Element association and morphology in some Middle Ordovician platform-equipped conodonts

Abstract

Conodont elements other than those with platform ledges along the processes have often been unrecognized or disregarded in reconstructions and phylogenetic discussions of Ordovician “platform conodont genera.” The platform elements are larger than the other element types and often exhibit more rapid evolutionary changes in morphology. Nevertheless, to understand the evolution of, and relationships between and among “platform genera,” it is necessary to also consider the associated ramiform and geniculate elements. Thus, on the basis of large collections, together comprising 10,800 elements from Sweden and southern China, all element types of Lenodus antivariabilis (An, 1981), L. variabilis (Sergeeva, 1963), Yangtzeplacognathus crassus (Chen and Zhang, 1993) and Eoplacognathus pseudoplanus (Viira, 1974) are described and discussed. All these species had seven morphologically distinct element types, and probably seventeen individual elements in each apparatus. Ancestors and descendants of these species are discussed more briefly and it is concluded that the ramiform and geniculate elements also are characteristic within each lineage.

Conodont dating of the Middle Ordovician breccia cap-rock limestone on Osmussaar Island, northwestern Estonia

“…However, M and S elements are recognized in apparatuses of the genus Lenodus (see Stouge & Bagnoli 1990), whose platform elements most certainly are the predecessors of the Eoplacognathus-type platforms. Löfgren & Zhang (2003) introduced equivalent M and S elements to the latter genera through transferring Lenodus pseudoplanus sensu Stouge & Bagnoli (1990) to Eoplacognathus (hence the use of L. {E.I)pseudoplanus herein). Moreover, in the currently investigated interval such elements are found also associated with stratigraphically later Eoplacognathus platforms (see below), which motivates a ftiture reassessment of the two genera Lenodus and Eoplacognathus.…”

“…According to Zhang (1999), the direct ancestordescendant relationship between these two taxa makes the presence of a secondary posterio-lateral process in the Pa platform elements of E. suecicus a key in discriminating it from late forms of L. {E.I) pseudoplanus. This was also discussed by Löfgren & Zhang (2003), where amendments on the E. suecicus Zone of Löfgren (1978) were made based on this morphological criterion. However, the use of this character seems to vary between workers, which might well be because it was not mentioned in the original description of Bergström (1971).…”

“…Continuous minor revisions were subsequently proposed by Löfgren (1993Löfgren ( , 1994Löfgren ( , 1995, Stouge (1989), Stouge & Bagnoli (1990) and Bagnoli & Stouge (1997), but for a little more than a decade, the biostratigraphical scheme established by Zhang (1998a) has been considered as the standard Baltoscandian conodont zonation for this interval (e.g. Löfgren 2000;Löfgren & Zhang 2003). The exception is Norwegian strata where conodont faunal composition is influenced by the near platform margin depositional environment, thus not being directly comparable to the faunas of inner-and mid-platform affinity (see Rasmussen 2001).…”

“…6D). By comparison, the Pa element pair shows a more apparent morphological evolution through the stratigraphie column (see Löfgren & Zhang 2003), and thus, such elements are also needed for a more precise age determination (i.e. to determine early-mid-late Kunda affinity).…”

Darriwilian species ofHistiodella(Conodonta) in the Argentine Precordillera

“…In Baltica, the middle Darriwilian conodont zonation includes four successive zones (Lo¨fgren 2000, 2004, Lo¨fgren & Zhang 2003: Lenodus variabilis, Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus (M. hagetiana and M. ozarkodella subzones) and Eoplacognathus suecicus (P. lunnensis and P. anitae subzones; Fig. 5).…”