The phylomorphogenesis of some early devonian platform conodonts

Abstract

The materials of all conodont workers including revised taxonomic systematics of the Early Devonian platform conodonts, which were previously included in the formal family PolygnathidaeUlrich &Bassler 1926, are summarized at some length. For the conodont systematics including a suprageneric classification we regard the Pa elements as the key elements, because it is these elements that belong to an apparatus of only one taxon in contrast to other elements that are included in the apparatus of several taxa. The Pa elements only can be the nominative elements of a multielement taxon (see alsoZiegler &Weddige 1999). Two main opinions of the origin of the genusPolygnathus and problems concerning the nomenclature of the family Polygnathidae are discussed. This family has comprised only pectiniform elements of carminiplanate type, according to the original diagnosis of the Polygnathidae byUlrich &Bassler (1926). We suggest to isolate some of the Devonian polygnathids, formerly assigned toPolygnathus, and group them in three genera:Ctenopolygnathus Muller &Muller 1957, withCt. angustidiscus (Youngquist 1945) as type species,Costapolygnathus gen. nov. withC. dubius (Hinde 1879) as type species, andLinguipolygnathus gen. nov. withL. linguiformis (Hinde 1879) as type species. Also,Ancyrognathus Branson &Mehl 1934a,Ancyrolepis Ziegler 1959,Polylophodonta Branson &Mehl 1934a,Pseudopolygnathus Branson &Mehl 1934b, ?Rhodalepis Druce 1969, ?Scaphignathus Helms 1959,Schmidtognathus Ziegler 1966, and ?Siphonodella Branson &Mehl 1944, are included in the family. Some Lower Devonian pectiniform elements of the carminiscaphate type are suggested to be assigned to the new family Eognathodidae with two genera:Gondwania gen. nov. withG. nevadensis (Clark &Ethington 1966) as type species,Eognathodus Philip 1965, and the new family Eopolygnathidae with three genera:Eoctenopolygnathus gen. nov. withE. pireneae (Boersma 1973) as type species,Eocostapolygnathus gen. nov. withE. kitabicus (Yolkin, Weddige, Izokh E 2)Amydrotaxis >Gondwania >Eognathodus >Eocostapolygnathus >Costapolygnathus; and 3)Amydrotaxis >Gondwania >Eognathodus >Eocostapolygnathus >Eolinguipolygnathus >Linguipolygnathus. The diagnostic features, on which the systematics of the elements is based, and the direction of the evolution and the main trends are outlined. The phylomorphogenesis and the proposed schemes of the phylomorphogenetic development ofPseudogondwania, Eognathidae, Eopolygnathidae, and Lower Devonian Polygnathidae are described. According to the phylomorphogenetic constructions, all the Pragian-Emsian conodont zonations suggested earlier are synthetical; they are links of particular phylomorphogenetic lineages. Alternative schemes based on the links of the unified phylomorphogenetic lineages, that meet all the requirements for the present zonations, are suggested. Among many lineages, those, in which the diagnostic features are most easily distinguished and the zonal index-species are mostly cosmopolitan, are chosen. In the zonations, the Lochkovian-Pragian boundary is drawn by the appearance of gondwanids, the Pragian-Emsian boundary by the entry of eopolygnathids. Within the Pragian, the distinct boundary between an early and late Pragian can be drawn by the appearance of eognathodids; whereEognathodus has not been found yet, this level is approximately determined by the entries ofPseudogondwania kindlei andPs. clarki. A very distinct boundary between an early and late Emsian is drawn by the appearance of polygnathids.