Cambrian Bradoriida and Phosphatocopida (Arthropoda) of the former Soviet Union

Some 40 bradoriid and phosphatocopid (Arthropoda) species are known from the Cambrian of the former Soviet Union. The faunas occur chiefly in Asia (mostly Siberia and Kazakhstan; also Kirghizia); west of the Urals bradoriid and phosphatocopid faunas are sparse, occurring in the Leningrad region, Belarus and Estonia. Most specimens are recovered as crack-out material from clastic and impure carbonate rocks; acid resistant valves from limestones are a minor component of the known faunas. Early Cambrian (Atdabanian-Botomian) faunas are widespread; middle and late Cambrian faunas are scarcer and are known largely from Siberia and Kazakhstan. Though many species are seemingly short-ranging, currently most have only local biostratigraphic significance, with only a few having practical international correlative value. Palaeogeographically, faunas west of the Urals show affinites with those of the Early Palaeozoic Baltica and Avalonia palaeocontinents (Olenellid trilobite realm). Siberian and central Asian (Kazakhstan, Kirghizia, Gorny–Altay–Mongolian belt) faunas show clear affinities with those of palaeocontinental South China and eastern Gondwana (Redlichiid trilobite realm).


INTRODUCTION
Bradoriids and phosphatocopids are small, bivalved, almost exclusively Cambrian arthropods which first appear coevally with, or slightly later than the first trilobites (see Siveter et af., 1996 and references therein). In spite of the fact that from the early Cambrian onwards bradoriids and phosphatocopids have worldwide distribution and that they often form abundant elements of Cambrian faunas (e.g. Hou & Bergstrom, 1991), their use in biostratigraphy and biogeography has largely been neglected. Several recent studies have highlighted their widespread occurrence in the Cambrian of especially Britain (see Siveter Rushton et al., in press; in press), North America (see Siveter & Williams, 1977), Australia (Jones & McKenzie, 1980;Hinz-Schallreuter, 1993a), the Baltic (Hinz-Schallreuter, 1993b, 1993c and China (see Shu, 1990aShu, , 1990b with nomenclatorial annotations by Malz, 1990;Huo et al., 1991 and references therein) and have demonstrated their biogeographical and regional and international correlative potential (e.g. Siveter et al., 1993;Williams et al., 1994b;. Bradoriids and phosphatocopids are also known from several parts of the former Soviet Union but a comprehensive account of their geographical occurrence and their biostratigraphic and biogeographic distribution and value remains to be elucidated; these are the primary aims of our paper. Most of the some 40 bradoriid and phosphatocopid species known from the former Soviet Union are from central and eastern Asia (Fig. 1). Many species were collected as part of geological field reconnaisance studies and are known from only a few specimens; thus, information regarding exact geographic and stratigraphic provenence is sometimes imprecise. As far as the material at our disposal allows we have, where appropriate, revised the generic assignment of all of the bradoriid and phosphatocopid species of the former Soviet Union.
Small bivalved arthropods of the Cambrian have traditionally been referred to the Order Bradoriida Raymond, 1935 and considered to include the oldest representatives of the ostracod crustaceans (Miiller, 1964(Miiller, , 1979Jones & McKenzie, 1980). Both major groups of bradoriids, the Bradoriina Raymond, 1935and the Phosphatocopina Miiller, 1964, were raised to ordinal level in Muller, 1982. Because of the problem of convergent evolution, resulting in the possession of a bivalved shell in many otherwise disparate groups of arthropods (as in ostracods, phyllocarids and some Burgess Shale taxa), recognition of the true affinities of such fossil forms ultimately depends on the preservation of appropriate soft parts (e.g. see Briggs, 1983;Muller & Walossek, 1991), rare examples of which are now known from both phosphatocopids and bradoriids (e.g. Muller 1979Muller , 1982Hou et al., 1996). Thus, some authors now consider the Phosphatocopida to be merely stem-group Crustacea (Muller & Walossek, 1991;Walossek & Miiller, 1992; but see also, for example, Him-Schallreuter, 1993b, 1993c, and the Bradoriida are regarded as merely a polyphyletic grouping of perhaps several arthropod groups which questionably includes some ancestral ostracods (e.g. Jones & McKenzie, 1980;Siveter et af., 1996;Hou et al., 1996).
Most of the phosphatocopid and bradoriid specimens from the former Soviet Union are preserved on rock slabs, as partly flattened valves and carapaces. Some secondarily(?) phosphatized specimens have been extracted from limestones from Siberia (Muller et al., 1995) and Kazakhstan (Melnikova & Taylor, unpublished). This material is the sort of provenance (see Miiller, 1979) which might yield phosphatocopids or bradoriids with soft tissues preserved.
HISTORY OF RESEARCH AND GEOGRAPHICAL DISTRIBUTION The earliest documentation of bradoriids and phosphatocopids are based on mid-to late 19th Century studies of British and North American faunas (see Rushton et al., in press;Siveter & Williams, 1997;Williams & Siveter, in press). In contrast, these groups were only quite recently recorded from the former Soviet Union, as a result of field and faunal studies in the Cambrian of Asia (Figs 1, 2). From the 1950s onwards finds were made in Siberia by palaeontologists such as N. P. Suvorova, V. Ye. Savitskii, N. P. Lazarenko, L. N. Repina and E. B. Romanenko. The earliest formal descriptions were of Siberian material from early Cambrian Atdabanian limestones of the Lena-Aldan region (Cambria sibirica Neckaja & Ivanova, 1956) and near Chekurovka in the Khara-Ulakh Mountains (Cambria melnikovi Ivanova, 1964) and of Anabarochilina Abushik, 1960 and 'Leperditia' from the early late Cambrian of the Kotui River area (Abushik, 1960). Melnikova (1983aMelnikova ( , 1983b documented additional taxa from Siberia.
Later, material was recorded from non-Siberian parts of the former Soviet Union. The middle Asian regions of Kazakhstan, Kirghizia and the Gorny Altay-Mongolian belt have all yielded Cambrian bradoriids and/or phosphatocopids, including the types of Ushkarella Koneva, 1978, Tscholponaella Melnikova, 1990, Uskutchiella Melnikova, 1992 and Altajanella Melnikova, 1992(Koneva, 1978Melnikova, 1988Melnikova, , 1990aMelnikova, , 1990bMelnikova, , 1992. In contrast, bradoriids and phosphatocopids have been noted only rarely in that part of the former Soviet Union west of the Urals. A few species occur in the Baltic state of Estonia and in the Leningrad region (Melnikova, 1984(Melnikova, , 1985(Melnikova, , 1987. Unpublished faunas are also known from borehole material from western Belarus. Most recent studies have either been short general summaries about the bradoriids and phosphatocopids of the former Soviet Union (Melnikova, 199Oc, 1990d) or detailed revisions of Siberian species which, in some cases, identify widespread biostratigraphical potential (e.g. Siveter et al., 1993Siveter et al., , 1994Siveter et al., , 1996Hinz-Schallreuter, 1993~;Williams et al., 1994a). The possibility of obtaining additional bradoriid and phosphatocopid material from the former Soviet Union is particularly well demonstrated by the acid-resistant faunas obtained from limestones in Siberia (Miiller et al., 1995) and from Kazakhstan during the 1980s by Melnikova and Taylor (unpublished manuscript; see herein PIS 3, 4).

BIOSTRATIGRAPHIC DISTRIBUTION
In the former Soviet Union bradoriids and phosphatocopids occur throughout the Cambrian but are known chiefly from the early parts of the system (Fig. 2). Many species are shortranging (Fig. 3), but currently most have only local biostratigraphic significance, with only a few having practical international correlative value.

Middle Cambrian (PIS 2,3)
Middle Cambrian bradoriid and/or phosphatocopid faunas contain relatively few taxa and are more sparsely distributed and often stratigraphically and geographically more imprecisely defined than the early Cambrian assemblages (Figs 2, 3). Anabarochilina primordialis (Linnarsson, 1869) is known from the late middle and late Cambrian of Siberia and the middle Cambrian of southern Britain and Scandinavia (see Siveter et ul., 1993). Verification of its full correlative value awaits systematic recollection of Siberian material. A. primordialis occurs in the late Cambrian Eyra Formation of the Kotui River region and probably occurs in the middle Cambrian Siligar Formation of the Malaya Kuonamka River region (Fig. 2). The record of A. primordialis from the late Cambrian dolomites of the Lapar Formation of the Olenek River region may be in error; more likely it is from limestones of the middle Cambrian Tyussala Formation (see also Melnikova, 1984;Abushik, 1960; herein Fig. 2). The only other named middle Cambrian taxon from Siberia is Vestrogothia? sp., represented by a single specimen from the Amgan Stage Sekten Formation near Chekurovka in the Khara-Ulakh Mountains Cpl. 3, fig. 10); additional finds comprise indeterminate phosphatocopids from the Kuonamka Formation (Miiller et al., 1995).
Several new species are known from the middle Cambrian of southern Kazakhstan (Melnikova & Taylor manuscript; herein P1. 3, figs 1, 5, 9). They include a late middle Cambrian (Lejopyge trilobite Biozone) Anabarochilna species, which resembles the middle Cambrian Anabarochilina australis (Hinz-Schallreuter, 1993c) from Australia, and a middle to late Cambrian form probably referable to the typically late Cambrian Chinese genus Euzepaera Shu (see Shu, 1990a).
West of the Urals the only known middle Cambrian species is Vojbokalina magnifica Melnikova, 1984, from the Sablinka Formation of the Leningrad Region.

Late Cambrian (Pls 3, 4)
Possible late Cambrian bradoriid and/or phosphatocopid faunas from west of the Urals are known only from as yet unstudied borehole material from western Belarus (Melnikova, personal observation; herein P1.4, fig. 8). In the Asiatic part of the former Soviet Union late Cambrian faunas are documented mostly from Kazakhstan and Gorny Altay (Figs 1-3).

BIOGEOGRAPHIC SIGNIFICANCE
In the Cambrian the Siberian and Estonian/western Russian areas of the former Soviet Union were positioned on the palaeocontinents of Siberia and Baltica, respectively (Fig. 4). Siberia lay at southerly tropical latitudes and Baltica at about 60° south, within the Bigotinid and Olenellid trilobite faunal realms, respectively (see Scotese & Mckerrow, 1990;Mckerrow et al., 1992). The sparse bradoriid and phosphatocopid faunas of Estonia and western Russia show some affinities with assemblages from Scandinavia (Baltica) and New Brunswick/Nova Scotia of the Canadian maritimes (situated on Avalonia, a southerly high latitude microcontinent on the northern margin of the Gondwana palaeocontinent). Those of Siberia show affinities particularly with South China, which in the Cambrian was an equatorial continental block on the northeastern margin of Gondwana.
Kazakhstan was once considered to be a discrete continental block or part of the Siberian plate (e.g. Scotese et al., 1979;Scotese & Mckerrow, 1990) but it is now thought to be an amalgamation of terranes, many of which may be seperated by ophiolites (Mckerrow et al., 1992). This so-called Altaid tectonic collage, which also includes Kirghizia and Gorny Altay, may have evolved dominantly along a subduction zone which, during the Cambrian, developed along the margin of a unified 'Baltica-Siberia' continent (Sengor et al., 1993). Early Palaeozoic faunas from different parts of Kazakhstan show affinities with Siberia, China and Europe (see summary in McKerrow et al., 1992, p. 603).

Siberia
The early Cambrian bradoriid and phosphatocopid faunas of eastern Siberia are dominated by cambriids, a palaeogeographically widespread group, which span parts of the Redlichiid, Bigotinid and Olenellid trilobite realms, but which were apparently restricted to within tropical/subtropical regions. This suggests possible distributional control by palaeo-latitudinal factors such as temperature (Siveter et al., 1994 and references therein). The Siberian bradoriid Cambria Neckaja & Ivanova, 1956 is also probably a common element of south Chinese Cambrian faunas (the full biogeographic significance of cambriid species awaits revision of Chinese material such as that of Huo et al., 1991). The presence of Kunmingella confirms this affinity with the Chinese faunas.
The middle and late Cambrian faunas of Siberia include the cosmopolitan Anabarochilina and a vestrogothiid species. Vestrogothids are more typical of the middle and late Cambrian faunas of Scandinavia (Baltica) and southern Britain (Avalonia).

Kazakhstan, Kirghizia and Gorny Altay
The bradoriid and phosphatocopid faunas of central and southern Kazakhstan and possibly Kirghizia suggest a faunal affinity with China and probably Australia, an area synonymous with the Cambrian Redlichiid trilobite realm. Early Cambrian bradoriid and phosphatocopid faunas from northeastern central Kazakhstan include Alutella, Tsunyiella and a possible cambriid species; southern Kazakhstan (Maly Karatau) and Kirghizia yield Dabashanella retroswinga (see Melnikova, 1990aMelnikova, , 1990b). This fauna is very similar to those of contemporaneous deposits in the Tarim and southern (Yangtze Platform) areas of China (see Zhang, 1987;Huo & Cui, 1989;Huo et al., 1991 and references therein). Middle and late Cambrian forms from Maly Karatau, detailed study of which is in preparation (Melnikova & Taylor), includes Monasterium and an Anabarochilina species with affinities to the Australian A. australis Hinz-Schallreuter The bradoriid and phosphatocopid faunas of the Eastern Trans-Baikal region, such as Liangshanella? and Alutella, show obvious affinities with faunas from China (Redlichiid trilobite realm). Ushkarella and Altajanella appear to be endemic to Kazakhstan and Gorny Altay, respectively.

COLLECTIONS
Nearly all bradoriid and phosphatocopid material from the former Soviet Union is housed in the Palaeontological Museum of the Palaeontological Institute (PIN) of the Russian Academy of Sciences, Moscow, Russia. The collections include material from Estonia, Belarus, the Leningrad Region, Siberia, Kazakhstan, Kirghizia and the Gorny Altay-Mongolian belt and are registered under the prefixes N1117, N2175, N3465, N4341-4344 and N4346.

CONCLUSIONS
A formal systematic and monographic treatment of the Bradoriida and Phosphatocopida of the former Soviet Union ideally requires much more material than is currently available. In particular, the use of acid preparation techniques on limestones should be targeted as a potential high yield mode of recovery of valves, and as a possible source of specimens with soft-part preservation. Such studies would facilitate enhanced evaluation of the taxonomic, biostratigraphic and biogeographic significance of the faunas.