In the original description of Mockina slovakensis (Kozur, 1972), the platform is about 2/3 as long as the entire conodont element, being almost the same width everywhere. The anterior platform margins have three to five long denticles, but the posterior part of the platform is smooth with a rounded end. Some denticles can be accompanied by an accessory denticle. The blade is very high in the front and the denticles are strongly fused. Approximately in the middle, the blade breaks off like a cliff and three to four small and widely separated nodes follow to the rear but within the platform. The basal pit is shifted far forward and is approximately centrally located. However, Budai and Kovács (1986) emended the original description, since they thought the original description of M. slovakensis was based on a single and pathologic form. In their new description, the carina is the highest in the middle, then gradually decreases in height in phylogenetically earlier forms, but sharply drops down without transition in advanced elements. The free blade is slightly inward bent and it comprises four to five completely fused denticles in adult. The wide keel extends along the whole length of the basal edge. The narrow, inverted pit is located near to center. Although they proposed some new diagnostic features, the abrupt decline of the blade and the strongly fused blade denticles were still the most distinctive characteristics of M. slovakensis. The new revision in Budai and Kovács (1986) optimized the description of M. slovakensis.
Furthermore, as more and more reports of M. slovakensis came out (e.g., Mazza et al., 2012; Rigo et al., 2012; Belvedere et al., 2008; Moix et al., 2007; Donofrio et al., 2003), we have found that there are large differences between certain individuals. We propose an improved description of the M. slovakensis population which is based on the integration data of the new investigation and the previous literature.
The platform of M. slovakensis is usually short. The shapes of the platform terminal are various and it could be pointed, rounded or blunt; the rounded and blunt platform terminal are likely to occur in the transitional forms; the pointed platform end almost only occurs in advanced form. The platform has 3–11 denticles (nodes) on the margins, generally with two high and sharp denticles on the external anterior margin and one on the internal anterior margin. The advanced form of M. slovakensis usually has 3–5 denticles on the anterior platform margins, and the posterior platform margins are nearly unornamented. In contrast, the posterior platform margins of the primitive forms or transitional forms between M. slovakensis and E. praeslovakensis are usually ornamented by small denticles or nodes. The pit is forward shifted and located in front of the middle of the platform, but it could backwardly lie in the central platform in the primitive forms. The keel end could be rounded or pointed but never bifurcated. The carina is composed of 2–6 denticles (nodes); generally, the carina of the advanced forms is shorter than those of the primitive and transitional forms. The posterior carina of the most M. slovakensis does not reach the end of the platform, but it could extend to the termination in some primitive and transitional forms, just like the E. praeslovakensis. The free blade usually consists of 2–6 strongly fused denticles and it has a wall-like termination where it descends abruptly to the carina. The last denticle of the blade is usually huge and projected backwards. In lateral view, the morphotype A of the M. slovakensis population has an obviously elevated platform, making the profile of the element stepped; the lateral profile of morphotype B is slightly elevated or nearly straight.
As more and more forms of the M. slovakensis population have been identified, Kovács and Nagy (1989) firstly divided the M. slovakensis into five morphotypes according to the shapes of platform and the distribution of ornamentation. But, in fact, the variation of the platform is diverse and it is not only five types.
During the investigation, we find that the shape of lateral profile shows two different forms in M. slovakensis population (Fig. 4). The profile of the majority is stepped because of the elevated platform, but there is a small number of M. slovakensis shows a nearly straight profile. Thus, we classify the M. slovakensis into two morphotypes, A and B, on the basis of shape of the lateral profile. Mockina slovakensis Morphotype A owns a stepped profile. In contrast, the lateral profile of M. slovakensis Morphotype B is nearly straight. These two morphotypes can also be found in the E. praeslovakensis.
In our study area, the two morphotypes are found together, but the occurrence of each morphotype is unclear in the other places. There are usually fewer figured specimens found in most previous literatures. In other literatures, M. slovakensis composes a monospecific fauna without any other age diagnostic fossils. Thus, the exact range of the two morphotypes is still unknown at present.
Although the M. slovakensis is very common species, it is misidentified frequently. The species owns some distinctive characteristics, especially the abrupt, wall-like posterior end of the blade, which is not present in any other Triassic platform conodont except for its ancestor E. praeslovakensis (Moix et al., 2007). Therefore, the E. praeslovakensis is the species most often thought of as the M. slovakensis in previous literatures. These two species resemble in the abrupt posterior end of the blade and the lateral profile of element. The main reason for the misidentification is the insufficient consideration of the lower side of the conodont element (Moix et al., 2007). The most prominent difference between these two species is the shape of the keel end, E. praeslovakensis owns a distinctive bifurcated keel end but M. slovakensis has a pointed or rounded keel terminal which is never bifurcated.
In addition to E. praeslovakensis, M. slovakensis is also easily confused with Mockina postera and Mockina zapfei. Mockina postera also has a short platform like M. slovakensis, but the platform terminal of M. postera is always pointed and the posterior platform margins are often asymmetrical. In contrast, the posterior platform end of M. slovakensis is mostly pointed or narrowly rounded, but some specimens have a broad, blunt posterior platform end. Mockina postera has a distinctive anterior denticulation commonly consisting of one prominent denticle on one platform margin and two on the other; M. slovakensis usually has three to five denticles on the anterior platform, but there could be also some small nodes occur on the posterior part of the platform, especially in the transitional forms. The blade descends relatively abruptly onto the platform in some M. postera specimen, but in general the last denticle is distinctly smaller than the preceding one; in other specimen, the blade descends gradually into the low carina. Moreover, the blade denticles of M. slovakensis are strongly fused, especially in the adults, which never occur on M. postera.
Mockina zapfei has a relatively short platform like other Mockina species, but the platform of M. zapfei is asymmetric due to the curvature of the carina to one side. The most prominent disparities between M. zapfei and M. slovakensis are that M. zapfei owns high blade and it descends gradually into the low carina and the last denticle of carina is projected beyond the platform.
Mockina slovakensis was found by Kozur firstly from beds immediately above the uppermost Norian Misikella hernsteini Zone and below the Rhaetian Misikella posthernsteini Zone. Thus, M. slovakensis was assigned to the uppermost Sevatian, but the age of Rhaetian also cannot be excluded (Moix et al., 2007; Kozur, 1972). Budai and Kovács (1986) assigned M. slovakensis to the middle Norian by comparing their fauna with Krystyn's unpublished middle Norian material from Timor Island. Subsequently, Kovács and Nagy (1989) also assigned M. slovakensis to the middle Norian mainly based on the unpublished data of Krystyn from Timor and the published data of Budai and Kovács (1986). But they did not exclude early Sevatian age because the occurrence of M. slovakensis together with Rhabdoceras suessi (Moix et al., 2007). Roghi et al. (1995) reported a conodont fauna which was marked by "Epigondolella" slovakensis in Dolomia di Forni. They assigned the "Epigondolella" slovakensis to the middle–late Alaunian (middle Norian) based on the unpublished data of Krystyn from Timor and the published data of Budai and Kovács (1986) and Kovács and Nagy (1989). Meço (1999) reported the occurrences of "Epigondolella" slovakensis at the upper part of Sevatian in Albania. Krystyn illustrated the "Epigondolella" slovakensis from Timor in Martini et al. (2000). In the caption of the plates, the "Epigondolella" slovakensis were assigned to the lower Norian, but in the main text they were in the age of middle Norian. The lower Norian could be a literal mistake, because the middle Norian (Alaunian) age was confirmed by the ammonites (Moix et al., 2007; Martini et al., 2000). Nevertheless, the three specimens (in Martini et al., 2000, pl. V, figs. 13–14, 17–18, 21–22) which were assigned to "Epigondolella" slovakensis are actually not the true M. slovakensis, cause the wall-like posterior end of the blade which is the most prominent character of M. slovakensis is missed. There is one species (pl. V, figs. 21–22) which has a similar shape of platform and it seems also own a bifurcated keel end from the upper view, but the blade denticles of this specimen is not strongly fused like M. slovakensis. In general, the conodonts presented in Martini et al. (2000) are not real M. slovakensis. Therefore, the approximately Alaunian 1 (middle Norian) age of their "slovakensis" which was confirmed by the ammonites is controversial. Donofrio et al. (2003) reported a conodont fauna which are dominated by M. slovakensis in sediments of the restricted intraplatform basin of the Seefeld Formation, Alps. The middle Norian age of the Seefeld Formation of the Alps is based on two further species, "Epigondolella" carinata and "Epigondolella" postera. However, Kozur held the opinion of that the "Epigondolella" carinata in Donofrio et al. (2003) are actually M. slovakensis, due to the two illustrated specimens determined as "E." carinata have a wall like posterior end of the anterior carina. Donofrio et al. (2003) assigned M. slovakensis to Alaunian 2 because that it is concomitant with "E. postera" in "Epigondolella" postera Zone. Channell et al. (2003) found that the M. slovakensis occurred in Silická Brezová (Slovakia), and it was assigned to the latest Alaunian. Moix et al. (2007) discussed and revised some of the misidentifications of M. slovakensis in previous literature, and then came up with a new species, E. praeslovakensis, by using paleontological and biostratigraphic criteria. The E. praeslovakensis was regarded as direct ancestor of M. slovakensis. Moreover, Moix et al. (2007) concluded that monospecific faunas which consist only of true M. slovakensis belong to the Sevatian up to the top of the M. hernsteini Zone, even if its first appearance datum (FAD) is confirmed in latest Alaunian (Channell et al., 2003), where it occurs together with advanced E. praeslovakensis. Monospecific faunas which consist exclusively of E. praeslovakensis belong to the middle to late, but not latest Alaunian. Belvedere et al. (2008) recovered a transitional form of M. slovakensis which characterized by a slightly bifurcated keel end along with a real M. slovakensis. They assigned the co-occurrence of E. praeslovakensis and M. slovakensis to latest Alaunian in age according to the discussion in Moix et al. (2007). Balini et al. (2010) found the occurrence of "Epigondolella" slovakensis in Sevatian (upper Norian) in Pizzo Mondello Section. Mazza et al. (2012) also recovered M. slovakensis in the same section and the occurrence of it ranges from Mockina bidentata Zone to Misikella hernsteini Zone (Sevatian 1 to Sevatian 3). Rigo et al. (2012) documented at Sasso di Castalda Section (Lagonegro Basin, Italy) the first occurrence (FO) of M. slovakensis after the M. serrulata and before the FO of M. bidentata, and its last occurrence after the FO of Parvigondolella andrusovi and before the FO of M. hernsteini. Mockina slovakensis is also found in the Pignola-Abriola Section in the Lagonegro Basin (Rigo et al., 2016) and Inuyama area, central Japan (Yamashita et al., 2018) in the age of Sevatian. Rigo et al. (2018) took M. slovakensis as the index species of the uppermost Alaunian M. slovakensis Zone in their new Upper Triassic conodont biozonations. The occurrence of M. slovakensis was assigned to the uppermost Alaunian to upper Sevatian.
Combining almost all the information of the distribution of M. slovakensis (Table 1), the occurrence of this species ranges from the base of M. slovakensis Zone (uppermost Alaunian) to the uppermost part of M. hernsteini Zone (uppermost Sevatian) (Rigo et al., 2018). It's worth to mention that M. slovakensis was apparently collected in the same layers of M. posthernsteini by Mastandrea et al. (1997). However, the illustrated specimens of M. posthernsteini are instead transitional forms with the ancestor (M. hernsteini) because they exhibit more than 3 denticles on the blade and/or they don't show a clear notch on the backside of the cusp, which are the main features of M. posthernsteini (Karádi et al., 2020; Rigo et al., 2018, 2016; Bertinelli et al., 2016; Giordano et al., 2010).
Year Authors Occurrence of M. slovakensis Place MA MB 1972 Kozur Upper Sevatian to lower Rhaetian Maly Mlynsky Vrch, Slovak Karst, Slovakia × 1986 Budai and Kovács Monospecific, exact age unclear Keszthely Hills, Transdanubian Range, Hungary × × 1989 Kovács and Nagy Monospecific, exact age unclear Pilis Hills, Transdanubian Range, Hungary × × 1993 Amodeo et al. Sevatian Vietri di Potenza, Lagonegro Basin, Italy No figure 1995 Roghi et al. Monospecific, exact age unclear Dolomia di Forni, Carnia, Southern Alps, Italy × 1997 Mastandrea et al. Late Norian to Rhaetain Colle del Crapio, Catena Costiera, Calabria, Italy × × 1999 Meço Upper Sevatian Ura Shtrenjte, Cukali zone, Albania × 2003 Donofrio et al. Supposedly upper Alaunian/lower Sevatian Seefeld Formation, Tirol, northern Calcarous Alps, Austria × × 2003 Channell et al. Sevatian Silická Brezová, Slovak Karst, Slovakia × × 2003 Mastandrea et al. Monospecific, exact age unclear Valle Corvino, Calabria, Italy × 2005 Onoue and Tanaka Presumably upper Alaunian Sambosan accretionary complex, Japan × × 2007 Moix et al. Uppermost Alaunian to upper Sevatian Gavuruçtuğu Block, Mersin Mélange, Turkey No figure 2008 Belvedere et al. Uppermost Alaunian Monte Pasubio, Southern Alps, Italy × 2010 Balini et al. Sevatian Pizzo Mondello, Sicani Basin, Sicily, Italy No figure 2012 Mazza et al. Sevatian 1 to Sevatian 3 Pizzo Mondello, Sicani Basin, Sicily, Italy × × 2012 Rigo et al. Alaunian 3 to Sevatian 2 Sasso di Castalda, Lagonegro Basin, Italy No figure 2016 Rigo et al. Sevatian Pignola-Abriola, Lagonegro Basin, Italy × 2018 Yamashita et al. Sevatian Inuyama area, Japan × MA. Morphotype A; MB. morphotype B. Cross mark only represents the occurrence, but not the exact range of the morphotypes in the relevant section.
Table 1. The occurrence of M. slovakensis in different areas
The Epigondolella praeslovakensis is the typical epigondolellid with strongly ornamented platform, considered as the progenitor of Mockina slovakensis (Moix et al., 2007). The adult forms of E. praeslovakensis have a typical lower side of genus Epigondolella with broad and always distinctly bifurcated keel end (Moix et al., 2007). The pit usually lies in behind the central platform. In contrast, the M. slovakensis is usually smaller and simpler than its ancestor. It shows the characteristic features of genus Mockina. The short platform is ornamented at the anterior part of platform margins, but the posterior platform margins is almost smooth. The pit of M. slovakensis usually lies in front of the central platform. The keel end could be pointed or rounded but never bifurcated. The evolutionary trend characterizing the morphocline between E. praeslovakensis and M. slovakensis consists of a decrease in the number of denticles and a simplification of the platform ornament and the keel end. The platform of E. praeslovakensis tends to simplify by losing ornamentation on the posterior part of the platform margins. The keel end evolves from bifurcated to pointed or rounded. Different transitional forms between E. praeslovakensis and M. slovakensis were commonly recorded, of which the most common ones maintain the main characteristics of E. praeslovakensis but without the typical bifurcated keel end, however, it is also different from the advanced M. slovakensis because of its strongly ornamented posterior platform margins.
In summary, the evolution trend from E. praeslovakensis (e.g., bifurcated keel end; ornamented posterior platform margins) to transitional forms (e.g., slightly bifurcated or rounded keel end; ornamented posterior platform margins) and then to M. slovakensis (e.g., pointed or rounded keel end; usually neat posterior platform margins) is a process of simplification and miniaturization. Coincidentally, the process of simplification and miniaturization from E. praeslovakensis to M. slovakensis is consistent with the trend of the whole conodont evolution during the Norian (Karádi et al., 2020; Rigo et al., 2018).
Mockina slovakensis was thought to solely live in restricted environments as it was usually found in the Upper Triassic intraplatform basins, such as the Rezi Dolomite Formation and Feketehegy Formation in Hungary (Kovács and Nagy, 1989; Budai and Kovács, 1986), Dolomia di Forni Formation in Italy (Roghi et al., 1995) and Seefeld Formation in Austria (Donofrio et al., 2003). Nevertheless, Meço (1999) reported the occurrences of "Epigondolella" slovakensis in a pelagic environment in Albania. Channell et al. (2003) stated that this species is rare in open sea environments, while it is very common in Norian intraplatform basins, which can be only dated by M. slovakensis. More and more reports indicate that the open marine environments are also suitable for subsistence of M. slovakensis (Rigo et al., 2016, 2012; Mazza et al., 2012; Balini et al., 2010), and also in bedded cherts deposited below the CCD (Yamashita et al., 2018). It was even found in shallow water limestone belong to carbonate platform condition (Belvedere et al., 2008). Mockina slovakensis was also analyzed for oxygen isotopic composition, the analyses of which suggest that this species throve in the warmer and upper part of the column water (Trotter et al., 2015).
In summary, M. slovakensis is surface dweller species living in different marine environments, from (hemi)pelagic deep ocean to shallow water carbonate platform, including intraplatform basins (Fig. 5).
The evolution of the Norian shallow-water carbonate platforms and adjacent basins is controlled by the rifting phases connected to the westward propagation of the Neotethys Ocean opening and to the early phases of the Ligurian-Piedmont Ocean opening in the west. Extensional tectonic phases in the Norian re-activated the NNE-SSW, NE-SW and E-W oriented faults, controlling the establishment and position of the Dolomia Principale shallow-water carbonate platform together with the development of the intraplatform basins represented by the Dolomia di Forni Formation (Cozzi, 2000). The presence of different outcrops of the Dolomia di Forni in the Carnian and Julian Prealps might be explained as portions of a single deeper basin, named Carnian Basin, fragmented during the Alpine orogeny (Scotti et al., 2002) or as isolated intraplatform basins developed due to the Triassic synsedimentary tectonic.
In the studied sections of the Dolomia di Forni Formation (i.e., Valmenone, Poschiedea-Rovaia area, Verzegnis, Seazza and Forchiar Creek, Rio Resartico area), M. slovakensis was documented as almost monospecific conodont association. This species was also documented in other coeval intraplatform basins, such as in the Seefeld Schichten, in the northern Calcareous Alps (southern Austria) (Donofrio et al., 2003), in the Rezi Dolomite Formation and the Feketehegy Formation in Hungary (Kovács and Nagy, 1989; Budai and Kovács, 1986) and in the lower part of the "metacarbonate unit" cropping out in the NW Anatolia (Kaya et al., 2001). Even if M. slovakensis represented the majority of the collected species from the intraplatform basins where it was documented, this species has occurred in the Dolomia di Forni and Seefeld Schichten along with other rare species, that are M. postera, M. carinata and E. praeslovakensis (Donofrio et al., 2003). Mockina slovakensis was also documented in open pelagic/hemipelagic basins, such as Lagonegro Basins (Rigo et al., 2012), Albania (Meço, 1999), Slovakia (Channell et al., 2003), Mount Cocuzzo basinal succession in southern Italy (Mastandrea et al., 1997), in Sicily at Pizzo Mondello (Mazza et al., 2012) and in the Inuyama area, Japan (Yamashita et al., 2018). Moreover, it was also documented in the very shallow water environment like the carbonate platform represented by the Dolomia Principale Formation (Belvedere et al., 2008).
The versatility of M. slovakensis can thus explain its spreading through different depositional environments and its prolificacy in intraplatform basins where other species probably suffered from the restricted conditions. However, the finding of other species, such as M. postera, M. carinata and its ancestor E. praeslovakensis, allows suggesting direct connections among the Tethys Ocean and the intraplatform basins of the Forni Dolomite and Seedfeld Schichten, at least for a limited period of geologic time. It is noteworthy that further to the west, the Lombardy intraplatform basins (e.g., Aralalta Group) have not provided any conodonts yet, and it needs to be investigated together with the Ortles and Quattervals Nappes in the central Austroalpine to understand if the Tethyan connection extended also to the western sectors. The spreading of this species might be fundamental to unravel the presence of a possible passageway from the Tethys Ocean to the western sector of the southern and northern Alps, at least during the Late Triassic (Fig. 6).