Ludlow neocucullograptid fauna from southern Tien Shan and its significance in biozonation  

Tatjana N. Koren^’1 and Anna A. Sujarkova¹         

Key words: Graptolites. Biozonation. Phylogeny. Ludlow. Central Asia.

A diverse Ludlow neocucullograptid fauna has been studied from complete sections of the Kursala and Pulgon Formations in the Peshkaut and Tashbulak Valleys and the Kyk Mountains of South Fergana, Tien Shan. The 150m–thick Kursala Formation (Peshkaut Valley sections) consist mostly of dark, richly graptolitiferous mudstones interbedded with thin sandstones and calcarenites. The Pulgon Formation (Tashbulak Valley and Kyk Mountains sections) consists of graptolite–bearing siltstone and mudstone beds less than 2 m thick, separated by coarser–grained strata from 50 to 200 m thick. Both formations range in age from the lundgreni/testis Biozone to the transgrediens Biozone. Collections were made bed by bed from the mudstones and shales at intervals as close as 10 to 30 cm, allowing for the establishment of a high–resolution graptolite biozonation within the Upper Silurian. Special attention was paid to the definition of the stratigraphic ranges of the members of the Bohemograptus, Egregiograptus and Polonograptus plexuses. Not less than 18 species and subspecies have been described, including some pandemic forms and 5 new taxa (Koren’ and Sujarkova, in press), many of which are recorded for the first time in southern Tien Shan. The neocucullograptids are associate with diverse and abundant species of Saetograptus, Colonograptus, Pristiograptus, Uncinatograptus, Pseudomonoclimacis, Cucullograptus, Neocucullograptus, Neolobograptus and some genera of the Plectograptinae Subfamily. The stratigraphically well–controlled graptolite assemblages are important in the understanding of Ludlow biostratigraphy, the evolutionary dynamics and the phyletic reconstructions of the neocucullograptid fauna.

The Ludlow succession in Tien Shan includes the following biozones, from the bottom up: the progenitor/colonus Biozone, the scanicus/chimaera Biozone, the leintwardinensis, tenuis Interzone, the podoliensis Biozone and the formosus/spineus Biozone. This succession serves as the regional standard for correlating graptolite–bearing terrigenous deposits in southern Tien Shan. The upper Wenlock (Homerian) strata are subdivided into the lundgreni/testis, praedeibeli/sherrardae, deubeli and ludensis Biozones (Koren’, 1994).

The base of Gorstian, or of the progenitor/colonus Biozone, is defined by the FAD of Plectograptus macilentus, Spinograptus spinosus, Lobograptus progenitor, Bohemograptus bohemicus, and Colonograptus colonus. The lower boundary of the succeeding scanicus/chimaera Biozone is defined by the first appearance of Lobograptus simplex and Saetograptus chimaera (Figure 1). The association of this biozone is the most diverse Ludlow succession studied, including not less than 35 species. Unusually early occurrences of numerous B. praecornutus, and some new species of Egregiograptus are recorded from its upper part.

Figure 1. Summary range chart of selected graptolites in the upper Gorstian–Ludfordian sequence of the Kursala and Pulgon Formations, Southern Tien Shan’.

The lower boundary of the leintwardinensis Biozone coincides with the FAD of the zonal species and the morphologically distinct Saetograptus linearis. The stratigraphic range of these species is within this biozone, and their overlapping ranges in all sections studied do not support the recognition of two succeeding leintwardinensis and linearis biozones, as suggested by Jaeger (1991). The leintwardinensis Biozone of central Asia contains the most diverse of all known assemblages of the biozone. It includes not fewer than 30 species and subspecies. Many of the elements are also common to the pristiograptid, monograptid, bohemograptid and saetograptid faunas of Arctic Canada, Poland, Bohemia and New South Wales. A special feature of the early Ludfordian graptolites in Tien Shan is a presence of new bohemograptid and egregiograptid species (Koren’ and Sujarkova, 1998; Koren’ and Sujarkova, in press). Some previously described species, such as Bohemograptus praecornutus and B. cornutus, known elsewhere not earlier than the post–leintwardinensis biozonal interval, appear much earlier in the Gorstian–Ludfordian boundary beds of Tien Shan (Figure 1).

The tenuis Interzone is defined in the continuous graptolite sequence as the full range of the index–species from the upper scanicus Biozone to the podoliensis Biozone, the associated species being common to both the underlying and the overlying zonal units. The lower boundary is easily defined by the final disappearance of the numerically abundant saetograptids. Compared to the taxonomically impoverished monograptid associations known from other regions, the much more diverse graptolite fauna of this interval of the study area includes about 20 species and subspecies of neocucullograptids and monograptids. The lower boundary of the podoliensis Biozone is defined by the FAD of the index species in association with Polonograptus aloisi, which spans most of the zonal interval (Figure 1). The biozone is characterized by 20 species and subspecies including some well known as well as new taxa. The assemblage includes abundant B. tenuis, B. praecornutus, B. cornutus, B. garratti and new species of Egregiograptus. The upper part of the podoliensis Zone is distinguished by the presence of numerous Polonograptus mirus. Only occasional finds of Neocucullograptus inexpectatus are known from the P. podoliensis stratigraphic range. Noteworthy is the absence of Neocucullograptus kozlowskii in the Tien Shan assemblages, a species common to the Siedlce beds of Poland, their equivalents in Volynia and to the Kopanina beds of Bohemia (Urbanek, 1970; Urbanek and Teller, 1997; Tsegelnjuk, 1976; Štorch, 1995). This could be due to the genus being restricted to inshore environments, while the polonograptids and bohemograptids were less bathymetrically controlled. Pristiograptids and pseudomonoclimacids derived from the Pristiograptus dubius stem are also abundant. In contrast to the Polish and Bohemian faunas, our fauna includes some of the hooded monograptids described by Tsegelnjuk (1976) from Volynia, and shares also some diagnostic polonograptids of the Bohemian assemblages. The unexpectedly high occurrences of the last plectograptines are also documented from this biozone.

In general, the upper Ludlow (Ludfordian) part of the graptolite sequence is the most difficult interval for detailed zonal correlation of the Upper Silurian. Graptolite–bearing deposits are less widely distributed and subdivided into locally recognizable graptolite units of limited stratigraphic extent. They also carry their own zonal nomenclature. This can partly be explained by the inadequacy of the graptolite record as known heretofore, but ecological or geographical differences in the taxonomic composition of contemporaneous assemblages cannot be excluded from consideration.

Regardless of the high diversity of the mid–Ludfordian fauna of the Southern Tien Shan, the overlapping stratigraphic range of the key species does not permit a subdivision of the interval between the top of the leintwardinensis and the base of the formosus/spineus Biozones into more than two graptolite units: the tenuis Interzone and the podoliensis Range Biozone.

The overlying uppermost Ludlow–Pridoli sequences and their graptolite fauna (the formosus/spineus Biozone to transgrediens Interzone) have also been studied in detail along the same sections in the Peshkaut Valley (Koren’ and Sujarkova, 1997).

The abundance, good preservation and well established stratigraphic succession of the Ludlow neocucullograptids in the southern Tien Shan have led us to recognize new facets of the taxonomic diversity, dynamics and time of appearance of the key species of the succession, which will be of critical important in the interpretation and understanding of their phylogeny (Koren’ and Sujarkova, in press).

The work was funded through the Russian Foundation for Fundamental Science, grant Nº 02 05–64 775.

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Received: February 15, 2003

Accepted: June 15, 2003