GEOLOGICA BALCANICA, 47 (2), Sofia, Nov. 2018, pp. 59–63.

Studia breviora

Advancement in ammonite zonal subdivision of the in Bulgaria

Evidence of Aalenian strata is provided by am- Aalenian biostratigraphy in Bulgaria. Based on the monites in the earliest subdivision of the stratigraphical distribution of the ammonite family System in Bulgaria (Zlatarski, 1908). Initially, these Graphoceratidae, four Aalenian ammonite zones rocks were regarded as Bajocian and this view has and nine subzones have been recognized: Leioceras been adhered to for a long time by many Bulgar- opalinum Zone (comprising the L. opalinum and L. ian stratigraphers. When introducing the Leioceras bifidatum subzones); Ludwigia murchisonae Zone opalinum Zone in Bulgaria, Sapunov (1959) con- (containing the Staufenia sehndensis–Ancolioceras sidered its base to be the dividing point between the opalinoides and Ludwigia murchisonae subzones); adjacent Lower Jurassic strata and the base of the Brasilia bradfordensis Zone (including the B. brad- Aalenian Stage of the above. Sub- fordensis and B. gigantea subzones); and Grapho- sequently, Sapunov (1964a) changed his opinion, ceras concavum Zone (embracing the G. cavatum, arguing that the Aalenian should be removed from G. formosum and Eudmetoceras amplectens sub- the scheme of Middle Jurassic stages of Bulgaria. zones). Other ammonites associated with the Aale- It was not until the publications of Sapunov (1968, nian are those of the family Hammatoceratidae, but 1969) that the Aalenian was formally separated in they have even more restricted distribution and are Bulgaria from the Bajocian as the lowest stage of of less biostratigraphical significance. This abridged the Middle Jurassic. Before this time and through report includes reduced faunal and reference lists to the present, a few contributions on the Aalenian and will be presented in more details elsewhere. have appeared in the literature, and most of them The Leioceras opalinum Zone is based on the contain only short lists of ammonite species cited range of the genus Leioceras. In most cases, the (e.g., Sapunov and Nachev, 1959; Sapunov, 1961; very base of the zone is too poor in ammonites and Nachev et al., 1963). This procedure continues to- we were unable to date the rocks more precisely. In day, mainly due to the rarity of ammonites in the particular, we are lacking in detail about the transi- Aalenian rocks. Despite the lack of common speci- tion between the upper Toarcian members of the ge- mens, we are able to propose here a sufficiently nus Pleydellia and the first graphoceratids (those of complete Aalenian ammonite zonation (Table 1). Leioceras). Thus, the lower limit of this zone (and The present biostratigraphical account is mostly of the Aalenian respectively) is drawn at the sudden based on the results and specimens from the papers appearance of small, suboxyconic, smooth or finely of Sapunov (1964b, 1968, 1969, 1970), Sapunov et ornamented examples of Leioceras. The latter are al. (1996), Metodiev (2000), Metodiev and Sapu- succeeded, at about the middle of the zone, by larg- nov (2001) and Metodiev et al. (2014), as well as er-sized Leioceras with stouter whorls and stronger, on recently obtained data. Our scheme matches ade- bundled or bifurcating ribbing. Around this level, quately with that for the current Aalenian zonal suc- the first appearance of the Leioceras-allied genus cession for Northwest Europe (Contini et al., 1997). Cylicoceras has been recorded in Bulgaria (Sapu- The efforts in dividing the Aalenian on the basis of nov, 1970). This evidence was used to divide the L. ammonites have been guided and facilitated by the opalinum Zone into two subzones: the L. opalinum use of the comprehensive results attained in the UK Subzone (to include the occurrence of the finely or- by Callomon and Chandler (1990), Chandler and namented Leioceras), which is almost certainly the Callomon (2009), Chandler (1997) and Chandler equivalent of the respective subzone in NW Europe, et al. (2006). These works were based on superbly and the L. bifidatum Subzone (to include the dis- preserved specimens, collected bed-by-bed, and tribution of the coarsely ribbed Leioceras and the provide a good foundation for the construction of occurrence of Cylicoceras), which possibly corre-

59 Table 1 Correlation between the Bulgarian Aalenian ammonite (sub)zones and the standard (sub)zones in NW Europe, according to Contini et al., 1997 (see also Callomon and Chandler, 1990; Chandler, 1997; Chandler et al., 2006).

sponds to the total extent of the L. comptum and L. were found in the lower subzone, whereas spo- bifidatum subzones sensu Contini et al. (1997). This radic specimens of the genus Csernyeiceras were division should remain provisional until more evi- detected in the upper subzone of the L. opalinum dence is obtained as coarse ribbed variants of Lei- Zone. oceras do occur rarely in the L. opalinum Subzone The Ludwigia murchisonae Zone as now inter- of NW Europe and persist well into the Ludwigia preted is less extensive in Bulgaria than the equiva- murchisonae Zone sensu stricto. The Leioceras bi- lent Aalenian ammonite zone of Contini et al. (1997) fidatum Subzone now fully replaces the earlier used in NW Europe. We follow the approach of the UK Leioceras comptum Subzone (Metodiev, 2000), in stratigraphers (e.g., Callomon and Chandler, 1990; agreement with the arguments advanced by Contini Chandler, 1997; Chandler et al., 2006), according to et al. (1997). In the present usage of the L. opali- whom the abundant Brasilia assemblages should be num Subzone, six characteristic Leioceras species, excluded from the extent of this zone and assigned to including the zonal and subzonal indices, have been the Brasilia bradfordensis Zone. The lower bound- determined (Sapunov, 1959, 1961, 1964, 1968; Sa- ary of the L. murchisonae Zone is taken at the tran- punov et al., 1996; Metodiev, 2000), and seem to sition where the ammonites of the genus Leioceras be relatively good matches of the corresponding co- are replaced by Ancolioceras (a late leioceratid), eval taxa in NW Europe (see Contini et al., 1997, and the first Ludwigia and Staufenia appear. Both pp. 37–38). In addition to the genera Leioceras and Ludwigia and Staufenia are present throughout the Cylicoceras, occasional representatives of the gen- total extent of the zone, but they have sparse occur- era Pseudammatoceras, Rhodaniceras and Bredyia rence in the lower part, and are much more common

60 into the upper part of the zone. Scattered hamma- the incorporation of Brasilia faunas into the Aal- toceratids, such as Planam­matoceras, Pseudamma- enian ammonite zonal scheme, and the Brasilia toceras and Spinammatoceras, also occur. No data bradfordensis Zone was initially incorporated as exist to support extending the range of the genus the topmost subzone of the L. murchisonae Zone. Cylicoceras into the L. murchisonae Zone of Bul- The results of a long period of research in the UK garia. This suggests that either the oldest Murchiso- (e.g., Cope et al., 1980; Callomon and Chandler, nae strata may be missing or the Cylicoceras group 1990; Chandler, 1997; Chandler et al., 2006) and did not reach the zone. The first abundant appear- other significant localities in Western Europe (e.g., ance of Brasilia delimits the upper boundary of the Contini, 1969; Rieber, 1963; Goy and Ureta, 1981; L. murchisonae Zone in Bulgaria. Rulleau et al., 2001) have perfectly demonstrated We have retained in our scheme (Table 1) a two- the evolutionary transition between the latest mem- fold division of the Ludwigia murchisonae Zone bers of the Ludwigia murchisonae group and those into the Staufenia sehndensis–Ancolioceras opali- of the genus Brasilia. This transition is so gradual noides Subzone and the L. murchisonae Subzone, that it is difficult to set apart these forms from each as proposed by Metodiev (2000). However, this other. The range of Brasilia, however, is prominent subdivision remains somewhat local and has not yet biostratigraphical marker and should be considered been widely recognized. The identification of the S. as a separate ammonite zone. We have, therefore, sehndensis–A. opalinoides Subzone is based on the removed the Brasilia assemblages from the L. mur- co-occurrence of the morphotypes of Staufenia and chisonae Zone and recognized it as a separate zone Ancolioceras, including Staufenia sinon (Bayle), S. in line with other authors (e.g., Chandler, 1997; sehndensis (Hoffmann) and A. opalinoides (Mayer), Chandler et al., 2006). The Brasilia bradfordensis as well as that of Ludwigia crassa Horn and several Zone is defined herein with two subzones: Brasilia poorly preserved Ludwigia ex gr. haugi Douvillé bradfordensis and Brasilia gigantea. Our records and Ludwigia ex gr. pustulifera Buckman. More from the B. bradfordensis Subzone include almost specimens are needed to recognize the presence of entirely small specimens (probably macroconchs this subzone in Bulgaria, and an alternative name and microconchs) ascribed to the genus Brasilia may need to be found as index. At the current state and to the subgenus Graphoceras (Ludwigella). The of knowledge, the S. sehndensis–A. opalinoides Brasilia gigantea Subzone contains mostly small- Subzone should be construed as no more than an ap- sized Ludwigia (Pseudographoceras) and Brasilia proximate correlation with both the Ludwigia haugi (Apedogyria) (microconchs), as well as big Brasilia Subzone and the Staufenia sehndensis–Ludwigia examples (macroconchs often attaining large sizes). obtustiformis horizons in NW Europe (see Table 1). We should point out that our evidence comes from The Ludwigia murchisonae Subzone has been rec- a single locality, and that the validity of this divi- ognized by the co-occurrence of two assemblages: a sion still needs verification from further sections. Ludwigia assemblage (including the index-species, In this framework, the Brasilia bradfordensis Sub- but also L. gradata Buckman and Ludwigia (Pseu- zone is based on the dominance of the index-species dographoceras) subtuberculata Rieber), as well as and several allied taxa, such as Brasilia falcifera an assemblage that consists of Staufenia discoidea (Althoff), B. deleta (Buckman), B. bayley (Buck- (Quenstedt) and S. staufensis (Hoffmann). We be- man), and B. subcava (Buckman). Examples of lieve that the decline of the genus Staufenia and Graphoceras (Ludwigella) ex gr. arcitenens (Buck- most of Ludwigia species precedes the first abun- man) and very scarce Ludwigia also occur, although dant appearance of the genus Brasilia, and therefore only at the base of the subzone. Hammatoceratids the extent of the L. murchisonae Subzone is slightly of the genera Pseudammatoceras and Accardia shifted up compared to the coeval subzone in NW were occasionally recorded. The Brasilia gigantea Europe. Accordingly, this subzone is comparable Subzone was recognized by the presence of the in- to the top of the L. murchisonae Subzone and the dex, associated with Brasilia nitens (Buckman), B. S. staufensis Horizon of the Brasilia bradfordensis similis (Buckman), B. decipiens (Buckman) and Subzone in the scheme of Contini et al. (1997). common Brasilia (Apedogyria) ex gr. rugosa–sub- The Brasilia bradfordensis Zone is defined by cornuta (Buckman) and Ludwigia (Pseudographo- the occurrence of characteristic species of the genus ceras) umbilicata (Buckman). That means that the Brasilia. Previously, the presence of these distinc- Brasilia bradfordensis Zone in Bulgaria matches tive ammonites was founded on isolated specimens the B. bradfordensis Horizon of the B. bradforden- from a few localities (e.g., Sapunov, 1969) and sis Subzone, whereas the Brasilia gigantea Subzone interpreted as a subgenus of the genus Ludwigia. accords with the B. gigantea Subzone of the L. mur- This concept was followed by Metodiev (2000) in chisonae Zone in NW Europe (Contini et al., 1997).

61 The Graphoceras concavum Zone forms the ally erected indices and need to be better evidenced: highest subdivision of the Aalenian in Bulgaria. As Graphoceras cavatum; G. formosum; and Eudme- elsewhere, it is characterized by the occurrence of toceras amplectens (Table 1). The lower subzone is the genus Graphoceras, which has been recorded at defined by the presence of G. cavatum (Buckman) several localities in Bulgaria (Sapunov, 1961, 1964a; and an association of Graphoceras (Ludwigella) Sapunov, 1969; Metodiev, 2000; Metodiev et al., ex gr. rudis–cornu Buckman and, therefore, it pre- 2014). This zone is not well developed or fossilifer- sumably corresponds to the G. concavum Subzone ous. Sufficiently detailed stratigraphy of the locali- in NW Europe (Contini et al., 1997). The middle ties is not yet available. The presence of Grapho- subzone is specified by examples of Graphoceras ceras concavum (J. Sowerby) itself still remains cf. formosum Buckman, G. cf. fallax Buckman and doubtful, as the examples previously cited in older an assemblage of Graphoceras (Ludwigella) ex gr. Bulgarian literature are either not available for re- latum–attenuatum (Buckman), and it appears to vision or no new ammonites have been collected. corresponding to the G. formosum Horizon of the Nevertheless, this zone is reliably recognizable and G. formosum Subzone as proposed by Contini et can be locally divided in the same manner as in NW al. (1997). The upper subzone comprises the oc- Europe. We recognize the G. concavum Zone by currence of Eudmetoceras amplectens Buckman, E. the presence of Graphoceras (Graphoceras) [M], klimakomphalum (Vacek), Parammatoceras boyeri despite being sporadic in occurrence. Except these Elmi, Graphoceras aff. decorum Buckman and Eu- ammonites, we have also recorded common occur- hoploceras spp., and it seems to be equivalent of the rence of Graphoceras [m], referred to the subgenus Euaptetoceras amplectens Horizon of the G. formo- Graphoceras (Ludwigella). Our material includes sum Subzone as used by Contini et al. (1997). The several characteristic species of well-dated strati- latter assemblage indicates an age close to the top of graphical distribution in NW Europe that we found the Aalenian (i.e., Aa-15 or Aa-16 of Callomon and in three levels, and allowed us to separate three suc- Chandler, 1990, from which the first sonniniids are ceeding subzones, which, however, have condition- known in the UK).

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Lubomir Metodiev, Ivo Sapunov Geological Institute, Bulgarian Academy of Sciences 1113 Sofia

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