CONODONT GENUS TERIDONTUS (MILLER, 1980) FROM THE EARLY ORDOVICIAN OF MONTAGNE NOIRE, FRANCE, THE

INTRODUCTION

THE CONODONT genus Teridontus was introduced in 1980 by Miller and was based on the Late Cambrian species Oneotodus nakamurai Nogami, 1967 from the Yencho Member of the Fengshan Fm. of northeast China. Teridontus was later reported from either the Upper Cambrian or Lower Ordovician (Landing et al., 1980; Miller, 1980; Landing and Barnes, 1981; Landing, 1983; An et al., 1983, 1985; Ni et al., 1983; Peng et al., 1983; Nowlan, 1985; Landing et al., 1986; Bagnoli et al., 1987; An, 1987; Buggisch and Repetski, 1987; Pohler and Orchard, 1990; An and Zheng, 1990; Seo and Ethington, 1993; Wang, 1993; Lehnert, 1994; Nicoll, 1994; Seo et al., 1994; Ji and Barnes, 1994; Taylor et al., 1996; Lehnert et al., 1997; Jia, 2000; Dubinina, 2000; PyIe and Barnes, 2002; Zeballo et al., 2005) sediments in numerous localities around the world, but a unanimous interpretation of the composition of the Teridontus apparatus organization was far from accepted.

In 2000 we were invited by J. J. Álvaro, E. P. Villas and D. Vizcaïno to join a working group on the stratigraphy of the Early Paleozoic of southern Montagne Noire (France). Our specific objective was the study of conodont faunas emerging from limestone intercalations in the Ordovician successions. Several samples from the Val d'Homs Formation collected in Sallèles-Caberdès, from Munio Formation, collected in Combes de Barroubio (St. Jean de Minervois) and, chiefly, from the St. Chinian Formation collected in La Regagnade Valley (St. Martial), produced an amazingly rich conodont fauna attributed to the P. deltifer Zone and, more precisely, to the lower part of the P. deltifer deltifer Subzone (Álvaro et al., 2005; Serpagli et al., 2007). The conodont assemblage has also yielded an extraordinarily abundant collection of well preserved elements belonging to a new species of Teridontus that is presented here. A more detailed analysis of the stratigraphie context, distribution and significance of the conodont fauna from southern Montagne Noire is the subject for another study and will be presented at a later date.

THE GENUS TERIDONTUS

Two emended diagnoses of the genus Teridontus were provided independently in 1994. Ji and Barnes, describing Early Ordovician conodonts from Western Newfoundland, Canada (1994, p. 64) suggested that four element morphotypes (a, b, c, e) constituted the Teridontus multielement structure. According to the authors, the a, b, and c elements (equivalent respectively to Sc, Sb and Sa elements) were respectively symmetrical-subrounded, transitional and suberect-symmetrical whereas the e element (equivalent to M) was compressed. No elements in P positions were identified; furthermore, the early stages of the apparatus might have lacked c, e (Ji and Barnes, 1990, 1994), as well as b elements (Ji and Barnes, 1994).

Nicoll (1994, p. 371), based on a Late Cambrian conodont fauna from Queensland, Australia, regarded Teridontus as a seximembrate apparatus bearing two P (Pa and Pb) and four S (Sa, Sb, Sc and Sd) element types, but lacking an element in the M position.

Criteria used to discriminate Teridontus elements in the two emendations are quite different. Ji and Barnes (1994) referred to the curvature of the cusp (suberect, reclined or recurved), cross section of the cusp and base, and presence of grooves or costae as main features to discriminate apparatus elements. According to Nicoll (1994), the length of the base (short in P and long in S elements) and the cross section of the basal margin were the major features used to segregate discrete element morphologies.

Zeballo et al. (2005), describing Tremadocian conodonts from Argentina, very recently provided a further emendation of Teridontus. The authors adopted the Ji and Barnes (1994) apparatus design, which better fitted with their material, but they included also a fifth morphotype (f) having a proclined cusp, straight anterior margin and triangular base profile. The difficulty in recognizing Nicoll's six morphotype apparatus structures was possibly attributed by the authors to the Late Cambrian age of Nicoll's and Nogami's original material, compared to the Early Ordovician age of their and of Ji and Barnes' material. On this basis, Zeballo et al. (2005) adopted a "sensu lato" designation for T. nakamurai, waiting for new data to come from further investigations.

Löfgren (1997a, 1997b, 1998, 1999a, 1999b) and Löfgren et al. (1999) in recent studies provided detailed reconstructions of several coniform apparatuses [e.g., Semiacontiodus (Miller, 1969); Cornuodus Fähraeus, 1966; Decoriconus peselephantis (Lindström, 1955); Variabiliconus Landing, Barnes and Stevens, 1986] adopting the apparatus design that Nicoll had also accepted for Teridontus. In all such apparatuses, and in Drepanodus and Cordylodus as well, two P (Pa = g; Pb = f) elements are always present. This fundamental sexi- or septimembrate pattern seems in fact common in latest Cambrian and earliest Ordovician conodonts, either in coniform or ramiform apparatuses (e.g., Cordylodus Nicoll, 1990; Jumudontus, Nicoll, 1992; Oepikodus, Stewart and Nicoll, 2003, Nicoll and Ethington, 2003; Erraticodon, Nicoll and Kelman, 2004). On this basis, we see no reason why these elements should apparently be missing in Teridontus.