vicinityof Baker's Gap, the lower (rubricosus) petered out, thoughthe upper (anglicus) persisted (Casey, 1961a, pp. 527-9).

To say that 'a succession of Hypacanthoplites ruhricosusand H. anglicus Subzones cannot be demonstrated .. .in anycontinuous sequence in England or Europe' (Ruffell &Owen, 1995, p. 8) is, in my opinion, wrong.

This same field evidence disposes of the suggestion thatthe differences in mode of preservation of the ammonitesare probably due to sediments of the same age fromdifferent localities being mixed together (Ruffell & Owen,1995). I do not see the basement-bed as a melange of multi­source material but rather as the product of winnowing ofsediment in situ by bottom currents which reduced theanglicus Subzone to a nodular concentrate but which failedto scour deep enough in the western part of the section toexhume the rubricosus concretions (Casey, 1939). All thisis consistent with the rapid lateral changes observed inother marginal areas of Lower Greensand deposition andwith the remarkable westward expansion of the jacohiZone, which passes into mainly unconsolidated and largelyunfossiliferous sediments inland from Folkestone (Casey,1961a). It is not inconceivable that the rubricosus Subzonetakes part in this expansion. This simple explanation for thelack of evidence of the rubricosus Subzone in the un­fossiliferous sands and silts below the anglicus Subzonesat Sandling Junction, though acknowledged by the officialsurveyors (Smart, Bisson & Worssam, 1966, p. 54), is notmentioned as a possibility by Ruffell & Owen.

Despite everything I have said to the contrary (Casey,1950, 1965), Dr Owen finds that the rubricosus andanglicus faunas 'have no significant difference; or are 'thesame' (Ruffell & Owen, 1995, p. 8). It is thereforenecessary to repeat that H. anglicus and allies showmorphological affinity with forms of the succeedingtardefurcata Zone, as clearly demonstrated by Marechal(1994) from occurrences in Normandy, whileH. rubricosusand its contemporaries are linked to species of the under­lying nolani Subzone (or Zone). Indeed, Spath (1939)had identified H. rubricosus with the TranscaspianH. nolanisimilis Breistroffer, a member of a closely-knitgroup which also includes H. subrectangulatus (Sinzow),H. pygmaeus (Sinzow) and H. mangyschlakensis

Glazunova, all of which had been regarded originally asmere varieties of A. nolani (see Casey, 1965, p. 438). Overlarge tracts of Kazakhstan and Turkmenistan thisrubricosus-mangyschlakensis group (perhaps a singlebiospecies) is found below the main development of thejacohi Zone with H. anglicus (= H. kopetdagensisGlazunova) (Casey, 1965, p. 428). The horizon of this grouphas been generally included in or mistaken for the nolaniZone/Subzone (e.g. Glazunova, 1953, p. 18). The conceptof a Zone of Acanthohoplites prodromus below this'nolaniZone', introduced by Tovbina (1968) and widely quoted,has always been difficult to accept by some workers in theTranscaspian and adjoining regions, the index speciesbeing scarcely distinguishable from A. nolani (Seunes) (fordiscussion see Tovbina, 1980). The explanation is that theprodromus Zone is the nearest equivalent to the true nolanihorizon of western Europe, while the overlying so-called'nolani Zone' in Transcaspia is wholly or in part therubricosus Subzone of the jacobi Zone. This conclusion hasbeen reinforced by recent study of the collections ofSinzow, Glazunova and Tovbina in St Petersburg and willbe developed in a future publication.

Far from being figments of my imagination, therubricosus and anglicus Subzones have turned out to becritical horizons for international correlation and under­standing of Upper Aptian biostratigraphy.

7. LOWER ALBIAN, ZONE OFLEYMERIELLA TARDEFURCATA

The Lower Albian section of the zonal scheme presented inthe authors' table I (Ruffell & Owen, 1995) is not new butnevertheless requires comment. In a previous publicationdealing with occurrences in southern England and northernFrance (Owen, 1992), the Subzones of Farnhamiafarnhamensis and Hypacanthoplites milletioides, proposedfor the basal and middle parts of the Zone of Leymeriellatardefurcata of the English succession (Casey, 1961a), werereplaced by the Subzones of Leymeriella schrammeni andL. acuticostata. taken from the north German succession.This marked a return to a 'standard' classification for thetardefurcata Zone I had rejected as impracticable for theLower Greensand because Leymeriella has not been foundat these levels in the regions concerned (including northernFrance). The geographical and ecological factorscontrolling the distribution of Leymeriella did not permitthe genus access to the Lower Greensand domain until afterthe regional unconformity/disconformity I termed the'mid-tardefurcata break' (Casey, 1961a, p. 501). At presentthis break cannot be fixed in terms of the 'standard' zonalscheme.

An error in Owen's paper (1992), as in an earlierpublication by the same author (Owen, 1988), was toconfuse Farnhamia with the genus Arcthoplites, the latternot known earlier than from the middle of the tardefurcataZone. In consequence, unwarranted assumptions were madeabout the position of the farnhamensis horizon, which

CORRESPONDENCE 73

Owen firmly correlated with the acuticostata Subzoneof Germany, thereby leading Amedro (1992, p. 213) topostulate a gap in the Lower Greensand sequence at thebase of the Albian. In the current paper (Ruffell & Owen,1995, p. 5, fig. 2), it is the milletioides Subzone which isnow equated with the acuticostata Subzone.

8. POSTSCRIPT

Advances in knowledge during the past 15 years compelsome modifications to the nomenclature I have used forLower Greensand ammonites at all taxonomic levels. It isnot surprising that the Aptian-Lower Albian zonal schemeconceived for the Lower Greensand some 35 years ago(Casey, 1961a) (Fig. 1) is also in need of review. In the areaunder discussion, for example, with wider experience of theammonites, I now have misgivings about generic separationof the nolani group (as Nolaniceras) from Acanthohoplites,and doubt whether the nolani Subzone should be taken aspart of the jacobi Zone rather than as a zone in its own right,which is the practice in FSU countries. However, a change

of status for the nutfieldiensis Zone and elimination of therubricosus and anglicus Subzones are not on the agenda.

Provincial differences in the ammonite faunas continueto bedevil attempts at long-range correlation. While wemay be confident that the Parahoplites melchioris Zone,recognized from the Caucasus to Central Asia, is more orless equivalent to the P. nutfieldiensis Zone of England, therelationship of its upper Subzone of Protacanthoplitesmonilis to the English Subzone of Parahoplites cunningtoniis by no means certain. The change from Aptian to Albiantime in western Europe was marked by a curious, almostparochial, endemism among the ammonites, with theemergence of Leymeriella, centred on north Germany,Farnhamia in southern England, and Bucaillella inNormandy. Integration of these regional occurrences into asingle zonal sequence will require much patient work. In themeantime, I see no merit in trying to impose uniformitywhere none can be discerned. Doubtless I will be consideredreactionary by those who think standardization is the nameof the game; nevertheless, I adhere to the view that a zonalscheme based on fossils actually present in the strata ispreferable to one based on fossils that are not.

REFERENCES

AMEDRO, F. 1992. L' Albien du bassin Anglo-Parisien:Ammonites, zonation phyletique, sequences. Bulletin CentresRecherches Exploration-Production Elf-Aquitaine. Boussens,16,187-233.

CASEY, R. 1939. The upper part of the Lower Greensand aroundFolkestone. Proceedings of the Geologists' Association, 50,362-378.

-- 1950. The junction of the Gault and Lower Greensand in EastSussex. and at Folkestone, Kent. Proceedings of the Geologists'Association, 61, 268-298.

-- 1961a. The stratigraphical palaeontology of the LowerGreensand. Palaeontology, 3, 487--621, pl. 77-84.

-- 1961b. A Monograph of the Ammonoidea of the LowerGreensand, Part III, 119-216, pl. xxvi-xxxv. PalaeontographicalSociety, London.

-- 1964. A Monograph of the Ammonoidea of the LowerGreensand, Part V, 289-398, pl. xliiii-lxvi,

-- 1965. A Monograph of the Ammonoidea of the LowerGreensand, Part VI, 399-546, pl. lxvii-xc.

-- 1980. A Monograph of the Ammonoidea of the LowerGreensand, Part IX, 663--660, pI. ci-cxii.

CONTE, G. 1985. Decouverte d'ammonites du Gargasien dansles 'Gres et calcaires 11 Discoides et Orbitolines' du synclinal dela Tave (Gard, France). Geobios, 18,203-209.

DELAMETTE, M. 1988. L'Evolution du domaine Helvetique(entre Bauges et Morcles) de l'Aptien superieur au Turonien:series condensees. phosphorites et circulations oceaniques.Publications Departement de Geologie et Paleontologic,Universite de Geneve, 5.

DRUSHCHITS, V. V. & GORBACHIK, T. N. 1979. The zonalsubdivision of the Lower Cretaceous of the southern USSRby ammonites and foraminifera (in Russian). Izvestiya AkademiiNauk SSSR, ser. geol. 12,95-105.

GLAZUNOVA, A. E. 1953. Ammonites of the Aptian and Albianof Kopet-Dag, Lesser and Greater Balkhans and Mangyshlak(in Russian). Trudy VSEGEI. Moscow.

KVANTALIANI, I. V. & SHARIKADZE, M. Z. 1982. Concerning

the phylogeny of the family Acanthohoplitidae (in Russian).Soobshcheniya Akademii Nauk Gruzinskoi SSR, 105,89-92.

MARECHAL, M. 1994. Les Hypacanthoplites Spath 1923 desArgiles 11 Bucaillella cayeuxi de Cauville et leur positionstratigraphique. Bulletin trimestriel de la Societe geologiquede Normandie et des Amis du Museum du Havre, 81 (2)83-96.

MIKHAILOVA, I. A. 1979. The evolution of Aptian Ammonoidea(in Russian). Paleontologicheskii Zhurnal, 13 (3), 267-274.

-- 1983. Systematics and phylogeny ofCretaceous Ammonoidea(in Russian). Nauka, Moscow.

OWEN, H. G. 1988. The ammonite zonal sequence and ammonitetaxonomy in the Douvilleiceras mammillatum Superzone (LowerAlbian) in Europe. Bulletin British Museum (Natural History),Geology series, 44, 177-231.

-- 1992. The Gault-Lower Greensand junction beds in thenorthern Weald (England) and Wissant (France). Proceedings ofthe Geologists' Association, 103, 83-110.

PRICE, F. G. H. 1874. On the Lower Greensand and Gault atFolkestone. Proceedings of the Geologists' Association, 4,135-150.

PROZOROVSKY, V. A. (ed.) 1989. Zones of the CretaceousSystem in the USSR. Lower Series (in Russian). Nauka,Leningrad.

RUFFELL, A. H. & BATTEN, D. J. 1994. UppeITI10st Wealdenfacies and Lower Greensand Group (Lower Cretaceous) inDorset, southern England: correlation and palaeoenvironment.Proceedings of the Geologists' Association, 105, 53--69.

-- & OWEN, H. G. 1995. The Sandgate FOITI1ation of the M20Motorway near Ashford, Kent, and its correlation. Proceedingsof the Geologists' Association, 106,1-9.

SHARIKADZE, M. Z. 1979. Procolombiceras - a new genus ofthe family Parahoplitidae from the Lower Aptian deposits ofGeorgia (in Russian). Soobshcheniya Akademii Nauk GruzinskoiSSR, 94 (2), 381-384.

SMART, J. G. 0., BISSON, G. & WORSSAM, B. C. 1966.Geology of the country around Canterbury and Folkestone

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(Sheets 289, 305, 306). Memoirs of the Geological Survey ofGreat Britain.

SPATH, L. F. 1939. Problems of ammonite nomenclature. V. OnAcanthohoplites jacobi (Collet) and the Jacobi Zone of theFolkestone Sands. Geological Magazine, 76, 276-279.

TOVBINA, S. Z. 1968. On the Zone of Acanthohoplites prodromusin the boundary deposits of the Aptian and Albian of Turkmenia(in Russian). Izvestiya Akademii Nauk Turkmenskoi SSR, ser.fiz.-tekhn., Khim. i geo\. 2, 100-108.

-- 1970. A new genus of the family Parahoplitidae (in Russian).Paleontologicheskii Zhurnal 3, 56-65.

-- 1980. Concerning the question of the Zone ofAcanthohoplites prodromus in the Upper Aptian of Turkmenia(in Russian). Izvestiya Akamdemii Nauk SSSR, ser. geo\., 7,142-144.

-- 1983. Zonal division of the Aptian and Albian stages ofTurkmenia (in Russian). lzvestiya Akademii Nauk SSSR, ser.geo\., 2, 62-71.

'Uppermost Wealden facies and Lower Greensand Group(Lower Cretaceous) in Dorset, southern England:

correlation and palaeoenvironment' by Ruffell & Batten (1994)and

'The Sandgate Formation of the M20 Motorway near Ashford, Kentand its correlation' by Ruffell & Owen (1995): reply.

H. G. Owen

Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD.

Dr Casey's comments on the above-mentioned papers andother work on the late Aptian and Lower Albian, raiseimportant issues concerning ammonite systematics andstratigraphical interpretation in the English LowerGreensand Group and elsewhere. His comments applyessentially to the identification and systematic grouping ofammonites and the revision of biostratigraphical zonationmade by Owen. In view of this, the reply is written by meand is restricted essentially to the comments made by Caseyon the two papers referred to above.

Most ornamented ammonite 'species' show morpho­logical variation between individuals when seen in a groupon a bedding plane and broad morphological changeswithin a succession of sediments through which they range.In the absence of zoological criteria indicating truespecificity, trivial-name nomenclature is subjective. In thecase of the Ammonoidea, which have been used exten­sively for the relative dating of sediments in the Mesozoic,the unnecessary allocation of distinct specific and genericnames to closely related morphotypes in a relatively shortstratigraphic time span, based purely on minor (sometimesminute) differences in ornament characters, hinders thecomparison of faunas and stratigraphical analysis. This is

especially the case where the number of individuals knownis few. In Dr Casey's monograph (1960-1980), there is amarked tendency to give distinct specific names toammonites showing relatively slight morphological differ­ences within genera. The ammonite literature of the FormerSoviet Union to which Dr Casey refers, is also characterizedby a marked tendency to excessive 'splitting' in the intro­duction of generic, subgeneric and trivial names.Ammonites together with micro-faunas and floras lendthemselves to the relative dating of sediment successions inthe Mesozoic and thus, to an understanding of regional andglobal palaeogeographic, sedimentological and tectonicprocesses. The zonal and subzonal schemes recognizedshould be applicable to the whole of the geographicalfaunal province in which the faunas/floras are endemic. Theprovince might be world-wide in its extent, but is usuallymore geographically limited; for example 'boreal', tethyan,european etc., in which cases interprovincial correlation canbecome more problematic.

In the paper by Ruffell & Batten, Casey objects to theidentifications I made of two ammonites. The first ofthese is Deshayesites punfieldensis Spath (BMNH C 93971)from the Punfield Marine Band exposure 3. Although an