Emsian ( Lower Devonian ) Polygnathids ( Conodont ) succession in the Spanish Central Pyrenees Sucesión de Polygnátidos ( Conodontos ) del Emsiense ( Devónico Inferior ) en los Pirineos Centrales

A comprehensive conodont study of five sections of Emsian age, carried out in the Spanish Central Pyrenees, has revealed an important succession of polygnathids that can be used for identifying basal boundaries of globally recognized biozones. This succession consists of Polygnathus excavatus excavatus, Po. gronbergi, Po. nothoperbonus, Po. mashkovae, Po. laticostatus, and the new species Polygnathus luciae that allows the identification of the excavatus, nothoperbonus and laticostatus Zones. The nothoperbonus Zone is further subdivided into Lower and Upper nothoperbonus subzones, relying upon the lowest occurrence of Po. mashkovae. These biostratigraphic data from the Spanish Central Pyrenees corroborate the succession of conodont indexes in other regions of Europe (north-western France and southern Italy), North Africa (Morocco), Central Asia, Australia, and North America (Alaska, Nevada and Canada).


Introduction
Conodonts have been widely used to subdivide the Devonian rocks into short intervals (or biozones) that can be identified globally.According to this, the boundaries and biozones of the Emsian Stage are based essentially upon the successive occurrence of, phylogenetically related, species of the genus Polygnathus (see for example Klapper and Johnson, 1975;Klapper, 1977;Weddige and Ziegler, 1977;Lane and Ormiston, 1979;Yolkin et al., 1994;Bardashev et al., 2002).Nevertheless, although the phylogeny of the Early Devonian polygnathids has been accepted as the basis for the conodont zonation of the Emsian Stage, this subdivision has been extensively discussed and diverse proposals have been put forward over recent decades (see discussion below and Mawson, 1995 for a historical review).Here we follow the zonal scheme for the Lower Emsian proposed by Yolkin et al. (1994), which includes the kitabicus, excavatus and nothoperbonus zones, although we consider the last one of these zones basically equivalent to the gronbergi Zone (see Mawson, 1995 and discussion below).
The gronbergi Zone was originally defined by Klapper and Johnson (1975) on the basis of the Early Devonian Polygnathus-bearing sequences found at Lone Mountain, Nevada (USA).This zone was subsequently subdivided into the Lower and Upper gronbergi subzones by Bultynck (1989), according to the polygnathid succession found in the La Grange Limestone of the Armorican Massif (north-western France), identifying the lower boundary of the Upper gronbergi subzone with the first occurrence of a new species, Polygnathus catherinae.However, some problems have since arisen over the definition of this new species, and consequently the proposed subdivision itself.
In the Pyrenees, the Devonian outcrops are relatively common and widespread.These Devonian rocks have been subdivided into four main facies-areas (the southern, central, western and northern facies-areas) (see Mey 1967aMey , b, 1968;;Hartevelt 1970).The material studied herein comes exclusively from the southern facies-area and, more precisely, from the Compte and Baliera subfacies-area of Mey (1967b) (Fig. 1).
Emsian rocks are abundant and well exposed on both sub-facies, although strong tectonics precludes continuity and, therefore, the Emsian column has to be compiled from various sections that can be precisely correlated by means of conodonts.Within this context, we have begun a comprehensive study of several sections that yielded, relatively, abundant conodont faunas in these subfacies (Compte and Baliera).Therefore, the main purposes of this paper are to present this polygnathid assemblage, to describe a new species of the genus Polygnathus, and to propose the subdivision of the nothoperbonus Zone on the basis of the conodont distribution found in the Pyrenees and other regions.
All the material in this study came from five sections that have been described within the more extensive Southern Facies-area of Mey (1967).Four of these sections belong to the Compte Subfacies-area (Hartevelt, 1970) (see Fig. 1): the La Guardia d'Àres sections (LGA and LGA-XI), located in the proximity of the village of La Guardia d'Ares; the Compte-I top section (CP-I top), Esta sucesión consiste en Polygnathus excavatus excavatus, Po. gronbergi, Po. nothoperbonus, Po. mashkovae, Po. laticostatus y la nueva especie Polygnathus luciae, lo que permite identificar las biozonas excavatus, nothoperbonus y laticostatus.Además, el primer registro de Po. mashkovae se usa para subdividir la Zona nothoperbonus en las Subzonas nothoperbonus Inferior y Superior.
Palabras clave: Conodontos; Devónico Inferior; Emsiense; Zona nothoperbonus; Pirineos Centrales Españoles the sections were sampled for conodonts.The positions of the samples in these five sections are shown in Figures 2 to 5 (black dots) together with the stratigraphical ranges of the conodont taxa selected.

Systematic Palaeontology
All the conodonts studied appear as isolated elements after the dissolution of carbonate rocks with formic acid (5-10%).The material recovered is housed in the Museum of Geology at the University of Valencia and is identified by the MGUV initials followed by their museum number.It is important to remark that only those important biostratigraphic conodonts around the nothoperbonus Zone are discussed (briefly) and/or illustrated .
In the Materials section we have used different abbreviations to indicate the section and bed of the material studied, and number of specimens from each bed.Thus, Bal 6, CP-I top, Vi-IB, LGA and LGA XI indicate the situated near the village of Gerri de la Sal; and the Villech-IB section (Vi-IB), close to the hamlet of Villech.Finally, the Baliera 6 section (Bal 6), located in the Baliera valley area, described and studied originally by Valenzuela-Ríos (1994, 2001), belongs to the Baliera Subfacies-area of Mey (1968) (Fig. 1).This last section has been described from strata of the Llaviero Mbr. of the Basibé Fm. characterized by blue-grey, platy limestone with thick bedded limestone and shale intercalations (see Valenzuela-Ríos, 1994 for a detailed description and geographical setting of the section).The LGA, LGA-XI, CP-I top and Vi-IB sections include rocks from the top of the Castanesa Fm., characterized by dark-grey, platy limestones, to the mid layers of the Villech Fm. (made up of red and green nodular limestones with intercalated red carbonaceous shales); they include the local Castells Beds Mbr., which is characterized by brownish, platy limestones with numerous millimetric to centimetric intercalations of brown shales at the lower part of the Villech Fm. (for a detailed description of the sections see Martínez-Pérez, 2010).All   Material 18 elements from the following sections and beds: Bal 6/3(1), Bal 6/14(1), Bal 6/37(1), Bal 6/43a(1), Bal 6/47(2), Bal 6/49(3), Bal 6/50 (1), LGA X/36(3), LGA X/42(1), LGA XI/9(1), LGA XI/11(1) and LGA XI/12(2).Material referred:  to MGUV-20.903.

Remarks
Po. exc.excavatus is characterized by a narrow platform with well developed but unequal adcarinal troughs, the outer one being more excavated and longer than the inner one; the carina is displaced towards the inner platform; platform surface ornamented by short ribs.Short tongue ornamented by interrupted transversal ridges.The elements present a relatively deep and open basal cavity, which occupies more than half of the length of the plat- form in lower view.This species is distinguished from its ancestors Po. pireneae and Po.kitabicus by the lanceolate shape of its platform (slightly curved internally), by a more restricted basal cavity, and by the development of clear adcarinal troughs.

Remarks
Diagnostic features of Po. gronbergi include a relatively large basal cavity, in lower view occupying most of the platform, which is flat or slightly inverted at the posterior end, and showing in upper view a short and moderately deflected posterior platform ornamented by few and short interrupted ridges.The specimens studied here are very similar to the holotype and paratypes of Klapper and Johnson (1975: pl. 1, figs. 1-18, 21-24, 27-28), as well as other specimens described in the literature (see for example Lane andOrmiston, 1979: pl. 6, figs. 6-7, 13 or Apekina andMashkova, 1978: pl. 75, fig. 6).This species is clearly discernible from other taxa of the Po.excavatus group, differing mainly in the incipient inversion of the basal cavity at the posterior end of Po. gronbergi.

Derivatio nominis
The species is dedicated to Lucía Martínez Lara.

Remarks
Po. laticostatus has a broad, straight platform that curves inwards in the posterior third and develops a large triangular tongue.The upper surface is orna- mented by numerous short transverse ridges; central carina and narrow, equally developed adcarinal troughs.The specimens studied have a completely inverted basal cavity with a relatively large, asymmetrical basal pit (with a more developed outer lip) located just before the inward deflection of the keel.This species is close in shape and ornamentation to Po. gilberti and Po.linguiformis, but the presence of an asymmetrical basal cavity, located more anteriorly in Po. laticostatus, and the morphological differences of their platforms, semicircular in Po. gilberti, and with a more developed tongue in Po. linguiformis, renders both taxa clearly distinguishable from Po. laticostatus.

Stratigraphical and geographical distribution
Po. laticostatus is an important biostratigraphic marker that has been used, together with Po. inversus, to indicate the lower/upper Emsian boundary and the base of the laticostatus Zone.Its stratigraphical distribution seems to be restricted to the laticostatus Zone.It has been found in the Pyrenees and Celtiberia (Carls and Valenzuela-Ríos, 2002) in Spain; in the Armorican Massif in France (Bultynck and Morzadec, 1979); in the Barrandian area in the Czech Republic (Klapper et al., 1978;Weddige and Ziegler, 1977); in Tajikistan (Bardashev and Ziegler, 1992) in central Asia; in Nevada (Klapper and Johnson, 1975;Johnson et al., 1980;Klapper and Johnson, 1980) in the USA; and in British Columbia (Pyle et al., 2003) in Canada.Carls and Gandl, 1969 MGUV-20.889;a1, lower view;a2, upper  The carina is slightly displaced towards the inner edge and composed by 4-5 rounded denticles laterally compressed.The edges of the platform are ornamented by ribs arranged perpendicular to the carina; some specimens show that several of these ribs are made up of aligned rounded denticles (Fig. 7f1).The ribs at the outer margin are more numerous.The adcarinal troughs are narrow and well developed, being the external slightly wider and longer than the inner one, running both up to the beginning of the tongue ornamentation.In lateral view the platform is practically straight, but slightly bent aborally in the posterior region.
In lower view the shallow basal pit is located centrally just before the inward deflection of the platform.The rest of the basal cavity is inverted and slightly asymmetrical, with strongly marked bands of lamellae on the inverted area.

Discussion
Po. luciae n. sp. is characterized by: its elongated Lshaped platform, having a more developed outer platform in its posterior region; and by a slightly asymmetrical and inverted basal cavity.Numerous specimens with these features have been described in the literature (see list of synonyms), and some of them have been identified as Po.mashkovae, probably because the morphological characteristics of Po. luciae n. sp. are very similar to Po. mashkovae (see Fig. 8).However, Po. luciae n. sp.differs clearly from Po. mashkovae in that the latter has a marked constriction in the anterior region of the platform, a wider outer platform at the posterior two-thirds and a well developed parapet-like structure, features that are absent or at an early stage of development in Po. luciae n. sp.Despite these clear morphological differences, Bultynck (1989) identified many specimens of Po. luciae n. sp. at La Grange (Armorican Massif) that he assigned to Po. mashkovae (see list of synonyms), even though the Type locality CP-I top section, in the Noguera-Pallaresa valley (Lérida, Spanish Central Pyrenees).

Diagnosis
Pa elements with the following combination of characters: L-shaped platform, with a nearly rectilinear outer edge just before the sharply inward deflection at its posterior third; outer platform more developed in its posterior half; slightly asymmetrical and inverted basal cavity centrally located.

Description
Short free blade, representing approximately one fifth of the total length of the element (Fig. 7d), although in some specimens it may be slightly longer (Fig. 7e).The platform is relatively long and straight, with more or less rectilinear outer edges.The element is slightly asymmetric, with a narrow platform in the anterior region and broadly expanded in the posterior two-thirds.Both platform edges turn sharply inwards at the posterior end, with the outer margin forming an angle of almost 90º (although in some specimens this angle is slightly wider), giving their characteristic L-shape to the element (see, for example, Figs 7d-e), and developing a triangular tongue ornamented by numerous and continuous transversal ridges.The lateral flanks are slightly raised compared to the central part of the element but do not develop a parapet-like structure.The inner and the outer anterior platform edges join the free blade at the same positions with angles close to 90°.

Remarks
Specimens from the Pyrenees assigned to Po. mashkovae have a characteristic asymmetric platform, with a narrow anterior third and a broadly expanded platform at the posterior two-thirds.In addition, the edge of the outer platform develops a constriction in the anterior half and a high, flange-like structure at the outer edge of its posterior half.The tongue, with continuous transverse ridges, is markedly deflected inwards.In lower view, the basal cavity is expanded beneath the platform, before the rounded elements that Bultynck (1989) identified as Po.mashkovae do not exhibit the characteristic constriction of their platforms in the anterior region, a feature that differs clearly between the two related species.
As far as the phylogenetic relationships of the new species is concerned, several phylogenetic lineages for Emsian polygnathids have been proposed in the second half of the twentieth century (see e.g., Bardashev, 1986;Bultynck, 1989;Yolkin et al., 1994).These lineages are based mainly on stratigraphic and morphological criteria such as the type of ornamentation, the basal cavity size, the development stage of the platform or the adcarinal troughs depth.In accordance with these criteria, Po. luciae n. sp.can be included at the base of the lateral branch Po. luciae n. sp.-Po.mashkovae -Po.apekinae/Po.vigierei -Po.serotinus, which has already been proposed by previous authors (see Bardashev, 1986;Bultynck, 1989;Yolkin et al., 1994) (Fig. 9).Thus, Po. luciae n. sp.would be the ancestor of Po. mashkovae.This lineage derives from the main stock of the Po.excavatus group and is characterized by very asymmetric platforms, with a broad outer platform with a characteristic outline and a high flange-like margin.In addition, the Pa elements of the different taxa belonging to this branch have a well developed tongue ornamented by continuous or interrupted transversal ridges.

Stratigraphical and geographical distribution
Po. luciae n. sp. is recorded within the nothoperbonus Zone, its upper range overlaps with the first record of Po. mashkovae in the Pyrenees, although according to Bultynck (1989), this taxon could reach the inversus/laticostatus Zone.

Remarks
Po. nothoperbonus has a long, narrow platform, with its outer and inner edges running more or less parallel in the anterior two-thirds, where there is no high flange-like development of the outer edge, and a short tongue deflected inwards and bearing either interrupted or continuous ridges.The platform surface is ornamented by short transverse ridges distributed perpendicularly to the platform edges.The basal cavity is variable in size, though the posterior end is always inverted or flattened.Mawson (1987) described Po.nothoperbonus as a new species on the basis of material from Victoria (Australia) and Lone Mountain in Nevada (USA).According to Mawson, Po. nothoperbonus is distinguishable from Po. perbonus mainly by the beginning of the inversion of the basal cavity in Po. nothoperbonus and the extra depth of this structure in Po. perbonus.In addition, Po. nothoperbonus differs from Po. luciae n. sp. and Po.mashkovae in the characteristic asymmetric platforms of these spe-or sharp inward deflection of the posterior platform, and is clearly inverted.
Our specimens are very similar to the original material described by Bardashev (1986: pl. 1, figs. 4-11).On the other hand, Bultynck (1989: pl. 2, figs. 1-12), described the new species Po. catharinae on the basis of material from La Grange (Armorican Massif) and, as he commented (Bultynck, 1989: p. 183), Po. catharinae resembles Po. mashkovae very closely.Both taxa present a characteristic asymmetric platform, with a broadly expanded platform at the posterior two-thirds, high flangelike structure at the outer edge and a well developed constriction in the anterior region of the platform.In the light of all the above, we believe that the forms that Bultynck (1989) identified as Po.catharinae belong in fact to Po. mashkovae, and therefore Po. catharinae is a junior synonym of this latter species.Furthermore, and after studying the material from the Pyrenees, we believe that the specimens that Bultynck (1989) identified as Po.mashkovae are actually the new species described here as Po.luciae n. sp., which is easily distinguishable from Po. mashkovae (see Po. luciae n. sp.discussion).

Stratigraphical and geographical distribution
Po. mashkovae is recorded from the mid nothoperbonus Zone to the inversus/laticostatus Zone.This taxon is used here to subdivide the nothoperbonus Zone into the Lower and Upper nothoperbonus subzones, indicating the first record of Po. mashkovae the base of the Upper nothoperbonus subzone (see the Discussion section below).
Among the diverse Emsian zonations, the one based on the successive occurrence of different species of the genus Polygnathus is the most commonly used for biostratigraphical subdivisions and world-wide correlations (see, for example, Klapper and Johnson, 1975;Klapper, 1977;Weddige and Ziegler, 1977;Lane and Ormiston, 1979;Yolkin et al., 1994, Bardashev et al., 2002).The phylogeny of the Early Devonian polygnathids has been accepted as the basis for this conodont zonation, however, this subdivision has been hotly debated and several cies compared to the straight and narrow platform of Po. nothoperbonus.

Stratigraphical and geographical distribution
Po. nothoperbonus is the conodont index of the nothoperbonus Zone, sensu Yolkin et al. (1994).It is recorded from the base of the nothoperbonus Zone to the inversus/ laticostatus Zone.
In the Pyrenees, due to the apparent lack of Po. perbonus, the scarcity of Po. gronbergi and the relative abundance of Po. nothoperbonus in the different sections studied, we use the nothoperbonus Zone to identify the interval between the excavatus and the laticostatus zones, including the Upper excavatus Subzone of Yolkin et al. (1994) within the nothoperbonus Zone (see also Mawson, 1995).

The nothoperbonus Zone Subdivision.
As commented above, the nothoperbonus Zone was defined by Yolkin et al. (1989Yolkin et al. ( , 1994) ) to replace partially the previous gronbergi Zone of Klapper and Johnson (1975).Additionally, Bultynck (1989) recognised the gronbergi Zone in section A at La Grange in the Armorican Massif, a biozone which he subdivided into the Upper and Lower gronbergi subzones, identifying the boundary between the two subzones with the first record of Polygnathus catherinae.Following the original idea of Bultynck (1989), we use Po.mashkovae s. s. (but not Po.catherinae or Po.mashkovae sensu Bultynck, 1989) for subdividing the nothoperbonus Zone into the Lower and Upper nothoperbonus subzones.
This subdivision is supported by the biostratigraphic dates from the Spanish Central Pyrenees (see Figs. 2-5).Among these records, we draw attention to the occurrence of different Polygnathus species (Po.excavatus excavatus, Po. gronbergi, Po. laticostatus, Po. mashkovae, Po. nothoperbonus and the new species described here, Po. luciae n. sp.), all of which, are important biostratigraphic markers (see the ranges of the conodont taxa selected for the interval studied in Figs.2-5).According to these records, Po. nothoperbonus has been registered in all five sections studied, marking in LGA-XI, CP-I top, and Bal 6 sections (Figs. 2, 3 and 5), the base of the Lower nothoperbonus Subzone.In the lower part of this subzone, in all the sections studied, Po. luciae n. sp.appears slightly above the first record of Po. nothoperbonus, so the lowest record of that taxon took place in the lower part of the nothoperbonus Zone.Po. mashkovae is first registered in the middle parts of the nothoperbonus proposals have been put forward by various authors over recent years (see Mawson, 1995 for a historical review).
Since the original proposal of Klapper and Johnson (1975), the most important and problematic changes in the Emsian zonal scheme have been suggested by Yolkin et al. (1994) and Bardashev et al. (2002).On the basis of the polygnathid sequences found in Zinzilban (central Asia), Yolkin et al. (1994) proposed a new zonation for the Early Emsian.This proposal substantially changed the lower Emsian zones, replacing the former dehiscens and gronbergi zones with the kitabicus, excavatus, and nothoperbonus zones.However, the most controversial proposal was that of Bardashev et al. (2002).On the basis of new polymorphogenetic lineages introduced by themselves, these authors suggested up to three alternative conodont-based zonations for the Pragian and Emsian stages.Their correlation bears big mistakes and conceptual errors that produced misleading results and impracticability.As a consequence, these zonations have been questioned by several authors (Mawson and Talent, 2003;Murphy, 2005;Slavík et al., 2007), who pointed out serious discrepancies in their concept of taxonomy and zonation.All these authors criticised the procedure of Bardashev et al. (2002), who chose only single-figured specimens from the literature without any biostratigraphic control, introducing, as a consequence, important errors into the stratigraphic position of some individual taxa.We agree with these criticisms and prefer to follow the scheme of Yolkin et al. (1994) to subdivide the lower Emsian in the Spanish Central Pyrenees.
This report is focused almost exclusively on the nothoperbonus Zone, which was defined by Yolkin et al. (1989Yolkin et al. ( , 1994) ) to replace partially the previous gronbergi Zone of Klapper and Johnson (1975).Mawson (1995) pointed out, however, that these two zones were virtually equivalent, a proposal upheld by the almost simultaneous occurrence of Po. gronbergi and Po.nothoperbonus in several regions and sections: both taxa appear for the first time at level 55 in locality 1 of Salmontrout (Alaska, USA) (Lane and Ormiston, 1979).In section A of the La Grange Limestone in the Armorican Massif in France, Bultynck (1989) described the first occurrence of Po. gronbergi in sample 0, and slightly above (15 cm), the first record of Po. nothoperbonus in sample 1, although, as he commented, the sequence is markedly condensed.The almost synchronicity of both taxa is also observed in the Pyrenean section Baliera-6, there, Po. gronbergi appears in Bed 48, and Po.nothoperbonus is registered slightly above (40 cm), in Bed 49 (Fig. 5).Additionally, as Mawson (1995) discusses, these two zones (gronbergi and nothoperbonus) are also equivalent to the Zone in LGA, CP-I top and Vi-IB sections (see Figs. 2, 3 and 4), and always above the first records of Po. luciae n. sp., although both taxa briefly overlap (see Fig. 2).This consistent distribution has already been reported in other regions such as France (Bultynck, 1989), Morocco (Bultynck andHollard, 1980) and Italy (Navas-Parejo pers.obs.) and strongly supports the nothoperbonus Zone subdivision proposed here.In the Pyrenees, the top of this zone has been identified only in the middle parts of the LGA section with the first record of Po. laticostatus, the index of the overlying conodont zone (Fig. 2).

Concluding remarks
We have recovered a significant conodont assemblage from five Pyrenean Lower Devonian (Emsian) sections (LGA, LGA-XI, CP-I top, Vi-IB and Bal 6), but only those conodonts with biostratigraphic relevance around the nothoperbonus Zone are discussed herein.Among these conodonts, the occurrence of different Polygnathus species, such as Po.excavatus excavatus, Po. gronbergi, Po. nothoperbonus, Po. mashkovae, Po. laticostatus and Po. luciae n. sp., stand out.According to these records, we have identified the Emsian excavatus, nothoperbonus and laticostatus zones and, subdivided the nothoperbonus Zone into the Lower and Upper nothoperbonus subzones by the entry of Po. mashkovae s.s.This subdivision is supported by the biostratigraphic data from the Spanish Central Pyrenees and corroborated by the consistent distribution of these conodont markers in other regions such as France (Bultynck, 1989), Morocco (Bultynck and Hollard, 1980) and Italy (Navas-Parejo pers.obs.).
In addition, the new species described in this paper, Polygnathus luciae n. sp., appears slightly above the first records of Po. nothoperbonus and, thus, the first record of that taxon would indicate a lower Emsian, early nothoperbonus, age.Moreover, Po. luciae n. sp.can be included at the base of the lateral branch of the following phylogenetically succession (Fig. 9): Po. mashkovae -Po.apekinae/Po.vigierei -Po.serotinus already proposed by previous authors (see Bardashev, 1986;Bultynck, 1989;Yolkin et al., 1994); all these conodonts are characterized by very asymmetric platforms.

Fig
Fig. 2.-Stratigraphic column of the LGA and LGA-XI sections showing the location of the levels sampled for conodonts (black dots) and ranges of selected conodont taxa of biostratigraphic relevance around of the nothoperbonus Zone.Fig. 2.-Columna estratigráfica de las seccionesLGA y LGA-XI mostrando la localización de los niveles muestreados para conodontos (puntos negros) y los rangos de los taxones seleccionados con importancia bioestratigráfica alrededor de la Zona nothoperbonus.

Fig. 3 .
Fig. 3.-Stratigraphic column of the CP-I top section showing the location of the levels sampled for conodonts (black dots) and ranges of selected conodont taxa of biostratigraphic relevance around of the nothoperbonus Zone.Fig. 3.-Columna estratigráfica de la sección CP-I top mostrando la localización de los niveles muestreados para conodontos (puntos negros) y los rangos de los taxones seleccionados con importancia bioestratigráfica alrededor de la Zona nothoperbonus.

Fig. 4 .
Fig. 4.-Stratigraphic column of the Vi-IB section showing the location of the levels sampled for conodonts (black dots) and ranges of selected conodont taxa of biostratigraphic relevance around of the nothoperbonus Zone.Fig. 4.-Columna estratigráfica de la sección Vi-IB mostrando la localización de los niveles muestreados para conodontos (puntos negros) y los rangos de los taxones seleccionados con importancia bioestratigráfica alrededor de la Zona nothoperbonus.

Fig. 5 .
Fig. 5.-Stratigraphic column of the Baliera 6 sections showing the location of the levels sampled for conodonts (black dots) and ranges of selected conodont taxa of biostratigraphic relevance around of the nothoperbonus Zone.Fig. 5.-Columna estratigráfica de la sección Bal-6 mostrando la localización de los niveles muestreados para conodontos (puntos negros) y los rangos seleccionados de taxones de conodontos de relevancia estratigrafíca en torno a la Zona nothoperbonus.
Po. perbonus (the index of the perbonus Zone or Upper exacatus Subzone of Yolin et al., 1994) is registered together with Po. nothoperbonus in Australia perbonus Zone.