GEOLOGIJA 59/2, 233-241 Ljubljana 2016 http://dx.doi.org/10.5474/geologija.2016.014
© Author(s) 2016. CC Atribution 4.0 License
Distephanopsis concavus Horvat: a revised silicoflagellate species from the Middle Miocene of the Central Paratethys
Revizija srednjemiocenske silikoflagelatne vrste Distephanopsis concavus Horvat
Aleksander HORVAT
Ivan Rakovec Institute of Palaeontology, Research Centre of the Slovenian Academy of Sciences and Arts, Novi trg 2, SI-1000 Ljubljana, Slovenia; e-mail: ahorvat@zrc-sazu.si
Prejeto / Received 18. 10. 2016; Sprejeto / Accepted 22. 11. 2016; Objavljeno na spletu / Published online 23. 12. 2016
Key words: silicoflagellates, biostratigraphy, Middle Miocene, Badenian, Central Paratethys
Ključne besede: silikoflagelati, biostratigrafija, srednji miocen, badenij, Centralna Paratetida
Abstract
The silicoflagellate species Distephanopsis concavus Horvat is revised based on further research of the type material. This species is characterized by having a basal ring with strongly concave sides which reduce the basal ring area and by four symmetry planes which are morphological characteristics not known in other Distephanopsis species. The species was found in Middle Miocene deposits of the Central Paratethys. Its occurrence at type locality in SE Slovenia is dated to late Middle Badenian and is correlated to the upper part of the nannoplankton biozone NN5.
Izvleček
V prispevku je na osnovi detajlnje študije tipskega materiala podana revizija silikoflagelatne vrste Distephanopsis concavus Horvat. Za vrsto so značilne izrazito konkavne stranice bazalnega obroča in štiri simetrijske ravnine, kar niso morfološke značilnosti ostalih poznanih vrst rodu Distephanopsis. Vrsta je najdena v srednjemiocenskih plasteh zahodnega dela Centralne Paratetide. Stratigrafska razširjenost vrste v tipskem najdišču je zgornji del srednjega badenija in je korelirana z zgornjim delom nanoplanktonske biocone NN5.
Introduction
After the isolation of the Paratethys from the Tethys, silicoflagellates became rare and their occurrences were limited to the Badenian and Sarmatian (Middle Miocene). Their rare occurrence in the Central Paratethys is due to the varying ecological conditions resulting from intense tectonics, eustatic changes and seaway connections to the Mediterranean sea and Indo-Pacific area. In the Central Paratethys silicoflagellates only appear in the sequences of eustatic maxima (Bachmann et al., 1963; Bachmann, 1971; Hajos, 1968; DuMiTRicA, 1978). This is also the case in Slovenia where Middle Miocene diatomaceous sediments with silicoflagellates have only been discovered in fragmentary sections in the Krško basin (Horvat, 2004).
So far the occurrences of silicoflagellates are well known only in the central part of the Pan-nonian basin, that is, in the Intracarpathian area and Vienna basin in Austria, Hungary and Chech Republic (Bachmann et al., 1963; Bachmann, 1971; Hajós, 1968; Dumitricá, 1978). From the western part of the Central Paratethys, only two localities with Badenian silicoflagellates have been reported so far (Horvat, 2004). Siliceous microfossils occur around the localities Šmarjeta and Šentjernej in the Krško basin (Fig. 1) where a silicoflagellate assemblage comparable to that of other Badenian Paratethys localities was found. This assemblage contained a new species Distephanopsis concavus (Horvat, 2004), which was only introduced by a short description in Slovenian with no type designation. The aim of this paper is to provide all the data necessary to establish the validity of
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this species in accordance with the International Code of Zoological Nomenclature. Biostrati-graphy and palaeoecoly of the assemblage with Distephanopsis concavus at its type locality are discussed as well.
Paleogeographic framework and geological setting
The Miocene sedimentary successions of western part of Central Europe are generally related to the Paratethys, which resulted from a process of final disintegration of the Tethys Ocean due to movement and subduction of Africa beneath the European plate and the counterclockwise rotation of the Eastern Alps (Rasser & Harzhauser, 2008). A new paleogeographic unit formed, called the Paratethys, whose development was at times independent of that of the Mediterranean. Within the Paratethys, two main sedimentary basins -the Central Paratethys in the west and the Eastern Paratethys in the east can be distinguished that were separated from each other by land. Their evolution was controlled by regional tectonic events, sea-level changes and by sediment
infill. The combination of these processes resulted in changes in marine, brackish and freshwater ecological conditions in the sedimentary basins as well as in the restricted connections of the Central Paratethys with the Mediterranean sea, and across it with the Atlantic to the west, and with the Eastern Paratethys and the Indo-Pacific to the east. Middle Miocene diatomaceous sedimentary rocks rich in silicoflagellates and diatoms have been recorded from the entire Central Paratethys. In spite of their wide geographical distribution, diatomaceous sediments in the Central Paratethys occur in temporally limited sequences, usually in sedimentary rocks with low carbonate content. Therefore, they are rarely found in continuous sections and generally can not be traced laterally over large distances and they are not easily attributed to defined formations. This is also the case in Slovenia where Middle Miocene diatomaceous sediments have only been discovered in fragmentary sections in the Krško basin and in the Tuhinj syncline (Horvat, 2004). All localities are situated south of the Sava and Donat faults (Fig. 1). Miocene sediments in the westernmost parts of the Pan-
Fig. 1. Simplified tectonic map with location (star symbol) of the Smarjeta section, Distephanopsis concavus type locality (from Horvat, 2004).
Distephanopsis concavus Horvat: a revised silicoflagellate species from the Middle Miocene of the Central Paratethys
235
nonian basin (sensu lato) unconformably overlie Mesozoic sedimentary successions of the Internal and External Dinarides.
The type section Šmarjeta was logged in the roadcut of the Šmarjeta - Zbure road (45°51'252"N; 15°14'48" E; elev. 208 m). The section consists of dark grey to green grey carbonate siltstone beds usually termed "Baden Tegel", which is a typical Badenian formation of the Central Paratethys (Fig. 2). The under- and overlying beds are not exposed in the area. In the succession frequent fragments of Badenian turritellids and rare remains of scaphopods, oysters, and very rare pereiraeas [Pereiraea gervaisi (Vezian)] also occur. Siliceous microfossils in all samples are rare. In general, only remains of sponge spicules and rare endoskeletal dinoflagellates occur. Modest diatom and silicoflagellate assemblage were found only in the sample Šm-5 (Fig. 2). Rare fragments of silicoflagellates are found also in the nearby Šentjernej location, where a rich Badenian malacofauna (Mikuž, 2009) and diatom flora (Horvat, 2003a) also occur.
Standard stages	Central Paratethys stages	Calcareous nannoplankton (Martini 1971)	Global 3rd order sequences (Hardenbol et al. 1998)	Lithology	1 Thickness (m)	Sample no.	Collection no.
				- — —	20 •	Šm -11	157
				—~~			
					•	Sm-10	156
				-z-z-	•	Šm - 9	155
Z	Z <				•	Šm - 8	154
< i—i	1—1 Z	lO	uo	-Z-Zlr			
<	LU o <	Z Z	(N	-Z—Z—	•	Sm-7	153
		i	OQ 1-		*	Šm - 6	152
->	CD	uo		— —			
		Z	1—	--i	*	Šm - 5	151
	0£		l/l 1-	■			
rf	LU	Z					
LU	CL CL			-Z—Z—	•	Sm -4	150
to	=3			-z-z-	•	Šm - 3	149
				"-T	•	Šm - 2	148
							
				-Z—Z—	• 0	Sm -1	147
dark grey to greenish grey carbonate siltstone with turritellids, scaphopods, oysters and pereiraeas
Fig. 2. Stratigraphie column of the Distephanopsis concavus type section (section Smarjeta). Shaded area marks the sample with the type material.
Materials and methods
For micropaleontological preparation 3 g of dry sediment were treated following the method of Schrader & Fenner (1976). For slides preparation 2 ml of suspension were placed on a glass slide. After the water was evaporated, the slide was covered by 18 x 18 mm cover-glass. The mounting medium was Entellan. Slides were examined using a Carl Zeiss-Amplival light microscope with planchromatic objective under 1000x magnification.
Systematic description
Systematic classification is given after Desi-kachary & Prema (1996), and two internet databases (Internet 1 and Internet 2).
Kingdom Chromista Corliss, 1994 Phyllum Heterokonthophyta Van de Hoek, Mann & Jahns, 1995 Classis Dictyochophyceae Silva, 1980
Order Dictyochales Haeckel, 1894 Family Distephanaceae Locker, 1974 Genus Distephanopsis Dumitrica, 1978 Type species Distephanopsis crux (Ehrenberg) Dumitrica, 1978
Distephanopsis concavus Horvat (Pl. 1, figs. 1-10)
2004 Distephanopsis concavus n. sp.; Horvat: 29, pl. 5, figs. 1-5.
Note: Because of the incomplete original descriptions in Horvat (2004) the species is revised and fully described herein.
Derivation of name: The name was given after characteristically concave shape of the basal ring.
Diagnosis: A species of Distephanopsis with concave sided basal ring.
Type designation: Holotype: Pl. 1, fig. 1, para-types: Pl. 1, figs. 2-10. Section Šmarjeta, SE Slovenia, sample Šm-5. The slide is stored at the Ivan Rakovec Institute of Palaeontology Research Centre of the Slovenian Academy of Sciences and Arts. The collection number of the slide is D151.
Type locality: Šmarjeta section (45°51'25"N; 15°14'48"E; elev. 208 m) on the northern part of the Krško basin, SE Slovenia.
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Aleksander HORVAT
PLATE 1
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1 - 10 Distephanopsis concavus Horvat
1 holotype; 2-10 paratypes, section Šmarjeta, sample Šm-5 (coll. no. D-151)
Distephanopsis concavus Horvat: a revised silicoflagellate species from the Middle Miocene of the Central Paratethys
237
Description: Large square basal ring with concave sides. Apical ring square, connected to basal ring in the places of maximum constriction and with sides parallel with the major and minor axes of skeleton. Basal spines are long and of equal length. The concavity of the basal ring and equal basal spines resulted in four symmetry planes. Sustaining spines very short and always on the right of lateral bars.
Dimensions: Length of basal ring without spines 33-46 ^m (holotype 33 ^m) and width of basal ring without spines 27-42 ^m (holotype 27 ^m).
Remarks: Distephanopsis concavus is distinguished from other Distephanopsis species by the advanced concavity of the basal ring, which reduces the basal ring area. The function of the basal ring is in controlling the sinking rate and orientation of the organism (Sarjeant et al., 1987; McCartney & Loper, 1989). McCartney & Loper (1989) indicated the importance of minimizing silicoflagellate surface area in minimizing the utilization of skeletal material but their model only considered the apical surface area while the basal ring was excluded. The reason for the geometric simplification of the protist skeletons that are so extremely sensitive to environmental changes has been explained by ecological stress whereas stable environment conditions favour evolution towards more complex geometries and increase of symmetry (Guex, 2006). In this sense Dss. concavus can be regarded as an advanced morphotype that evolved from Dss. crux during times of ecological stability.
Dss. crux Ehrenberg and other Distephanop-sis species have straight or slightly convex basal ring. In addition, Dss. crux Ehrenberg usually has differentiated radial spines, those lying in the major axis being longer than those lying in the minor axis, while Dss. concavus has radial spines of equal length.
A basal ring of Distephanopsis stradneri (Jerkovic) Desikachary & Prema is sometimes slightly concave in the middle part of the skeleton (Jerkovic, 1965: figs. 43-45; Jerkovic, 1969: pl. 2, fig. 2; Perch-Nielsen, 1985: figs. 20/16, 17) but this concavity is less pronounced than that of Dss. concavus. Dss. stradneri also has a smaller apical ring and radial spines of different length.
By size and concavity of the basal ring Distephanopsis concavus is similar to Distephanus norvegiensis Perch-Nielsen (Perch-Nielsen, 1985, fig. 19/2), but Ds. norvegiensis has 5-sided basal ring, 5 radial spines, and smaller apical ring.
Age: Late Middle Badenian (Middle Miocene), upper part of the nannoplankton biozone NN5.
Discussion
Stratigraphy
The following silicoflagellate assemblage (Pl. 2, figs. 1-11) was observed and described from the Middle Miocene sections in the Krško basin (Horvat, 2004): Septamesocena apiculata apiculata (Schulz) Bachmann, Mesocena diodon diodon Ehrenberg, Dictyocha fibula Ehrenberg, Distephanopsis crux crux (Ehrenberg), Distephanopsis longispinus (Schulz) Desikachary & Prema, Distephanopsis crux parvus (Bachmann) Bukry, Distephanopsis crux scutu-latus (Bukry) Desikachary & Prema, Distephanopsis stradneri (Jerkovic) Desikachary & Prema, Distephanus speculum (Ehrenberg) Haeckel, Cannopilus hemisphaericus (Ehrenberg) Haeckel, Paracannopilus picassoi (Stradner), and Distephanopsis concavus Horvat. The assemblage is comparable to those of other Badenian localities in the Paratethys (Bachmann et al., 1963; Bachmann, 1971; Hajos, 1968; Dumitrica, 1978).
The standard silicoflagellate biozonation of Martini (1971, 1972) attributes the upper part of the Lower Miocene and the Middle Miocene to the unique Corbisema triacantha biozone. Bukry (1981) and Perch-Nielsen (1985) divided Martini's Corbisema triacantha biozone into two subzones: the lower Cannopilus schulzii subzone (corresponding to the NN4 and NN5) and the upper Distephanus stauracanthus subzone (corresponding to NN6 and NN7). Dumitrica (1978) distinguished two silicoflagellate assemblages in the Central Paratethys: the Lower Badenian 'Paracannopilus picassoi assemblage' and the Upper Badenian 'Distephanopsis stauracanthus assemblage'. The assemblage from the Šmar-jeta section can be compared with Dumitrica's 'Paracannopilus picassoi assemblage' (Horvat, 2004).
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Hajos (1986) proposed for the Badenian the Dyctyocha fibula - Distephanus crux v. longispina biozone that chronologically corresponds the late Early Badenian and Middle Ba-denian.
Based on calcareous nannofossil dating, the Smarjeta section can be placed in the upper part of the biozone NN5 which can be correlated with the upper part of the Middle Badenian. Nanno-fossils in all samples are moderately to well preserved. Individual samples contain 9-21 species (15 on average). The presence of very rare specimens of Sphenolithus heteromorphus Deflandre in most samples, including the top sample, together with the absence of Helicosphaera amp-liaperta Bramlette &Wilcoxon, allow the assignment of the studied interval to the biozone NN5. The presence of rare specimens of the large morphotype (>7^m) of Reticulofenestra pseudou-mbilica (Gartner) Gartner in samples Sm-1, Sm-4, and Sm-7 as well as rare specimens of Sphenolithus abies Deflandre in samples Sm-4 and Sm-10 is consistent with the upper part of the nannoplankton biozone NN5 (Bartol, pers. com.).
Despite the absence of Corbisema triacantha as the type species of the upper part of Middle Badenian and comparison of the Dumitrica's (1978) Central Paratethys Lower Badenian assemblage the age of Distephanopsis concavus could not be Lower Badenian but late Middle Ba-denian.
Paleoecology
The paleoecology of the Badenian silicoflagel-lates of the Krško basin could be deduced from co-occuring diatoms. The Badenian diatom assemblage in the Krško basin has a transgressive character. It is dominated by resting spores of Chaetoceros spp. and Thalassionema nitzschio-ides (Grunow) Mereschkowsky (Horvat, 2003b, 2012) which are usually thought to reflect stronger coastal upwelling conditions and high seasonal primary production (Barron, 1986; BArcena & Abrantes, 1998; BArcena et al., 2001; Burckle, 1984; Maynard, 1976; Abrantes, 1991; Sancet-ta, 1999, Nave et al., 2001; Booth et al., 2002). The structure of the diatom assemblage of the Krško basin suggests nutrient-enriched waters, which were transported laterally from the center of upwelling that was located to the west of the study area, presumably in the zone of mixing of warm proto-Mediterranean and cool Atlantic water masses (Horvat, 2004, 2012; Bartol et al., 2014).
Acknowledgements
The work was part of the Research Program P1-0008 financially supported by the Slovenian Research Agency. I am grateful to Miloš Bartol for calcareous nannoplankton analyses. I wish to thank Paulian Dumitrica and Kevin McCartney for reviewing the early version of the manuscript and for their constructive comments and suggestions and two anonymous reviewers for their comments, which greatly improved the manuscript.
PLATE 2
Silicoflagellate assemblage of the Distephanopsis concavus type section Smarjeta (sample Šm-5).
1	Cannopilus hemisphaericus (Ehrenberg) Haeckel
2	Distephanopsis crux crux (Ehrenberg)
3	Mesocena diodon diodon Ehrenberg
4	Distephanus speculum (Ehrenberg) Haeckel
5	Septamesocena apiculata apiculata (Schulz) Bachmann
6	Distephanopsis crux scutulatus (Bukry) Desikachary & Prema
7	Dictyocha fibula Ehrenberg.
8	Distephanopsis crux parvus (Bachmann) Bukry
9	Distephanopsis stradneri (Jerkovic) Desikachary & Prema
10	Paracannopilus picassoi (Stradner)
11	Distephanopsis longispinus (Schulz) Desikachary & Prema Scale bar 10 ^m except fig. 3 (5 ^m)
Distephanopsis concavus Horvat: a revised silicoflagellate species from the Middle Miocene of the Central Paratethys	239
PLATE 2
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