Gondolellidae Lindström, 1970

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

Family Gondolellidae Lindström, 1970 Genus Metapolygnathus Hayashi, 1968

Type species: Metapolygnathus communisti Hayashi, 1968 , from chert at the base of the Adoyama Formation within a mixed Middle–Upper Triassic fauna: Ashio Mountains, Central Japan .

Original diagnosis (from Hayashi 1968: 72).—“The present genus has intermediate biocharacters between the genera Polygnathus, Gondolella , and Gladigondolella . Platform shape square, growing short to long, posterior to anterior, the platform has subparallel lateral margins in the mature stage. Furrows lateral to carina are generally deep and well developed. Lower surface bears a stout, wide, flat and sharply edged keel, the posterior margin is always sharply squared and frequently forks off into two branches, groove narrow and striated extends along the length of unit, basal cavity located near the centre of the unit, more or less invisible or narrow and deep crack.”

Emended diagnosis.—The genus is characterised on the lower side by a very narrow basal cavity, hardly visible, centrally located in primitive specimens and anteriorly shifted with respect to the middle of the platform in advanced forms. The keel shows a strong posterior prolongation behind the pit. The cusp is undistinguished in size and it is followed by two or more carinal nodes. The platform is robust but reduced and generally lacks ornamentation. In some species a few nodes may be present, but are always confined to the anterior platform margins or to the geniculation point. A distinct free blade is present, with highly fused denticles. The keel end is often, but not necessarily, bifurcated.

Remarks.—We believe that a new description of the genus is necessary, given the progress in the studies of the morphology and systematic of the Late Triassic conodonts achieved from the institution of the genus in 1968. In the original diagnosis of Metapolygnathus ( Hayashi 1968) , in fact, there are no remarks about a very diagnostic morphological feature that characterizes the type species of this genus: the presence of a pronounced posterior prolongation of the keel behind the pit. Remarks concerning the occurrence of some sort of ornamentation on the platform margins are also absent. The image of the holotype ( Hayashi 1968: pl. 3: 11; the holotype is re−illustrated in Fig. 2B of the present paper), does not seem to show any kind of ornamentation on the platform margins. Nevertheless, analyses of rich populations of true Metapolygnathus communisti from the Pizzo Mondello section reveal the occurrence of 1–2 tiny nodes on the platform margins (see also Noyan and Kozur 2007). These nodes, however, are always confined to the geniculation point (see Sweet 1981 for morphological terminology). According to these observations and to the original diagnosis, we restrict the assignment of the genus Metapolygnathus only to those species that have the following combination of three characters: a well evident posterior prolongation of the keel end, a centrally located pit and the absence of ornamentation or, at least, the presence of tiny nodes but always confined to the anterior platform margins.

As a consequence, the stratigraphic distribution of the genus Metapolygnathus is here restricted to the Tuvalian (Carnian)–lower Lacian (Norian). In the stratigraphically older specimens of Metapolygnathus (Tuvalian) , the pit may be located not exactly at the centre of the platform but slightly behind it, while in the more advanced (i.e., stratigraphically higher) specimens (Lacian) the pit may be still more anteriorly shifted with respect to the middle of the platform. Since the Julian and early Tuvalian metapolygnathids do not have the posterior keel prolongation and the centrally located pit, we assign them to other genera: Paragondolella and Carnepigondolella .

The genus Paragondolella is probably the direct forerunner of the genus Metapolygnathus , and it differs from the latter mainly in the subterminal position of the pit, the absence of the posterior prolongation of the keel, the long platform which covers almost the entire length of the element allowing the development of only a very short free blade, and the complete absence of nodes on the platform margins.

The genus Carnepigondolella is the forerunner of Epigondolella ( Kozur 2003) and is characterised, on its lower side, by a pit which is placed behind the middle of the platform, but never centrally located as in Metapolygnathus , and slightly anteriorly shifted with respect to the keel end. Although in Carnepigondolella a forward shifting of the pit with respect to the keel end is recognizable, it is never so pronounced that it generates a posterior prolongation as in Metapolygnathus , which is set between 1/4 to half of the total keel length in the Lacian forms. Furthermore, the genus Carnepigondolella shows strong ornamentation on the platform margins, which is absent in Metapolygnathus . This ornamentation consists of large nodes or low denticles, which may occur also on the lateral margins of the platform and not only at the anterior margins or the geniculation point.

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).