Epifaunal Anchored Brachiopods

During the Early Cambrian, brachiopods inhabited shallow subtidal environments. The earliest brachiopods (family Cryptotretidae, order Paterinida) occur in the Tom-motian and Atdabanian of the Anabar-Sinsk Basin of the Siberian Platform and the Zavkhan Province of Mongolia. Cryptotretids occurred on calcareous-argillaceous interreefal substrates of shallow warm epicontinental seas, often under high-energy

Figure 16.3 A, Dictyonina sp., PIN 4290/206, ventral valve, Middle Cambrian, Marjuman stage (Kotuy River, Siberian Platform), scale bar 1 mm. B and C, Stilpnotreta inaequalis Ushatinskaya, pitted larval shell surface, Middle Cambrian, Marjuman stage (Siberian Platform, Kotuy River); B, PIN 4290/141, scale bar 0.5 mm; C, PIN 4511/76, scale bar 0.2 mm. D, Linnarssonia rowelli Pel'man, PIN 3848/3001, ventral valve, Early Cambrian, Botoman Perekhod Formation (Ulakhan-Kyyry-Taas, middle Lena River, Siberian Platform), scale bar 1 mm. E, Eoobolus sp., PIN 4290/252, ventral valve interior, Early Cambrian, Atdabanian Krasnyy Porog Formation (Sukharikha River, Siberian Platform), scale bar 0.5 mm. F, Fossuliella linguata (Pel'man), PIN 4290/252, adult shell surface, Middle Cambrian, Amgan Stage (Siberian Platform, Olenek River), scale bar 0.1 mm. G, Acrothele

sp., PIN 4290/253, posterior part of dorsal valve exterior, thin spines on the larval shell serving as adaptation to soft substrate, Middle Cambrian, Marjuman stage (Olenek River, Siberian Platform), scale bar 0.1 mm. H, Semi-treta sp., PIN 4511/41, iceberg-type brachio-pod shell of quasi-infaunal lingulate with highly conical ventral valve, inhabiting soft muddy substrate, Middle Cambrian, Marjuman stage (Kotuy River, Siberian Platform), scale bar 1 mm. I, Batenevotreta formosa Ushatinskaya, holotype PIN 4377/124, ventral valve, Middle Cambrian, Amgan Sladkie Koren'ya Formation (Batenevsky Ridge, Altay-Sayan Foldbelt), scale bar 1 mm. J, Quadrisonia simplex Koneva, Popov, and Ushatinskaya, PIN 4321/1, posterior part of ventral valve, Late Cambrian, Steptoean stage (Olenty-Shiderty Province, northeastern Kazakhstan), scale bar 0.5 mm.

Figure 16.4 Reconstruction of the shell of the lingulate Salanygolina, possessing a large pedicle opening and high flattened pseudoarea on the ventral valve, Early Cambrian, Atdabanian Stage (Zavkhan Province, Mongolia).

conditions (Zhuravleva 1966; Wood et al. 1993). These genera, Aldanotreta, Crypto-treta, and Dzunarzina, together with some later Salanygolina, possessed a large pedicle opening and flattened high pseudoarea on the ventral valve. The latter feature might have provided additional support on substrates (figure 16.4). These forms were probably anchored to small pebbles and shell fragments, which were abundant in the vicinity of reefs.

Obolellids appeared in the Anabar-Sinsk Basin during the late Tommotian, and ni-susiids developed there during the middle Atdabanian. Both groups retained an opening throughout life and were typical anchored forms. Obolellids were confined to interreef and reef habitats. They were common elements of reefal cryptic communities from the early Atdabanian (Kobluk and James 1979; Kobluk 1985). Obolella and other brachiopods attached at their posterior margin, with the shell opening into the cavity. Attachment was effected by a short stout pedicle, probably bearing papillae

Figure 16.5 Cryptic cavity formed by archaeo-cyathan secondary skeleton in a calcimicrobial-archaeocyathan reef containing the calciate Obolella sp. attached by its rear, with the aperture opening into the cavity, X 10, thin section PIN 3848/710, Early Cambrian, Atdabanian

Pestrotsvet Formation (Oi-Muran village, middle Lena River, Siberian Platform). The carbonate substrate is dissolved slightly where papillae rooted into it (arrow). Source: Photograph courtesy of Andrey Zhuravlev.

Figure 16.5 Cryptic cavity formed by archaeo-cyathan secondary skeleton in a calcimicrobial-archaeocyathan reef containing the calciate Obolella sp. attached by its rear, with the aperture opening into the cavity, X 10, thin section PIN 3848/710, Early Cambrian, Atdabanian

Pestrotsvet Formation (Oi-Muran village, middle Lena River, Siberian Platform). The carbonate substrate is dissolved slightly where papillae rooted into it (arrow). Source: Photograph courtesy of Andrey Zhuravlev.

on its surface. Carbonate substrates are dissolved slightly where papillae rooted (figure 16.5). A similar feature is common among Recent attached brachiopods (Bromley and Surlyk 1973). Jackson et al. (1971) described Recent Thecidellina and Aegyro-theca, which are similar in size to Cambrian cryptic brachiopods, from coral reefs. Many of these forms anchor on the upper surface of reefal caves but are absent from the floors. Thus, the animals occurred well above the water-sediment interface, minimizing occlusion by mud. Such a strategy was perhaps exploited by Early Cambrian obolellids that inhabited calcimicrobial-archaeocyath reefs. Obolella shell pavements several meters long and 0.5-1.0 cm thick are preserved in middle Atdabanian calcareous-argillaceous mudstones. This might have resulted from local transport and redeposition of valves adjacent to a mass settlement of brachiopods. In the late Middle

Cambrian Bol'shoy Kitat Formation of Kuznetsky Alatau (Altay-Sayan Foldbelt), banks formed by densely spaced in situ Diraphora occur (Aksarina 1983). The brachiopods were anchored at their posterior margin to the originally muddy carbonate substrate.

Initially, Lingulida and Acrotretida were mainly restricted to Atdabanian siliciclas-tic sediments in higher latitudes. The earliest of them are found in thin limestones at Comley (British part of Avalonia) and from siltstones in the western Baltica (Keller and Rozanov 1979; Hinz 1987; Jendryka-Fuglewicz 1992). Simultaneously, or shortly after, during the late Atdabanian-early Botoman, lingulids and acrotretids appeared in argillaceous carbonate facies on the Siberian Platform (Perekhod and Krasnyy Porog formations) and Laurentia (Sekwi Formation) (Voronova et al. 1987; Astashkin et al. 1991) These regions were shallow basins with low water energies and slow submergence (Rushton 1974; Rozanov and jydka 1987). The brachiopods were characterized by small (<5 mm), thin shells. An opening for the pedicle, by which the animals appeared to be anchored, had the form of either a foramen or a groove and was located near the apex (see figures 16.3D,E).

Clustered accumulations of very small (1-2 mm) lingulates occur in the late Early to early Middle Cambrian calcareous-argillaceous Kuonamka facies of the Siberian Platform. A single 200-300 g sample contains up to 200-300 well-preserved valves, and sometimes complete shells. In these clusters, species number is generally two or three and up to five. Acrotretids dominate and lingulids are less common. Extremely fine grain and homogeneous structure of the Kuonamka facies indicate calm conditions (Bakhturovet al. 1988). The abundance of cyanobacterial fossils indicates a relatively shallow depth within the photic zone, approximately 50-100 m, for the basin (Zhegallo et al. 1994). Dominantly soft silty substrate, and the presence in both the acrotretids and lingulids confined to this facies of a pedicle opening that functioned throughout life, suggest restriction of these brachiopods to algal thickets. These algae could be Margaretia, comprising abundant carbonaceous beds of the Kuonamka facies (Barskov and Zhuravlev 1988). The clustered distribution of brachiopod settlements might be related to sporadic occurrence of algal thickets. The Kuonamka facies (Sinsk and Kuonamka formations) accumulated in anoxic conditions (Zhuravlev and Wood 1996). Thus, attachment of lingulates to benthic algae allowed them to rise above the anaerobic bottom water layer. Such attached shells have been discovered in situ recently on Margaretia thalli from the Sinsk Formation (Ivantsov et al. 2000). On the other hand, hemerythrin molecules that are responsible for oxygen transport in brachiopod blood impart relatively low oxygen requirements. Some Recent brachio-pods can survive periods of anoxia and are capable of both aerobic and anaerobic metabolism (Brunton 1982). In the Early Triassic, for instance, lingulids were typical of lower dysaerobic assemblages (Hallam 1994). Thus, abundant lingulate assemblages confined to anoxic strata of the Kuonamka facies were probably well adapted to dysaerobic-anaerobic conditions. Similarly, lingulates were common elements of the Late Cambrian Olenid community that existed in stagnant bottom conditions in

Figure 16.6 Calciate Nisusia sp. attached to the spicular sponge Pirania muricata Walcott, lectotype USNM 66459, Middle Cambrian, Amgan Stephen Formation, Burgess Shale

(Mount Stephen, British Columbia, North America). Scale bar equals 1 cm. Source: Reprinted with permission from Rigby 1986: plate 20, figure 1.

Figure 16.6 Calciate Nisusia sp. attached to the spicular sponge Pirania muricata Walcott, lectotype USNM 66459, Middle Cambrian, Amgan Stephen Formation, Burgess Shale

(Mount Stephen, British Columbia, North America). Scale bar equals 1 cm. Source: Reprinted with permission from Rigby 1986: plate 20, figure 1.

Wales (McKerrow 1978). Relatively low metabolic rates in brachiopods were also significant for survival in such conditions (James et al. 1992).

The Middle Cambrian lingulate Dictyonina and the calciate Nisusia may have attached to the large spicular sponge Pirania from the Burgess Shale of Laurentia (Walcott 1920; Whittington 1980; Conway Morris 1982; Rigby 1986) (figure 16.6). The sponge skeletons were complete, and the sponges probably alive, when brachiopods attached to their spicules in order to capture higher, and thus stronger, currents. Conway Morris (1986) suggested brachiopod-sponge commensalism.

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