БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2017“ BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2017” Ammonites of the family Sphaeroceratidae from the Bajocian in Bulgaria Амонити от семейство Sphaeroceratidae от Байоския етаж в България To the memory of Andrew (Walrus) England (May 26, 1954 – September 8, 2017) Lubomir Metodiev Любомир Методиев Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 24; 1113 Sofia; E-mail: [email protected] Keywords: ammonites, Sphaeroceratidae, Bajocian, Bulgaria. The ammonite family Sphaeroceratidae consists of continuous nature of the exposures and the difficulty in small- to medium-sized ammonites, with characteris- following them both vertically and laterally. Only a few tic sphaeroconic to eccentric coiling, contracted body- taxo nomically and biostratigraphically aimed studies chamber, deep and narrow to occluded umbilicus and have been published to date (Tzankov, Bončev, 1934; fine wiry ribbing that occur in the Bajocian (Middle Kamenov, 1936; Sapunov, 1961; Metodiev et al., 2014). Jurassic). The number of specimens collected from Thus, this family remains little known in Bulgaria. The Bajocian strata in Bulgaria is small. The geographi- taxonomic re-evaluations of older collections, as well cal occurrence of Bulgarian sphaeroceratids, in terms as the study of some newly excavated localities, have of both localities and regions, is associated mainly recently revealed the presence of several species of the with the Etropole Formation of the Western Balkan genera Labyrinthoceras, Frogdenites, Sphaeroceras Mts. Several fields, associated with the Ozirovo, Bov and Chondroceras. As stated by Parsons (1980), the and Polaten Formations (W Balkan and the East Fore- morphological divergence that separates these genera is Balkan Mts.), have also yielded ammonites from this slight. Besides, the Sphaeroceratidae and the Otoitidae, group (see Fig. 1). However, most specimens were such as Emileia and Otoites, are extremely alike. collected during different geological studies over the Perhaps, for this reason, the older Bulgarian literature years and precise stratigraphical information for this often contains citations of Emileia species referred to material is almost lacking. The main drawback for ad- these sphaeroceratid genera and vice versa. Indeed, vancing the knowledge about these faunas is the dis- these forms strongly resemble each other, but the bulk Fig. 1. The ammonites of family Sphaeroceratidae (lower Bajocian, Middle Jurassic), displayed with their geographical and stratigraphical occur- rences, on the sketch-map of distribution of the Jurassic rocks in Bulgaria (scale bars: 1 cm). 93 of Sphaeroceratidae includes notably small ammonites, Strazha (Polaten Fm., East Fore-Balkan), indicating with more depressed whorls and more accentuated con- the S. niortense and the G. garantiana zones. Authentic traction of the body-chamber, that differ from Emileia record of an unusually large (32 mm diameter) but in ribbing style. good example of S. brongniarti has been recovered The genera Labyrinthoceras and Frogde nites pro- from the Bov Formation of the Zimevitsa Plateau vide the earliest record of the family Sphaeroceratidae (Metodiev et al., 2014). In this locality, this species in Bulgaria. In terms of their stratigraphic position, they associates with Chondroceras, such as C. evolvescens occur in a narrow interval of the lower Bajocian, from (Waagen) and C. polypleurum (Westermann) (see the upper W. laeviuscula Zone to the lower O. sauzei Fig. 1), all of them lying at the level of the lower Bajocian Zone. The Bulgarian Labyrintho ceras specimens S. humphriesianum Zone. The genus Chondroceras have previously been labeled as “Labyrinthoceras has not been recorded in Bulgaria before. It is based on perexpansum S. Buckman” (Sa punov, unpublished small sphaerocones with a narrow but open umbilicus. material). According to Parsons (1980) and Sandoval Chondroceras evolvescens display typical ornament and Chandler (2015), this species is a junior subjective of sharp prorsiradiate primary ribs that divide into synonym of Waagen’s species Labyrinthoceras two or three forwardly inclined secondaries, which meniscum. The ammonite identified by Kamenov gently sweep on the broad venter. The mouth-border is (1936, pl. III, Fig. 3) as “Emileia brogniartii, marked by a prominent liplike extension of the ventral d’Orbigny”, from the type-area of the Etropole area. Chondroceras polypleurum revealed similarities Formation, represents a perfect match of the paratype in many respects to C. evolvescens but differs in of Labyrinthoceras meniscum (Waagen), refigured by having wider umbilicus and whorl section, and much Sandoval and Chandler (2015, Fig. 2) (see Fig. 1). A higher rib density. few more specimens of this species, from the same Parsons (1980) considered the sphaeroceratids to area, have also been recorded (Sapunov, unpublished be specialized ammonites with some form of facies data), and this material has a lower O. sauzei Zone control and restricted geographical distribution, due to dispersal. Another good example of Labyrinthoceras their specific morphology and relatively small number was found in the Ozirovo Formation of the Ponor– of fields, from which they are known. Conversely, Gradets Jurassic Strip (Kamenov, unpublished Sandoval and Chandler (2005) regarded them, although material). It was tentatively identified as “Emileia sp.”, not being abundant, as widely distributed throughout but it is Labyrinthoceras dietzei Sandoval & Chandler the western Tethys and having a dispersal that was (Fig. 1), which probably belongs to an interval facilitated by a continuous sea-level rise at the earliest that corresponds to the top of the W. laeviuscula Bajocian. Interestingly, this assumption is confirmed Zone. According to Sandoval and Chandler (2015), from the sedimentary record in Bulgaria, because the Labyrinthoceras appear in the lower W. laeviuscula occurrence of Sphaeroceratidae coincides with the Zone, but they do have their maximum development advent of a regional highstand maximum flooding in the O. sauzei Zone. Hence, the occurrence of this depositional setting that approximately corresponds to genus in Bulgaria, although sporadically recorded, the Aalenian/Bajocian stage boundary interval. confirms this statement. The Bulgarian examples of the genus Frogdenites have previously been assigned References to Labyrinthoceras (Sapunov, unpublished material), Kamenov, B. 1936. La géologie des environs d’Etropole. – Rev. but they closely match the holotype for Frogdenites Bulg. Geol. Soc., 8, 2, 30–137, 3–7 (in Bulgarian with a spiniger Buckman (Sandoval, Chandler, 2015, French abstract). Fig. 6/3a–d, refigured holotype; see also Fig. 1). Parsons, C. F. 1980. Systematic Revision of the Bajocian Ammo nite Subfamily Sphaeroceratinae. PhD Thesis (un- Outside Bulgaria, this genus is most common in the published), University of Keele, 90 p. W. laeviuscula Zone and is known to have mainly a Sandoval, J., R. Chandler. 2015. Labyrinthoceras and Frog­ pre-Labyrinthoceras occurrence (Sandoval, Chandler, denites (Sphaeroceratidae, Ammonitina) from Western 2015). Bulgarian material does not suggest, however, Tethys: The origin of the Sphaeroceratidae. – Geobios, 48, such stratigraphical position and, until obtaining new 39–56. data, Frogdenites should be considered as an affiliate Sapunov, I. 1961. Stratigraphy of the Jurassic in the Etropole and Teteven areas (Central Balkan Range). – Trav. géol. of the O. sauzei Zone. Bulg., Sér. stratigr. et tect., 3, 93–137 (in Bulgarian with The genus Sphaeroceras was found in Bulgaria by Russian and English abstracts). Tzankov and Bončev (1934), based on small globular Metodiev L., E. Koleva-Rekalova, D. Ivanova, D. Dochev, S. ammonites with rapid retraction of the umbilical seam Velev, I. Dimitrov. 2014. Middle Jurassic fossil and depo- in proximity to the aperture. However, these ammonites sitional record from the area of the Zimevitsa Plateau (West were misidentified as Sphaeroceras brongniarti Bulgaria). – In: Proceedings of the National Conference (Sowerby), and they belong to different species having “GEOSCIENCES 2014”. Sofia, BGS, 65–66. Tzankov, V., E. Bončev. 1934. Quelques céphalopodes fossiles much higher stratigraphic position than S. brongniarti du Bajocien des environs du village Stratidja, arrondisse- – Sphaeroceras auritum (Parona) (see Fig. 1). They ment d’Eski-Djumaia (Bulgarie du nord). – Ann. Univ. came from a highly condensed upper Bajocian Sofia, Fac. Phys. et Mathém., 3–sci.­nat., 30, 235–248 (in assemblage of the well-known ammonite locality Bulgarian with a French abstract). 94.