Latest Triassic conodonts of the Slovenian Basin and some remarks on their evolution Zgornjetriasni konodonti Slovenskega bazena in njihov evolucijski razvoj

A stratigraphical importance of some latest Triassic conodont taxa from the Slovenian Basin with special regard on their distribution in the Slatnik Formation of the Mt. Kobla section (Julian Alps) is presented. Description of a new species Misikella buseri n. sp. is given. It is marked by a reduced segminate element and it represents a distinct stage at the decline of Misikella evolution.


Introduction
The phylum Conodonta was one of the groups that became extinct at the Triassic-Jurassic boundary (TJB) when one of the five largest mass extinctions of the Phanerozoic marine record occurred (sePkoski & rauP, 1986). There are very few reports of Jurassic survivals and kozur (1993) documented Neohindeodella detrei from the earliest Hettangian of Cso ˝vár in Hungary. During the latest Triassic conodonts therefore suffered major decline in diversity and their extinction can be interpreted as the cummulative result of several causal factors and not of a single catastrophic event (clark, 1983). There is an evident decline of conodont taxa from Carnian-Norian boundary strata. Many gondolellids last appeared in the Norian, but a few range into the lower Rhaetian, for example E. bidentata as the last epigondolellid representative. Only few gondolellids, such as Norigondolella and the platform-less Parvigondolella and Misikella, persisted during the latest Triassic.
The study of conodonts has resulted in the recognition of several stratigraphically significant conodont species belonging to the genera: Budurovignathus, Epigondolella, Gladigondolella, Metapolygathus, Misikella, Neogondolella, Nicoraella, Norigondolella and Paragondolella in the Middle and Late Triassic stata of the Slovenian Basin (Buser et al., 2007. The youngest Triassic conodont assemblages are marked by the presence of Misikella, a genus already described from western Slovenia of the Pokljuka plateau and from the Mt. Šija area (kolar-jurkov{ek et al., 1983; kolar-jurkov{ek, 1994).
The Slovenian Basin extends in an east-west direction of central Slovenia and became established during the Ladinian following disintegration of the Slovenian Carbonate Platform and it persisted continuously until the Late Cretaceous (Buser, 1989;Buser et al., 2007. The basin pinched out in the So~a river valley and was not connected to the Belluno Basin (Buser, 1986). From the eastern part of present Slovenia, it continues across Croatia north of Zagreba~ka gora to Hungary. The present exposed parts of the Slovenian Basin measure in length about 170 km and in width about 40 km (Buser et al., 2007. After the basic research by Buser (1986Buser ( , 1987Buser & oGorelec, 2008) the Mt. Kobla section was sampled and measured also by roži~ et al. (2009). They discriminated the non-dolomitized uppermost part of the Ba~a Dolomite Formation by formalizing this unit and introducing the Slatnik Formation formed of hemipelagic limestone alternating with resedimented limestone indicating a progradation of sedimentary environments from the the basin plain to the lower slope (roži~, 2008). The Slatnik Formation is preserved in the northern part of the basin where the latest Triassic succession experienced less intense diagenetic alteration. The Late Triassic part of the Mt. Kobla section is documented by three upward prograding high-frequency cycles. The Triassic part of the section is followed by the Krikov Formation of Jurassic age. The authors also provided conodont data and they recognized three latest Triassic conodont zones: Epigondolella bidentata, Parvigondolella andrusovi-Misikella hernsteini and

Geological setting
The Mt. Kobla section (x = 5121,590, y = 5420,550, z = 1498 m) is located on the southern brim of the the Julian Alps (Fi g. 1, 2) and preserves the Upper Norian -Lower Jurassic succession of the Slovenian Basin (roži~ et al., 2009). This area is part of the Tolmin Nappe that together with the overlying Julian Nappe structurally forms the eastern part of the Southern Alps (Placer, 1999(Placer, , 2008. The Slovenian Basin is one of the three paleogeographic units that exis ted during the Late Triassic in the territory of Slovenia. These are (from north to south): the Julian Carbonate Platform, the Slovenian Basin and the Dinaric Carbonate Platform (Buser, 1986(Buser, , 1989(Buser, , 1996 (Fig. 1).

Significant conodont taxa in the latest Triassic
Norigondolella is represented by the long-ranging species N. steinbergensis. It is an extremely facies dependant species that can be found in fully pelagic sediments of open sea (cherty limestone) of the latest Triassic and therefore it is not stratigraphically important outside this facies (kozur & Mock, 1991). In the Mt. Kobla section in Slovenia, the LAD of N. steinbergensis is documented in the upper Misikella posthernsteini A. Z. (= Misikella koessenensis -Misikella posthernsteini Subzone) at 88,7 m.
Genus Parvigondolella evolved from Epigondolella bidentata as the ultimate stage in epigondolellid evolution. The genus however is quite rare as can be explained by its existence at the end of evolution of an important stock and it is represented by only few species: P. andrusovi, P. ? lata, P. rhaetica, P. vrielyncki. Some authors regard the platform-less gondolellid forms as juvenile forms of E. bidentata named as the andrusovi stage (krystyn, 1980; krystyn et al., 2007). However, such a view was rejected based on different stratigraphic occurrences, as Parvigondolella ranges up high in the Rhaetian interval that is beyond the highest occurrence of E. bidentata or any other platform-bearing metapolygnathid (kozur & Mock, 1991). The latter authors also supported their statement by noting the size of the different stages and that certain parameters (size and number of denticles, height of blade) of younger onotogenetic stages cannot exceed the parameters of the largest adult specimens. kozur & Mock (1991) documented their view based on reports of the quite widespread appearance of Parvigondolella in the Rhaetian of Alps, Carpathians, Hungary, Lagonegro Basin that is also confirmed now with data from Slovenia.
Some ramiform elements are also present but are rather rare. Nevertheless in a few cases they permit apparatus reconstruction. Among them kozur & Mostler (1991) reported also a stratigraphically most important species Neohindeodella detrei as an index species for the youngest strata above the last occurrence of Misikella and these authors stated the form seems to be homeomorph with forms known from older strata of the Late Triassic.
Zieglerioconus has been known to appear with only one representative, Z. rhaeticus, but new Norian species have yet not been described (channell et al., 2003). It is a secondary single cone conodont with a widely expanded and excavated basal cavi ty. Zieglerioconus was first described from the Rhaetian (M. koessenensis Subzone) of Cso ˝vár in Hungary, and it is rare species that has been so far reported only from Great Britain what means it appears in sediments of the pelagic Tet hys and non-pelagic of outer Tethys in the Germanic Basin (kozur & Mock,1991;sWiFt, 1989;PálFy et al., 2007). The hitherto known occurrences of the species are quite typical for the M. koessenensis Subzone, but Zieglericonus begins in the uppermost Carnian and occurs throughout the Norian (channell et al., 2003;kozur, pers. comm.). New data from Slovenia confirm the existence of Zieglerioconus in the late Sevatian. In the Kobla section, Zieglericonus ranges from Parvigondolella andrusovi-Misikella hernsteini A. Z. to lower Misikella koessenensis Sz. of the Misikella posthernsteini Zone. The origin of Zieglerioconus is still yet not known, but kozur & Mock (1991, p. 277) sugges ted its forerunner could be a Misikella of the M. longidentata group or Oncodella paucidentata. According to recent data from Slovenia, Zieglerioconus and M. hernsteini made their first appearance together in the Misikella hernsteini-Parvigondolella andrusovi A. Z. in absence of Oncodella and thus, documenting the existence of Zieglerioconus in the latest Norian strata ( Fig. 2; Pl. 1, Fig. 2). The forerunner of Zieglerioconus and M. hernsteini is therefore supported to be linked with the M. longidentata group.
Oncodella paucidentata is a characteristic element that already appeared in the uppermost Sevatian and it is also a facies controlled species (kozur & Mock, 1991). In Slovenia, it ranges from the upper part of the Parvigondolella andrusovi-Misikella hernsteini A.Z. up to an un-named Misikella Zone that corresponds to the known range of the species elsewhere.
Genus Misikella is most important for the stratigraphy of the Late Sevatian-Rhaetian interval. Some biostratigraphic schemes have been reported so far and they are slightly different (Mostler et al., 1978;GazDzicki et al., 1979;krystyn, 1980kozur, 2003;krystyn et al., 2007). In 1991, kozur and Mock made a report on stratigraphic value of certain conodont taxa. The main intent was to eliminate M. rhaetica (it appeared already in the uppermost Sevatian) as an index form for the zone introduced by Mostler et al. (1978) as it is strongly facies controlled and occurs already at levels with frequent M. hernsteini. From the upper part of the M. posthernsteini A.Z. they gave a description of the Misikella koessenensis Subzone (originally described by Kozur 1989) as a substitute of the M. rhaetica Zone of earlier schemes, and the author noted also to a shorter range of Misikella koessenensis. The first appearance of Misikella posthernsteini is an important biomarker used to define the Norian-Rhaetian boundary (kozur & Mock, 1974b(kozur & Mock, , 1991kozur, 1996;krystyn, 1980BirkenMajer et al., 1990). On the other hand, M. ultima is a facies independant species and therefore it is very good boundary marker of the M. ultima A. Z. (the lower boun dary is defined by FAD of the index species, but the upper boundary is marked by the dissapea rance of Parvigondolella and Misikella) (kozur & Mock, 1991).

Evolution of Misikella
The evolutionary trend M. hernsteini -M. posthernsteini -M. ultima is well known (kozur & Mock, 1991). The phylomorphogenetic line between M. hernsteini and M. posthernsteini was documented first by Mostler et al. (1978) and was recently studied by GiorDano et al. (2010) in which transitional forms are arranged in three evolutionary steps. Misikella hernsteini is characterized by a long blade composed of 5-6 denticles and an appropriatelly long and narrow basal dropshaped basal cavity. The step 1 in the evolution of M. hernsteini -M. posthernsteini is marked by a shorter blade with a decreased number of denticles (four in number); the basal cavity is enlarged and has a markedly expanded posterior part with a rounded or triangle-like outline but is devoid of any incision. The increase of the height of denticles is obvious. The length : height ratio (L : H) in M. hernsteini is 2 : 1,5 whereas in the step 1 of the H. hernsteini -H. posthernsteini evolutionary trend is almost 1 : 1. This step corresponds descrip-tion of H. koessenensis. In general, there are very rare Misikella specimens bearing denticles behind the cusp and all of them belong to M. rhaetica. The size of M. koessenensis is comparable to the size of M. hernsteini, whereas in the M. posthernsteini an increase in height may be seen. In this study, short and high elements that reveal no posterior denticle, composed of 3-5 denticles and an appropriate ratio L : H are included to M. koessenensis sensu lato rather than make a compromise to regard these forms as Misikella n. sp. B, as suggested by Kozur (pers. comm.). Mostler originally wrote that most specimens of M. koessenensis have slender denticle behind the cusp. Thus, the Slovenian specimens fit the original diagnosis of the taxon. However, most conodont workers so far included only specimens with posterior denticle(s) to this species, and therefore an emendation of M. koessenensis should be done. In the faunas of the investigated Kobla section such elements make their first apperance at 58 m, 3 metres above the first occurrence of M. hernsteini. The following two evolutionary steps 2 and 3 leading towards M. posthernsteini are marked by a progressive degree of incision in the posterior part of the basal cavity that causes a heart-shaped lower outline, and parallel to this also a reduction of denticles ending in three-dentate elements with a noticeable larger size.
Two lines can be seen in evolution of Misikella. They are reprepresented by M. hernsteini and M. posthernsteini. The M. hernsteini evolutionary line is marked by a slight reduction of overall unit size, whereas in the M. posthernsteini line an enlargement of overall size is evident. Both lines are marked by a reduction in the number of denticles. The evolutionary trend in the two lines is practically identical if one takes into consideration the outline of the lower side that is drop-shaped in M. hernsteini and heart-shaped in M. posthernsteini, respectivelly.
We agree with observation of GiorDano et al.  et al. (2007) claiming that the lower area is important for recognition of Sevatian-Rhaetian taxa is in general supported. They argued the lower side of M. rhaetica does not extend to the full length of the lower side of the element but extends only 2/3. Therefore they assigned »M.« rhaetica to Parvigondolella. The original diagnosis of M. rhaetica includes elements bearing 1-3 smaller denticles behind the cusp and the basal cavity extends only 2/3 of the lower side in most specimens (Mostler et al. 1978). In this study, the elements of Misikella with 1-2 denticles of smaller size behind the cusp and with a widely opened basal area extending nearly the full length are attributed to M. rhaetica as illustrated in roži~ et al. (2009, Fig. 9e). Thus, the species name Misikella rhaetica sensu lato is used herein. Similar specimens are regarded as long M. hernsteini by Kozur (pers. comm.) or are assigned to Misikella n. sp. A by Muttoni et al. (2010). Emendation of M. rhaetica is therefore needed as the original diagnosis includes also specimens with longer basal area. However, based on the material from Slovenia, we may conclude that there exists a strong homeomorphism in morphology of Misikella rhaetica and Parvigondolella rhaetica.
The known origin of Misikella started with M. hernsteini, as its forrunner is yet still not known. However, it could be linked with M. longidentata. M. hernsteini appeared during the late Sevatian in the M. hernsteini -P. andrusovi A. Z. Soon after appearance of M. hernsteini two lines of evolution can be noted that markedly differ in outline of the basal area. One line retained the drop-shaped basal area, whereas a heart-shaped basal area is evident in the second: The two species have been so far reported as a paralel step in Misikella evolution as they both reveal a secondary bar, but they retained the similar outline of its ancestor. Based on available data it seems that M. rhaetica was a parallel step in Misikella evolution. The final step in evolution of both lines is represented by M. buseri and M. kovacsi with forms characterized by very reduced elements bearing only two denticles. As the following expressed trend in evolution of Misikella its ultimate stage could be expected in secondary single cone forms, similar to Zieglerioconus. However, their direct relationship is excluded due to their co-occurrences in the Slovenian section. Available data suggest the origin of Zieglerioconus and Misikella from a common and still unknown ancestor (Fig. 3).
The evolution of the last representatives of gondolellid stock in the latest Triassic (platform type Epigondolella and non-platform types Misikella and Parvigondolella) share certain similarities. The evolutionary steps of Epigondolella have already been studied by traMMer (1974) who documented a trend in reduction of element size, reduction of platform length, decrease in number of lateral denticles along with increasing height of denticles in the carina. A strong reduction in size and total reduction of the platform was shown in the Middle Triassic lineage Neogondolella -Celsigondolella by Kozur (1968). The end form, Celsigondolella watznaueri is very similar to Parvigondolella. A similar evolutionary trend can be observed also in the non-platform types, Misikella and Parvigondolella. Our study documents that a strong homeomorphism occurred in the evolution of the three genera leading to very reduced forms. This phenomenon was observed in the evolution of certain Late Triassic genera as has already been pointed out by kozur & Mock (1974a). The specimen here illustrated (Pl. 1, Fig. 1) reveals a narrow basal area that is not typical for representatives of the genus Misikella at this level and morphologically stands close to Parvigondolella, therefore this element is here assigned to the genus »Misikella«.

Systematic paleontology
Genus Misikella Kozur & Mock 1974 Type species: Spathognathodus hernsteini Mostler 1967 Misikella buseri n. sp. Pl. 1, Fig. 3 Derivatio nominis. In honour of Prof. Dr. Stan ko Buser, a Slovenian geologist for his outstan ding contribution to the knowledge of the Slovenian Basin and the Slovenian geology in general.
Diagnosis. The P1 element is a very reduced segminate element comparable to other species in Misikella marked by a big cusp and one more small denticle, a basal cavity rounded posteriorly and devoid of any incision.
Description. This species is characterized by very reduced segminate element with a large cusp and one more small denticle: in the holotype the denticle next to the cusp is very reduced. The basal area is very opened and has an oval posterior edge. The basal cavity is widely expanded appropriatelly to the large size of the cup.  The illustrated conodont specimens were photographed at the JEOL JSM 6490LV Scanning Electron Microscope at the Geological Survey of Slovenia / Geolo{ki zavod Slovenije and are housed in the micropaleontological collection of the same institution.

Acknowledgements
The author is indebted to Merlynd Nestell (Arlington, Texas) for editing the manuscript and wishes to extend her thanks to Heinz Kozur (Budapest, Hungary) for helpful suggestions and communications on conodonts. Milan Sudar (Belgrade, Serbia) and Bojan Ogorelec (Ljubljana, Slovenia) provided constructive reviews of the manuscript. Facilities and technical staff of the Geological Survey of Slovenia are gratefully acknowledged. The investigation was financially supported by the Slovenian Research Agency (programme number P1-0011).