New Lower Devonian Polygnathids ( Conodonta ) from the Spanish Central Pyrenees , with comments on the early radiation of the group

The comprehensive study of six Pragian-lower Emsian (Lower Devonian) sections from the Spanish Central Pyrenees has yielded a rich assemblage of conodont faunas, highlighting an important succession of Polygnathus species. Among them, the presence of the biostratigraphical markers P. pireneae, P. kitabicus, P. excavatus excavatus and P. exc. 114 stands out. This conodont succession allows the identification of the Pragian/Emsian boundary and the early Emsian zones and subzones: the kitabicus Zone and the Lower and Middle excavatus subzones. It is also remarkable the presence of three new polygnathid species: P. aragonensis n. sp., P. carlsi n. sp. and P. ramoni n. sp. Additionally, P. pannonicus and P. sokolovi are also reported for first time in the Iberian Peninsula. According to all these records, the abundance and diversity of these species suggest that during this time interval an evolutionary radiation of the group took place, identifying two pulses during the early radiation of the group. The first one during the pireneae-kitabicus Zone, and a second one during the Lower excavatus Zone. The new data presented herein increase considerably the known paleobiodiversity of the genus during the Pragian-early Emsian interval in the Spanish Central Pyrenees and corroborates the presences of the most important early Emsian conodont markers used in other regions such us Europe, North Africa, Central Asia, Australia, and North America.


Introduction
Conodonts, owing to their wide geographical distribution and their fast morphological evolution have been applied with great success for dating and correlating marine rocks from the Cambrian to the Triassic Periods.For specific time intervals, some groups of conodonts have demonstrated to be extremely good biomarkers.In that sense, Devonian polygnathid species have been extensively used in establishing boundaries between different stages and substages, being especially useful for the Emsian conodont-based zonal subdivision (see for example Klapper and Johnson, 1975;Klapper, 1977;Weddige and Ziegler, 1977;Lane and Ormiston, 1979;Yolkin et al., 1994;Mawson, 1995;Bardashev et al., 2002, Martínez-Pérez et al., 2011).
During the last years, we have conducted an exhaustive research of the lower Emsian rocks from the Spanish Central Pyrenees.This study has yielded a significant assemblage of conodont faunas, which allows the recognition of an important Polygnathus radiation in the region, with the presence of different cosmopolitan and endemic species.Therefore, the main purposes of this paper are to describe this pyrenean lower Emsian polygnathid assemblage including the description of three new species of the genus Polygnathus, and to discuss the radiation this genus underwent during its early evolutionary stage.
Other four sections have been described within the Compte Subfacies-area (Mey, 1967a;Hartevelt, 1970) (see Fig. 1): La Guardia d'Àres sections (LGA-X and LGA-XI) located in the proximity of La Guardia d'Ares; the upper part of Compte-I section (CP-I top) situated near the village of Gerri de la Sal; and the Villech-IA section (Vi-IA) close to the hamlet of Villech.These sections include rocks from the top of the Castanesa Fm., characterized by dark-grey, platy limestones, and the lower part of the Villech Fm. (made up of red and green nodular limestones with intercalated red carbonaceous shales).The sections LGA-X, LGA-XI and Vi-IA also expose the local Castells Beds unit in the lower part of the Villech Fm., which is characterized by brownish, platy limestones with numerous millimetric to centimetric intercalations of brown shales (see Martínez-Pérez, 2010a for a detailed stratigraphical and geographical descriptions).All the sections were sampled for conodonts.The positions of the samples in these six sections are shown in figures 2 to 7 (black dots) together with the stratigraphical ranges of the conodont taxa studied.

Systematic Palaeontology
Conodonts are characterized by a complex oral apparatus commonly composed of 13-17 individual elements.This apparatus is divided into an anterior set of structurally similar elements designated as S elements, a related set with a different structure called the M elements, and a posterior and more robust P elements (Pa and Pb respectively).Our work is entirely focused in the Pa elements, which rapid evolution has become in the base for the systematic of the group, including the genus Polygnathus.
All specimens discussed herein appear as isolated elements after the dissolution of carbonate rocks with formic acid (5-10%), and they are deposited at the Museum of Geology of University of Valencia (MGUV).
The abbreviations used indicate the section, bed and number of specimens of the material studied.Thus, Bal-6, Isb-1, CP-I top, Vi-IA, LGA-X and LGA-XI indicate the sections (see abbreviations in the Material Provenance section), the following number separated by a slash ("/") indicates the bed, and the numbers in parentheses correspond to the numbers of elements in this bed.The synonym list just shows the most important references.Eichenberg, 1930Order Ozarkodinida Dzik, 1976Family Polygnathidae Bassler, 1925Genus Polygnathus Hinde, 1879 Polygnathus aragonensis n. sp.

Derivatio nominis
Named after the Crown of Aragón, Spain, as the horizon and type locality of the species are near the origin-town of the Aragón Kingdom.
Type horizon Bed 10M, described in the Llaviero Member of the Basibé Fm. from the Basibé Sufacies-area (Southern-facies area, Aragonian Pyrenees).
Diagnosis Pa element with the following combination of characters: flat platform with poorly developed adcarinal troughs in the anterior half; marked inner inflexion on platform posterior half; platform ornamented by rounded nodes; inner anterior platform margin joints the free blade in a markedly posterior position with respect to the outer margin; and always with angles greater than 90º.Description Pa element upper view: free blade about 1/4 of the element total length (Fig. 8d).Anterior carina formed by at least 9 rounded denticles, laterally compressed and partially fused in the anterior part of the platform (Fig. 8d).Posterior carina located at the centre of the platform with 4-7 rounded denticles that reaches the end of the element.Platform clearly bent inward with a sharp angle in the outer platform margin (Figs.8a-c).The platform anterior margins meet the free blade at angles clearly greater than 90°.Inner margin always meets free blade in a position markedly anterior than the outer one.Oral surface platform margins ornamented by rounded nodules vaguely thicker than those of the central carina (Figs. 8a1,b1,c2); in some specimens are slightly compressed laterally (Fig. 8d2).Weak adcarinal troughs only in the most anterior region of the platform.Pa element lateral view: element predominantly straight, bent aborally in the posterior region.Pa element lower side: slightly asymmetrical and deep basal cavity, occupying most of the lower surface, being the outer lip more developed than the inner.Basal cavity margins do not protrude the platform margins in most specimens; but in some specimens show slightly protrusion (Fig. 8b).Basal cavity continues anterior and posteriorly by narrow and deep edge slightly concave in the anterior region and convex in the mid-posterior region.The carina is positioned closer to the inner side on the tongue.Description Pa element upper side: short free blade, representing approximately between 1/4 and 1/5 of the element total length.Anterior carina with, at least, 8 denticles strongly compressed laterally, even forming a ridge in the platform area.Main body slightly lanceolate, asymmetrical, with great development of the outer platform, especially in the posterior half.The element has a characteristic outline, with a straight inner margin, which is turned slightly inwards at its most posterior end, and an outer sinuous margin, slightly concave in the anterior part and convex in the middle-posterior area, ending in a sharp tongue bearing interrupted ridges (Fig. 8e1,f1,h2,g2).Anterior platform region narrower than the middle part of the element; anterior margins joint the free blade approximately at the same height, with angles greater than 90º, although in some specimens this angle is close to 90° (Fig. 8e1).Posterior carina formed by, at least, 8 rounded denticles situated in a central position, but gradually approaching the inner margin in the posterior region; in some specimens they reach the inner edge.Platform upper surface ornamented with small ridges arranged perpendicular to the margins.These ridges are more or less aligned with the carina denticles, being more numerous in the outer platform.Adcarinal troughs well developed, being the outer slightly deeper and longer.
Pa element lateral side: straight main body, bending aboraly in the posterior region.
Pa element lower side: deep and asymmetrical basal cavity, occupying most of the lower surface, the tip of the basal cavity is located approximately in the middle of the element; asymmetrical lips, being more developed the outer one, but without protruding beyond the platform margins; basal cavity extends toward the anterior and posterior ends of the element in open, deep furrows.

Discussion
The great basal cavity development and the shape of the anterior region of the platform indicate that P. carlsi n. sp. is a primitive species within the genus.P. carlsi n. sp.differs from P. kitabicus for the further development of the adcarinal troughs.Besides, the carina in P. kitabicus is positioned in the centre while in P. carlsi n. sp. it is closer to the inner side, even contacting with the inner edge in the most posterior region of the platform of some specimens.The minor development of adcarinal troughs and the characteristic platform outline separate P. carlsi n. sp.from P. exc.excavatus.Some specimens of P. aragonensis n. sp.have morphologies similar to those of P. carlsi n. sp.(Fig. 8d).However, P. carlsi n. sp. has ridges and well developed adcarinal troughs, in contrast with the hemispherical nodules and poor developed or absent adcarinal troughs of P. aragonensis n. sp.

Stratigraphical and geographical distribution
The stratigraphical distribution of P. carlsi n. sp. in the Spanish Central Pyrenees is restricted from the Lower exca-furrows, with the tip of the basal cavity sited in the anterior part of the element.
Discussion The morphological features above described placed P. aragonensis n. sp.within the stock of primitive species within the genus, very close to other taxa such as P. pireneae, P. kitabicus and P. sokolovi, all characterized by the absence or poorly development of adcarinal troughs, by the position and angles of the margins-platform junction, by its ornamentation represented basically by individual nodes, and by the great development of the basal cavity, being fully expanded.
P. aragonensis is close to P. pireneae.However, the former has a stronger inner bent, and smaller basal cavity with no protrusion of basal margins.
Type horizon Bed 10M, described in the Llaviero Member of the Basibé Fm. from the Basibé Sufacies-area (Southern-facies area, Aragonian Pyrenees).
Remarks P. excavatus 114 has a smaller and shallower basal cavity and a tongue ornamented by semi-crossed instead of interrupted ridges, in contrast with that of P. exc.excavatus.The basal cavity of the closely related P. gronbergi shows a slight inverted basal cavity at its most posterior part as opposed to that of P. excavatus 114.
Remarks P. exc.excavatus shows a lanceolated narrow platform with well developed adcarinal troughs, being the outer more excavated and longer than the inner; carina displaced towards the inner margin; platform surface bears short ribs; short tongue with interrupted transversal ridges; relatively deep and open basal cavity, which occupies more than half of the platform length.
This species is distinguished from other early polygnathids such as P. pireneae, P. aragonensis n. sp. and P. kitabicus by the lanceolate-shaped platform (slightly curved internally), by a more restricted basal cavity, and by the development of clear adcarinal troughs, being the external one more excavated.It also differs from P. carlsi n. sp. by the characteristic outline of the later (see above).
Stratigraphical and geographical distribution This taxon is the conodont index of the Lower excavatus Subzone of Yolkin et al. (1994)
Remarks P. kitabicus has a narrow and elongated platform that slightly curves inwards in its posterior half.Upper surface bears transverse ribs, slightly more numerous in the outer margin; weakly developed and narrow adcarinal troughs, being slightly more excavated in the anterior region, both reach the platform posterior end.Asymmetrical, deep, and completely expanded basal cavity, occupying most of the lower surface.Basal cavity expansions reach the platform margins; in some specimens, slightly exceed their limits (Fig. 9e).
P. kitabicus differs from P. pireneae and P. aragonensis n. sp. by the clear development of adcarinal troughs and by its ornamentation composed of short ribs.P. rosae presents a more elongated platform with a characteristic constriction in the anterior region, in contrast with P. kitabicus.The platform marked inflexion in the posterior half and the presence of isolated nodes in the adcarinal space separates P. sokolovi from P. kitabicus.
Remarks Elements of P. pireneae have a characteristic narrow and flat platform, with the central carina reaching the posterior end of the element.Upper surface typically ornamented by rounded denticles separated from the carina by the adcarinal space, which in some specimens seems to develop an incipi-ent depression, but never forms well developed adcarinal troughs (Fig. 9c1).Completely open and excavated basal cavity, with asymmetrical lateral expansions that in many specimens overpasses the platform margins (Fig. 9c1, d1).
P. pireneae differs from Pa elements of the similar species P. aragonensis n. sp., by the presence of a smaller basal cavity and the characteristic platform inflexion.Pa elements of the closer taxa P. kitabicus, P. rosae and P. excavatus have smaller basal cavities, more developed adcarinal troughs and an ornamentation based on ribs instead of nodes.P. pireneae differs from P. sokolovi, by exhibiting clear adcarinal troughs with nodes, and the characteristic inflexion of the platform in its posterior half.
Type horizon Bed 26, described in the Llaviero Member of the Basibé Fm. from the Basibé Sufacies-area (Southern-facies area, Aragonian Pyrenees).
Diagnosis Pa element with the following combination of characters: long platform, slightly narrower in the anterior third with clear development of the outer platform in its posterior half, producing a sub-rounded outer margin, while the inner margin is almost straight or gently bent inwards in the posterior region.Deep and completely open basal cavity.Description Pa element upper side: short free blade, representing approximately less than 1/4 of the total length of the element; anterior carina with 8-9 denticles laterally compressed; straight main body, slightly lanceolate, markedly narrower in the anterior third (Fig. 8i1, j1, k1); outer platform further developed in the posterior half producing a sub-rounded outer margin contour; in contrast, the inner margin in this region of the platform is almost straight or bent gently inward; both margins are innerly curved at the posterior end of the element developing a sharp tongue; platform margins meet free blade at the same position with an angle of 90º or slightly higher (Fig. 8k1), although in some specimens the inner margin meets more posteriorly.The posterior carina bears 6-7 individualized hemispherical central denticles, approaching to the platform inner margin in the posterior half, and contacting with the inner edge in some specimens (Fig. 8i1); upper surface ornamented by ridges that are arranged perpendicular to the platform margin, being more numerous in the outer margin; inner margin often bears small rounded nodules; well-developed adcarinal troughs, outer slightly wider; both extend to the posterior third of the element.
Pa element lateral side: straight main body or slightly bent aborally in its posterior region.
Pa element lower side: deep and fully expanded basal cavity, occupying most of the element; almost symmetrical lateral expansions, restricted to the limits of the platform, placing the tip of the basal cavity in the anterior half.Basal cavity extends into the anterior and posterior region of the element as narrow, open furrows; anterior parallel sided and posterior subtriangular.

Discussion
The large and fully expanded basal cavity, well-developed adcarinal troughs and the ornamentation based on short ribes or nodes places P. ramoni n. sp.within the stock of primitive polygnathids.Most of these features are also present in P. exc.excavatus, P. excavatus 114, P. kitabicus and P. carlsi n. sp.However, the general platform outline with a further development of the outer platform in the posterior half separates P. ramoni n. sp. for this set of taxa.Pa elements of P. kitabicus have less-developed adcarinal troughs and a minor development of the platform ornamentation.Pa elements of P. exc.excavatus and P. excavatus 114 show more excavated external trough and a wider platform in the anterior region.P. carlsi n. sp.shows a "sinuous" outer margin, opposed to a more rounded outer contour of P. ramoni n. sp.

Material
A single broken Pa element from Isb-1/10J (MGUV-21.086).Remarks P. sokolovi has a flat and narrow platform with a distinct angularity of the outer platform margin just behind its midlength in contrast to the slight curvature of the inner one.Upper platform surface ornamented by rounded nodules on its inner edge and very slightly laterally compressed nodules on the outer edge.Adcarinal troughs weakly developed, with isolated hemispheric nodules at the adcarinal space (Fig. 9h1).Deep and fully expanded basal cavity, occupying most of the lower platform surface.
P. sokolovi differs from other related taxa by the combination of the sharp platform inflexion and the presence of isolated hemispherical nodules in the adcarinal spaces.
Stratigraphical and geographical distribution In the Zinzilban stratotype (Uzbekistan), P. sokolovi ranges from the upper part of the pireneae Zone into the lower part of the kitabicus Zone.However, the pyrenean record cal distribution seems to be restricted to the Spanish Central Pyrenees.Remarks Polygnathus rosae has a very characteristic long, narrow and straight flat platform with equally-developed weak adcarinal troughs in the anterior part of the platform; a slightly asymmetrical element with a characteristic constriction in the anterior third of the platform; a central carina reaching the posterior end of the element; platform margins ornamented by small ribs that are arranged perpendicularly to the carina, although some small and rounded denticles can be identified in the inner margin, where the ribs are more numerous; and an asymmetric basal cavity unusually long, narrow and shallow, but not inverted.

Polygnathus rosae
P. rosae has a number of features common on primitive polygnathids.The narrower and shallower basal cavity together with the development of weak adcarinal troughs and ornamentation separates P. rosae from P. pireneae, P. rosae has a much more elongated platform, with a characteristic constriction, than P. kitabicus.The greater development of the adcarinal troughs, their asymmetrical depth and the carina displacement towards the inner platform margin distinguished P. excavatus from P. rosae.

Stratigraphical and geographical distribution
The stratigraphical distribution of Po. rosae is restricted to a short interval around the Lower and Middle excavatus comes from the Lower excavatus Subzone, extending its known range upwards.It has been recorded in Spain (Spanish Central Pyrenees), Central Asia (Zinzilban) (Apekina and Mashkova, 1978;Yolkin et al., 1989Yolkin et al., , 1994) ) and Australia (Flood, 1969).
For decades, the earliest Polygnathus species described was P. pireneae from the Lower Devonian strata of the Spanish Central Pyrenees (Boersma, 1973).However, some authors have recently included two Pragian Eognathodus taxa, E. trilinearis zeravshanicus (Bardashev and Ziegler, 1993) and E. trilinearis Cooper, 1973 as primitve Polygnathus (P.trilinearis and P. zeravshanicus respectively) (Mawson et al., 1992, Mawson, 1998).These two taxa ara characterized by bear a third row of denticles, which represents the novel fea-a third row of denticles, which represents the novel feature that allows distinction between polygnathids and eognathodids (with just two rows of denticles) (Mawson, 1995(Mawson, , 1998)); besides, they show a more developed platform.In any case, all these basal Polygnathus species appeared during the Pragian (Valenzuela-Ríos, 1997;Slavik et al., 2007) and it was not until the uppermost Pragian-early Emsian where the genus underwent its great diversification, becoming in very important components of the conodont fauna.The comparison between the scarce species recorded in the Pragian with the high number of taxa described during the lower Emsian shows an extraordinary increase in their palaeodiversity, which is represented in the Spanish Central Pyrenees by the record of up to 10 species in the excavatus Zone.The stratigraphical distribution of these species shows a Polygnathus radiation during the Lower excavatus Zone (see Fig. 10).This radiation is documented by the strong change in the fossil record content, with the first record of seven different taxa during the Lower excavatus Zone (Fig. 10).This phenomenon had already been described by Yolkin et al. (1994Yolkin et al. ( , 2008) ) and Izokh et al. (2011) in the Emsian stratotype area in Zinzilban (Uzbekistan, Central Asia), however some differences stand out.
The Zinzilban record shows an early radiation of the group involving mainly the sokolovi-tamara branch and the pannonicus branch during the late pireneae-kitabicus zones (Yolkin et al., 1994(Yolkin et al., , 2008;;Izokh et al., 2011).Conversely, the Pyrenean Polygnathus record shows an important radiation of the genus during the Lower excavatus Zone, recording for the first time during this interval up to five new taxa (P.exc.excavatus, P. ramoni n. sp., P. carlsi n. sp., P. rosa and P. aragonensis n. sp.), together with the first record in the Pyrenees of P. sokolovi and P. pannonicus (Fig. 10).
This data suggest a two-pulse radiation of the genus during the Pragian/Emsian interval.However, it is important to remark that although some taxa are shared by both regions (P.pireneae, P. kitabicus, P. exc.excavatus, P. excavatus 114, P. pannonicus and P. sokolovi), other taxa involved seem to be endemic for each region and/or pulse.In the Iberian Peninsula we have identified four endemisms, P. rosae, P. ramoni n. sp., P. carlsi n. sp. and P. aragonensis n. sp.characterizing the radiation during the Lower excavatus Zone.Meanwhile, in Central Asia, the endemic taxa P. hindei, P. tamara and P. pannonicoformis (Izokh et al., 2011) characterized the early radiation of the group during the kitabicus inverval in the Zinzilban region.This two-pulse radiation of the genus is also supported by a similar pattern described in the Icriodus record in the Pyrenees (Martínez-Pérez, 2010a, b).
Therefore, these data suggest that this time interval represents an important evolutive and adaptative moment for the genus, where the group started to innovate, and different morphologies appeared.However, geographical and/or environmental factors made that the same group of species evolved in different ways, characterizing each geographic area with a few different endemic species.

Concluding remarks
A significant conodont assemblage from six Pyrenean Lower Devonian sections (LGA-X, LGA-XI, CP-I top, Vi-IA, Isb-1 and Bal-6) is recovered; but only those polygnathids species around the Pragian/Emsian boundary are discussed herein.Among these conodonts, the occurrence of P. pireneae, P. kitabicus, P. exc.excavatus, P. excavatus 114, P. sokolovi, P. pannonicus and P. rosae stands out.P. pannonicus and P. sokolovi are recorded outside Asia for the first time.In addition, three new Polygnathus species are described: P. aragonensis n. sp., P. carlsi n. sp. and P. ramoni n. sp.Although the biostratigraphical implications of these faunas are out of the present work, they allow us to identify the beginning of the kitabicus Zone, Lower excavatus and Middle excavatus subzones by the entries of their index taxa.This abundance and diversity of polygnathid species suggests that during the late Pragian-early Emsian a Polygnathus evolutionary radiation took place, increasing considerably the number of species of the genus.Although this phenomena was already described in Emsian stratotype area in Zinzilban (Uzbekistan, Central Asia), the study of the Spanish material has allowed us to describe accurately this radiation, identifying two different pulses in the different regions studied, one during the kitabicus Zone in Zinzilban and a second one during the Lower excavatus Zone in the Spanish Central Pyrenees.This radiation is also corroborated by a similar evolutive pattern described for the conodont genus Icriodus from the Spanish Central Pyrenees.
This early polygnathid record suggests that this group was a very successful genus, becoming theirs species in important part of the conodont faunas during the late Pragian-early Emsian ecosystems and the base of the Emsian conodontzonation.

Fig. 1 .
Fig. 1.-Geological scheme of the study area in the Central Spanish Pyrenees, indicating the four Subfacies areas contained in the Palaeozoic outcrops in the Southern Facies-area of Mey (1967): I) Sierra Negra Subfacies, II) Baliera Subfacies, III) Renanué Subfacies and IV) Compte Subfacies; together with the location of the seven sections studied: Isábena I section (Isb 1), Baliera 6 section (Bal), the LGA X and LGA XI sections (LGA), Compte-I top section.(CP-I top) and Villech IA section (Vi-I) (Based on Valenzuela-Ríos, 1994).

Fig. 2 -
Fig. 2-.Stratigraphic column of the Baliera 6 section showing the location of the levels sampled for conodonts (black dots) and ranges of selected conodont taxa of biostratigraphic relevance.Polygnathus nothoperbonus is just figured to show the upper boundary of the Middle excavatus Zone.

Fig. 3 -
Fig. 3-.Stratigraphic column of the Isábena 1 section showing the location of the levels sampled for conodonts (black dots) and ranges of selected conodont taxa.

Fig. 4 -
Fig. 4-.Stratigraphic column of the LGA X section showing the location of the levels sampled for conodonts (black dots) and ranges of selected conodont taxa.The age of the bottom of the section is given by unpublished data from the genus Icriodus.

Fig. 5 -
Fig. 5-.Stratigraphic column of the LGA-XI section showing the location of the levels sampled for conodonts (black dots) and ranges of selected conodont taxa.Polygnathus nothoperbonus is just figured to show the upper boundary of the Middle excavatus Zone.The age of the bottom of the section is given by lithological correlation with the top of the LGA-X section.

Fig. 7 -
Fig. 7-.Stratigraphic column of the Vi-IA section showing the location of the levels sampled for conodonts (black dots) and ranges of selected conodont taxa.
. sokolovi and P. kitabicus have better developed adcarinal troughs and more restricted basal cavities than P. aragonensis n. sp.Besides, P. kitabicus bears well individualized rounded nodules, in contrast with the small ridges of P. aragonensis n. sp.Stratigraphical and geographical distribution P. aragonensis n. sp.ranges from the Lower excavatus Subzone to the nothoperbonus Zone (lower Emsian).It has been only found in the Spanish Central Pyrenees.
, and is recorded from the base of the excavatus Zone to the Lower nothoperbonus Zone.It is worldwide distributed (see Martínez-Pérez et al., 2011 for a recent list of occurrences).