Metapolygnathus praecommunisti, Mazza & Rigo & Nicora, 2011

Mazza, Michele, Rigo, Manuel & Nicora, Alda, 2011, A new Metapolygnathus platform conodont species and its implications for Upper Carnian global correlations, Acta Palaeontologica Polonica 56 (1), pp. 121-131 : 124-129

Metapolygnathus praecommunisti sp. nov.

Figs. 2C–J , 3 .

1991 Metapolygnathus communisti ; Orchard 1991a: 191: pl. 2: 18–20. 2007 Metapolygnathus communisti ; Moix et al. 2007: 308: pl. 1: 11, 12. 2007 Metapolygnathus sp. nov. Y; Orchard 2007b: 141: pl. 2: 37–39.

2007 Metapolygnathus communisti ; Rigo et al. 2007: 195, fig. 5/4. 2010 Metapolygnathus praecommunisti nomen nudum sp.; Mazza et al. 2010: 133: pl. 1: 6.

Etymology: Forerunner of Metapolygnathus communisti .

Type material: Holotype: the platform specimen ( MPUM 10103 ) illustrated in Fig. 2C . Morphotypes: specimens illustrated on Fig. 2D, E (asymmetric morphotype) and on Fig. 2F, G (rounded morphotype).

Type locality: Pizzo Mondello section (Monti Sicani, Western Sicily, It−

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aly), the abandoned quarry (“la Cava” locality) on the southwestern slope of Pizzo Mondello Mountain.

Type horizon: Bed NA12 of the Pizzo Mondello section, a white micritic calcilutite with black−brown cherty nodules and rich in halobiids, named as “Calcari con Selce” (i.e., cherty limestones) ( upper Tuvalian [Carnian], Upper Triassic) .

Material.— 82 specimens from the Pizzo Mondello section and 29 from Pignola 2 section.

Diagnosis.—A primitive Metapolygnathus with a weakly to unornamented platform having subparallel margins, extended for two thirds of the entire element, and a wide anterior trough margin which leaves a short free blade of 1–2 denticles. The pit is centrally located or slightly posteriorly shifted with respect to the middle of the platform and it is anteriorly shifted with respect to the keel end, which is prolonged behind the pit and often irregular. The blade is composed of highly fused denticles, is very high and descends gradually into a low carina without steps. Behind the cusp, which is unremarkable in size from the other denticles, a carinal node larger than the others is always present, often in terminal position.

Description.— Metapolygnathus praecommunisti is characterised by a pit that is centrally located or slightly shifted posteriorly with respect to the entire length of the platform; the pit is narrow and surrounded by a prominent loop. Since M. praecommunisti is here defined as the transitional species between the genus Paragondolella and the Metapolygnathus communisti group (see “Remarks”), the species may show some intraspecific variation. In the stratigraphically lower specimens (the most primitive forms closer to the genus Paragondolella ), the pit is slightly shifted posteriorly with respect to the centre of the platform and, thus, it is located in the posterior half. In the stratigraphically higher specimens (advanced forms), which are much closer to the genus Metapolygnathus , the pit is centrally located, following an evolutionary trend typical of the Upper Triassic pectiniform elements (see Mazza and Rigo 2008). A very important morphological character for the definition of this species is the prolongation of the keel behind the pit. We define the keel as posteriorly prolonged when that part is at least 1/4 of the entire length of the keel. This prolongation is less evident in the primitive specimens and longer in the advanced ones, but always present. The keel termination may be pointed, squared or rounded and it is often deformed, thus presenting a typical asymmetric shape.

The platform is relatively short (two thirds of the entire element length) and generally slender, with sub−parallel margins and a rounded posterior end.

The geniculation point is placed at two thirds of the platform length and it bears three low and separated nodes on the inner side and 1–2 nodes on the outer side. The number of nodes may be higher in advanced specimens, while they are absent in the most primitive and juvenile forms. At the geniculation point the platform margins decrease abruptly in a wide anterior trough margin, which leaves a real free blade of only 1–2 denticles. Platform margins are thick and both margins and nodes are covered by an intense microcrenulation. In specimens of late adult growth stages the platform is longer, the posterior end is blunter and the platform nodes coalesce ( Fig. 3G, H ). The blade is very high, typically truncated in its anterior part and composed of highly fused denticles. It descends gradually without steps in a low and long carina which reaches the posterior end of the platform. The cusp, which is the denticle exactly above the pit, is unremarkable in shape and dimension and it is followed by 2–3 accessory carinal nodes. Behind the cusp, in terminal or subterminal position, a larger carinal node is always present.

Two morphotypes are recorded from the type locality of M. praecommunisti , the Pizzo Mondello section. The first, very common, is characterised by a typical posterior asymmetry, due to the development of one posterior−lateral lobe ( Fig. 2D, E ); the second is identified by a slight enlargement in the posterior third of the platform, which gives the platform a more rounded shape ( Fig. 2F, G ).

Remarks.— Metapolygnathus praecommunisti was used as nomen nudum (according to arts. 16.4 and 72.3 of the International Code of Zoological Nomenclature) in Mazza et al. (2010), where this species was briefly introduced but not officially instituted. The holotype of this species is here illustrated and described for the first time. Metapolygnathus praecommunisti is here defined as the transitional species between Paragondolella noah and M. communisti . Primitive specimens of M. praecommunisti , in fact, are very close to P. noah . They are characterised by a similar platform shape, an abrupt step of the platform margins at the geniculation point and a large last node of the carina. As a difference, in M. praecommunisti the pit is more shifted to the anterior, a posterior prolongation of the keel is present, the platform is shorter and the step between blade and carina is lacking, resulting in a more continuous feature. The anterior platform margins do not bear any nodes, but only an internal crenulation on the lateral margins not visible in profile, which develops in nodes in the advanced forms (i.e., stratigraphically higher specimens).

Advanced specimens of M. praecommunisti present instead a central pit with respect to the platform and the keel is well prolonged behind the pit. Some nodes, as mentioned above, develop on the anterior platform margins, the abrupt step at the geniculation point decreases, the platform becomes shorter and the carina becomes longer.

In M. communisti the pit is strongly shifted to the anterior with respect to the centre of the platform and the posterior prolongation of the keel reaches almost half of the platform length. The platform is shorter than in M. praecommunisti and the last large denticle of the long carina disappears, but M. communisti retains the same blade profile, the shape of the platform and the anterior platform nodes of its forerunner, M. praecommunisti .

The forward shifting of the pit toward the centre of the element, the shortening of platform margins and the occurrence of nodes at the geniculation points are well−known evolutionary patterns of Late Triassic conodonts ( Orchard 2007a, b; Mazza and Rigo 2008). Thus the morphological variations here illustrated are clearly transitional to M. communisti and place M. praecommunisti as the transitional species between P. noah and M. communisti ( Kozur 2003; Mazza and Rigo 2008; Mazza et al. 2010) ( Fig. 4 ).

Metapolygnathus praecommunisti is similar to Carnepigondolella nodosa , but they differ because in C. nodosa the pit is more posteriorly shifted, the long posterior prolongation of the keel is lacking, the posterior platform is always symmetrical, the nodes on the platform are more numerous and they often reach the middle of the lateral margins.

Stratigraphic and geographic range.—The new species is common in the Neotethys of western Sicily and in the Lagonegro Basin (our data; Kozur 2003 and references therein), as in the whole Neotethyan province. Metapolygnathus praecommunisti is widespread also in the northern Tethys province, where specimens of M. communisti have been recovered in Carnian strata below the FAD of Epigondolella quadrata . These specimens do not belong to M. communisti in the strict sense, but to the new species M. praecommunisti as here defined (e.g., in the Carnian Epigondolella orchardi Zone of sec−

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tion C of the Kocatabur Block, Mersin Melange; see Moix et al. 2007). M. praecommunisti occurs also in North America (Black Bear Ridge section) in the Metapolygnathus sp. nov. Y Zone (upper Tuvalian), and it is probably a homeomorph of Metapolygnathus sp. nov. Y Orchard ( Orchard 2007b: pl. 2: 28–30, 37–39). The Metapolygnathus sp. nov. Y Zone is in fact established on the previous M. communisti Zone and the new species that gives the name to the zone corresponds to the transitional metapolygnathids recognised by Kozur (2003). Furthermore, analysis of the published material and the original conodonts from the collection of the Black Bear Ridge section (through the kind permission of Michael J. Orchard) confirms that Metapolygnathus sp. nov. Y and M. praecommunisti are the same species.

Metapolygnathus praecommunisti ranges from the lower Tuvalian to the upper Tuvalian. In the lower Tuvalian the species is common in its primitive form, closer to Paragondolella noah and with rare or absent nodes on the platform margins. Advanced forms of this species occur in the upper Tuvalian.

Metapolygnathus praecommunisti was recovered at the Pizzo Mondello section in beds with Halobia simplex , Halobia superba , and with abundant Halobia lenticularis and Halobia radiata . The Carnian ammonoid Dicotropites plinii was also found 5 metres below the level from which the holotype of M. praecommunisti derives (NA12).

Figures

Fig. 2. SEM photographs of gondolellid conodonts. A. Holotype of Paragondolella noah (Hayashi, 1968). B. Holotype of Metapolygnathus communisti Hayashi, 1968 from the original plates of Hayashi (1968: 79, pl. 3). Both specimens are from chert at the base of the Adoyama Formation within a mixed Middle–Upper Triassic fauna (Ashio Mountains, Central Japan). C–J. Metapolygnathus praecommunisti sp. nov. from the Upper Triassic of “Calcari con Selce”, Pizzo Mondello section (Sicani Mountains, Sicily).C. Holotype (sample NA12). D, E. Asymmetric morphotypes, samples NA14 and NA18 respectively. F, G. Rounded morphotypes, samples PM11A and NA19 respectively. H, I. More advanced forms with short platform (sample NA19). J. Advanced form, NA19 (from Mazza et al. 2010).

Fig. 3. SEM photographs of gondolellid conodonts Metapolygnathus praecommunisti sp. nov. from the Calcari con Selce of the Pizzo Mondello section (Sicani Mountains, Sicily) (A, C, D, F–H) and from the Upper Triassic of Calcari con Selce of the Pignola 2 section (southern Apennines, Basilicata) (B, E). A–C. Primitive forms, closer to Paragondolella noah (Hayashi, 1968), samples NA8, P25 and FNP53 respectively. D, E. Late juvenile growth stage, samples NA15A and P34 respectively. F. Advanced form, sample PM11A. G, H. Extremely mature stages, samples NA11 and PM11A.

Fig. 4. A schematic figure illustrating the evolution of Paragondolella noah (Hayashi, 1968) into its descendant species. The main evolutionary trends from P. noah to the last representative of the lineage, Metapolygnathus parvus Kozur, 1972, are observable: the gradual shortening of the platform, the forward shifting of the pit, the posterior lengthening of the keel termination and the development of nodes at the geniculation point. In M. parvus the nodes disappear for the extreme reduction of the platform. A, B. Metapolygnathus parvus Kozur, 1972 (sample NA37, Lower Norian, PMs). C, D. Metapolygnathus communisti Hayashi, 1968 (samples NA39 and PM27 respectively, Lower Norian, PMs). Metapolygnathus praecommunisti sp. nov. from the Upper Carnian of the PMs and P2s: E, F. Very advanced specimens (samples NA22 and NA19 respectively, PMs). G, H. Intermediate specimens (samples NA12 and NA14 respectively, PMs). I, J. Primitive specimens (sample P25 of P2s and sample NA8 of PMs respectively). K, L. Paragondolella noah (sample NA2 of PMs and sample P21 of P2s respectively). Abbreviations: PMs, Pizzo Mondello section; P2s, Pignola 2 section.