The new genus Walliserognathus and the origin of Polygnathoides siluricus (Conodonta, Silurian)

The new genus Walliserognathus is described to host the species posthamatus (Walliser). The genus is monospecific. The shape of P1 and P2 elements of Walliserognathus posthamatus, characterized by a small platform along the whole elements, and its stratigraphic distribution limited to the Ancoradella ploeckensis Zone suggest that the species may have been the bridge in the phylogenesis from Wurmiella to Polygnathoides.


INTRODUCTION
Walliser (1964) described the morphospecies Spathognathodus inclinatus posthamatus from the Ludlow of Cellon (Carnic Alps) to include carminate elements with a narrow platform along the entire length of both sides of the elements. In terms of multielement taxonomy, the taxon was later named Ozarkodina excavata posthamata, as were the other form subspecies of Spathognathodus inclinatus. It should be noted that at that time the apparatus was still not known, and only a possible P2 element had been tentatively suggested by Jeppsson (1974). After the revision of late Silurian ozarkodinids by Murphy et al. (2004), all representatives of the 'excavata Group' were placed in the genus Wurmiella, and therefore in all recent papers the taxon under discussion is called Wurmiella posthamata (Walliser, 1964). However, the P2 element of the species does not fit with the characteristics of Wurmiella, mainly because its basal cavity is restricted to the lower part of the element and is not visible in lateral view as in the P2 elements of Wurmiella. Therefore we introduced here the new genus Walliserognathus to host the species posthamatus.
Polygnathoides siluricus Branson & Mehl, 1933 is one of the more widespread and better known conodont taxa of the Silurian. Its P1 element with a wide platform of rhomboidal shape is diagnostic and very easy to recognize, and its global distribution in a limited interval of the Ludfordian makes it a perfect index species. As a result, the P. siluricus Zone has been included in all the Silurian zonation schemes as a range zone, since Walliser (1964) introduced it in the first biozonation scheme of the Silurian. In all the succeeding schemes the P. siluricus Zone always possesses the same definition with the lower and upper boundaries defined by the First Appearance Datum (FAD) and by the Last Appearance Datum of the species, respectively (e.g. Aldridge & Schönlaub 1989;Corradini & Serpagli 1999;Jeppsson et al. 2006;Cramer et al. 2011;Corradini & Corriga 2012;Corradini et al. 2015). For a summary on Silurian conodont biostratigraphy see Corradini et al. (2015). However, despite the numerous elements collected from all over the world, the ancestor of P. siluricus is still not known and is generally referred to an unidentified ozarkodinid (e.g. Sweet 1988, pp. 93-94).
In this paper we propose that P. siluricus evolved from Walliserognathus posthamatus (Walliser, 1964). The latter is a characteristic but relatively rare taxon documented only from the Ancoradella ploeckensis Zone that became extinct just before the FAD of P. siluricus and has some morphologic features intermediate between Wurmiella excavata and P. siluricus.

MATERIAL
This study is based on collections from several sections and localities in the Carnic Alps, Sardinia and Morocco and on a thorough bibliographical investigation. The figured specimens come from various sections in those areas: Cellon (Walliser 1964;Corradini et al. 2015), Rauchkofel Boden (Schönlaub 1980;Schönlaub et al. 2017), Hoher Trieb and Cuestalta (authors' unpublished data) in the Carnic Alps, Genna Ciuerciu (Barca et al. 1995;Corradini et al. 1998Corradini et al. , 2009b and Su Forreddu (authors' unpublished data) in Sardinia and Atrous 7 (Corriga et al. 2014a(Corriga et al. , 2014b in Morocco. The material is stored in the Museo Friulano di Storia Naturale (MFSNgp), Museo di Paleontologia 'Domenico Lovisato' of Cagliari University (MDLCA), Museo di Paleontologia of Modena and Reggio Emilia University (IPUM) and at the Geoscience Centre of Georg-August University Göttingen (GZG). Precise information and repository numbers of illustrated specimens are reported in the caption of Fig. 1.

SYSTEMATIC PALAEONTOLOGY
The taxa are presented in stratigraphic and phylogenetic order.
Diagnosis. An ozarkodinid characterized by a small platform developed on both sides of the blade in P1 (carminate to carminiplanate) and P2 (angulate to anguliplanate) elements and with the ovoidal/polygonal partly inverted basal cavity.
Remarks. The genus Walliserognathus differs from all the Silurian genera previously attributed to Ozarkodina ('Ozarkodina', Ozarkodina, Wurmiella and Zieglerodina) in the presence of the small but well-developed platform in P1 and P2 elements. The P1 element also differs from those of Ozarkodina and Zieglerodina in its denticulation pattern, which is made of denticles of more or less equal size along the entire length of the element, and in the partially inverted basal cavity. The symmetrical and partially inverted basal cavity of the P2 element does not fit with those of the P2 elements of Wurmiella, which have an asymmetrical basal cavity more developed in the inner side, visible in lateral view. The development of the platform in Walliserognathus may be somewhat similar to the unrelated Devonian genus Tortodus (Weddige, 1977). The latter has a small well-developed platform in the posterior part of the element and bears a free blade anteriorly.
Wurmiella? hamata (Walliser, 1964) has a very small partial platform in the holotype (Walliser, 1964, pl. 18, fig. 26), but it is not well developed in all the specimens. Therefore this species is not included in Walliserognathus, although it may be reconsidered when the P2 element is described.
Emended diagnosis. An ozarkodinid with P1 (carminate to carminiplanate) and P2 (angulate to anguliplanate) elements characterized by a small platform developed along the whole element on both sides of the blade.
Description. Only P1 and P2 elements are known with certainty. The P1 element of Walliserognathus posthamatus is a spathognathodiform element with the upper and lower margins of the blade more or less parallel. Most specimens are laterally curved close to the posterior end, whereas others are almost straight. The denticles are robust, closely spaced and laterally expanded, more or less of equal size along the whole element and may be slightly smaller close to the posterior end; the cusp is sometimes only slightly larger than the adjacent denticles. The blade is characteristically expanded at the level of insertion of the denticles, forming an evident narrow platform extending along the whole element that gradually tapers to the extremities. The partially inverted basal cavity is larger under the cusp and continues as a deep groove under the whole element.
The P2 element is ozarkodiniform (angulate to anguliplanate), with an angle of about 130° between the two robust and straight processes. The cusp is high, laterally compressed and posteriorly reclined. The strong denticles are closely spaced, laterally compressed and posteriorly reclined. An evident ledge is present along the whole element at the level of the insertion of the denticles. The small, partially inverted, symmetrical basal cavity is located below the cusp where it has a more or less rhomboidal outline. It is not connected with the ledge margins and continues as deep grooves under the processes.
Remarks. The narrow platform of the P1 elements is very diagnostic and distinguishes Wa. posthamatus from all the other Silurian ozarkodinids. Most specimens have the posterior part laterally deflected, whilst others are almost straight. Ebner (1976, pl. 3, figs 10, 11) illustrated a specimen with transitional features between Wu. excavata and Wa. posthamatus, bearing an incipient small ledge that does not reach the distal parts of the element. Wurmiella? hamata may have a small ledge, but differs from Wa. posthamatus in the strongly curved posterior end and the presence of a lateral process.
Jeppsson (1974) suggested that the P2 element figured by Walliser (1964) in pl. 26, fig. 26 may belong to the apparatus of 'S.' posthamatus, due to the evident ledge along the whole element. Also the specimen figured by Walliser (1964) in pl. 26, fig. 30 shows the same features and may belong to Wa. posthamatus. Both these specimens came from sample C19, together with the holotype (P1) of the species. The only P2 element, so far attributed to the species with certainty, was illustrated by Barrick et al. (2010, fig. 4H).
The shape of the P2 element excludes a possible attribution of the species to Wurmiella. In fact, P2 elements of Wurmiella have 'an asymmetrical basal platform that is higher on the inner side so that the basal cavity is seen in inner lateral view' (Murphy et al. 2004, p. 10). The basal cavity of Walliserognathus posthamatus is limited below the element and does not reach the inner part of the element; therefore it is not visible in lateral view.
A small enlargement of the central part of the platform may occur in a few P2 elements (Fig. 1H), which somewhat resembles that of the P2 element of P. siluricus, but is definitely much less developed and without the characteristic triangular outline of the latter. Walliser (1964) reported the occurrence of some elements of 'Polygnathoides emarginatus' (now the P2 element of P. siluricus) from the A. ploeckensis Zone (samples 18A, 18B). However, the restudy of the original collection (Corradini et al. 2015) has demonstrated that those P2 elements have a small platform without the characteristic triangular flange of P2 of P. siluricus and actually are P2 elements of Wa. posthamatus, whose P1 occur in these samples.
The other elements of the apparatus are still unknown, even if in our material samples with Wa. posthamatus contain a few broken and poorly preserved M elements similar to those of P. siluricus. Similar elements are present in the Walliser (1964) collection of the Cellon section from the upper part of the A. ploeckensis Zone and likely may be associated with Wa. posthamatus. Stratigraphic range. The species occurs only within the A. ploeckensis Zone (Ludlow), having its FAD in the lower part of the zone and ranging up to its top.
Remarks. The complete apparatus is still not known.
It is now widely accepted that the P2 element is represented by the form species Polygnathoides emarginatus (Branson & Mehl, 1933) (e.g. Klapper & Murphy 1975;Slavik et al. 2010;Corriga et al. 2014b). Jeppsson (1983, fig. 1A-E) gave a tentative incomplete reconstruction of the apparatus based on an association from Gotland where the S0 element was missing. Sweet (1988, fig. 5.40) presented this reconstruction without elaboration in the text. However, as Slavik et al. (2010) already pointed out, Jeppsson's reconstruction cannot be confirmed in associations from other localities and is disregarded. Uyeno (1981) suggested that the M element could have been represented by elements assignable to the form species Neoprioniodus latidentatus Walliser, 1964, andCorriga et al. (2014b, fig. 5I) recorded a similar M element from Morocco. Slavik et al. (2010, fig. 3D, G, T) illustrated possible S1 elements from Bohemia. The M elements in our collections from Sardinia and the Carnic Alps, corresponding to the form species Neoprioniodus latidentatus, are always associated with P1 and P2 elements of P. siluricus. Also a few elements similar to the S1 figured by Slavik et al. (2010) are present. Therefore we may support his suggestion, even if more investigation of the S elements of the apparatus is necessary.
The P1 element of P. siluricus shows a wide variability in the shape of the platform. In upper view it has a more or less rhomboidal shape, with the minor axis variable in length. A few specimens are narrow, whereas others have a very wide platform in their central part. The upper surface can be more or less flat, or may be strongly undulated in the central part, where a distinct crease may occur, often more pronounced on one side. In lower view, the basal pit is transversely extended, and short secondary keels are present in larger elements.
Stratigraphic range. The species is exclusive to the eponymous P. siluricus Zone (Ludfordian).

PHYLOGENY
Elements of Wu. excavata with a small enlargement of the blade at the base of the denticles are well known from several stratigraphic levels within the Ludlow. In Wa. posthamatus this ledge is very wide and a true platform is developed at both sides of the blade along the whole element. It likely evolved from the elements of Wu. excavata characterized by an enlarged blade near the base of the A. ploeckensis Zone (Fig. 2). The development of the platform resulted in that the basal cavity became partially inverted with a somewhat polygonal outline. In the P2 element of Wa. posthamatus the development of the small platform constrained the small, partially inverted basal cavity to the lower part of the element and made it not visible in lateral view, differing from that of Wurmiella.
Polygnathoides siluricus has a wide rhomboidal platform that may have derived from Wa. posthamatus by the lateral enlargement of the platform in the central part of the element (Fig. 2). The denticulation became stronger and the basal cavity changed from partially inverted in Wa. posthamatus to fully inverted in P. siluricus. The transition is evidenced by the occurrence of early forms of P. siluricus from the base of the P. siluricus Zone, characterized by a narrow and/or almost not undulated platform (e.g. Walliser 1964, pl. 17, fig. 6). In these elements the basal cavity is similar to that of Wa. posthamatus, and its lateral extensions are not always present: they are more developed in specimens with a large, undulated platform.
A similar evolution is observable also in the P2 element, where the small platform of Wa. posthamatus enlarged laterally to the cusp into the more developed  Corradini et al. (2015). Sketch drawings of P1 elements after Walliser (1964). Abbreviations: A., Ancoradella; K., Kockelella; Oz., Ozarkodina; Pe., Pedavis; P., Polygnathoides; Wa., Walliserognathus; Wu., Wurmiella. subtriangular one of P. siluricus. It should be noted that a few P2 elements of Wa. posthamatus already have a slightly enlarged platform in the inner part of the element. As in the P1 element, the denticulation became stronger and the lower part of the element suffered a similar evolution, with the basal cavity not modified, but the grooves changed to keels.

CONCLUSIONS
The main results of this paper on middle Ludlow conodonts may be summarized as follows:  the new genus Walliserognathus is introduced to host the species Wa. posthamatus (Walliser, 1964);  only the P1 and P2 elements of Wa. posthamatus are known with certainty;  an M element of the apparatus of P. siluricus is suggested, and the proposal of the S1 element by Slavik et al. (2010)