GEOLOGIJA 54/1, 81-90, Ljubljana 2011 doi:10.5474/geologija.2011.006 Latest Triassic conodonts of the Slovenian Basin and some remarks on their evolution Zgornjetriasni konodonti Slovenskega bazena in njihov evolucijski razvoj Tea KOLAR-JURKOVŠEK Geološki zavod Slovenije, Dimiceva ulica 14, SI-1000 Ljubljana; e-mail: tea.kolar@geo-zs.si Prejeto / Received 7. 4. 2011; Sprejeto / Accepted 5. 5. 2011 Key words: conodonts, Late Triassic, biostratigraphy, Mt. Kobla, Slovenian Basin Klju~ne besede: konodonti, zgornji trias, biostratigrafija, Kobla, Slovenski bazen Abstract 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. Izvle~ek Prikazan je stratigrafski pomen nekaterih pomembnih zgornjetriasnih konodontih rodov v Slovenskem bazenu s sposebnim poudarkom na njihovem pojavljanju v Slatenski formaciji profila Kobla (Julijske Alpe). Opisana je nova vrsta Misikella buseri n. sp., za katero je značilen zmanjšan segminatni element in predstavlja eno od zaključnih stopenj ob koncu razvoja rodu Misikella. 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 Csovar 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 epigondolel-lid representative. Only few gondolellids, such as Norigondolella and the platform-less Parvigon-dolella 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: Bu-durovignathus, Epigondolella, Gladigondolella, Metapolygathus, Misikella, Neogondolella, Nico-raella, Norigondolella and Paragondolella in the Middle and Late Triassic stata of the Slovenian Basin (Buser et al., 2007, 2008). 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. Sija area (Kolar-Jurkovsek et al., 1983; Kolar-Jurkovsek, 1994). The aims of this paper are to give a brief review of the Upper Triassic conodont genera known from Slovenia (Kolar-Jurkovsek, 1991; Buser et al., 2007, 2008), and to describe a new Misikella species, Misikella buseri n. sp., from the Rhaetian strata of the Slovenian Basin, sampled in the Mt. Kobla section (Fig. 1). Previous research The existence of Late Triassic deep-water sediments in western Slovenia was recognized by many authors (Winkler, 1923; Aubouin, 1960; Cousin, 1973). They were later object of several basic studies conducted by Buser (Buser, 1986, 1987, 1989, 1996, 2003; Buser & Debeljak, 1996; Buser et al., 2007, 2008; Buser & ogorelec, 2008) Fig. 1. Actual position of geotectonic units in western Slovenia with extension of sediments of the ancient Julian and Dinaric Carbonate Platforms and the intermediate Slovenian Basin with the position of the Mt. Kobla section (star). Modified by Buser et al. (2007). and are currently object of multidisciplinary study carried out by Rožic and co-workers (Rožic, 2008, 2009; Rožic & Kolar-Jurkovšek, 2007; Rožic et al., 2009; Gale, 2010). 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, 2008). The basin pinched out in the Soca 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 Zagrebacka 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, 2008). After the basic research by Buser (1986, 1987; Buser & ogorelec, 2008) the Mt. Kobla section was sampled and measured also by Rožic et al. (2009). They discriminated the non-dolomitized uppermost part of the Baca Dolomite Formation by formalizing this unit and introducing the Slat-nik Formation formed of hemipelagic limestone alternating with resedimented limestone indicating a progradation of sedimentary environments from the the basin plain to the lower slope (Rožic, 2008). The Slatnik Formation is preserved in the northern part of the basin where the latest Trias-sic succession experienced less intense diagenetic alteration. The Late Triassic part of the Mt. Ko-bla section is documented by three upward pro-grading 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 Misikella hernsteini- Misikella posthernsteini Zones (Rožic et al., 2009). A more detailed sampling of the interval with the finding of M. post-hernsteini provided new data and enabled a more precise documentation of the latest zone. Thus, the Misikella posthernsteini Assemblage Zone can be divided into two units: the Misikella hernsteini-Misikella posthernsteini and the Misikella koes-senensis Subzones (Gale et al., submitted). The recovered conodont faunas include representatives of Epigondolella, Misikella, Norigondolella, Oncodella, Parvigondolella and Zieglerioconus. The new species of Misikella was collected in the interval from 76,0 to 77,8 m of the Mt. Kobla section (Fig. 2). A detail description of the investigated section along with biostratigraphic conclusions based on conodonts and foraminifers is given in Gale et al. (submitted). Fig. 2. Geological column of the Mt. Kobla section with conodont distribution. Modified after Gale et al. (submitted). 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 (Fig. 1, 2) and preserves the Upper Norian - Lower Jurassic succession of the Slovenian Basin (Rožic 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, 2008). The Slovenian Basin is one of the three paleogeographic units that existed during the Late Triassic in the territory of Slovenia. These are (from north to south): the Julian Carbonate Platform, the Slovenian Basin and the Dina-ric Carbonate Platform (Buser, 1986, 1989, 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. stein-bergensis 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 epigon-dolellid 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 Rhae-tian 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 strati-graphically most important species Neohindeo-della detrei as an index species for the youngest strata above the last occurrence of Misikella and these authors stated the form seems to be homeo-morph with forms known from older strata of the Late Triassic. Zieglerioconus has been known to appear with only one representative, Z. rhaeticus, but new No-rian species have yet not been described (Channell et al., 2003). It is a secondary single cone cono-dont with a widely expanded and excavated basal cavity. Zieglerioconus was first described from the Rhaetian (M. koessenensis Subzone) of Csovar 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 Tethys and non-pelagic of outer Tethys in the Germanic Basin (Kozur & Mock,1991; Swift, 1989; Palfy et al., 2007). The hitherto known occurrences of the species are quite typical for the M. koessenen-sis Subzone, but Zieglericonus begins in the uppermost Carnian and occurs throughout the No-rian (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 post-hernsteini Zone. The origin of Zieglerioconus is still yet not known, but Kozur & Mock (1991, p. 277) suggested its forerunner could be a Misi-kella 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 No-rian 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 Se-vatian and it is also a facies controlled species (KozuR & Mock, 1991). In Slovenia, it ranges from the upper part of the Parvigondolella andruso-vi-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, 1980, 1987; Kozur, 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 biomar-ker used to define the Norian-Rhaetian boundary (Kozur & Mock, 1974b, 1991; Kozur, 1996; Krystyn, 1980, 1987; Birkenmajer 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 boundary is defined by FAD of the index species, but the upper boundary is marked by the dissapearance 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 drop-shaped 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. koessenen-sis 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 Misikel-la. 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. posthern-steini, respectivelly. We agree with observation of Giordano et al. (2010) that the outline of the basal side is important and it is used as the main criterion to distinguish M. hernsteini and M. posthernsteini. Their evolution from units with an oval basal area in M. hernsteini is documented with a series of transitional forms leading to elements with an evident inflexion ending in a heart-shaped basal side of M. posthernsteini. Arguments of Moix 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 Rožic 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. lon-gidentata. 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: 1. Evolutionary line: M. hernsteini - (M. rhaetica) - M. koessenensis - M. buseri. Already in the lower M. hernsteini - P. andrusovi A. Z. two species, M. koessenensis and M. rhaetica evolved. According to data from Slovenia the two taxa appeared simultaneously where specimens of M. koessenensis with no denticles posterior to the cusp were collected. Such forms gave rise to M. buseri that represents a final step in this evolutionary line marked by obvious reduction of the overall size, decrease in number of denticles and proportional increase of the height of denticles. 2. Evolutionary line: M. hernsteini - M. posthernsteini - M. ultima - M. kovacsi: This evolutionary line is marked by the evolution of a heart-shaped basal area in M. posthernsteini that was followed by M. ultima and M. kovasci. Misikella posthernsteini evolved from M. hernsteini through several transitional forms and marks the lower bounday of the M. posthernsteini A. Z. The evolutionary trend M. posthernsteini - M. ultima - M. ko-vasci is characterized by a decrease of overall size and in a marked reduction of number of denticles. The two phylogenetic lineages share similar stages of evolution. M. koessenensis and M. post-hernsteini evolved after common ancestor of both lines. At this stage two lines with a distinct forming of the basal area are evident. According to the Slovenian data, M. koessenensis and M. rhae-tica evolved simultaneously from M. hernsteini. The intermediate stage is represented by M. rhae-tica and M. ultima. The two species have been so far reported as a paralel step in Misikella evolution as they both reveal a secondary bar, but they Stage Substage Conodont Zone Neohindeododella detrei Zone £ TO Misikella ultima Zone