The Ecology of the Cambrian Radiation - Andrey Zhuravlev - Chapter 16 potx

CHAPTER SIXTEEN Galina T. Ushatinskaya Brachiopods All brachiopods are sessile benthic organisms; in feeding style they are ciliary sus- pension feeders. Cambrian brachiopods show several types of substrate relationships: pedicle-anchoring, free-lying, cemented epifaunal, infaunal, quasi-infaunal, and in- terstitial, as well as possibly pseudoplanktic. The earliest brachiopods are known from Early Cambrian carbonates of the Siberian Platform. Lingulates appeared at the beginning of the Tommotian, and calciates arose in the middle Tommotian. Addi- tional lingulate orders appeared during the late Atdabanian in siliciclastic sediments in northern European areas. The acquisition of a mineralized skeleton by brachio- pods at the beginning of the Cambrian may have been connected with changes in ocean water chemistry. Differences in diet probably defined distinctions in skeletal composition: lingulates could consume phytoplankton, but calciates preferred animal proteins. BRACHIOPODS BELONGto thesubkingdom Eumetazoa andare characterized bytwo unique features. First, they have an intermediate protostomian-deuterostomian em- bryology. It is likely that brachiopods separated from other Bilateralia prior to proto- stomian-deuterostomian differentiation (Malakhov1976, 1983). Second, the Brachio- poda is the only phylum that produces both calcium carbonate and phosphatic shells. By the second half of the twentieth century, the brachiopod systematics developed by Huxley (1869) became widely accepted (Sarycheva 1960; Williams and Rowell 1965). This subdivided brachiopods into two classes, Articulata and Inarticulata, based on presence or absence of valve articulation, respectively. Therefore, the In- articulata included brachiopods with calcium carbonate shells, as well as those with phosphatic shells. During the last decades, studies of Recent brachiopods have re- vealed that forms possessing shells of different composition and microstructure are also distinguished in their embryology and molecular phylogeny (e.g., Williams 1968; 16-C1099 8/10/00 2:17 PM Page 350 BRACHIOPODS 351 Acrotretida Siphonotretida Discinida Lingulida Paterinida Trimerellida Craniopsida Craniida Obolellida Kutorginida all 'articulate' orders Lingulata Calciata Lingulatea Craniformea 'Articulata' ? ? ? Figure 16.1 Subdivision of the phylum Brachiopoda into classes Lingulata and Calciata and orders, according to Popov et al. (1993). Malakhov 1976; Jope 1977, 1986; Ushatinskaya 1990; Nielsen 1991; Williams and Holmer 1992). They therefore appear to belong to different lineages. As a result, a new systematic scheme was proposed by Gorjansky and Popov (1985) and later modified by Popov et al. (1993). According to this scheme, the phylum Brachiopoda includes the classes Lingulata and Calciata (figure 16.1). The former contains brachiopods with phosphatic shells only, and the latter those with calcareous shells. Recently, this supra- ordinal classification has been developed by Williams et al. (1996). CAMBRIAN BRACHIOPOD RADIATION AND DIVERSITY Both classes are known from the Early Cambrian. In the Cambrian, Calciata were rep- resented by the orders Obolellida, Kutorginida, Craniopsida, Naukatida, and Chileida, in addition to traditional articulates (Orthida and Pentamerida orders). However, their diversity was low, and they became abundant only during the Ordovician to Devo- nian. Lingulates were much more diverse, and all five orders were present in the Cambrian (figure 16.2). Three of these—Paterinida, Lingulida, and Acrotretida—ap- peared during the Early Cambrian, the Siphonotretida appeared in the Middle Cam- brian, and the Discinida arose at the end of the Cambrian. 16-C1099 8/10/00 2:17 PM Page 351 352 Galina T. Ushatinskaya 60 50 40 30 20 10 0 calciates genera tom atd bot toy amg mar ste sun dat lingulates Figure 16.2 Generic diversity of calciate and lingulate brachiopods in the Cambrian. Stages: tom ϭ Tommotian; atd ϭ Atdabanian; bot ϭ Botoman; toy ϭ Toyonian; amg ϭ Amgan; mar ϭ Marjuman; ste ϭ Steptoan; sun ϭ Sunwaptan; dat ϭ Datsonian. LARVAL ECOLOGY AND BRACHIOPOD DISTRIBUTION General brachiopod paleoecology has often been discussed (e.g., Lochman 1956; Sa- rycheva 1960; Ivanova 1962; Rudwick 1965, 1970; Gorjansky 1969; Rowell and Krause 1973; McKerrow 1978; Percival 1978; Williams and Lockley 1983; Bassett 1984; Holmer 1989; Popov et al. 1989; Wright and McClean 1991; Popov 1995), and the following information is based on this research. All brachiopods are passive suspension feeders that use the lophophore, a variously looped or coiled extension of the mesocoelom, for water and food uptake. Chuang (1959) showed that the alimentary system of Lingula unguis contains fermenters that allow the animal to digest phytoplankton. The remaining Recent lingulates do not differ from it in this respect. In contrast, Recent calciates lack such fermenters and feed, mainly, on bacterial aggregates and dissolved nutrient matter (Atkins 1960; McCammon 1969; Rhodes and Thompson 1993). All brachiopods belonging to sessile benthos spread at the larval stage. Recent cal- ciates have a lecithotrophic larva that is free-living from several hours to 1–2 days. After that, the larva settles on a substrate. Recent lingulates possess a planktotrophic larva that floats in the water column for from several days to a month (Malakhov 1976). In some cases, under unfavorable conditions, a complete morphogenesis is observed, and a pedicle and a lophophore, bearing numerous cirri, are formed that appear just like those in anchored animals (Zezina 1976). These features allow lin- gulates exclusive facilities for migration. Many Recent lingulates are widespread in shallow waters of the Indian and western Pacific oceans, and Pelagodiscus atlanticus (order Discinida) is a cosmopolitan species. Some Cambrian species of the Paterinida, 16-C1099 8/10/00 2:17 PM Page 352 BRACHIOPODS 353 Acrotretida, and Lingulida orders have a wide, sometimes global distribution. This phenomenon may be due to a protracted pelagic phase ( Jablonski 1986; Rowell 1986). Indirect evidence for this is that larval shell sizes do not exceed one-third to one-fifth of the whole shell (figures 16.3A,B,D). In addition, the larval shell surface of all acrotretids and most Cambrian lingulids bears numerous minute pits (fig- ures 16.3C,G). Many researchers ascribe this feature to a vesicular structure of the periostracum or to an entirely organic larval shell (Biernat and Williams 1970; Wil- liams and Curry 1991). Popov et al. (1982) regarded the pitted microornament of the umbonal area in acrotretids as a negative impression of the entirely organic larval shell and suggested that the acrotretid shell acquired mineralization only after settle- ment. In any case, such a structure may increase buoyancy and probably is an adap- tation to a pelagic lifestyle. Prolongation of the pelagic larval stage might serve to enhance the distribution of some acrotretids. The acceleration in foramen development might indirectly substan- tiate such a suggestion (Popov and Ushatinskaya 1992). Early Cambrian Linnarssonia possessed only a vestigial delthyrium on the posterior margin of the larval shell. De- velopment of this delthyrial opening into a foramen occurred after the settlement of the animal. Later genera, Homotreta and Hadrotreta, had a well-defined delthyrium that turned into a foramen soon after settlement. Several genera (Neotreta, Quadri- sonia, Angulotreta, Rhondellina) had larval shells with well-developed foramens (fig- ures 16.3I,J). Some Middle Cambrian paterinids (Paterina, Micromitra, Dictyonina) from the Siberian Platform had larval valves of about one-half to one-third of the en- tire shell size, although usually the size of larval valves was about one-fifth to one- tenth of the adult shell. This phenomenon probably also indicates prolongation of the larval stage. SETTLEMENT AND INTERACTION WITH THE SUBSTRATE Although overall brachiopod diversity was low during the Cambrian, most of the eco- logic types already existed. These included epifaunal anchored, cemented, and free- lying forms, as well as infaunal, quasi-infaunal, interstitial, and possibly pseudo- planktic brachiopods. 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 16-C1099 8/10/00 2:17 PM Page 353 354 Galina T. Ushatinskaya 16-C1099 8/10/00 2:17 PM Page 354 BRACHIOPODS 355 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 prob- ably anchored to small pebbles and shell fragments, which were abundant in the vi- cinity 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 open- ing throughout life and were typical anchored forms. Obolellids were confined to interreef and reef habitats. They were common elements of reefal cryptic communi- ties 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 16.3 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 Plat- form), scale bar 1 mm. E, Eoobolus sp., PIN 4290/ 252, ventral valve interior, Early Cam- brian, 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, Si- berian 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 Ushatin- skaya, holotype PIN 4377/124, ventral valve, Middle Cambrian, Amgan Sladkie Koren’ya Formation (Batenevsky Ridge, Altay-Sayan Foldbelt), scale bar 1 mm. J, Quadrisonia sim- plex 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. 16-C1099 8/10/00 2:17 PM Page 355 356 Galina T. Ushatinskaya 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 aper- ture opening into the cavity, ϫ 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: Photo- graph courtesy of Andrey Zhuravlev. on its surface. Carbonate substrates are dissolved slightly where papillae rooted (fig- ure 16.5). A similar feature is common among Recent attached brachiopods (Brom- ley 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, mini- mizing occlusion by mud. Such a strategy was perhaps exploited by Early Cam- brian obolellids that inhabited calcimicrobial-archaeocyath reefs. Obolella shell pave- ments 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 16-C1099 8/10/00 2:17 PM Page 356 BRACHIOPODS 357 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-Fugiewicz 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 sub- mergence (Rushton 1974; Rozanov and jydka 1987). The brachiopods were charac- terized by small (Ͻ5 mm), thin shells. An opening for the pedicle, by which the an- imals 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 condi- tions (Bakhturov et al. 1988). The abundance of cyanobacterial fossils indicates a rela- tively 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 assem- blages 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 16-C1099 8/10/00 2:17 PM Page 357 358 Galina T. Ushatinskaya 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: Re- printed with permission from Rigby 1986: plate 20, figure 1. Wales (McKerrow 1978). Relatively low metabolic rates in brachiopods were also sig- nificant 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 Mor- ris (1986) suggested brachiopod-sponge commensalism. Free-Lying Forms Some paterinids, Micromitra and Paterina, inhabiting both siliciclastic and siliciclastic- carbonate Early Cambrian substrates, did not have a separate pedicle opening. They 16-C1099 8/10/00 2:17 PM Page 358 BRACHIOPODS 359 possessed slightly convexo-convex shells with gaps between the valves in the area of the pseudodelthyrium. The possibility cannot be excluded that these brachiopods were attached only when young, as in later strophomenids. Kutorginids may also have been free-lying forms restricted to carbonate substrates. They had large shells (up to 2–3 cm) bearing concentric wrinkling and thickened posterior margins that allowed them to maintain a stable position on the sea floor. Infaunal Brachiopods For long it was accepted that the majority of Cambrian lingulids were infaunal forms inhabiting shallow nearshore conditions similar to their Recent representatives (e.g., Pel’man 1982). Indeed, both Recent genera, Lingula and Glottidia, are burrowing ani- mals adapted to unstable intertidal environments. They have a set of features that pro- vide good adaptation to life on shifting sands-silts in shallow conditions. First, they anchor by a pedicle whose distal end produces a sticky substance that binds loose substrate (Thayer and Steele-Petrovic 1975). Second, they have developed a more effective mechanism for protecting and cleaning the mantle cavity and lophophore from foreign particles than have other brachiopods (Chuang 1961). Nonetheless, Rudwick (1965), Gorjansky (1969), and Krause and Rowell (1975) doubted whether all Early Paleozoic lingulids were infaunal. Analysis of umbonal morphology in middle Late Cambrian lingulids from the Leningrad region (Baltica) revealed that they were typical epifaunal forms inhabiting the entire shelf (Popov et al. 1989). Most of them had pseudoareas projecting far from the rear margin of the valves. Such projections may have prevented movement of valves from causing dipping into the substrate. In addition, complete closing of the pedicle groove was sometimes observed in adoles- cent and gerontic forms. The very small size and thin shells of many Early Cambrian lingulids suggest that they were unable to burrow. Recently, however, Jin et al. (1993) described Botoman lingulids in the Chengjiang fauna from fine siltstone in southern China. The posterior parts of shells and very long pedicles are preserved in these brachiopods (Burzin et al., this volume: figure 10.1:9). In Recent brachiopods, such a pedicle serves for attachment in deep burrowings. Nonetheless, L. Ye. Popov (pers. comm., 1996) believes that these lingulids were epi- benthic, with a shell supported by a long pedicle, for these reasons: (1) the shell and the main part of the pedicle are invariably preserved on a bedding surface, and only the distal end of the pedicle is embedded in the sediment; (2) there are no bioturba- tion features in the rock; and (3) soft-bodied preservation suggests anaerobic condi- tions within the sediment and possibly in the lower part of the water column. On the contrary, Erdtmann et al. (1990) developed a scenario in which infaunal elements of the Chengjiang fauna migrated to the sediment surface during short temporal anoxic events. Some Cambrian lingulate burrows from the Botoman Bradore Formation (Labrador) were evidently from infaunal forms (Pemberton and Kobluk 1978). 16-C1099 8/10/00 2:17 PM Page 359 [...]... in the early stages of the evolution of the order These features allowed them to occupy wide belts of mobile sandy substrate of Baltica, from which the first abundant occurrences of lingulids are known (Middle Cambrian Sablinka Formation) Such occurrences are dominated by one or two species (Popov et al 1989) Popov and Khazanovich (1988) ascribed the growth of lingulate biomass at the end of the Cambrian. .. seawater is the principal source of Ca2ϩ in a brachiopod shell Ca2ϩ ions enter the lophophore from the surroundings by diffusion Phosphorus is transported with the food, which is the major source of PO43Ϫ ions for shell formation The appearance of mineralized skeletons at the advent of the Cambrian was due to a unique combination of environmental conditions and the stage of development of the organic.. .1 6- C1099 8/10/00 2:17 PM Page 360 360 Galina T Ushatinskaya Quasi-Infaunal Adaptations The abundance of soft muddy substrates during the Middle and Late Cambrian possibly fostered the appearance of the quasi-infaunal lifestyle among brachiopods (Henderson 1974; Percival 1978) Brachiopods that lived in such conditions developed highly conical ventral valves turned to the substrate As a result, the. .. the substrate surface during the entire life of the brachiopod, resembling an iceberg (see figure 16. 3H) Thin spines on the acrothelid larval shell served as another soft substrate adaptation Once settled, the larva used spines to keep its anterior and lateral margins slightly above the surface The presence of spines on both valves allowed freshly settled animals to survive even being toppled onto their... N Repina, A Yu Rozanov, Yu Ya Shabanov, A Yu Zhuravlev, S S Sukhov, and V M Sundukov 1991 Cambrian System on the Siberian Platform International Union of Geological Sciences, Publication 27 Atkins, D 1960 The ciliary feeding mechanism of the Megathyridae (Brachiopoda) and the growth stages of the lophophore Journal of the Marine Biological Association of the United Kingdom 39 : 459– 479 Bakhturov, S... [Microorganisms in the LowerMiddle Cambrian Kuonamka Formation rocks on the Siberian Platform (Molodo River)] Litologiya i poleznye iskopaemye 1994 (5) : 123–127 Zhuravlev, A Yu and R A Wood 1996 Anoxia as the case of the mid-Early Cambrian (Botomian) extinction event Geology 24 : 311–314 Zhuravlev, A Yu., B Hamdi, and P D Kruse 1996 IGCP 366: Ecological aspects of the Cambrian radiation field meeting... Ultrastruc- 1 6- C1099 8/10/00 2:17 PM Page 364 364 Galina T Ushatinskaya ture of the protegulum of some acrotretide brachiopods Palaeontology 13 : 491–502 Brasier, M D 1991 Nutrient flux and the evolutionary explosion across the Precambrian -Cambrian boundary interval Historical Biology 5 : 85–93 Brett, C E., W D Liddell, and K L Derstler 1983 Late Cambrian hard substrate communities from Montana / Wyoming: The. .. Description of Lingula parva Smith (Brachiopoda) from the coast of tropical West Africa Atlantida Reports 6 : 161 168 Conway Morris, S., ed 1982 Atlas of the Burgess Shale London: Palaeontological Association Conway Morris, S 1986 The community structure of the Middle Cambrian Phyllopod Bed (Burgess Shale) Palaeontology 29 : 423– 467 Cook, P J 1992 Phosphogenesis around the Proterozoic-Phanerozoic... end of the Middle Cambrian, provided new opportunities for brachiopod ecologic radiation The earliest forms cemented to rigid substrates are observed among orthids in the late Middle Cambrian Mila Formation of northern Iran (Zhuravlev et al 1996) and in the Late Cambrian Snowy Range Formation of Montana and Wyoming (Brett et al 1983) Possible Pseudoplanktic Brachiopods The question of possible pseudoplanktic... Lethaia 16 : 281–289 Bromley, R G and F Surlyk 1973 Borings produced by brachiopod pedicles, fossil and Recent Lethaia 6 : 349–365 Brunton, C H C 1982 The functional morphology and paleoecology of the Dinantian brachiopod Levitusia Lethaia 15 : 149 167 Chuang, S H 1959 The structure and function of the alimentary canal in Lingula unguis (L.) (Brachiopoda) Proceedings of the Zoological Society of London . Lingulida, and Acrotretida—ap- peared during the Early Cambrian, the Siphonotretida appeared in the Middle Cam- brian, and the Discinida arose at the end of the Cambrian. 1 6- C1099 8/10/00 2:17 PM Page. Ushatinskaya Quasi-Infaunal Adaptations The abundance of soft muddy substrates during the Middle and Late Cambrian pos- sibly fostered the appearance of the quasi-infaunal lifestyle among brachiopods (Hen- derson. only the distal end of the pedicle is embedded in the sediment; (2) there are no bioturba- tion features in the rock; and (3) soft-bodied preservation suggests anaerobic condi- tions within the