26 



L.E. POPOV, L.R.M. COCKS AND I.F. NIKITIN 



5. Parastrophina-Kellerella Association (Diversity Index 3.17, 

 observed range 1.61-5.42, N=10) is closely associated with carbon- 

 ate build-ups and also belongs to BA-3. These build-ups were 

 interpreted by Nikitin et al. (1974) as a chain of bioherms with a 

 frame built by the cyanobacterians Girvanella and Renalcis: how- 

 ever, micritic limestone usually comprises the most significant part 

 of the volume of the rock in the core of a build-up. According to 

 Nikitin et al. (1974), these build-ups form a low ridge, raised about 

 1.5-3 m above surrounding areas with fine clastic sedimentation. 

 Fossils are usually concentrated in pockets of bioclastic limestone 

 between individual bioherms and mounds (Samples 948, 2538, 

 8223a, 8256). Composition of this association is essentially similar 

 to the Acculina-Dulankarella Association, with more than 80% of 

 recorded species in common. However, the abundance of 

 camarelloideans increases up to 49% (Sample 948) and the archaic 

 spire-bearing brachiopods Pectenospira pectenata, Kellerella misiusi 

 and Nikolaispira guttula constitute a significant part of the associa- 

 tion (21—42% in the most representative samples), whereas in the 

 Acculina-Dulankarella Association they are less than 2% (Table 5). 

 The relative abundance of strophomenides decreases significantly 

 and such genera as Acculina, Dulankarella, Mabella and Shlyginia 

 disappear completely. Christiania is represented by the species C. 

 aff. sulcata, which is closely linked to this association. Diminution in 

 the sizes of the strophomenides might reflect the predominance of 

 hard grounds. Taxonomic composition of the association is modified 

 in the bedded bioclastic limestone which has large ooids (up to 1 cm 

 across) on the top and flanks of the core (Samples 8214, 8217, 8223, 

 8223b). Brachiopods are relatively rare and dispersed through the 

 rock. Relative abundance of strophomenides, and especially 

 Christiania aff. sulcata, increases, whereas spire-bearing brachiopods 

 become rare or completely disappear in some samples (8214, 8223, 

 9223b). According to the Principal Component Analysis these oc- 

 cupy an intermediate position between the cluster formed by samples 

 of the Acculina-Dulankarella Association and samples 948, 2538, 

 8256, which represent the fully developed Parastrophina-Kellerella 

 Association (Fig. 8B). 



The associated trilobite fauna is only partly known and includes 

 such taxa as Illaenus sp., Acrolichas punctata, Amphilichas 

 punctata, Eokosovopeltis romanovskyi, Glaphurina weberi, 

 Mesotaphraspis spinosus, Pliomerina sulcifrons, Selenoharpes sp. 

 and Sphaerexochus aff. hisingeri (Weber 1948, Chugaeva 1958, 

 Kolobova & Popov, 1986). Crinoid columnals usually represent the 

 most important source of bioclasts in the rock. They mostly belong 

 to Webericrinus variabilis, Ordinacrinus ordinaris, Malovicrinus 

 depressus, Tatjanicrinus crusciformis, Flexicrinus flexus, 

 Communicrinus communis and Multifidocrinus mulrifidus ( Stukalina 

 1988). 



In the eastern part of the Anderkenen-Akchoku section (Sample 

 8226), isolated carbonate build-ups up to 16 m thick appear within 

 Unit 2, which is mostly cross-bedded sandstone containing lingulide 

 and mollusc associations. It is likely that these build-ups formed 

 almost intertidally, but, except for a much lower diversity, the 

 brachiopod assemblage retains a relative abundance of spire-bearers 

 (Kellerella misiusi) and camerelloideans (Parastrophina p/e/ia) typi- 

 cal of the Parastrophina-Kellerella Association, whereas trilobites 

 such as Acrolichas sp., Eokosovopeltis romanovskyi , Sphaerexochus 

 aff. hisingeri and Illaenus sp. also show close similarity to the 

 assemblage from the carbonate unit in the upper part of the Anderken 

 Formation. 



6. Zhilgyzambonites-Foliomena Association (Diversity Index 1.21; 

 observed range 1.14-130, N=3) is known from three samples col- 

 lected from the unit of mudstones and siltstones in the uppermost 



Table 6 Composition of Zhilgyzambonites-Foliomena Association from 

 the Anderken Formation showing number of complete shells, ventral and 

 dorsal valves respectively. 



Sample number 



2531 



8251 



8255 



Number of specimens 



4 



4 



15 



Diversity index 



1.44 



0.72 



1.48 



Foliomena prisca 



1:0:0 



0:1:0 



1:3:3 



Olgambonites insolita 







0:3:2 



Zhilgyzambonites extenuata 



0:2:1 



0:3:3 



2:5:5 



Kassinella? sp. 







0:0:1 



Chonetoidea sp. 







1:0:0 



Anisopleurella sp. 



0:1:1 







Anderken Formation in the Anderkenyn-Akchoku section (Figs 3, 5, 

 Unit 6, Table 6). Brachiopods are a minor part of a trilobite-domi- 

 nated benthic assemblage, which includes Amphitrion cf. radians, 

 Ampixinella sp., Birmanites almatiensis, Bronteopsis extraordinaris, 

 Cheraurus kassini, Cybele weberi, Dionide kazachstanica, 

 Dindymene sp., Hamtnatocnemis sp., Microparia speciosa, Ovalo- 

 cephalus sp., Granulatagnostus granulatus and Sphaerognostus sp. 

 (Chugaeva 1958, Nikitin et al. 1974). Co-occurrence of agnostids, 

 cyclopygids and Ovalocephalus allows us to refer this assemblage to 

 the Ovalocephalus fauna of Fortey (1997) which is characteristic of 

 outer shelf trilobite biofacies corresponding to BA 4-5. Co- 

 occurrence of the Foliomena and the Ovalocephalus faunas is 

 common in late Caradoc to early Ashgill deep water benthic 

 communities in South China (Rong et al. 1994). The Zhilgyzam- 

 bonites-Foliomena Association includes only six strophomenide 

 genera. Two of them (Olgambonites and Zhilgyzambonites) are 

 Kazakhstan endemics, whereas Anisopleurella and Kassinella are 

 characteristic of early Foliomena faunas in the Caradoc of China 

 (Rong et al. 1999) and elsewhere. 



OVERALL PALAEQECQLOGY 



The sequence of brachiopod associations in the Anderken Formation 

 shows an onshore-offshore pattern with a monotaxic lingulide com- 

 munity inhabiting mobile sand nearshore, low diversity mollusc and 

 brachiopod associations of BA-2 on a shallow clastic shelf, medium 

 to high diversity faunas of BA-3 linked with carbonate build-ups, 

 and a deep water Foliomena fauna as part of a trilobite-dominated 

 benthic assemblage in BA 4-5. In terms of abundance, diversity and 

 taxonomic composition, the five associations formed by 

 rhynchonelliformean brachiopods can be subdivided into three 

 groups, which show little interaction and apparently evolved inde- 

 pendently. They are: 1, low-diversity strophomenide-dominated 

 Tesikella and Mabella-Sowerbyella Associations of shallow clastic 

 shelf corresponding to BA-2; 2, medium to high diversity Acculina- 

 Dulankarella and Parastrophina-Kellerella Associations closely 

 linked to carbonate build-ups; and 3, a deeper-shelf 

 Zhilgyzambonites-Foliomena Association representing an early 

 Foliomena Fauna. 



The predominance of strophomenides in environments corre- 

 sponding to BA-2, which is usually dominated by rhynchonellides 

 and spire-bearing taxa in the late Ordovician and Silurian (Boucot 

 1975; Ziegler et al. 1968), is a distinctive feature of the Anderken 

 sequence of communities. The Tesikella and Mabella-Sowerbyella 

 Associations share eight brachiopod species but Tesikella necopina 

 (the index species of the former association) does not usually co- 

 occur with Mabella conferta and Shlyginia fragilis. The Tesikella 

 necopina Association also demonstrates significant variations in 



