The Middle Ordovician conodonts Eoplacognathus robustus Bergström and E. lindstroemi (Hamar): Taxonomy and apparatus reconstruction

Susana Heredia,Ana Mestre

doi:10.5027/andgeov46n3-3189

Abstract

The Middle-Upper Ordovician Ponón Trehué Formation cropping out in the San Rafael Block (Argentina) provided a large collection of conodonts which have been published years ago. The species Eoplacognathus robustus and Eoplacognathus lindstroemi recovered from these strata have been revised and restudied. In this contribution we describe M and S elements that were assigned previously to Baltoniodus prevariabilis and Baltoniodus variabilis as belonging to the E. robustus and E. lindstroemi apparatuses. The morphological characters of the P, S and M elements as part of these late Eoplacognathus apparatus were analyzed and support the proposal of a new reconstruction of it. We have compared this apparatus with the Baltoniodus apparatus and suggest the inclusion of the Eoplacognathus genus into the Balognathidae family. The new architecture of the Eoplacognathus apparatus is comparable to the genus Lenodus. This analysis improves the knowledge about this index conodont group allowing a new insight on the evolution, phylogeny and paleobiology of this linage.

Highlights

The Eoplacognathus apparatus was first described and their species defined as zonal and subzonal index conodonts by Bergström (1971), supporting this proposal on the P elements
The main purpose of this contribution is to analyze the morphological characters and taxonomy of P elements of Eoplacongathus robustus and E. lindstroemi, and include S and M elements as part of the Eoplacognathus apparatus, thereby proposing a new reconstruction of it; we compare it to the Baltoniodus apparatus
The Eoplacognathus robustus and E. lindstroemi apparatuses from Ponón Trehué were compared to Eoplacognathus pseudoplanus and E. suecicus apparatuses from the Precordillera (Heredia, 2012; Mestre, 2012; Mestre and Heredia, 2013) (Table 1, Fig. 5)

Summary

Introduction

The Eoplacognathus apparatus was first described and their species defined as zonal and subzonal index conodonts by Bergström (1971), supporting this proposal on the P elements. This original description is only based on the oral shape of these P elements. The Middle-Upper Ordovician strata of Ponón Trehué outcrops, exposed in the San Rafael Block (Fig. 1), provided a large collection of conodonts. Previous contributions on this conodont fauna were carried out by Heredia (1982, 1998, 2001, 2002) and Lehnert et al (1999). The main purpose of this contribution is to analyze the morphological characters and taxonomy of P elements of Eoplacongathus robustus and E. lindstroemi, and include S and M elements as part of the Eoplacognathus apparatus, thereby proposing a new reconstruction of it; we compare it to the Baltoniodus apparatus

Geological setting

Previous Considerations On Taxonomy

Taxonomy

Remarks

Conclusions